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Fire regimes and woody biomass dynamics in Australian savannas
Abstract
Aim
Many tropical savannas are undergoing a trend of increasing woody biomass, or ‘woody thickening’. Management to reduce fire frequency and intensity in savannas could substantially increase the amount of carbon stored in woody biomass. We addressed two questions: (1) are northern Australian savannas thickening; and (2) to what extent, and by what demographic processes, does fire affect woody biomass accumulation?
Location
Three large national parks, covering 24,000 km ² , in monsoonal northern Australia.
Methods
We examined changes in woody biomass carbon stocks – inferred from tree basal area and the density of woody understorey plants – over a 10‐year period in 136 savanna monitoring plots. We statistically assessed these changes in relation to fire frequency and severity. We used a meta‐analysis to identify general trends in woody cover in Australian savannas over the last half‐century.
Results
Woody biomass carbon stocks were relatively stable across the three national parks, but rates of change were statistically indistinguishable from earlier findings of a weak thickening trend. Change was negatively correlated with fire frequency, particularly the frequency of severe fires. High frequencies of severe fires decreased rates of accumulation of biomass by existing trees (through reductions in tree growth and death of individual stems), rather than whole‐tree mortality and suppression of recruitment. However, across northern Australia, our meta‐analysis identified a general, albeit weak, trend of woody thickening.
Main conclusions
The drivers of northern Australia's weak thickening trend are uncertain, but likely candidates include increasing atmospheric CO 2 concentration and water availability, and pastoral intensification. We demonstrate that changes to fire management have the potential to either increase or decrease rates of woody thickening relative to any underlying trend. Understanding how savanna fires affect woody biomass, and how fire effects are mediated by climate and CO 2 , are essential research priorities to predict the fate of savannas.
... It has been suggested that woody thickening may partially be a function of the steadily increasing CO 2 content of the atmosphere [24,25], a supposition consistent with remote sensing calculations of increasing biomass [25]. There is an even stronger suspicion that reductions in fire frequency might be responsible for much woody thickening [25,26], a suspicion reinforced by the effectiveness of planned burning regimes in preventing shrub and tree takeover of some temperate grassy ecosystems, such as those in the Surrey Hills, Tasmania [27]. ...
... It has been suggested that woody thickening may partially be a function of the steadily increasing CO 2 content of the atmosphere [24,25], a supposition consistent with remote sensing calculations of increasing biomass [25]. There is an even stronger suspicion that reductions in fire frequency might be responsible for much woody thickening [25,26], a suspicion reinforced by the effectiveness of planned burning regimes in preventing shrub and tree takeover of some temperate grassy ecosystems, such as those in the Surrey Hills, Tasmania [27]. ...
... It has been suggested that woody thickening may partially be a function of the steadily increasing CO 2 content of the atmosphere [24,25], a supposition consistent with remote sensing calculations of increasing biomass [25]. There is an even stronger suspicion that reductions in fire frequency might be responsible for much woody thickening [25,26], a suspicion reinforced by the effectiveness of planned burning regimes in preventing shrub and tree takeover of some temperate grassy ecosystems, such as those in the Surrey Hills, Tasmania [27]. ...
Woody thickening is a widespread phenomenon in the grassy woodlands of the world, often with deleterious effects on nature conservation values. We aimed to determine whether increasing the frequency of planned fire prevented woody thickening and improved conservation values in a Eucalyptus viminalis grassy woodland in the process of invasion by Allocasuarina verticillata (henceforth Allocasuarina) in Hobart, Tasmania, Australia. We used a before–after control intervention design. Ten plots from which detailed vegetation data were collected in 2018 (before the burns), 2019 (between burns), 2020 (between burns) and 2021 (after the burns) were randomly located in each of four blocks. Two of the blocks were burned in both 2018 and 2021. One block was burned only in 2021, and another was not burned at all. Mechanical thinning of Allocasuarina took place in 2021 in six plots in one unburned block and in three twice-burned plots. The fires were low intensity and patchy, reflecting the reality of planned burns in this environment. Thus, there were unburned plots mixed with burned plots in each of the three burned blocks. We compared changes in vegetation and cover attributes between a preburn survey in 2018 and a postburn survey in 2021, between five fire history/thinning classes (unburned, no thinning; unburned, thinning; twice burned; burned in 2018 only; burned in 2021 only). Fires in both 2018 and 2021 resulted in lower litter cover and higher exotic species richness than one fire in 2021. Exotic species richness increase between 2018 and 2021 was greater after fires in 2018 and 2021 than after a fire in 2021 alone. Exotic species richness was lowest six years after fire and highest one to three years after fire. The basal area of Allocasuarina was, counter-intuitively, less reduced by two fires in four years than by one. Mechanical thinning reduced shrub layer cover, which largely consisted of small trees, but did not affect basal area. Our data suggested that grass cover increased until five years after a fire, declining back to a low level by eight years. The implications of the results for conservation management are that the mechanical removal of young Allocasuarina may be successful in preventing its thickening and that burning at a five-year interval is likely to best maintain understorey conservation values. The counter-intuitive results related to Allocasuarina basal area emphasise the importance of understanding cumulative effects of fire regimes on fuel cycles and the consequent effects on tree mortality.
... decadal periods, with minor evidence of woody thickening, and that biomass is negatively correlated with fire frequency (Murphy et al., 2013). However, a key finding from this study was that the observed decreases in tree biomass following severe fires were not driven by mortality of individual trees, but rather through by decreases in the rates of biomass accumulation of Figure 5. Correlation between in fire intensity and difference in woody canopy cover (red) and aboveground carbon storage (green), relative to unburnt plots. ...
... There is increasing interest in understanding the effect of different fire regimes on carbon stored in Australian savannas (Murphy et al., 2013;Cook et al., 2015) and recent studies (Cook et al., 2016) have shown higher carbon stocks in dead organic matter under lower fire frequencies. At the Territory Wildlife Park fire plots the early biennial fire caused a reduction of 10 t C ha −1 compared to unburnt plots, whereas late biennial fires almost doubled that reduction to 19 t C ha −1 (Figure 5). ...
... These patterns are consistent with the trend of lower greenhouse gas emissions under early dry season fires, relative to late fires (Meyer et al., 2012) and point to the importance of available fuel load and its characteristics (greater herbaceous volume and lower moisture content late in the dry season) in understanding fire induced structural change in savannas. Murphy et al. (2013) suggested that the moderation of fire regimes in northern Australia is likely to increase carbon storage 5 in woody biomass, but the extent to which woody biomass can increase in these savannas is highly uncertain. Our results reduce some of this uncertainty, by providing quantification of the degree carbon stored in unburnt plots deviates from a range of different fire frequencies. ...
Fire regimes across the globe have been altered through changes in land-use, land management and climate conditions. Understanding how these modified fire regimes impact vegetation structure and dynamics is essential for informed biodiversity conservation and carbon management in savanna ecosystems. We used a long-term fire experiment at the Territory Wildlife Park (TWP), northern Australia, to investigate the consequences of altered fire regimes for habitat structure and aboveground carbon storage. We mapped vegetation three-dimensional (3D) structure in high spatial resolution with airborne LiDAR, across 18 replicated 1 ha plots of varying fire frequency and season treatments. We used LiDAR-derived canopy height and cover metrics to extrapolate field-based measures of woody biomass to the full extent of the experimental site (R² = 0.82, RMSE = 7.35 t C ha−1), and analysed differences in aboveground carbon storage and 3D structure among treatments. Woody canopy cover and biomass were highest in the absence of fire (76 % and 39.8 t C ha−1) and lowest in plots burnt late in the dry season on a biennial basis (42 % and 18.2 t C ha−1). Woody canopy vertical profiles differed among all six fire treatments, with greatest divergence in height classes
... Despite these observed fire impacts, recent assessments suggest that contemporary regional fire regimes have had limited effects on vegetation structural attributes at landscape scales. For example, a recent meta-analysis incorporating a number of local and regional studies suggests there has been a general, if weak, trend of woody thickening in regional savannas in recent decades [29], especially in higher rainfall locations [10]. ...
... Current evidence indicates that across the relatively unmodified savanna landscapes of northern Australia (where tree clearing has, to date, been limited), there has been a general, if weak, trend of woody thickening in recent decades [29]. Such an assessment, however, disguises considerable regional variability, for example: little evidence for woody biomass change under lower rainfall conditions [10,68]; marked woody biomass tree death and resultant woody biomass stochasticity under drought conditions especially in lower rainfall savannas [69]; and substantial variability in regional woody biomass dynamics across the 426,000 km 2 mesic (>1000 mm y −1 ) northern savannas associated with contemporary fire severity patterning [28]. ...
... A large body of ethnographic and ecologic evidence suggests that contemporary fire regimes over the Arnhem Plateau arose through the latter half of the twentieth century associated with Aboriginal depopulation and the replacement of former intensive fire management practices with a boom and bust wildfire cycle [5,25,26,39,40,57,[60][61][62]77]. Such observations stand in contrast to many other savanna regions, including in Australia, where key drivers influencing woody thickening and encroachment trends have involved reduced or no burning typically in combination with intense pastoral utilisation, enhanced rainfall and evapotranspiration conditions, and possible CO 2 fertilisation [11,29,75,[89][90][91]. The present study serves as a useful reminder that, in fire-prone savanna landscapes, severe fire regimes can substantially outweigh the woody growth-enhancing effects of other regional and global-scale drivers. ...
Although contemporary fire regimes in fire-prone Australian savannas are recognised as having major impacts on an array of biodiversity and environmental values, a number of studies have observed significant monsoon rainforest expansion in recent decades. Here we assess the status of a locally extensive endemic monsoon rainforest type, dominated by Allosyncarpia ternata (Myrtaceae), restricted to sandstone terrain including in the World Heritage property, Kakadu National Park. We undertook assessments of: (1) geographic correlates of Allosyncarpia forest distribution; (2) change in canopy cover at 40 representative forest patches at topographically exposed sites with reference to a 60-year aerial photo and fine-scale image archive, and fire mapping data; and (3) structural characteristics associated with sites exhibiting stable, contracting, and increasing canopy cover. Mean canopy cover at sampled forest patches declined by 9.5% over the study period. Most canopy loss occurred at the most fire-susceptible patches. Assessment of structural characteristics at sampled sites illustrated that canopy expansion represented vegetative recovery rather than expansion de novo. The study (1) confirms the vulnerability of exposed margins of this forest type to fire incursions; (2) illustrates the magnitude of, and describes solutions for addressing, the regional conservation management challenge; and (3) serves as a reminder that, in savanna environments, severe fire regimes can substantially outweigh the woody growth-enhancing effects of other regional (e.g., increased rainfall) and global-scale (e.g., atmospheric CO2 fertilisation) drivers.
... A global trend in 'woody thickening', or increasing tree cover in savannas and woodlands has been observed [1] and ascribed to a range of causes, from rising CO 2 levels [2][3][4], to changing rainfall and fire frequency [4][5][6][7][8][9][10][11][12][13] and the impact of large herbivores [14]. ...
... A range of field studies have examined the dynamics of tropical savanna vegetation in Australia. Murphy et al. [11] analysed tropical savanna vegetation change via three on-ground surveys at five-yearly intervals in three Northern Territory national parks. Direct measurements of live tree basal area were converted via an allometric to tree carbon mass. ...
A global trend of increasing tree cover in savannas has been observed and ascribed to a range of possible causes, including CO2 levels, changing rainfall and fire frequency. We tested these explanations in the Australian tropical savanna, taking 96 savanna ‘cool burning’ projects from Australia’s emissions offset scheme as case studies. We obtained readings of tree cover and explanatory variables from published remote sensing or spatial data sources. These were analysed using time-series linear regression to obtain coefficients for the influence of severe fire occurrence, annual rainfall and prior percentage tree cover. Although statistically significant coefficients for the key variables were found in only half (severe fire) or one quarter (rainfall) of the individual project models, when comparing all the model coefficients across the rainfall gradient, ecologically coherent explanations emerge. No residual trend was observed, suggesting rising CO2 levels have not influenced tree cover over the study period. Our approach models tree cover change by separating ecological drivers from human-controlled factors such as fire management. This is an essential design feature of national emissions inventories and emissions offsets programs, where crediting must be additional to the expected baseline, and arise from human activity.
... Rainforest on CYP (BVG 1) has expanded into adjoining woodlands and grasslands (Bowman and Fensham 1991;Russell-Smith et al. 2004a, 2004b. The structure of Eucalypt-dominated woodland (BVG3) and floodplain forest (BVG4) appears to have remained fairly stable across northern Australia (Crowley and Garnett 1998;Murphy et al. 2014;Prior et al. 2020). Teatree woodland on CYP (BVG 8) has markedly thickened under a fire regime that has been influenced by cattle grazing (BVG 8, Crowley et al. 2009), and encroached on adjoining grasslands (BVG13; Neldner et al. 1997;Crowley and Garnett 1998). ...
... Fire in the previous year reduced APG, with EDS fires having a greater impact than LDS fires in all vegetation types. A negative effect of fire on APG was to be expected, as fires in Australian tropical savannas may scorch the canopy, or kill branches and whole trees to the point of elimination of fire sensitive species (Prior et al. 2009;Bowman et al. 2014;Murphy et al. 2014). Nevertheless, it seems counterintuitive that EDS fires had the greatest impact on foliage cover. ...
Carbon accounting in tropical savannas relies on a good understanding of the effects of atmospheric carbon dioxide (CO2) and land management on foliage projective cover (FPC) and vegetation structure. We used generalised additive modelling to track changes in Autumn Persistent Green (APG, a satellite-image-derived measure of FPC) in six vegetation types on Cape York Peninsula, Australia, over an 18-year period, and examined the influence of fire and grazing land tenure. We then used field monitoring and variography (analysis of spatial autocorrelation) in a smaller study area to determine whether changes in APG reflected vegetation structural change. APG increased through the 18-year study period and was significantly influenced by vegetation type, recent fire history and grazing land tenure. Residual year-on-year increases suggest CO2 fertilisation was the main driver of APG increase. APG was reduced by fires in the previous year, with early dry season fires having greater impact than late dry season fires, particularly in grassland and rainforest. This is consistent with leaves being most fire sensitive early in the year, when they are actively growing, than in the late dry season, when they are dormant. As seedlings and suckers would be particularly fire-sensitive, early fires may therefore be more effective than late fires at preventing woody encroachment. We demonstrated that variography provides a good indication of whether APG increases are caused by increases in FPC alone, or by an increase in tree density. We found support for increased woody plant density in grasslands, and that this increase was most pronounced on grazing lands. Conversely, we found no support for stem density increases in the dominant eucalypt woodland, despite APG increases being highest in this vegetation type. Hence, increases in FPC cannot always be equated to increases in woody biomass, and may occur in their absence. This conclusion has serious implications for global carbon accounting.
... While resources, such as mean annual precipitation (MAP), can set limits on vegetation growth, disturbances such as fires can constrain savanna vegetation structure. Fires consume above-ground biomass (AGB), alter tree-grass coexistence and modify the size class distribution of trees (Donaldson et al., 2022;Higgins et al., 2007;Murphy et al., 2014), preventing vegetation from reaching its resource driven potential and creating a substantial discrepancy between observed and climatically defined potential vegetation biomass Bond & Keeley, 2005). However, vegetation structural responses to fire over large spatial extents are difficult to ascertain, due in part to a lack of spatially expansive fine-scale data on vegetation structure. ...
... Most insights into the effects of fire regimes on savanna vegetation stem from field-based vegetation measurements (e.g. Devine et al., 2015;Higgins et al., 2007;Murphy et al., 2014). However, field-based methods are limited in spatial extent and thereby do not adequately quantify context-dependent effects of fire. ...
Fire plays an integral role in shaping the vegetation structure of savanna ecosystems. However, the effects of fire regime characteristics, such as frequency and season of burn, on savanna vegetation structure, biomass and tree abundance across landscape types are largely unknown.
We used high‐resolution airborne light detection and ranging (LiDAR) to investigate the long‐term effects of fire manipulation on savanna vegetation in Kruger National Park, South Africa. We analysed the effects of fire exclusion and experimental burns every 1, 2, 3, 4 and 6 years and during different seasons on aboveground biomass (AGB), tree stem densities and vegetation vertical height profiles across a rainfall gradient and on contrasting geologies.
Across savanna types, and especially in drier savannas, fire season was more influential for constraining AGB than fire frequency. Plots experiencing fires during the late‐ and mid‐dry season had 44.50% and 43.60%, respectively, lower AGB relative to unburnt plots than wet‐season fires. However, in mesic savannas, fire frequency interacted with fire season to influence AGB: plots subjected to high frequency, dry‐season fires had 55.35% lower AGB than unburnt plots, whereas plots burnt in the wet season at lower frequencies had lower AGB (24.40% lower than unburnt plots) than plots subjected to high frequency, wet‐season fires (13.74% lower AGB than unburnt plots).
Fire regimes had variable effects on tree densities, and effects varied with the savanna type. Woody vertical vegetation profiles showed the largest differences in response to dry‐season fires, with the greatest divergence in vegetation height classes <5 m.
Synthesis and applications. Understanding the influence of fire regimes on vegetation structure has important implications for the management of savanna heterogeneity and for predicting trajectories of change in savanna vegetation as fire regimes vary with climate change. We show that the magnitude of the effect of fire on woody vegetation structure varies with savanna context. Our results suggest that heterogeneous vegetation structure can be achieved by applying fires in the dry season in mesic savannas, whereas in dry savannas, variation in fire regimes is less consequential for constraining biomass accumulation and altering vegetation structure.
... In the recent past, the vast savanna landscape has become prone to frequent and extensive wildfires, emitting, on average, >16Mt of accountable greenhouse gases (GHG), annually, contributing 2-4% of Australia's total GHG emissions (Cook and Meyer 2009;Murphy et al. 2014;Edwards et al. 2015;Fig. 2). ...
... Applying available data from various north Australian studies (e.g. Ansell et al. 2020;Evans and Russell-Smith 2020;Edwards et al. 2015;Murphy et al. 2014;Loughran et al. 2004), we assume that, since 2013, SB has contributed to GHG emissions abatement, increased soil carbon and tree biomass, and overall resulted in better management of lands across the savanna landscape due to less wildfires . ...
The savannas of tropical northern Australia, covering 1.9M km², are relatively unmodified and support a very sparse human population (0.5 person/km²). Largely marginalised and impoverished Indigenous communities are key stakeholders in the region with legal rights to >60% of the land. Colonisation in the late 19th century significantly impacted long-standing Indigenous land management practices, resulting, until recently, in fire regimes dominated by extensive wildfires emitting, on average, >16Mt of greenhouse gases (GHG) per annum. To manage these emissions, the Australian Government in 2013 enacted an incentivised scheme—the Savanna Burning Methodology (SBM) under the Carbon Farming Initiative Act (2011)—to reduce wildfires through strategically applied prescribed burning. This paper assesses the value of ecosystem services (ES) delivered by fine-scale fire management under the SBM that is now applied to 25% of the 1.2 M km² regulatory eligible savanna area, abating >7 Mt of GHG emissions per annum. While this scheme delivers and maintains a diverse range of ES supporting (i) the well-being of local Indigenous people, estimated at $189 million/yr (using a substitute value of government expenditure on Indigenous welfare), and (ii) many off-site ES for regional and global populations, the realised market value for GHG emissions abatement represents < 1% (i.e. USD 74.6 million since 2013) of the total value of ES. This assessment emphasizes the: (i) need to recognise the many benefits derived from SB; (ii) challenges associated with valuing ES for regional savanna stakeholders; (iii) further development of incentivised mechanisms for maintaining the flow of ES across sparsely settled northern Australian savannas. This assessment has broader implications globally where Indigenous and local communities aspire to sustainably manage their lands
... The importance of fire in modifying tree cover in savannas in Australia (e.g. Bowman & Panton 1995;Yates et al. 2008;Murphy et al. 2014;Edwards et al. 2018) and elsewhere (Sankaran et al. 2005;Lehmann et al. 2014) is universally recognised. In our study, fire in the year of image acquisition had a statistically important but small, negative effect on cover (Figs 5b,6a; Appendix S7). ...
... Our finding of long-term stability in tree cover is unlike results from many other studies in northern Australia, which had larger geographic coverage but more limited temporal replication than this study. These studies have shown both expansion of closedcanopy rainforest into savanna (Banfai & Bowman, 2005Tng et al. 2011;Ondei et al. 2017) and thickening of the savanna itself (Fensham & Fairfax 2003;Vigilante & Bowman 2004;Crowley et al. 2009;Lehmann et al. 2009;Murphy et al. 2014). Lehmann et al. (2009) found an overall 4.9% increase in tree cover in Kakadu's savannas between 1964 and 2004, but there was high spatio-temporal variability. ...
Previous analyses of historical aerial photography and satellite imagery have shown thickening of woody cover in Australian tropical savannas, despite increasing fire frequency. The thickening has been attributed to increasing precipitation and atmospheric CO2 enrichment. These analyses involved labour‐intensive, manual classification of vegetation, and hence were limited in the extent of the areas and the number of measurement times used. Object‐based, semi‐automated classification of historical sequences of aerial photography and satellite imagery has enabled the spatio‐temporal analysis of woody cover over entire landscapes, thus facilitating measurement, monitoring and attribution of drivers of change. Using this approach, we investigated woody cover change in 4000 ha of intact mesic savanna in the Ranger uranium lease and surrounding Kakadu National Park, using imagery acquired on 10 occasions between 1950 and 2016. Unlike previous studies, we detected no overall trend in woody cover through time. Some variation in cover was related to rainfall in the previous 12 months, and there were weak effects of fire in the year of image acquisition and the antecedent 4 years. Our local‐scale study showed a mesic eucalypt savanna in northern Australia has been resilient to short‐term variation in rainfall and fire activity; however, changes in canopy cover could have occurred in other settings. When applying this semi‐automated approach to similar studies of savanna dynamics, we recommend maximising the time depth and number of measurement years, standardising the time of year for image acquisition and using many plots of 1 ha in area, rather than fewer, larger plots.
... In assessing the drivers for change, they found that there was a synergistic impact involving mainly fire and the presence of feral animals. Conversely, recent research shows that thickening of woody cover in northern Australian tropical savanna may not be occurring and that fire frequency and intensity plays important role in preventing woody cover expansion by suppressing woody growth in savanna (Murphy et al. 2014). ...
... By grouping the 1 ha cells of proportional cover into classes, the distribution of cover was able to be readily observed, and although there was spatial variability in cover between dates, the proportional distribution of cover over the site was consistent. This aligns with recent research in the region (Murphy et al. 2014). The dynamism of woody cover shown is consistent with other findings (Lehmann et al., 2008a). ...
Savanna landscapes are characterised by a canopy of discontinuous tree cover overlying an understorey of shrubs and continuous grass cover. The distribution of trees (woody cover) is variable both spatially and temporally. Analysis of woody cover dynamics can provide a spatial and temporal envelope encompassing variability is useful for informing mine closure criteria. With the impending closure of Ranger uranium mine, ecologically appropriate closure criteria for ecosystem restoration are being developed through a framework of rehabilitation standards. One such closure criteria is canopy cover and historical woody cover is being used to derive the range in woody cover that can be expected over time once the mine site has been revegetated. This study reports on the development and testing of a technique for extracted woody cover from remotely sensed data (in the form of historical aerial photography and satellite imagery) in the areas adjacent to Ranger uranium mine in the World Heritage listed Kakadu National Park, northern Australia. An object-based image analysis technique was applied to four data sets from four different dates: greyscale, true colour and colour infrared aerial photo mosaics (from 1964, 1976 and 1981 respectively) and a high spatial resolution satellite image (from 2010). Overall accuracies of woody cover from each of the data sets exceeded 94%. In addition, proportional cover derived from this method displays linear relationships to cover derived from visual estimates. Due to the success of the technique, it will be applied to more data sets from different dates over the study area to assess the variability of woody cover over time to inform ecosystem restoration criteria for the mine closure.
... Researchers on every continent have called for more information on life histories and population dynamics of savanna trees to improve theory of savanna structure (physiognomy) and function (House et al. 2003, Sankaran 2005, Midgley et al. 2010. By substituting space for time, some field studies or reviews of the Australian humid savannas have concluded that disturbance (e.g., fire and/or storms and/or grazing) is indeed an important driver that explains savanna physiognomy (e.g., different degrees of woody cover and/or size distributions of larger trees; e.g., Petty et al. 2007, Prior et al. 2006, Lehmann et al. 2009, Murphy et al. 2014. A basic understanding of the ecological mechanisms, however, has been more elusive; for example, Lehmann et al. (2009) ignored juvenile stages entirely (Midgley et al. 2010). ...
... Regionally, outside of protected or aboriginal lands, eliminating late dry season fires entirely is likely impossible, mainly due to unauthorized and/or careless ignitions. With the very frequent and extensive fires occurring today near highways, some remote communities, and large cities such as Darwin (Russell-Smith et al. 2007, Elliott et al. 2009, Murphy et al. 2014, previously forested tracts of land will most likely become almost treeless, or indeed, grassy or shrubby savannas in the long run, due to loss of both firesensitive plants as well as the canopy eucalypt populations. In many locations in the Top End (including parts of KNP), the stage distributions of the eucalyptus canopy tree populations of today, compared to 30 yr ago, are consistent with a trajectory toward extirpation (P. A. Werner, personal observation). ...
Fire is a major disturbance driving the dynamics of the world's savannas. Almost all fires are set by humans who are increasingly altering fire timing and frequency on every continent. The world's largest protected areas of savannas are found in monsoonal northern Australia. These include relatively intact, tall, open forests where traditional indigenous fire regimes have been largely replaced in the past half century by contemporary patterns with trees experiencing fire as often as three out of five years. Eucalypt canopy trees form the basic structure of these savannas and changes to the canopy due to fire regimes cascade to affect other plants and animals. In this study, we used data from nearly three decades of field studies on the effects of fire on individual trees to define eight life‐history stages and to calculate transition rates among stages. We developed a stage‐based matrix population model that explicitly considers how fire season and understory influence growth, survival, and recruitment for each life‐history stage. Long‐term population growth rates and transient population dynamics were calculated under five different fire regimes, each in two understory types, using both deterministic and stochastic simulations of seasonal timing of fires. We found that fire was necessary for long‐term persistence of eucalypt canopy tree populations but, under annual fires, most populations did not survive. Population persistence was highly dependent on fire regime (fire season and frequency) and understory type. A stochastic model tended to yield higher population growth rates than the deterministic model with regular, periodic fires, even under the same long‐term frequency of fires. Transient population dynamics over 100 yr also depended on fire regime and understory, with implications for savanna physiognomy and management. Model predictions were tested in an independent data set from a 21‐yr longitudinal field study in Kakadu National Park. This study is a novel and integrative contribution to our understanding of fire in savanna biomes regarding the potential for long‐term persistence and transient dynamics of savanna canopy tree populations. The model is relatively simple, generalizable, and adaptable for further investigations of the population dynamics of savanna trees under fire.
... Large woody trees have particular traits and characteristics which give them high ecological value. Trees provide many vital functions such as carbon sequestering (Murphy et al. 2014 Smith & Reid 2013), soil enrichment from woody litter (Casals et al. 2014;Sauer et al. 2012;Scalon et al. 2014;Sitters et al. 2013;Vourlitis et al. 2014), providing shelter, food and habitat to a variety of animals (Gibbons & Lindenmayer 2002;Kerle et al. 1992;Lindenmayer et al. 2000;Lindenmayer et al. 2014;Rayner, et al. 2014) and reducing loss of soil moisture through shading of the ground layer with canopy and leaf litter on the ground (Holdo et al. 2014). There is also a substantial body of research addressing the importance of deep rooted species in lifting moisture from deep in the sub soil to higher points in the soil horizon where shallower rooted plants can access it (Bucci et al. 2004;Gotsch et al. 2010;Hao et al. 2008;O'Grady et el. ...
... Testing whether differences in ground cover, especially of introduced grasses, are affecting river red gums was beyond the scope of my project however I anticipated that end users interested in river red gums were also very likely to be interested in concurrently monitoring other aspects of the environment likely to impact on the trees. In tropical savannas of Australia, the link between rain, grass, fire and tree form is widely recognised (Andersen et al. 2012;Bond et al. 2012;Murphy et al. 2014;Setterfield et al. 2010 showed that there were significant differences in the distribution of these grasses within sites, and that sandy river channels did not support their growth to the same extent as other landscape types. It was apparent that the largest diameter trees in the Roe Creek system also occurred in the river channel which suggests a possible interaction between where large trees occur and the density of invasive grasses although this requires further testing to confirm. ...
In central Australia, river red gums (Eucalyptus camaldulensis sub sp. arida) occupy several thousand kilometres of ephemeral river channels which creates a network of habitat rich in moisture, nutrients and shelter in an environment otherwise dominated by woody shrubs. Very little is currently known about river red gums in central Australia but there is concern that they are being impacted by more frequent and intense fires. This thesis is concerned with developing a robust methodology for accurately assessing the condition of river red gum populations in central Australia so that land managers can acquire baseline information on them.
I firstly conducted detailed ground based assessments of river red gums within eight 500m sections of river in two river systems to gain an understanding of how the characteristics of river red gums vary at different spatial scales. This information was then used as a reference for testing whether assessments of river red gum characteristics from oblique aerial images could be accurately made. I then developed and tested a method for assessing broad sections of rivers using a combination of free high resolution satellite imagery and oblique photographs. The broad assessment was based on determining the abundance of particular tree types which would represent variability within populations, and that were likely to be sensitive to picking up the overall health of the population and broad habitat values. An assessment of a 25 km length of river was made by first identifying representative patches of trees along the river for assessment from satellite imagery, assessing all trees within these patches from oblique aerial photos and then collecting ground based data to determine accuracy.
River red gum density, size and form were highly variable among sites within and between river systems with no clear patterns emerging. However tree density and characteristics were significantly influenced by the location of the tree in the landscape. Trees were most dense on the edges of river channels, with trees on the floodplain tending to be more scattered. In one river system the largest trees tended to occur in the channel. The accuracy with which tree types could be determined from aerial oblique imagery was variable but improved substantially with practice. Ultimately, some key tree characteristics could be identified with reasonable levels of accuracy despite some issues with positional accuracy. In the broad assessment accuracy was high and shows oblique aerial imagery could be used to accurately assess the relative abundance of particular tree types which can indicate health and tree form. The use of oblique aerial imagery as a method for assessing river red gums requires further testing but offers the opportunity to conduct relatively rapid and efficient assessments and with a range of readily available tools.
... Nonetheless, despite lower intensities across the board compared to larger experiments like those obtained at the Kapalga experiment, our findings are in agreement with the diminished basal areas observed there under very-high-intensity late season fires ( Andersen et al., 2003). There is increasing interest in understanding the effect of different fire regimes on carbon stored in Australian savannas ( Murphy et al., 2013;Cook et al., 2015) and recent studies have shown higher carbon stocks in dead organic matter under lower fire frequencies (Cook et al., 2016). At the Territory Wildlife Park fire plots the early biennial fire caused a reduction of 10 t C ha −1 on average compared to unburnt plots, whereas late biennial fires almost doubled that average reduction to 19 t C ha −1 (Fig. 5). ...
... This is further emphasized by the variation in response to fire along the environmental gradient of the experimental site. Murphy et al. (2013) suggested that the moderation of fire regimes in northern Australia is likely to increase carbon storage in woody biomass, but the extent to which woody biomass can increase in these savannas is highly uncertain. Our results reduce some of this uncertainty, by providing quantification of the degree to which carbon stored in unburnt plots deviates from a range of different fire frequencies. ...
Fire regimes across the globe have been altered through changes in land use,
land management, and climate conditions. Understanding how these modified fire
regimes impact vegetation structure and dynamics is essential for informed
biodiversity conservation and carbon management in savanna ecosystems. We
used a fire experiment at the Territory Wildlife Park (TWP), northern
Australia, to investigate the consequences of altered fire regimes for
vertical habitat structure and above-ground carbon storage. We mapped
vegetation three-dimensional (3-D) structure in high spatial resolution with
airborne lidar across 18 replicated 1 ha plots of varying fire frequency
and season treatments. We used lidar-derived canopy height and cover metrics
to extrapolate field-based measures of woody biomass to the full extent of
the experimental site (R2=0.82, RMSE = 7.35 t C ha−1) and
analysed differences in above-ground carbon storage and canopy structure among
treatments. Woody canopy cover and biomass were highest in the absence of
fire (76 % and 39.8 t C ha−1) and lowest in plots burnt late in
the dry season on a biennial basis (42 % and 18.2 t C ha−1).
Woody canopy vertical profiles differed among all six fire treatments, with
the greatest divergence in height classes <5 m. The magnitude of fire effects
on vegetation structure varied along the environmental gradient underpinning
the experiment, with less reduction in biomass in plots with deeper soils.
Our results highlight the large extent to which fire management can shape
woody structural patterns in savanna landscapes, even over time frames as
short as a decade. The structural profile changes shown here, and the
quantification of carbon reduction under late dry season burning, have
important implications for habitat conservation, carbon sequestration, and
emission reduction initiatives in the region.
... Another issue is that most eddy covariance towers are installed in fire-free regions or report on fire-free periods. Therefore, models calibrated against these data will not be able to represent the observed impact of fire on ecosystem functional properties (Bond & Keeley, 2005;Pausas & Schwilk, 2012) and limitations on biomass accumulation (Murphy et al., 2014). There is a risk of overfitting data when transferring model parameters from fire-free conditions to regions of high fire frequency. ...
... The reduction of biomass ( Figure 5) for higher MABF agrees with aDGVM model simulations of fire suppression over African savannas (Scheiter & Higgins, 2009) and fire management for northern Australia (Scheiter et al., 2015) as well as worldwide observations from in situ fire exclusion experiments (Higgins et al., 2007;Tilman et al., 2000). Furthermore, the larger decrease in biomass from the NOFIRE to the FIRE simulations is in agreement with observations that indicated a negative correlation between fire frequency and biomass in Australian tropical savannas, for example (Murphy et al., 2014;Russell-Smith et al., 2003;Williams et al., 1999). There is also an increase in differences between prior biomass information and values retrieved in the FIRE experiment as MABF increases ( Figure S3). ...
Fire is a major component of the terrestrial carbon cycle that has been implemented in most current global terrestrial ecosystem models. Here we use terrestrial carbon cycle observations to characterize the importance of fire regime gradients in the spatial distribution of ecosystem functional properties such as carbon allocation, fluxes, and turnover times in the tropics. A Bayesian model-data fusion approach is applied to an ecosystem carbon model to derive the posterior distribution of corresponding parameters for the tropics from 2000 to 2015. We perform the model-data fusion procedure twice, that is, with and without imposing fire. Gradient of differences in model parameters and ecosystem properties in response to fire emerge between these experiments. For example, mean annual burned fraction correlates with an increase in carbon use efficiency and reductions in carbon turnover times. Further, our analyses reveal an increased allocation to more fire-resistant tissues in the most frequently burned regions. As fire modules are increasingly implemented in global terrestrial ecosystem models, we recommend that model development includes a representation of the impact of fire on ecosystem properties as they may lead to large differences under climate change projections.
... Woody encroachment, the increase in woody biomass, stem densities or woody cover, is a symptom of an alteration in ecosystem processes (Parr et al., 2012); it has been documented across continents and biomes (Myers-Smith et al., 2011;Murphy et al., 2014;O'Connor et al., 2014). However, as an open-canopied tropical biome, savannas appear particularly vulnerable to woody encroachment (Parr et al., 2014), where encroachment can have negative consequences for soil carbon storage (Berthrong et al., 2012), ground water recharge, tourism (Gray & Bond, 2013), grazing potential (Angassa & Baars, 2000) and biodiversity (Ratajczak et al., 2012). ...
... Australian savannas are characterized by low nutrient soils, highly connected landscapes with little topographic barriers that generate fire regimes of frequent, intense and large fires (Archibald et al., 2013). Frequent fire retards tree growth, biomass and establishment (Murphy et al., 2014) and could buffer encroachment in landscape that has densities of <1 person per km 2 (Australian Bureau of Statistics, 2012). While there are regional differences within Australia in both rainfall seasonality and fire regimes (Archibald et al., 2013), Australian savannas generally have extreme seasonality and longer dry seasons than either Africa or Australia (Lehmann et al., 2011;Staver et al., 2011). ...
Tropical savannas are a globally extensive biome prone to rapid vegetation change in response to changing environmental conditions. Via a meta-analysis, we quantified savanna woody vegetation change spanning the last century. We found a global trend of woody encroachment that was established prior the 1980s. However, there is critical regional variation in the magnitude of encroachment. Woody cover is increasing most rapidly in the remaining uncleared savannas of South America, most likely due to fire suppression and land fragmentation. In contrast, Australia has experienced low rates of encroachment. When accounting for land-use, African savannas have a mean rate annual woody cover increase two and a half times that of Australian savannas. In Africa, encroachment occurs across multiple land uses and is accelerating over time. In Africa and Australia, rising atmospheric CO2 , changing land management and rainfall are likely causes. We argue that the functional traits of each woody flora, specifically the N-fixing ability and architecture of woody plants, are critical to predicting encroachment over the next century and that African savannas are at high risk of widespread vegetation change. This article is protected by copyright. All rights reserved.
... Fire exclusion experiments demonstrate that fire exclusion often results in an increase in woody biomass (Higgins et al., 2007;Murphy et al., 2010;Singh et al., 2018) and canopy cover (Bird et al., 2000;Devine et al., 2015;Pinheiro et al., 2021;Singh et al., 2018;Smit et al., 2010). However, the rates are not similar, with the fastest woody invasion rates in the savannas of South America and the lowest rates in Australia (Murphy et al., 2014;Stevens et al., 2017). ...
Woody plant encroachment (WPE) is a global trend that occurs in many biomes, including savannas, and accelerates with fire suppression. Since WPE can result in increased storage of soil organic carbon (SOC), fire management, which may include fire suppression, can improve ecosystem carbon (C) sequestration in savannas.
At our study site in Kruger National Park, South Africa, we used a long‐term (~70 year) fire experiment to study the drivers and consequences of changes in woody cover (trees and shrubs) on SOC sequestration. We surveyed four fire manipulation treatments, replicated at eight locations within the park: annual high‐intensity burns, triennial high (dry season) and low‐intensity (wet season) burns, and fire exclusion, to capture the range of fire management scenarios under consideration. The changes in woody cover were calculated over a period similar to the experiment's duration (~80 years) using aerial photographs (1944–2018). Soils were analysed to 30 cm depth for SOC and δ¹³C, under and away from the tree canopy to isolate local‐ and landscape‐level effects of WPE on SOC.
The largest increases in woody cover occurred with fire exclusion. We found that plots with higher increases in woody cover also had higher SOC. However, trees were not the only contributor to SOC gains, sustained high inputs of C4‐derived C (grasses), even under canopies in fire suppression plots, contributed significantly to SOC. We observed little difference in SOC sequestration between cooler triennial (wet season) burns and fire suppression.
Synthesis. Grass input to soil organic carbon (SOC) remained high across the full range of woody cover created by varying burning regimes. The total SOC stocks stored from tree input only matched grass‐derived SOC stocks after almost 70 years of fire exclusion. Our results point to C4 grasses as a resilient contributor to SOC under altered fire regimes and further challenge the assumption that increasing tree cover, either through afforestation schemes or fire suppression, will result in large gains in C sequestration in savanna soils, even after 70 years.
... Dry grasslands and savannah ecosystems across the world have been negatively affected due to the rapid expansion of woody invasive species (White et al., 2000;Murphy et al., 2014;Stevens et al., 2016). An increase in woody cover in these landscapes has been attributed to a range of environmental and anthropogenic factors such as fire suppression, change in herbivore community, and an increase in atmospheric temperature and CO 2 . ...
... Although C4 grasses may have multiple species traits that promote species dominance (Edwards et al., 2010), anthropogenic influence has caused widespread changes in spatial processes including fire regimes and mega-herbivore grazing, which affect the relative dominance of grasses and trees in savanna vegetation. Deliberate lighting or suppression of fires, loss of herbivores due to hunting or disturbance by roads and fencing will influence woody encroachment by affecting tree recruitment (Murphy et al., 2014;Archer et al., 2017). ...
The Terai-Duar Savanna and Grassland ecoregion (hereafter Terai), located at the base of the Himalayas in the Indian subcontinent, support diverse ecosystems with pure grasslands, savanna, and forests in varying proportions. There are indications that grasslands and savanna are being lost to woody encroachment across the ecoregion. A significant fraction of plant and animal taxa are endemic to grasslands, so the decline of grasslands is likely to impact biodiversity and ecosystem function. We assessed large-scale vegetation changes to Terai habitats over three decades (1989-2019) and quantified the role of environmental factors in driving the observed changes. Selecting eight large protected wildlife habitats (four each from India and Nepal), we performed supervised vegetation classification for three-time points. We then quantified grassland-woodland transitions and used Bayesian Conditional Autoregressive spatial models to test the influence of climatic, environmental, and anthropogenic factors on observed changes. Including the protected sites with substantial grassland areas, we found that the total extent of grasslands was 1417 km 2 (28 % of total area) in 1988-1989, which declined by 34.4 % to 923 km 2 in three decades, while woodland area increased by 8,7% from 3235 km 2 to 3516 km 2. Grasslands were also converted to cropland or inundated by flooding, but to a lesser extent. Dry season grass fire had the strongest influence on grassland persistence, followed by anthropogenic impacts. However, the eight sites differed with respect to the nature and extent of the changes in vegetation transitions, perhaps reflecting site differences in climatic conditions and anthropogenic influence.
... Fire is a prevalent disturbance agent in savanna ecosystems, where it influences ecosystem composition and structure across a wide range of spatial scales and levels of ecological organization (Bond & Keeley, 2005;Hoffmann et al., 2002;Smit et al., 2010). The codominance of grass and woody vegetation, a defining feature of savannas, is often attributed to fire, which limits the recruitment of woody vegetation, enabling grasses to persist (Bond & Keeley, 2005;Govender et al., 2006;Levick et al., 2012;Murphy et al., 2014;Smit et al., 2010Smit et al., , 2016. Effects of fire on open ecosystem vegetation also interact with herbivores, which are attracted to recently burned areas (Allred et al., 2011;Burkepile et al., 2013Burkepile et al., , 2016Fuhlendorf et al., 2017), and abiotic variables such as soil and rainfall. ...
Fire regimes are expected to change with climate change, resulting in a crucial need to understand the specific ways in which variable fire regimes impact important contributors to ecosystem functioning, such as mound‐building termites. Termite mounds and fire are both important agents of savanna ecosystem heterogeneity and functioning, but there is little understanding of how they interact across savanna types.
We used very high‐resolution LiDAR remote sensing to measure the size and distribution of termite mounds across approximately 1300 ha of experimental burn plots in four South African savanna landscapes representing a wide range of fire treatments differing in seasonality and frequency of burning.
In nutrient‐poor granitic savannas, fire had no impact on termite mound size, densities and spatial distributions. In nutrient‐rich basaltic savannas with high mammalian herbivore abundance and intermediate rainfall, very frequent fires caused a decrease in termite mound size, whereas in arid nutrient‐rich basaltic savannas, fires that occurred at intermediate frequencies and in transitional seasons (i.e. late dry season and late wet season) decreased the degree of spatial overdispersal exhibited by mounds.
Overall, our results suggest that termite mounds are resistant to variation in fire seasonality and frequency, likely indicating that ecosystem services provided by mound‐building termites will be unaffected by changing fire regimes. However, consideration of changes to termite mound size and distribution could be necessary for land managers in specific savanna types, such as nutrient‐rich soils with high mammalian herbivore abundance.
... With recognition of Indigenous fire management practices in the early 2000s, and inclusion of Savanna Burning as an accountable activity for Annex 1 (Advanced Economy) countries under the Kyoto Protocol, a collaboration between scientists and Indigenous Elders was undertaken through the 2000s to develop a first-of-its-kind, market-based Savanna Burning (SB) GHG emissions accounting methodology (Russell-Smith et al. 2009;. The SB approach accounts for and incentivizes the undertaking of prescribed burning in the EDS period, under relatively mild fire-weather conditions, to reduce the risk of extensive LDS wildfires and resultant GHG emissions (Murphy et al. 2014). In 2011, SB was formalized under Australian Commonwealth Law through the Carbon Farming Initiative Act (2011), involving the establishment of an accredited accounting methodology for the calculation of GHG emissions reductions from registered projects (Commonwealth of Australia 2013). ...
An ecosystem is healthy if it is active, maintains its organization and autonomy over time, and is resilient to stress. Healthy ecosystems provide human well‐being via ecosystem services, which are produced in interaction with human, social and built capital. These services are affected by different ecosystem stewardship schemes. Therefore, society should be aiming for ecosystem health stewardship at all levels to maintain and improve ecosystem services. We review the relationship between ecosystem health and ecosystem services, based on a logic chain framework starting with (1) a development or conservation policy, (2) a management decision or origin of the driver of change, (3) the driver of change itself, (4) the change in ecosystem health and (5) the change in the provision of ecosystem services and (6) the change in their value to humans. We review two case studies to demonstrate the application of this framework. We analysed 6,131 records from the Ecosystem Services Valuation Database (ESVD) and found that in approximately 58% of the records data on ecosystem health was lacking. Finally, we describe how the United Nations’ System of Environmental‐Economic Accounting (SEEA) incorporates ecosystem health as part of efforts to account for natural capital appreciation or depreciation at the national level. We also provide recommendations for improving this system.
... In the latter regions, the common eucalypts are thin-barked shrub or mallee eucalypts that either resprout basally (multi-stemmed) or are killed by fire (Schubert et al. 2016). In regions with low productivity, the grass fuel load is sparse, resulting in lowintensity surface fires (Murphy et al. 2013(Murphy et al. , 2014Miller and Murphy 2017). Under these conditions, the cost to eucalypts of maintaining thick bark outweighs the advantage, especially given the post-fire resilience conferred by basal resprouting (Clarke et al. , 2015, accounting for their relatively thin bark and negative bark allometry (Lawes et al. 2011a;Schubert et al. 2016). ...
Context: Positive bark allometry (hyperallometry), characterised by rapid early bark growth, is expected where fire selects for thicker bark to resist cambial damage and topkill. Aim: We examine this prediction for 52 Australian eucalypt species from different habitats, fire regimes and environmental conditions. Methods: An effective bark allometric coefficient (α) was estimated from the first segment of breakpoint regression, which included fire-affected young trees. Key results: Eucalypts presented a negative–positive bark allometry continuum. Contrary to expectation, 73% of species (n = 38) displayed negative effective bark allometry. Early rapid bark growth was observed (ᾱ = 0.92 ± 0.04, x̄ ± SE, n = 168 sites), but was slower than isometry and bark was thinner overall than expected. Fire type (crown-fire propensity) and net primary productivity (resource availability) most influenced bark allometry. Productive ecosystems experienced crown-fire and bark was thicker at standardised diameter (x̄ ± 1SE = 2.04 ± 0.20 cm) than in less productive ecosystems under surface-fire (x̄ ± 1SE = 1.68 ± 0.18 cm). Bark morphology types did not differ in their stem diameter (x̄ ± SE = 21.47 ± 1.06 cm) or bark thickness (x̄ ± SE = 1.88 ± 0.08 cm) thresholds, representing putative stable alternative evolutionary solutions. Conclusions: Fundamentally, many eucalypts possess negative bark allometry with relatively thin bark, because unique bark bud traces sustain post-fire resilience through epicormic resprouting. Implications: The resource allocation trade-off to thin bark versus height growth, selected by fire and resource limitation, accounts for eucalypt persistence and domination of Australia’s flammable ecosystems.
https://www.publish.csiro.au/BT/justaccepted/BT21150
... Dry grasslands and savannah ecosystems across the world have been negatively affected due to the rapid expansion of woody invasive species (White et al., 2000;Ratajczak et al., 2012;Murphy et al., 2014;Stevens et al., 2016). An increase in woody cover in these landscapes has been attributed to a range of environmental and anthropogenic factors such as fire suppression, change in herbivore community, and an increase in atmospheric temperature and CO 2 . ...
Encroachment by woody invasive plants is a major threat to grasslands and savannah ecosystems worldwide. Rodents, being primary consumers, are likely to be the first to respond to changes in the structure and composition of native vegetation. We examined the effect of an invasive shrub Prosopis juliflora (hereafter Prosopis) on the native rodent community of an arid grassland system of Western India. Our sampling plots were divided into five categories representing different stages of Prosopis invasion and other land cover types. These consisted of restored native grassland, agriculture fallow, open brushland, sparse-Prosopis plots, and Prosopis-dominated plots. We also examined the impact of woody invasion on the response of native rodents toward moonlight and temperature. As hypothesized, we found a significantly higher abundance of rodent species in the native grassland habitat compared to sparse-Prosopis habitats. However, there was no significant difference in rodent abundance and diversity between the grassland and Prosopis-dominated habitats. Thus, species richness and abundance of rodents were the highest in the restored grasslands and dense Prosopis thickets, and the lowest in the sparse Prosopis, potentially showing a “U” shaped response to Prosopis invasion. We observed a species-specific effect of Prosopis on the activity of Tatera indica, Bandicota bengalensis, and Millardia meltada. Habitat type mediated the effect of different environmental factors (moonlight and temperature) on the activity of the most commonly ocurring species T. indica while activity of M. meltada showed a weak association with environmental factors. B. bengalensis was the most generalist species showing similar activity across all habitat types. Thus, the impact of Prosopis invasion on the rodent community was uneven, and depended on species as well as on local environmental characteristics.
... Although C4 grasses may have multiple species traits that promote species dominance (Edwards et al., 2010), anthropogenic influence has caused widespread changes in spatial processes including fire regimes and mega-herbivore grazing, which affect the relative dominance of grasses and trees in savanna vegetation. Deliberate lighting or suppression of fires, loss of herbivores due to hunting or disturbance by roads and fencing will influence woody encroachment by affecting tree recruitment (Murphy et al., 2014;Archer et al., 2017). ...
The importance of grasslands for the sustenance of global biodiversity is paramount. Grassland ecosystems support rich and unique diversity at all trophic levels, are remarkably productive, and resilient to environmental changes. Grasslands in the Indian subcontinent are among the most threatened due to habitat loss, sparking renewed interest in the ecology of the different grasslands found here. We studied land cover dynamics of woodland-grassland mixtures that are part of the Terai ecosystems located at the base of the Himalayan mountain ranges. The vegetation in this region is known to be extremely dynamic even within short time scales, but the patterns and processes associated with this dynamism are not well understood. We analyzed the landcover changes at eight protected wildlife conservation areas from the region (four from India and four from Nepal) that occurred over the last three decades. We used the random forest classifier and an ensemble-based classification technique to carry out supervised classification of the land cover, which was dominated by vegetation. Landsat data, verified with a set of ground measurements and Google earth imagery, were used to generate the landcover types. Using the time series of land cover data, we quantified the observed transitions over decadal timescales. We then used Linear Discriminant functions and Bayesian spatial models to determine the relative importance of environmental variables influencing land cover transitions. We found that the area occupied by grasslands have reduced across all the protected areas we studied. In the last 30 years, the overall natural grassland area decreased by 24 percent, while the agricultural area doubled. The woodland cover increased by 28 percent as a result of ecological succession. Distance from human settlements was found to be the most crucial factor affecting the transitions, followed by topography and distance to water bodies. The grasslands are being widely transformed or degraded to early successional woodland and farmlands, and show increased alien plant invasions. Human encroachment and an increase in human activities have a major influence on these transitions. The impact of these changes on biodiversity and ecosystem function needs to be studied and the urgent attention of managers to stop further degradation is needed. Powered by TCPDF (www.tcpdf.org)
... However, vegetation type/structure and moisture are more important in dry vegetation types, where fire is often limited by biomass fuel loads and connectivity (Bradstock, 2010;Cochrane, 2009;Daniau et al., 2012;Pausas and Paula, 2012). High fuel connectivity in dry woody vegetation is a function of biomass accumulation (increased vegetation growth), which can be caused by one or more of the following: (1) high moisture, (2) high CO 2 and (3) insufficient disturbance, such as infrequent fires (Murphy et al., 2014;Pausas and Paula, 2012). A large percentage of the SEAM's forest vegetation falls into the category of dry, contiguous, woody vegetation, making the region one of the most fire-prone places on Earth (Keith, 2017). ...
The rapid increase in severe wildfires in many parts of the world, especially in temperate systems, requires urgent attention to reduce fires’ catastrophic impacts on human lives, livelihoods, health and economy. Of particular concern is southeast Australia, which harbours one of the most flammable vegetation types on Earth. While previous studies suggest climate and European activities drove changes in southeast Australian fire regimes in the last 200 years, no study has quantitatively tested the relative roles of these drivers. Here, we use a Generalized Linear Modelling to identify the major driver(s) of fire regime change in the southeast Australian mainland during and prior to European colonization. We use multiple charcoal and pollen records across the region and quantitatively compare fire history to records of climate and vegetation change. Results show low levels of biomass burned before colonization, when landscapes where under Indigenous management, even under variable climates. Biomass burned increased markedly due to vegetation/land-use change after colonization and a major decline in regional precipitation about 100 years later. We conclude that Indigenous-maintained open vegetation minimized the amount of biomass burned prior to colonization, while European-suppression of Indigenous land management has amplified biomass accumulation and fuel connectivity in southeast Australian forests since colonization. While climate change remains a major challenge for fire mitigation, implementation of a management approach similar to the pre-colonial period is suggested to ameliorate the risk of future catastrophic fires in the region.
... Thus, under crown fire, which would damage the smooth bark, E. miniata is expected to be disadvantaged relative to E. tetrodonta, which has dense outer bark cover to the branch tips. Both surface and canopy fires occur at moderate frequency (fire return time = 18-36 months) in the seasonally wet-dry tropics of north Australia (Murphy et al. 2010;Murphy et al. 2014) ensuring that E. miniata and E. tetrodonta coexist at high densities. ...
Half-butt eucalypts (genera: Eucalyptus and Corymbia) have both thick outer bark at the stem base (half-butt) conferring resistance to surface fire, and thin photosynthetic canopy bark that reduces moisture stress. Here we examine how the functional ecology of dual outer bark types influences the wide distribution of Australian half-butt species. We evaluate the proposition that half-butts should predominate in semi-arid environments prone to surface fires. We measured the bark thickness, butt height relative to flame/fire char height and tree height, height of first branch, and the location and prevalence of epicormic resprouting of co-occurring Eucalyptus miniata (half-butt) and E. tetrodonta (fibrous bark only) individuals, across 15 sites with contrasting fire frequencies (2000-2015) in the Darwin region. Total tree height was compared with butt height for all E. miniata individuals. The survival of half-butt and other eucalypt species, as well as non-eucalypts, was investigated at three sites affected by intense gamba grass (Andropogon gayanus) fire. The proportion of half-butt species in each of Australia's 85 bioregions was calculated from geographic distribution records of 618 eucalypt species. Mean annual fire frequency (1997-2010), fire type (crown or surface fires) and climate in each bioregion was determined from satellite-derived records. Butt height at a site, including gamba grass sites, was not induced by flame height or affected by fire frequency and was approximately half the canopy height of the tree, suggesting it is internally regulated. The half-butt E. miniata and full-bark eucalypts were similarly resilient (survival) under surface fire conditions. Half-butt species predominated in arid and semi-arid bioregions characterised by surface fire, consistent with our proposition that half-butt bark is an adaptation to surface fire, and thin photosynthetic outer canopy bark reduces moisture stress, accounting for the wide distribution of half-butt eucalypts in arid and seasonally dry regions of Australia.
... Whether savanna burning projects will have longer-term, potentially perverse outcomes on woody component dynamics is difficult to predict-especially given the confounding effects of CO 2 fertilisation on enhancing woody plant growth (Bond and Midgley, 2012;Donohue et al., 2013), contrasted with predicted substantial increase in severe fire-weather conditions, especially sustained high temperatures, under global and regional warming scenarios (Pechony and Shindell, 2010;Moise et al., 2015). Recent studies report limited woody thickening in north Australian savannas (Murphy et al., 2014;Stevens et al., 2017), although woody invasion of grassland systems (especially by Melaleuca spp.) is occurring over substantial areas of Cape York Peninsula associated with heavy cattle grazing and limited seasonally intense burning (Crowley and Garnett, 1998;Russell-Smith et al., 2014). Earlier we noted that savanna burning projects are not eligible for nor compatible with management of productive grassland systems prone to woody thickening. ...
Savannas are the most fire-prone of Earth's biomes and currently account for most global burned area and associated carbon emissions. In Australia, over recent decades substantial development of savanna burning emissions accounting methods has been undertaken to incentivise more conservative savanna fire management and reduce the extent and severity of late dry season wildfires. Since inception of Australia's formal regulated savanna burning market in 2012, today 25% of the 1.2M km² fire-prone northern savanna region is managed under such arrangements. Although savanna burning projects generate significant emissions reductions and associated financial benefits especially for Indigenous landowners, various biodiversity conservation considerations, including fine-scale management requirements for conservation of fire-vulnerable taxa, remain contentious. For the entire savanna burning region, here we compare outcomes achieved at ‘with-project’ vs ‘non-project’ sites over the period 2000–19, with respect to explicit ecologically defined fire regime metrics, and assembled fire history and spatial mapping coverages. We find that there has been little significant fire regime change at non-project sites, whereas, at with-project sites under all land uses, from 2013 there has been significant reduction in late season wildfire, increase in prescribed early season mitigation burning and patchiness metrics, and seasonally variable changes in extent of unburnt (>2, >5 years) habitat. Despite these achievements, it is acknowledged that savanna burning projects do not provide a fire management panacea for a variety of key regional conservation, production, and cultural management issues. Rather, savanna burning projects can provide an effective operational funded framework to assist with delivering various landscape-scale management objectives. With these caveats in mind, significant potential exists for implementing incentivised fire management approaches in other fire-prone international savanna settings.
... Most of these studies are limited in either spatial coverage or temporal resolution. Recent research shows that there may be no thickening of savanna woody cover, with fire frequency and intensity preventing expansion through suppression of woody growth (Murphy et al. 2014, Prior et al. 2020. ...
Abstract The maturity of remote sensing and ecosystem restoration science provides new opportunities to monitor and assess ecosystem indicators at finer resolutions and at suitable scales. A key link in joining these fields is creating a framework to apply these rich datasets to ecosystem restoration projects. Savanna woodland ecosystems have discontinuous tree cover that is an important, but potentially highly variable, structural component of these ecosystems. Woody cover in savannas is also well suited to being measured with remote sensing techniques. We extracted woody cover from a 66‐yr time series of aerial imagery covering 1718 ha of mesic savanna ecosystem in northern Australia, adjacent to the Ranger uranium mine and encompassing portions of Kakadu National Park. This ecosystem is proposed as a reference ecosystem that may be used to comparatively assess and monitor restoration trajectories of the mine site in the coming decades. The spatiotemporal patterns of change in woody cover were assessed at spatial extents similar to the mine site. We were able to construct a robust distribution of canopy cover values and associated spatial heterogeneity that can be used to set closure criteria, inform restoration trajectories, and guide monitoring activities for the restored mine site.
... The Cerrado is one of the most susceptible ecosystems to future changes in climate and land use (Murphy et al. 2014) as it is a hotspot of biodiversity (Myers et al. 2000). Some scenarios of climate change indicate reductions of approximately 25% of the Cerrado area and a decrease in the distribution range of many tree species by more than 50% (Siqueira & Peterson 2003). ...
The increase in atmospheric CO2 concentration ([CO2]) due to climate change may alter the composition of communities and the functioning of ecosystems. Owing to greater nitrogen use efficiency, leguminous plants present more effective responses to increasing [CO2] than non‐legumes. The objective of this study was to evaluate the morphophysiological and growth responses of two species from the Cerrado, a native C3 herbaceous legume (Stylosanthes capitata Vogel.) and an invasive C4 grass (Melinis minutiflora P. Beauv) growing alone or in competition, in ambient (350 ppm) and high (1000 ppm) [CO2]. Competition with S. capitata reduced leaf development and dry matter production in M. minutiflora individuals, regardless of [CO2]. Stylosanthes capitata increased root system growth under high [CO2] and competition. This change allows greater soil exploitation, decreasing nutrient limitation to S. capitata development. The leguminous species S. capitata showed advantages in the competition for resources under high [CO2] concerning the invasive grass. Although invasive C4 species are initially better in the colonisation of the invaded areas, the C3 native species of Cerrado may adjust morphophysiological traits when growing at high [CO2] and develop strategies that enable it to more effectively compete for resources.
... with trends in woody encroachment, a process widespread in grassy ecosystems across the globe (Van Auken 2009; Naito and Cairns 2011;Stevens et al. 2017). Woody cover and biomass have increased in many savannas over recent decades, raising concerns for wildlife conservation and human livelihoods that depend on the maintenance of open, grassy ecosystems (Silva et al. 2001;Wigley et al. 2009b;Murphy et al. 2014;Mitchard and Flintrop 2013;O'Connor et al. 2014;Stevens et al. 2016Stevens et al. , 2017. The potential causes for this widespread trend are various, including fire suppression (Case and Staver 2017), declines in herbivore populations (Daskin et al. 2015), and increasing atmospheric carbon dioxide (Buitenwerf et al. 2012). ...
Droughts are expected to become more frequent and severe, due to climate change, with uncertain consequences for savanna vegetation. Drought-driven tree mortality has been observed in some savannas, but little is known about how tolerant savanna trees are to drought, or what determines differences among species in drought vulnerability. Here, we examine which characteristics best distinguished tree species that were most negatively affected by a recent drought in Kruger National Park, South Africa. Woody-encroaching, root-suckering species, which tended to have relatively low investments in non-structural carbohydrates, had the highest mortality rates. Of particular interest was Dichrostachys cinerea, a notable contributor to widespread woody encroachment across southern African savannas, which suffered high drought mortality. Using rainfall maps and vegetation survey data, we show that D. cinerea distributions in Kruger National Park are limited to areas with higher mean annual rainfall and lower past drought frequency, and stem densities are lower where droughts have occurred more frequently, suggesting that past droughts have had lasting impacts on the distribution and abundance of this species. Woody encroachers, such as D. cinerea, may face a trade off between traits promoting proliferation vs. drought tolerance, and more frequent and severe droughts could increasingly limit their spread.
... Increased air temperature and elevated throughfall reaching the ground due to the removal of forest canopy can stimulate the decomposition of detritus, leading to consequent increases in soil C emissions [7,37]. However, the role of forest as a C source after fire would transform into a C sink as pioneer tree species and understory shrub grow vigorously in the long term [38]. ...
Fire is the predominant natural disturbance that influences the community structure as well as ecosystem function in forests. This study was conducted to examine the soil properties, loss of aboveground biomass, and understory plant community in response to an anthropogenic fire in a coniferous (Pinus massoniana Lamb.) and broadleaf (Quercus acutissima Carruth.) mixed forest in a subtropical–temperate climatic transition zone in Central China. The results showed that soil pH, NO3--N concentration, and microbial biomass carbon (C) increased three months after the fire; however, there were no significant differences in soil organic C, total nitrogen (N), NH4+-N concentration, or microbial biomass N between the burned and unburned observed plots. The total aboveground biomass was 39.0% lower in the burned than unburned plots four weeks after fire. Direct biomass combustion (19.15 t ha−1, including understory shrubs and litters) was lower than dead wood biomass loss (23.69 t ha−1) caused by the fire. The declining trends of tree mortality with increasing diameter at breast height for both pine and oak trees suggest that small trees are more likely to die during and after fires due to the thinner bark of small trees and tree and branch fall. In addition, burning significantly stimulated the density of shrub (160.9%) and herb (88.0%), but it also affected the richness of shrub and herb compared with that in the unburned plots two months after the fire. The rapid recovery of understory plants after fires suggest that the diversity of understory species could benefit from low-severity fires. Our findings highlight that the decomposition of dead wood and understory community recovery should be considered for offsetting C emissions after fires for further research.
... Burrows et al. 2002;Cowley et al. 2014;Crowley 2015), we have no in-principle opposition to this suggestion since, in effect, it could support the development of complementary ecosystem service market opportunities especially in pastorally marginal situations. There is, however, evidence for only slight woody thickening across our focal savanna region (Murphy et al. 2014), with the notable exception of former grassland and open-woodland systems in Cape York (Crowley and Garnett 1998). (2019) worry that we 'double count' the costs of land degradation, where costs for deteriorating 'B' and degraded 'C' land conditions (following Tothill and Gillies 1992) were estimated (very conservatively) from losses in pasture production and subsequent cattle returns, i.e. ...
In a recent paper we set out a case for extending current and emerging ecosystem services enterprise opportunities to support sustainable land sector development in far northern Australia (Russell-Smith and Sangha 2018: The Rangeland Journal 40, 315–330. doi:10.1071/RJ18005). In that paper we illustrate very significant economic viability and environmental sustainability issues associated with the current dominant land use, the extensive rangeland beef cattle industry. Our beef enterprise economic assessments drew heavily on reports by Ian McLean, Phil Holmes and colleagues, as well as various other authoritative studies. In a detailed response, McLean and Holmes outline their concerns that, in various instances, we misrepresented their data and that our assessment ‘does not accurately portray the economic performance and contribution of the pastoral sector in northern Australia, nor justify the conclusion that fundamental land sector change is required’ (Comment by McLean and Holmes 2019: The Rangeland Journal, 41, 157–160. doi:10.1071/RJ18098). We acknowledge the singular contributions of those authors for our understanding of the enterprise characteristics and challenges faced by northern beef producers, but further, we: (a) for context, demonstrate the magnitude of the economic and sustainability challenges faced by the majority of northern beef producers as described in a range of pertinent studies including their own; (b) provide a detailed refutation of all eight of their listed concerns; and (c) conclude that available evidence does in fact strongly support the need for exploring diversified enterprise opportunities towards developing a sustainable and inclusive far northern land sector.
... This endeavor is also motivated by the critical need of large scale woody plant cover dataset of savanna ecosystems, which cover about 20% of the earth's terrestrial surface and are home to a large proportion of the world's human population, wildlife, and livestock (Scholes and Archer, 1997;Ramankutty and Foley, 1999). For instance, woody plant cover is an essential element in woody plant biomass estimation (Murphy et al., 2014;Poulter et al., 2014;Krofcheck et al., 2016), productivity prediction (Anadón et al., 2014;Stevens et al., 2017), and ecological gradients modeling (Bucini and Hanan, 2007;Yang et al., 2016). Moreover, the coexistence of woody plants and herbaceous vegetation in savanna ecosystems lends woody plant cover to fractional representation (Sankaran et al., 2005(Sankaran et al., , 2008Alofs and Fowler, 2013). ...
... The reasons for the recent thickening are debated but include land use change, a trend to increasing rainfall in northern Australia and/or a CO 2 fertilization effect (eg Murphy and Bowman, 2012;Murphy et al., 2014;Scheiter et al., 2015;Ondei et al., 2017). For the purposes of this study we simply note that all three isotope proxies yield a consistent direction of change across multiple, geographically separated sites, suggesting that the proxies themselves do faithfully encode an interpretable environmental signal that is, under favourable conditions, Fig. 8. ...
This study reports palynological and geochemical results for modern and ancient sediments from 19 lakes on a rainfall gradient (784e1880 mm), across a range of savannas in Northern Australia. All proxies varied significantly across the range of sites examined, providing a robust envelope of values that can reliably be employed to identify a savanna signature in the sedimentary record. While the results indicate it is possible to identify a savanna, we found only three statistically significant relationships between any proxy measured in surface sediments and the major climate driver of savanna vegetation composition (rainfall amount). This is because edaphic factors play a dominant role in determining vegetation composition and also potentially because of the impact of land use change. Measures of fire determined by charcoal counting were positively correlated with geochemical measures of pyrogenic carbon abundance, suggesting both record a similar signal. While measures of fire incidence were not correlated with rainfall, there was a significant positive correlation between charcoal abundance and number of fires early in the dry season, suggesting that charcoal abundance is controlled more by the number/timing of fires than climate. There was also a significant correlation between the d 13 C-value of pyrogenic carbon and tree:grass ratio derived from palynological indicators, indicating that the d 13 C-value of PyC is a reliable indicator of savanna 'woodiness'. Comparison of the carbon isotope composition of total organic carbon, PyC and n-alkanes between modern and Holocene sediments suggest that the savannas of the region have remained either similar in 'woodiness' or have thickened over the last millennia.
... The contributions of the "initial" and "relay" floristic components, or the persistent and invading species, in the community assembly is a combination of deterministic mechanisms including environmental filtering, biological interaction and population growth, as well as the dispersal limit as a stochastic driver (Leibold et al., 2004). In spite of numerous studies addressing relative contributions of these mechanisms in shaping beta diversity patterns of post-fire communities, existing results have been divergent and sometimes conflicting (Gotelli and Mccabe, 2002;Murphy et al., 2014;Shryock et al., 2015;Catano et al., 2017). ...
Post-fire succession is an ideal case for studying effects of disturbance on community assembly, and the key is to disentangle the contributions of assembly processes to the variation of community composition, namely beta diversity, and the contingent scales. The central Yunnan Plateau of Southwest China is characterized by monsoon related seasonal drought, and frequent forest fires. We sampled five fire sites burned in different years and a middle aged forest, measured species composition dissimilarity and its species turnover and nestedness components, within each fire site and across all sites. Results indicated species turnover as the primary component of beta diversity within all communities. There was no trend of change with year-since-fire (YSF) in beta diversity among early post-fire communities, but beta diversity in the middle aged community was significantly higher. Species turnover patterns across fire sites revealed a weak dispersal limit effect, which was stronger at lower than upper slope position for woody plants, and reverse for herbs. At the site scale, the species dissimilarity and turnover both enlarged with increasing slope position difference, especially in the middle-aged community, but the species nestedness had no consistent trend among sites, except a decreasing trend in the middle-aged forest. (Partial) Mantel tests indicated habitat filtering [primarily indicating total nitrogen (TN) and slope position] played a much stronger role than dispersal limit and YSF (indicating competition intensity) for the post-fire forest assembly at the landscape scale, for both woody and herbaceous layers. However, at the site scale, Mantel tests indicated a diminishing effect of soil nutrient filtering with increasing YSF, while effects of topography and spatial distance in the middle aged community was stronger. This divergence suggests the primary assembly mechanism gradually shift away from the soil constraint. While the seasonal drought and the mountain topography dominate the environmental legacy, our results imply that fires may reinforce a priority effect in the forests assembly in this region, by creating a habitat filtering (e.g., moisture and nitrogen limitation) effect on species composition in post-fire communities.
... This is consistent with previous findings from Kidman Springs where increases in woody cover were lowest with more frequent LDS treatments in both habitats (Cowley et al. 2014). Frequent, intense fire in the LDS can inhibit woody species growth and stem survival (Williams et al. 1999;Higgins et al. 2007;Murphy and Russell-Smith 2010;Murphy et al. 2014). This may influence understorey composition via influences of shading, development of seed barriers and resource competition (Bowman 2000;Cook 2003;Woinarski et al. 2004). ...
A long-term (1993-2016) fire experiment in the grazed semi-arid savanna of the Northern Territory was used to investigate the relative impacts of soil properties and fire history on vegetation composition and diversity in grassland and woodland habitats. Subtle variation in soil texture influenced vegetation composition and abundance independently of fire variables and was generally a more important control on floristic patterns. Total species richness, lifeform richness and the abundance and presence of many individual plant species declined with increasing clay content. Linear mixed effect models with combined habitat data, showed total richness and richness of annual and perennial forbs, annual grasses and legumes increased with more frequent fire. Perennial grass abundance and richness was not influenced by fire. Total and lifeform mean richness did not vary between two and four yearly or early and late burnt treatments. Richness and abundance was generally significantly higher on burnt blocks than unburnt blocks regardless of fire season or interval. These results suggest greater diversity after burning is a result of an increase in ephemeral species. However, the overall influence of fire on floristic patterns is relatively moderate and fire regimes may therefore be manipulated for other management imperatives, such as fauna conservation, carbon sequestration and pastoral productivity without substantial impacts on botanical values in semi-arid tropical savannas.
... While our analysis could be improved by the Rudge, 2015). This is at the upper end of reported tree growth for north Australian savannas (Beringer et al., 2007;Cook et al., 2005;Lehmann, Prior, & Bowman, 2009;Murphy, Lehmann, Russell-Smith, & Lawes, 2014) and is consistent with reported site net ecosystem productivity , site disturbance history and increasing site rainfall . A key finding of Rudge (2015) was that increasing biomass primarily occurred in the middle to high tree size classes and that there was little change in size class distribution (i.e. ...
Tree–grass savannas are a widespread biome and are highly valued for their ecosystem services. There is a need to understand the long-term dynamics and meteorological drivers of both tree and grass productivity separately in order to successfully manage savannas in the future. This study investigated the interannual variability (IAV) of tree and grass gross primary productivity (GPP) by combining a long-term (15 year) eddy covariance flux record and model estimates of tree and grass GPP inferred from satellite remote sensing. On a seasonal basis, the primary drivers of tree and grass GPP were solar radiation in the wet season and soil moisture in the dry season. On an interannual basis, soil water availability had a positive effect on tree GPP and a negative effect on grass GPP. No linear trend in the tree–grass GPP ratio was observed over the 15-year study period. However, the tree–grass GPP ratio was correlated with the modes of climate variability, namely the Southern Oscillation Index. This study has provided insight into the long-term contributions of trees and grasses to savanna productivity, along with their respective meteorological determinants of IAV.
... Moreover, fire is also a factor that controls the vegetation dynamics and it frequently affects forest-shrubby/herbaceous ecotonal areas (Bond and van Wilgen, 1996). In addition, fire affects the increasing or decreasing woody biomass within a landscape (Murphy et al., 2014). Fire regimes are influenced by both climatic factors controlling the fuel production and human activity (Bond and van Wilgen, 1996;Grau and Veblen, 2000). ...
Fire has shaped the environment and has been important for human cultural development. In this paper, we propose to study past fire events using ecological modelling. For instance, the ecology of fire can help us to understand and interpret archaeological problems related to past settlement patterns or environmental scenarios. Variation in fire regimens are related to both, vegetation and precipitation fluctuations. Recently, we have model past ecosystem dynamics using remote sensing in the Ambato Valley (Catamarca NW Argentina) ranging from 442 to 1998 CE. Our aim here is to use remotely sensed vegetation data to enhance our understanding of environmental disturbance in the period 2000–2011. We characterised the spatial-time dynamics of the annual NDVI as an indicator of vegetation functioning. Then we related the NDVI dynamics to precipitation and fire events in an arid highland environment in the area. Further, we analysed the vegetation data (monthly NDVI, MODIS/TERRA satellite, 1km² pixels), and the climate data: annual precipitation. Then we calculated the NDVI annual average of every pixel and the NDVI anomalies of every year over the studied period. Lastly, we related NDVI data with annual precipitation and compared the NDVIs prior to and after known fire events in this period. On a spatial scale, the results show that the NDVI values were (a) low in shrublands and in cultivated areas, (b) medium in grasslands and piedmont forest with anthropic impact, and (c) high in highland forests. Within the studied time-period, extreme positive and negative anomalies were detected. The precipitation inter-annual variations were greater than the NDVI inter-annual variations, thus demonstrating that in some areas of the valley the horizontal precipitation can make important contributions to the ecosystem humidity. Extreme negative anomalies were observed the year of fire and fire scars at least for the next two years. These results demonstrate the relation between structure and function of vegetation, precipitation and fire. Understanding these relations can enable us to explain results when hindcasting (“predicting” what happened during past episodes of climate change) palaeoenvironmental conditions and fire events, thus helping us to interpret different archaeological contexts related to fire events.
... Vegetation should have been replaced by closed environment and mosaics that had been maintained by herbivore pressure should have given way to uniform or zonal vegetation patterns. Actually, tropical savanna is dominated by a combination of C 4 grasslands and woody species (Ratnam et al. 2011), whose dispersals can differ over time and space (Sankaran et al. 2005;Murphy et al. 2014). It has been speculated that the processes controlling the occurrence of these trees are similar throughout the major savanna in Africa, Australia, and South America (Knapp et al. 2004;Bond 2008). ...
The causes of Late Pleistocene megafauna extinctions remain controversial, with major phases coinciding with both human arrival and major climate change. This event revealed slightly different extinction patterns between areas of South America. In this chapter we evaluate a database of paleontological and archaeological sites with horses. The archaeological data suggest a low archaeological signal ca 14.5 ka BP and increase to a moderate and continuous signal during early Holocene. The data from Pampas and Patagonia region are sufficiently robust to assess synergy interaction among last appearance records of horses, first appearance records of humans, and the Younger Dryas to Holocene climatic transition in South America.
... As such, the IPCC assumption that CO 2 emissions from the previous year's burning are recovered by the following year's wet season growth may have some validity for regional-scale GHG accounting. This assumption at plot-to-catchment scales may not be valid, as localised interannual variability in rainfall, site history and fire management can result in either net accumulation or loss of carbon Murphy et al., 2014Murphy et al., , 2015b. Assuming year-to-year CO 2 emitted from burning is resequestered, assessment of the non-CO 2 only emissions from savanna burning with deforestation is useful. ...
The clearing and burning of tropical savanna leads to globally significant emissions of greenhouse gases (GHGs); however there is large uncertainty relating to the magnitude of this flux. Australia's tropical savannas occupy the northern quarter of the continent, a region of increasing interest for further exploitation of land and water resources. Land use decisions across this vast biome have the potential to influence the national greenhouse gas budget. To better quantify emissions from savanna deforestation and investigate the impact of deforestation on national GHG emissions, we undertook a paired site measurement campaign where emissions were quantified from two tropical savanna woodland sites; one that was deforested and prepared for agricultural land use and a second analogue site that remained uncleared for the duration of a 22-month campaign. At both sites, net ecosystem exchange of CO2 was measured using the eddy covariance method. Observations at the deforested site were continuous before, during and after the clearing event, providing high-resolution data that tracked CO2 emissions through nine phases of land use change. At the deforested site, post-clearing debris was allowed to cure for 6 months and was subsequently burnt, followed by extensive soil preparation for cropping.
During the debris burning, fluxes of CO2 as measured by the eddy covariance tower were excluded. For this phase, emissions were estimated by quantifying on-site biomass prior to deforestation and applying savanna-specific emission factors to estimate a fire-derived GHG emission that included both CO2 and non-CO2 gases. The total fuel mass that was consumed during the debris burning was 40.9 Mg C ha⁻¹ and included above- and below-ground woody biomass, course woody debris, twigs, leaf litter and C4 grass fuels. Emissions from the burning were added to the net CO2 fluxes as measured by the eddy covariance tower for other post-deforestation phases to provide a total GHG emission from this land use change.
The total emission from this savanna woodland was 148.3 Mg CO2-e ha⁻¹ with the debris burning responsible for 121.9 Mg CO2-e ha⁻¹ or 82 % of the total emission. The remaining emission was attributed to CO2 efflux from soil disturbance during site preparation for agriculture (10 % of the total emission) and decay of debris during the curing period prior to burning (8 %). Over the same period, fluxes at the uncleared savanna woodland site were measured using a second flux tower and over the 22-month observation period, cumulative net ecosystem exchange (NEE) was a net carbon sink of −2.1 Mg C ha⁻¹, or −7.7 Mg CO2-e ha⁻¹.
Estimated emissions for this savanna type were then extrapolated to a regional-scale to (1) provide estimates of the magnitude of GHG emissions from any future deforestation and (2) compare them with GHG emissions from prescribed savanna burning that occurs across the northern Australian savanna every year. Emissions from current rate of annual savanna deforestation across northern Australia was double that of reported (non-CO2 only) savanna burning. However, if the total GHG emission, CO2 plus non-CO2 emissions, is accounted for, burning emissions are an order of magnitude larger than that arising from savanna deforestation. We examined a scenario of expanded land use that required additional deforestation of savanna woodlands over and above current rates. This analysis suggested that significant expansion of deforestation area across the northern savanna woodlands could add an additional 3 % to Australia's national GHG account for the duration of the land use change. This bottom-up study provides data that can reduce uncertainty associated with land use change for this extensive tropical ecosystem and provide an assessment of the relative magnitude of GHG emissions from savanna burning and deforestation. Such knowledge can contribute to informing land use decision making processes associated with land and water resource development.
... Rather, savannahs are projected to remain stable in 2070 under the given climate change and emission scenario. Limited evidence exists for a coherent trend of forest expansion into Australian savannahs [54]. Here parametrizing Australian trees as more sensitive to shading than African savannah trees and reducing the shading effect of trees on grasses in Australia is partially responsible for the future resilience of Australian savannahs. ...
The extent of the savannah biome is expected to be profoundly altered by climatic change and increasing atmospheric CO 2 concentrations. Contrasting projections are given when using different modelling approaches to estimate future distributions. Furthermore, biogeographic variation within savannahs in plant function and structure is expected to lead to divergent responses to global change. Hence the use of a single model with a single savannah tree type will likely lead to biased projections. Here we compare and contrast projections of South American, African and Australian savannah distributions from the physiologically based Thornley transport resistance statistical distribution model (TTR-SDM)—and three versions of a dynamic vegetation model (DVM) designed and parametrized separately for specific continents. We show that attempting to extrapolate any continent-specific model globally biases projections. By 2070, all DVMs generally project a decrease in the extent of savannahs at their boundary with forests, whereas the TTR-SDM projects a decrease in savannahs at their boundary with aridlands and grasslands. This difference is driven by forest and woodland expansion in response to rising atmospheric CO 2 concentrations in DVMs, unaccounted for by the TTR-SDM. We suggest that the most suitable models of the savannah biome for future development are individual-based dynamic vegetation models designed for specific biogeographic regions.
This article is part of the themed issue ‘Tropical grassy biomes: linking ecology, human use and conservation’.
... The CO 2 effect is accompanied by decreased fire, primarily due to increased dry-season fuel moisture (figure S4), further enabling the expansion of tree cover (woody thickening) in savanna ecosystems and increasing vegetation water use in the north. This is in agreement with historical observations, which have suggested woody thickening in northern Australian savannas is primarily controlled by climate and CO 2 , with fire imposing a secondary effect (Murphy et al 2014). By contrast, in southeastern Australia CO 2induced increase in vegetation water use efficiency, and modest decreases in green vegetation cover, alleviate the runoff declines predicted (by the Budyko approach) from direct climate effects. ...
Future environmental change is expected to modify the global hydrological cycle, with consequences for the regional distribution of freshwater supplies. Regional precipitation projections, however, differ largely between models, making future water resource projections highly uncertain. Using two representative concentration pathways and nine climate models, we estimate 21st century water resources across Australia, employing both a process-based dynamic vegetation model and a simple hydrological framework commonly used in water resource studies to separate the effects of climate and vegetation on water resources. We show surprisingly robust, pathway-independent regional patterns of change in water resources despite large uncertainties in precipitation projections. Increasing plant water use efficiency (due to the changing atmospheric CO2) and reduced green vegetation cover (due to the changing climate) relieve pressure on water resources for the highly populated, humid coastal regions of eastern Australia. By contrast, in semi-arid regions across Australia, runoff declines are amplified by CO2-induced greening, which leads to increased vegetation water use. These findings highlight the importance of including vegetation dynamics in future water resource projections.
... forest-encroached savannahs;[7,[54][55][56]). There is also growing evidence that woody encroachment is accelerating globally[57,58](but see[59]) as elevated atmospheric carbon dioxide concentrations shift the balance between C 4 grasses and woody species with C 3 photosynthesis[60]. A clear contrast exists between the role of fire in savannahs and its role in most tropical forests: savannah biodiversity depends on frequent fires, whereas forests are typically degraded by fire[3,9]. ...
Ancient tropical grassy biomes are often misrecognized as severely degraded forests. I trace this confusion to several factors, with roots in the nineteenth century, including misinterpretations of the nature of fire in savannahs, attempts to reconcile savannah ecology with Clementsian succession, use of physiognomic (structural) definitions of savannah and development of tropical degradation frameworks focused solely on forests. Towards clarity, I present two models that conceptualize the drivers of ecosystem degradation as operating in both savannahs and forests. These models highlight how human-induced environmental changes create ecosystems with superficially similar physiognomies but radically different conservation values. Given the limitation of physiognomy to differentiate savannahs from severely degraded forests, I present an alternative approach based on floristic composition. Data from eastern lowland Bolivia show that old-growth savannahs can be reliably distinguished by eight grass species and that species identity influences ecosystem flammability. I recommend that scientists incorporate savannahs in tropical degradation frameworks alongside forests, and that savannah be qualified as old-growth savannah in reference to ancient grassy biomes or derived savannah in reference to deforestation. These conceptual advances will require attention not only to tree cover, but also to savannah herbaceous plant species and their ecologies.
This article is part of the themed issue ‘Tropical grassy biomes: linking ecology, human use and conservation’.
... The relative importance of environmental controls in structuring these systems varies across these geographical regions, and relative to the environmental niche of each region [14]. For example, the high rainfall Australian savannahs dominated by tall, fast growing, narrow-canopied evergreen Eucalyptus species are less sensitive to fire than the wide-canopied deciduous Brachystegia and Julbernardia species that dominate a savannah region equivalent in area across southern Africa [57,58]. Thus, for a given set of environmental conditions, similar fire frequencies and intensities could produce different vegetation structures, and the difference in sensitivity to fire of these floras is highly likely underpinned by the functional traits of the plant species themselves [56]. ...
Tropical grassy biomes (TGBs) are changing rapidly the world over through a coalescence of high rates of land-use change, global change and altered disturbance regimes that maintain the ecosystem structure and function of these biomes. Our theme issue brings together the latest research examining the characterization, complex ecology, drivers of change, and human use and ecosystem services of TGBs. Recent advances in ecology and evolution have facilitated a new perspective on these biomes. However, there continues to be controversies over their classification and state dynamics that demonstrate critical data and knowledge gaps in our quantitative understanding of these geographically dispersed regions. We highlight an urgent need to improve ecological understanding in order to effectively predict the sensitivity and resilience of TGBs under future scenarios of global change. With human reliance on TGBs increasing and their propensity for change, ecological and evolutionary understanding of these biomes is central to the dual goals of sustaining their ecological integrity and the diverse services these landscapes provide to millions of people.
This article is part of the themed issue ‘Tropical grassy biomes: linking ecology, human use and conservation’.
... Increases in woody cover have been documented across the globe [1][2][3] with the largest increases occurring in the open ecosystems (grasslands [4], tundra [3], savannahs [1]). Woody encroachment in savannahs is most frequently attributed to changes in land management, particularly the alteration of fire and herbivory [1,5,6]. ...
Woody encroachment in ‘open’ biomes like grasslands and savannahs is occurring globally. Both local and global drivers, including elevated CO 2 , have been implicated in these increases. The relative importance of different processes is unresolved as there are few multi-site, multi-land-use evaluations of woody plant encroachment. We measured 70 years of woody cover changes over a 1020 km ² area covering four land uses (commercial ranching, conservation with elephants, conservation without elephants and communal rangelands) across a rainfall gradient in South African savannahs. Different directions of woody cover change would be expected for each different land use, unless a global factor is causing the increases. Woody cover change was measured between 1940 and 2010 using the aerial photo record. Detection of woody cover from each aerial photograph was automated using eCognitions' Object-based image analysis (OBIA). Woody cover doubled in all land uses across the rainfall gradient, except in conservation areas with elephants in low-rainfall savannahs. Woody cover in 2010 in low-rainfall savannahs frequently exceeded the maximum woody cover threshold predicted for African savannahs. The results indicate that a global factor, of which elevated CO 2 is the likely candidate, may be driving encroachment. Elephants in low-rainfall savannahs prevent encroachment and localized megafaunal extinction is a probable additional cause of encroachment.
This article is part of the themed issue ‘Tropical grassy biomes: linking ecology, human use and conservation’.
Background Tropical savannas represent a large proportion of the area burnt each year globally, with growing evidence that management to curtail fire frequency and intensity in some of these regions can contribute to mitigation of climate change. Approximately 25% of Australia’s fire-prone tropical savanna region is currently managed for carbon projects, contributing significantly to Australia’s National Greenhouse Gas Inventory. Aims To improve the accuracy of Australia’s national carbon accounting model (FullCAM) for reporting of fire emissions and sequestration of carbon in savanna ecosystems. Methods Field data from Australian savannas were collated and used to calibrate FullCAM parameters for the prediction of living biomass, standing dead biomass and debris within seven broad vegetation types. Key results Revised parameter sets and improved predictions of carbon stocks and fluxes across Australia’s savanna ecosystems in response to wildfire and planned fire were obtained. Conclusions The FullCAM model was successfully calibrated to include fire impacts and post-fire recovery in savanna ecosystems. Implications This study has expanded the capability of FullCAM to simulate both reduced emissions and increased sequestration of carbon in response to management of fire in tropical savanna regions of Australia, with implications for carbon accounting at national and project scales.
Forests are vital for biodiversity, carbon storage and ecosystem services, but can be potentially threatened by fires. Given the significance of forests and fire in a changing climate, research into the long‐term effects of fire on forests plays an important role in understanding the global carbon cycle by the forests functioning as a large terrestrial carbon sink or source. In this study, we used aerial photography from 1975 and 2013 to count the change in the number of trees in 560 dry sclerophyll plots (40 × 40m) in the Blue Mountains of Australia. We analysed the relationship between the number of fires and severe fires in that period on the change in numbers of trees. We found that the average response was an increase of 1 tree per plot over 38 years. The number of fires had a small positive effect on tree numbers; plots with 2 or 3 severe fires had 1 and 2 extra trees, respectively, than those without fire. One exception was a severe fire in 2001 that did not show this positive effect, probably because it corresponded with extensive drought. Our findings suggest that number of forest canopy trees is resilient to the number of fires and number of severe fires.
Natural temperate grasslands are endangered throughout their range, largely because of their almost complete conversion to agriculture, and by changes in fire and grazing regimes. Woody encroachment by shrubs is a global threat to the structure, function and composition of grasslands. We wished to determine the characteristics of fire regimes that prevent or reverse woody plant encroachment, maintain native species richness and minimise exotic plant invasion in temperate grassland at the Surrey Hills Tasmania, where a fire management plan with variable prescriptions had been implemented for two decades. We collected floristic, fire regime and environmental data from 105 quadrats (1 × 10 m) in 2016/2017, and compared the data from 2016/2017 to that from a 1994 survey using the same methods. A high frequency and cover of native shrubs characterised areas unburned for at least 20 years before 2017, but not those unburned in the 20 years before 1994. Shrub cover began to strongly increase after a decade without fire and was greater on larger plains. Native species richness declined with an increasing minimum interval between fires and increased with elevation. It began to decline at the quadrat scale when shrub cover attained 40%. In the data set as a whole, 17 of the 67 most abundant taxa were absent from all quadrats unburned for 20 years before 2017, indicating a high potential for loss of species at a landscape scale in the absence of fire. Exotic species cover was randomly distributed in relation to fire regimes and environment. The current fire management regime has largely been at a sufficient frequency and minimum interval (approximately a decade between fires) to maintain the grassiness and native species richness of treated plains, which is fortunate given that recent land‐use change appears to have resulted in increased frequency of shrubs independent of their cover.
Questions
Fire and livestock grazing are regarded as current threats to biodiversity and landscape integrity in northern Australia, yet it remains unclear what biodiversity losses and habitat changes occurred in the 19–20th centuries as livestock and novel fire regimes were introduced by Europeans. What baseline is appropriate for assessing current and future environmental change?
Location
Australia's Kimberley region is internationally recognized for its unique biodiversity and cultural heritage. The region is home to some of the world's most extensive and ancient rock art galleries, created by Aboriginal peoples since their arrival on the continent 65,000 years ago. The Kimberley is considered one of Australia's most intact landscapes and its assumed natural vegetation has been mapped in detail.
Methods
Interpretations are based on a continuous sediment record obtained from a waterhole on the Mitchell River floodplain. Sediments were analysed for geochemical and palynological proxies of environmental change and dated using ²¹⁰ Pb and ¹⁴ C techniques.
Results
We show that the present‐day vegetation in and around the waterhole is very different to its pre‐European counterpart. Pre‐European riparian vegetation was dominated by Antidesma ghaesembilla and Banksia dentata , both of which declined rapidly at the beginning of the 20th century. Soon after, savanna density around the site declined and grasses became more prevalent. These vegetation shifts were accompanied by geochemical and biological evidence for increased grazing, local burning, erosion and eutrophication.
Conclusions
We suggest that the Kimberley region's vegetation, while maintaining a ‘natural’ appearance, has been altered dramatically during the last 100 years through grazing and fire regime changes. Landscape management should consider whether the current (impacted) vegetation is a desirable or realistic baseline target for biodiversity conservation.
Wildfires burn large parts of the tropics every year, shaping ecosystem structure and functioning. Yet the complex interplay between climate, vegetation and human factors that drives fire dynamics is still poorly understood. Here we show that on all continents, except Australia, tropical fire regimes change drastically as mean annual precipitation falls below 550 mm. While the frequency of fires decreases below this threshold, the size and intensity of wildfires rise sharply. This transition to a regime of Rare-Intense-Big fires (RIB-fires) corresponds to the relative disappearance of trees from the landscape. Most dry regions on the globe are projected to become substantially drier under global warming. Our findings suggest a global zone where this drying may have important implications for fire risks to society and ecosystem functioning.
(1) Three plots in Northern Guinea savanna were enumerated and then clear-felled in 1950. Since then one plot has been completely protected, a second has been burnt annually early in the dry season, and a third plot has been burnt annually late in the dry season. (2) In 1976-77 there were 202 trees ha$^{-1}$ ($\geqslant$ 30 cm girth) on the protected plot, forty-two trees ha$^{-1}$ on the early burnt plot and twenty trees ha$^{-1}$ on the late burnt plot. Corresponding figures for basal area are 3.43, 0.51, 0.24 m$^2$ ha$^{-1}$. (3) The basal area of grass on both the burnt plots has remained constant at about 13% since 1960 whereas the basal area of grass on the protected plot has continued to decline and was 6.3% in 1976. Grass biomass at the end of the growing season in 1976 was 182 g m$^{-2}$ on the protected plot, and 260 g m$^{-2}$ and 144 g m$^{-2}$ on the early and late burnt plots respectively. (4) There were seventy-three species of vascular plants on the protected plot in 1977, and fifty-three and forty-four respectively on the early and late burnt plots. (5) Only slight differences in the soils were observed, though the protected plot had significantly more organic matter and total nitrogen. (6) The results are compared with those of similar experiments elsewhere in Africa, and recommendations are made for improved experimental design.
The biomass of component parts of individuals of three dominant canopy tree
species in the northern savannas of Australia was determined from field
populations in World Heritage Kakadu National Park. Forty individual trees of
Eucalyptus tetrodonta F. Muell.,
E. miniata Cunn. ex Schauer and
E. papuana F.Muell., representing a range in size from 4
to 50 cm diameter at breast height (DBH), were felled for dry biomass of
leaves, branches, woody stems and bark. Forty-seven other trees of
E. tetrodonta and E. miniata were
excavated for belowground biomass, by using trenching methods. The average
proportion of aboveground biomass in foliage was 3–5%, to
branches 20–32%, and trunk wood 77–59%, with little
change over the size of a tree. Water content of foliage decreased with size
of tree in all species, indicating an increasing xeromorphy as the trees age.
Gross morphology of roots was bimorphic, with 70% of biomass at
<20-cm soil depth, and large roots running horizontally on top of the
shallow (0.3–1.4 m) ferricrete layer. There was no evidence of roots
having access to water below this layer. Patterns of heights, percentage
biomass allocation, percentage water content, and bark thickness of the three
species were consistent with the rank order of their distributions across a
topographic gradient, reflecting relative capacities to withstand drought,
belowground competition and fire. By using tree diameter as the independent
variable (x in cm DBH), allometric relationships were
calculated to provide a method for calculating growth and productivity by
using non-destructive repeat measures of sizes of trees. The total aboveground
biomass (y in kg) of individual trees is
y =
0.2068x2.3191 for
E. tetrodonta, y =
0.1527x2.390 for
E. miniata and y =
0.0356x2.8567 for
E. papuana. Total belowground biomass per tree for
E. tetrodonta is y =
31.150e0.0601x and for E. miniata,
y = 28.753e0.0644x. As a
tree grows, the aboveground biomass increases as a power function and
belowground biomass as an exponential function of DBH, producing a decreasing
proportion of total biomass below ground, e.g. the root/shoot ratio of
E. tetrodonta is 0.50 for trees <10 cm DBH, 0.40 for
trees 20 cm DBH, and 0.25 for trees 40–55 cm DBH. The overall proportion
of total biomass below ground in Kakadu is well below 50%, contrary to
the commonly accepted notion that the majority of biomass in savannas is below
ground.
The need for accurate quantification of the amount of carbon stored in the environment has never been greater. Carbon sequestration has become a vital component of the battle against global climate change, and monitoring and quantifying this process are major challenges for policymakers. Plant allometric equations allow managers and scientists to quantify the biomass contained in a tree without cutting it down, and therefore can play a pivotal role in measuring carbon sequestration in forests and savannahs. These equations have been available since the beginning of the 20th century, but their usefulness depends on the ability to estimate the error associated with the equations - something which has received scant attention in the past. This paper provides a method based on the theory of linear regression and the lognormal distribution to derive confidence limits for estimates of biomass derived from plant allometric equations. Allometric equations for several southern African savannah species are provided, as well as the parameters and equations required to calculate the confidence intervals. This method was applied to data collected from a sampling campaign carried out in a savannah landscape at the Skukuza flux site, Kruger National Park, South Africa. Here the error was 10% of the total site biomass for the woody biomass and 2% for the leaf biomass. When the data were split into individual plots and used to estimate site biomass (as would occur in most sampling schemes) the error increased to 16% and 12% of the woody and leaf biomasses, respectively, as the sampling errors were added to the errors in the allometric equation. These methods can be used in any discipline that applies allometric equations, such as health sciences and animal physiology.
Savannas are the major biome of tropical regions, spanning 30% of the Earth's land surface. Tree:grass ratios of savannas are inherently unstable and can be shifted easily by changes in fire, grazing, or climate. We synthesize the history and ecological impacts of the rapid expansion and eradication of an exotic large herbivore, the Asian water buffalo (Bubalus bubalus), on the mesic savannas of Kakadu National Park (KNP), a World Heritage Park located within the Alligator Rivers Region (ARR) of monsoonal north Australia. The study inverts the experience of the Serengeti savannas where grazing herds rapidly declined due to a rinderpest epidemic and then recovered upon disease control. Buffalo entered the ARR by the 1880s, but densities were low until the late 1950s when populations rapidly grew to carrying capacity within a decade. In the 1980s, numbers declined precipitously due to an eradication program. We show evidence that the rapid population expansion and sudden removal of this exotic herbivore created two ecological cascades by altering ground cover abundance and composition, which in turn affected competitive regimes and fuel loads with possible further, long-term effects due to changes in fire regimes. Overall, ecological impacts varied across a north-south gradient in KNP that corresponded to the interacting factors of precipitation, landform, and vegetation type but was also contingent upon the history of buffalo harvest. Floodplains showed the greatest degree of impact during the period of rapid buffalo expansion, but after buffalo removal, they largely reverted to their prior state. Conversely, the woodlands experienced less visible impact during the first cascade. However, in areas of low buffalo harvest and severe impact, there was little recruitment of juvenile trees into the canopy due to the indirect effects of grazing and high frequency of prescribed fires once buffalo were removed. Rain forests were clearly heavily impacted during the first cascade, but the long term consequences of buffalo increase and removal remain unclear. Due to hysteresis effects, the simple removal of an exotic herbivore was not sufficient to return savanna systems to their previous state.
In the Wet Chaco region of Argentina, increasing shrub encroachment in savannas over the last few decades has led to important changes in the structure and functioning of the landscape. Some sectors of this territory are characterized by the appearance of circular clusters of woody patches, dispersed throughout the grassland matrix. The increasing size of these patches leads to a gradual change from grassland to dense shrubland. We studied these circular woody patches in the eastern region of the Argentine province of Formosa and characterized the variation in terms of floristic composition, diversity and predominant seed dispersal mode in different size patches. We observed an increase in species richness, diversity and compositional heterogeneity among patches with increasing patch size. Seed dispersal by animals, especially birds, is an important factor in the expansion of these woody vegetation patches within the grassland matrix.
The stock, rates of sequestration and allocation of carbon were estimated for trees in 14 0.1-ha plots at Kapalga in Kakadu National Park, Northern Territory, using new allometric relationships of carbon stock to stem cross-sectional area and measured growth rates of trees. Carbon stocks of trees ranged from 12 to 58 t ha-1, with sequestration representing ∼9% of the total stocks. More than half of the sequestered carbon is allocated to leaves and twigs and ∼20% to wood. Only ∼25% is retained in the live trees with leaf and twig fall accounting for 80-84% of the total transfers to the environment. An alternative method of calculating sequestration rates from consideration of water use and carbon-isotope discrimination data had a close to 1:1 match with estimates from allometric relationships. We developed and applied algorithms to predict the impacts of fire on carbon stocks of live trees. This showed that the reduction in live carbon stocks caused by single fires increased with increasing intensity, but the impact was highly dependent on the tree stand structure.
Although biomass burning of savannas is recognised as a major global source of greenhouse gas emissions, quantification remains problematic with resulting regional emissions estimates often differing markedly. Here we undertake a critical assessment of Australia's National Greenhouse Gas Inventory (NGGI) savanna burning emissions methodology. We describe the methodology developed for, and results and associated uncertainties derived from, a landscape-scale emissions abatement project in fire-prone western Arnhem Land, northern Australia. The methodology incorporates (i) detailed fire history and vegetation structure and fuels type mapping derived from satellite imagery; (ii) field-based assessments of fuel load accumulation, burning efficiencies (patchiness, combustion efficiency, ash retention) and N : C composition; and (iii) application of standard, regionally derived emission factors. Importantly, this refined methodology differs from the NGGI by incorporation of fire seasonality and severity components, and substantial improvements in baseline data. We consider how the application of a fire management program aimed at shifting the seasonality of burning (from one currently dominated by extensive late dry season wildfires to one where strategic fire management is undertaken earlier in the year) can provide significant project-based emissions abatement. The approach has wider application to fire-prone savanna systems dominated by anthropogenic sources of ignition.
Surveys over a period of 38 years have shown a rapid successional change in the remaining areas of cerrado vegetation in western São Paulo State. Cerradão (the dense, tall, forest form of cerrado (sensu
lato)) and cerrado/Atlantic forest ecotonal vegetation have replaced more open forms (such as campo cerrado) during this period. An aerial photographic survey in 1962 showed 75% cerrado (sensu
stricto), 16% campo cerrado, and only 9% cerradão, while a survey combining Landsat imaging of 1992 with aerial photography of 1984 gave 69%, 0.6%, and 30.5% respectively for the same physiognomies. Visiting 10% of the sites of the latter survey in 2000 showed that cerradão had become the dominant vegetation of 68% of them. In a particular 180 ha site in Assis municipality, cerradão increased from 12.0 to 41.4% of the area in 22 years. Reduction of anthropic pressures, such as fire and cattle-grazing, is considered responsible for these rapid changes. Conservation issues and research priorities related to these changes are discussed and proposed.
Although local increases in woody plant cover have been documented in arid and semiarid ecosystems worldwide, there have been few long-term, large-scale analyses of changes in woody plant cover and aboveground carbon (C) stocks. We used historical aerial photography, contemporary Landsat satellite data, field observations, and image analysis techniques to assess spatially specific changes in woody vegetation cover and aboveground C stocks between 1937 and 1999 in a 400-km2 region of northern Texas, USA. Changes in land cover were then related to topo-edaphic setting and historical land-use practices. Mechanical or chemical brush management occurred over much of the region in the 1940–1950s. Rangelands not targeted for brush management experienced woody cover increases of up to 500% in 63 years. Areas managed with herbicides, mechanical treatments or fire exhibited a wide range of woody cover changes relative to 1937 (−75% to + 280%), depending on soil type and time since last management action. At the integrated regional scale, there was a net 30% increase in woody plant cover over the 63-year period. Regional increases were greatest in riparian corridors (33%) and shallow clay uplands (26%) and least on upland clay loams (15%). Allometric relationships between canopy cover and aboveground biomass were used to estimate net aboveground C storage changes in upland (nonriparian) portions of regional landscapes. Carbon stocks increased from 380 g C m−2 in 1937 to 500 g C m−2 in 1999, a 32% net increase across the 400 km2 region over the 63-year period. These plant C storage change estimates are highly conservative in that they did not include the substantial increases in woody plant cover observed within riparian landscape elements. Results are discussed in terms of implications for ‘carbon accounting’ and the global C cycle.
Abstract The impact of feral Asian water buffalo (Bubalus bubalis) and season of fire on growth and survival of mature trees was monitored over 8 years in the eucalypt savannas of Kakadu National Park. Permanently marked plots were paired on either side of a 25-km-long buffalo-proof fence at three locations on an elevational gradient, from ridge-top to the edge of a floodplain; buffalo were removed from one side of the fence. All 750 trees ≥ 1.4 m height were permanently marked; survival and diameter of each tree was measured annually; 26 species were grouped into four eco-taxonomic groups. The buffalo experiment was maintained for 7 years; trees were monitored an additional year. Fires were excluded from all sites the first 3 years, allowed to occur opportunistically for 4 years and excluded for the final year. Fires were of two main types: low-intensity early dry season and high-intensity late dry season. Growth rates of trees were size-specific and positively related to diameters as exponential functions; trees grew slowest on the two ends of the gradient. Eucalypt mortality rates were 1.5 and 3 times lower than those of pantropics and of arborescent monocots, respectively, but the relative advantage was lost with fires or buffalo grazing. Without buffalo grazing, ground level biomass was 5–8 t ha−1 compared with 2–3 t ha−1, within 3 years. In buffalo-absent plots, trees grew significantly slower on the dry ridge and slope, and had higher mortality across the entire gradient, compared with trees in buffalo-present plots. At the floodplain margin, mortality of small palms was higher in buffalo-present sites, most likely due to associated heavy infestations of weeds. Low-intensity fires produced tree growth and mortality values similar to no-fire, in general, but, like buffalo, provided a ‘fertilization’ effect for Eucalyptus miniata and Eucalyptus tetrodonta, increasing growth in all size classes. High-intensity fires reduced growth and increased mortality of all functional groups, especially the smallest and largest (>35 cm d.b.h.) trees. When buffalo and fires were excluded in the final year, there were no differences in growth or mortality between paired sites across the environmental gradient. After 8 years, the total numbers of trees in buffalo-absent plots were only 80% of the number in buffalo-present plots, due to relatively greater recruitment of new trees in buffalo-present plots; fire-sensitive pantropics were particularly disadvantaged. Since the removal of buffalo is disadvantageous, at least over the first years, to savanna tree growth and survival due to a rebound effect of the ground-level vegetation and subsequent changes in fire-vegetation interactions, process-orientated management aimed at reducing fuel loads and competitive pressure may be required in order to return the system to a previous state. The ‘footprint’ of 30 years of heavy grazing by buffalo has implications for the interpretation of previous studies on fire-vegetation dynamics and for current research on vegetation change in these savannas.
A 35-year controlled burning experiment in Minnesota oak savanna showed that fire frequency had a great impact on ecosystem carbon (C) stores. Specifically, compared to the historical fire regime, fire suppression led to an average of 1.8 Mg·ha 1 ·yr 1 of C storage, with most carbon stored in woody biomass. Forest floor carbon stores were also significantly impacted by fire frequency, but there were no detectable effects of fire sup-pression on carbon in soil and fine roots combined, or in woody debris. Total ecosystem C stores averaged 110 Mg/ha in stands experiencing presettlement fire frequencies, but 220 Mg/ha in stands experiencing fire suppression. If comparable rates of C storage were to occur in other ecosystems in response to the current extent of fire suppression in the United States, fire suppression in the USA might account for 8–20% of missing global carbon.
Aim (1) To estimate the local and global magnitude of carbon fluxes between savanna and the atmosphere, and to suggest the significance of savannas in the global carbon cycle. (2) To suggest the extent to which protection of savannas could contribute to a global carbon sequestration initiative.
Location Tropical savanna ecosystems in Africa, Australia, India and South America.
Methods A literature search was carried out using the ISI Web of Knowledge, and a compilation of extra data was obtained from other literature, including national reports accessed through the personal collections of the authors. Savanna is here defined as any tropical ecosystem containing grasses, including woodland and grassland types. From these data it was possible to estimate the fluxes of carbon dioxide between the entire savanna biome on a global scale.
Results Tropical savannas can be remarkably productive, with a net primary productivity that ranges from 1 to 12 t C ha ⁻¹ year ⁻¹ . The lower values are found in the arid and semi‐arid savannas occurring in extensive regions of Africa, Australia and South America. The global average of the cases reviewed here was 7.2 t C ha ⁻¹ year ⁻¹ . The carbon sequestration rate (net ecosystem productivity) may average 0.14 t C ha ⁻¹ year ⁻¹ or 0.39 Gt C year ⁻¹ . If savannas were to be protected from fire and grazing, most of them would accumulate substantial carbon and the sink would be larger. Savannas are under anthropogenic pressure, but this has been much less publicized than deforestation in the rain forest biome. The rate of loss is not well established, but may exceed 1% per year, approximately twice as fast as that of rain forests. Globally, this is likely to constitute a flux to the atmosphere that is at least as large as that arising from deforestation of the rain forest.
Main conclusions The current rate of loss impacts appreciably on the global carbon balance. There is considerable scope for using many of the savannas as sites for carbon sequestration, by simply protecting them from burning and grazing, and permitting them to increase in stature and carbon content over periods of several decades.
Savannahs are a mixture of trees and grasses often occurring as alternate states to closed forests. Savannah fires are frequent where grass productivity is high in the wet season. Fires help maintain grassy vegetation where the climate is suitable for woodlands or forests. Saplings in savannahs are particularly vulnerable to topkill of above-ground biomass. Larger trees are more fire-resistant and suffer little damage when burnt. Recruitment to large mature tree size classes depends on sapling growth rates to fire-resistant sizes and the time between fires. Carbon dioxide (CO(2)) can influence the growth rate of juvenile plants, thereby affecting tree recruitment and the conversion of open savannahs to woodlands. Trees have increased in many savannahs throughout the world, whereas some humid savannahs are being invaded by forests. CO(2) has been implicated in this woody increase but attribution to global drivers has been controversial where changes in grazing and fire have also occurred. We report on diverse tests of the magnitude of CO(2) effects on both ancient and modern ecosystems with a particular focus on African savannahs. Large increases in trees of mesic savannahs in the region cannot easily be explained by land use change but are consistent with experimental and simulation studies of CO(2) effects. Changes in arid savannahs seem less obviously linked to CO(2) effects and may be driven more by overgrazing. Large-scale shifts in the tree-grass balance in the past and the future need to be better understood. They not only have major impacts on the ecology of grassy ecosystems but also on Earth-atmosphere linkages and the global carbon cycle in ways that are still being discovered.
Aim Comparative analyses of fire regimes at large geographical scales can potentially identify ecological and climatic controls of fire. Here we describe Australia’s broad fire regimes, and explore interrelationships and trade-offs between fire regime components. We postulate that fire regime patterns will be governed by trade-offs between moisture, productivity, fire frequency and fire intensity. Location Australia. Methods We reclassified a vegetation map of Australia, defining classes based on typical fuel and fire types. Classes were intersected with a climate classification to derive a map of ‘fire regime niches’. Using expert elicitation and a literature search, we validated each niche and characterized typical and extreme fire intensities and return intervals. Satellite-derived active fire detections were used to determine seasonal patterns of fire activity. Results Fire regime characteristics are closely related to the latitudinal gradient in summer monsoon activity. Frequent low-intensity fires occur in the monsoonal north, and infrequent, high-intensity fires in the temperate south, demonstrating a trade-off between frequency and intensity: that is, very high-intensity fires are only associated with very low-frequency fire regimes in the high biomass eucalypt forests of southern Australia. While these forests occasionally experience extremely intense fires (> 50,000 kW m⁻¹), such regimes are exceptional, with most of the continent dominated by grass fuels, typically burning with lower intensity (< 5000 kW m⁻¹). Main conclusions Australia’s fire regimes exhibit a coherent pattern of frequent, grass-fuelled fires in many differing vegetation types. While eucalypts are a quintessential Australian entity, their contribution as a dominant driver of high-intensity fire regimes, via their litter and bark fuels, is restricted to the forests of the continent’s southern and eastern extremities. Our analysis suggests that the foremost driver of fire regimes at the continental scale is not productivity, as postulated conceptually, but the latitudinal gradient in summer monsoon rainfall activity.
This paper is the botanical report of an expedition to Dogon Kurmi, near Jos in central Nigeria, in August 1955, with comments on the soil and fauna. Seedlings of twenty-two species of both forest and savanna type were examined to throw light on whether forest plants were invading the `derived' savanna or whether the reverse was true. The trees in the forest attain 20 m or even 40 m, in the savanna about 5 m; at Dogon Kurmi the forest and savanna are found together and intergrading. When studied in the wet season the savanna grasses stood 1-2 m high; in the dry season their dry culms are fired. Fifteen transects, from six different situations, covered both gradual and abrupt transitions from kurmi to savanna in varying soils and also pure savanna and pure stands of Uapaca togoensis. These were mapped showing old and young plants of the selected species and the extent of every tree's shade. The relative `success' of a given species in differing conditions of shade and firing was estimated using numbers and height as a criterion. The need to choose common and easily identifiable species and for complete mapping, curtailed work on the high forest; in practice also, seedlings were often not distinguishable from suckers or fired saplings. Analysis of the twenty-two species (sixteen are individually discussed) show three to be randomly dispersed by wind, and five, for various reasons, clumped. Middle-sized trees of five species are scarce; in two of these the heavy seeds which fall to the ground almost entirely fail to germinate. Five fire-tolerant species attain a larger size in open situations; of apparent shade-lovers four are limited more by fire and one by its mode of dispersal. Edaphic factors only occasionally influence the distribution of forest and savanna plants, though five plants show weak correlation with clay content and only one tolerates soils of small effective depth. Though some shrubs and climbers may protect trees against fire, Uapaca alone of such trees helps forest trees to grow under it; there is little to suggest that the forest is encroaching on the savanna and the reverse is happening in one transect.
Savanna burning for greenhouse gas abatement presents an opportunity for remote Aboriginal communities of northern Australia to engage with the mainstream economy while fulfilling cultural obligations for land stewardship. The recently established Tiwi Carbon Study aims to identify the biophysical and economic potential of fire management for greenhouse gas abatement on the Tiwi Islands north of Darwin, as a basis for possible livelihood opportunities for the Aboriginal Tiwi people. Recent (2001–2010) fire history for the Tiwi Islands based on AVHRR satellite imagery shows that on average 35% (187 700 ha) of its savanna woodlands and open forests are burned every year, with 72% of burning occurring late in the dry season (August to November). Non‐CO2 greenhouse gas emissions from Tiwi fires average 68 000 t CO2‐e year−1 and we discuss scenarios for greenhouse gas abatement through management of these fires by Tiwi people, consistent with the savanna burning methodology approved under the Federal Government's Carbon Farming Initiative. Changed fire management scenarios produced emissions abatement of up to 46 000 t CO2‐e year−1, with highest savings under a change in both fire frequency and intensity. In addition to abatement of non‐CO2 emissions, fire management has the potential to alter rates of carbon sequestered in soil and vegetation. Current ecosystem C stocks (excluding roots) on the Tiwi Islands range from 60 to 160 t C ha−1. The Tiwi Carbon Study features a long‐term, landscape‐scale fire experiment for informing full carbon accounting in relation to different fire management options, and for understanding their implications for biodiversity. We discuss potential co‐benefits and trade‐offs of fire management for greenhouse gas emissions abatement in relation to biodiversity and Tiwi cultural requirements and livelihood aspirations.
Using a detailed fire history collected over a 10-year period throughout a savanna landscape in northern Australia, we have addressed the question of whether fire severity, inferred from a semiquantitative fire severity index, increases with time since previous fire. There was a clear trend of fires becoming much more severe with increasing time since previous fire. Between 1 and 5 years following a fire, the probability of a subsequent fire being classified as 'severe' increased from 3 to 8% for early dry-season fires, and from 21 to 43% for late dry-season fires. It was clear that the strong increase in fire severity was not confined to the first 2-3 years following the previous fire, as previously suspected. These findings highlight the difficulty of reducing both fire frequency and severity in northern Australian savanna landscapes, as they imply that a negative feedback process exists between the two; that is, reducing fire frequency is likely to increase the severity of fires that do occur.
Fires burn vast areas of the monsoonal savannas of northern Australia each
year. This paper describes the contemporary fire regimes of two ecologically
similar, relatively large national parks (Litchfield—1464
km2; Nitmiluk—2924 km2) in
the Top End of the Northern Territory, over 8 and 9 years, respectively. Fire
histories for both parks were derived from interpretation of LANDSAT TM
imagery, supplemented with NOAA-AVHRR for cloudy periods at the end of the
7-month dry season (c. April–Oct). Data concerning
seasonality, extent and frequency of burning were analysed with respect to
digital coverages for the park as a whole, landscape units, vegetation types,
infrastructure and tenure boundaries. Ground-truth data established that
interpreted accuracy overall, for 2 assessment years, ranged between 82 and
91% for both parks. Over 50% of Litchfield and 40% of
Nitmiluk was burnt on average over this period, with Litchfield being burnt
substantially in the earlier, cooler, and moister, dry season, and Nitmiluk
mostly in the parched late dry season, after August. On both parks the current
frequency of burning in at least low open woodland / heath habitats is
ecologically unsustainable. Both parks are prone to extensive fire incursions.
The data support earlier regional assessments that the average fire return
interval is around 2 years in at least some areas of northern Australia.
Nevertheless, comparison of contemporary fire regimes operating in three major
regional national parks shows distinct differences, particularly with respect
to the extent and seasonality (hence intensity) of burning in relation to
different landscape components. Management implications are considered in
discussion.
Aim
Comparative analyses of fire regimes at large geographical scales can potentially identify ecological and climatic controls of fire. Here we describe A ustralia's broad fire regimes, and explore interrelationships and trade‐offs between fire regime components. We postulate that fire regime patterns will be governed by trade‐offs between moisture, productivity, fire frequency and fire intensity.
Location
Australia.
Methods
We reclassified a vegetation map of A ustralia, defining classes based on typical fuel and fire types. Classes were intersected with a climate classification to derive a map of ‘fire regime niches’. Using expert elicitation and a literature search, we validated each niche and characterized typical and extreme fire intensities and return intervals. Satellite‐derived active fire detections were used to determine seasonal patterns of fire activity.
Results
Fire regime characteristics are closely related to the latitudinal gradient in summer monsoon activity. Frequent low‐intensity fires occur in the monsoonal north, and infrequent, high‐intensity fires in the temperate south, demonstrating a trade‐off between frequency and intensity: that is, very high‐intensity fires are only associated with very low‐frequency fire regimes in the high biomass eucalypt forests of southern A ustralia. While these forests occasionally experience extremely intense fires (> 50,000 kW m ⁻¹ ), such regimes are exceptional, with most of the continent dominated by grass fuels, typically burning with lower intensity (< 5000 kW m ⁻¹ ).
Main conclusions
Australia's fire regimes exhibit a coherent pattern of frequent, grass‐fuelled fires in many differing vegetation types. While eucalypts are a quintessential A ustralian entity, their contribution as a dominant driver of high‐intensity fire regimes, via their litter and bark fuels, is restricted to the forests of the continent's southern and eastern extremities. Our analysis suggests that the foremost driver of fire regimes at the continental scale is not productivity, as postulated conceptually, but the latitudinal gradient in summer monsoon rainfall activity.
We assess a replicated fire plot experiment undertaken between 1973 and 1996 in two Eucalyptus-dominated savanna vegetation formations (open forest, woodland), at Munmarlary, in monsoonal northern Australia. Four treatments, each with three replicates, were imposed on each vegetation type: annual early dry-season burning; annual late dry-season burning; biennial early dry-season burning; and unburned controls. Treatments were imposed faithfully, with noted exceptions, on 1-ha plots. Fire intensities were typically low (<1000 kW/m) to moderate (1000-2500 kW/m), varied significantly between treatments, and generally were greater in woodland. In both woodland and open forest, pH was significantly lower and NO3-N was significantly higher in unburned plots. Organic C was not significantly greater in unburned treatments. Effects of fire regime on other soil chemical properties differed between open forest and woodland sites. Among the grasses, invariant frequent burning led to the dominance of a small number of annual species, notably regionally dominant Sorghum. In the absence of burning, annuals declined generally, whereas some perennials increased while most decreased. These responses usually were apparent within the first five years of the experiment. At the relatively small spatial scale of the grass sampling regime, there was high turnover of both annual and perennial grasses. Under low- to moderate-intensity, frequent burning regimes, woody vegetation dominated by mature eucalypts is structurally stable. In the absence of burning for at least five years, there was release of the non-eucalypt, woody component into the midstory; this occurred more rapidly in open forest. Accession of rain forest species occurred on some woodland plots, especially the unburned treatment. In contrast, eucalypts were not released significantly from the understory. Rather, as suggested by other studies, recruitment of eucalypts into the canopy appears to involve significantly reduced root competition through death of dominant eucalypts. Although the Munmarlary experiment provides invaluable quantitative data for exploring relationships between fire regimes and the responses of north Australian savanna systems, it has been less successful in meeting the complex information requirements of regional fire managers. Replicated experimental fire plot designs, no matter how elegant and rigorously implemented, may substantially fail the test of management relevance, given the fundamental requirement for savanna biodiversity managers to experience the integrated effects of fire regimes that vary idiosyncratically over multiple time and spatial scales. We suggest that such information requirements are better met through modest, targeted "adaptive management" studies, involving collaborative partnerships between managers and researchers.
Ten years of photo and associated data records from an extensive fire and vegetation effects monitoring programme established in two large north Australian National Parks were used to ( 1) develop a simple-to-use semiquantitative fire severity index based on observed fire impact on vegetation, particularly leaf-scorch height; and ( 2) explore relationships between seasonality and fire severity in different landform and vegetation types. Using a three-tiered fire severity scale, data for 719 fires recorded from 178 plots over the period 1995-2004 indicate that the great majority of early dry season (pre-August) fires were of very low severity (fire-line intensities << 1000kW m(-1)), whereas fires later in the dry season were typically of substantially greater severity. Similar trends were evident for vegetation occupying all landform types. The utility and limitations of the fire severity index, and implications for ecologic, greenhouse inventory, and remote sensing applications are discussed.
In a landscape-scale experiment, fires were lit in replicate catchments 15-20 km2 in area, either early in the dry season (June) or late in the dry season (September) between 1990 and 1994. For each fire, Byram-intensity was determined in representative one ha areas of Eucalyptus miniata – E. tetrodonta open-forest, with a ground stratum dominated by annual grasses. Fuel weights were measured by harvest, fuel heat content was assumed to be constant, and the rate of spread was determined using electronic timers. Fuels consisted primarily of grass and leaf litter, and ranged from 1.5 to 13 t ha-1; in most years, average fuel loads were 2-4 t ha-1. Rates of spread were generally in the range of 0.2-0.8 ms-1. The mean intensity of early dry season fires (2100 kW m-1) was significantly less than that of the late dry season fires (7700 kW m-1), primarily because, in the late dry season, there was more leaf litter, fuels were drier, and fire weather was more extreme. Crown fires, a feature of forest fires of high intensity in southeastern Australia, were not observed in the Kapalga fires. Fire intensity was a very good predictor of both leaf-char height and leaf-scorch height for fires between 100 kW m-1 and 10,000 kW m-1, the range in which the majority of experimental fires fell.
Trends in Australian rainfall over the period 1901 to 2002 are analysed with the aim of evaluating and assessing long-term trends. In particular, this study examines long-term trends in Australian rainfall with the aim of identifying any continental- scalepatternsthatcouldbedescribedas'unusual'.All-Australia annual average rainfall and all-Australia average decile time series indicate a positive long-term trend over the full period. Trend maps indicate that much of this trend is the result of increases in summer half-year rainfall over western, northern and central Australia that have occurred over the latter part of therecord1952-2002.Whilesignificantnegativetrendsinwinter half-year rainfall over southwest WesternAustralia are evident, there is little evidence that they are part of any continental-scale trends, at least not on 100 or 50-year time-scales. Empirical Orthogonal Teleconnection patterns (EOTs) of annual rainfall provide a means for delineating independent spatial modes. These indicate that much of the variance in all-Australian rain - fall can be attributed to the first two modes that cover much of central easternAustralia and central westernAustralia. In addi - tion, the pattern of positive trends comprises at least two modes, which, being linearly independent, indicate that the large-scale pattern of increases is itself unusual in a historical context.
A fire-mediated recruitment bottleneck provides a possible explanation for the coexistence of trees and grasses in mesic savannas. The key element of this hypothesis is that saplings are particularly vulnerable to fire because they are small enough to be top-killed by grass fires, but unlike juveniles, they take several years to recover their original size. This limits the number of recruits into the adult size classes. Thus savanna vegetation may be maintained by a feedback whereby fire restricts the density of adult trees and allows a grass layer to develop, which provides fuel for subsequent fires. Here, we use results from a landscape-scale fire experiment in tropical Australia, to explore the possible existence of a recruitment bottleneck. This experiment compared tree recruitment and survival over 4 y under regimes of no fire, annual early and annual late dry-season fire. Stem mortality decreased with increasing stem height in the fire treatments but not in the unburnt treatment. Tree recruitment was 76-84% lower in the fire treatments than the unburnt treatment. Such fire-induced stem loss of saplings and reduced recruitment to the canopy layer in this eucalypt savanna are consistent with the predictions of the fire-mediated recruitment bottleneck hypothesis.
Aerial photographs were used to assess changes in woody vegetation cover at 122 locations within a sandstone-plateau savanna woodland in the Victoria River region, Northern Territory, Australia. Despite locally variable vegetation responses, there has been little change in total woody vegetation cover since 1948. Thirty-three locations were also surveyed on the ground. It was found that sites for which vegetation cover had changed over the 50-y period were not significantly different from stable sites in terms of floristic composition, recent fire history, demographic stability among the dominant tree species, or edaphic setting. However, two of the dominant overstorey tree species – Eucalyptus
tetrodonta and Eucalyptus
phoenicea – showed significantly higher mortality on sites that had experienced vegetation cover decline since 1948. We suggest that observed changes in woody vegetation cover are a consequence of natural cycles of die-back and recovery of at least these two species in response to spatially heterogenous variables such as dry-season moisture stress. Although the widespread decline of fire-sensitive Callitris
intratropica populations clearly indicates a historical shift from lower- to higher-intensity burning conditions within the study area, we reject the hypothesis of a landscape-wide process such as changing fire regimes or climatic change as the driving factor behind large-scale vegetation changes detected by aerial photographic analysis.
For the past century, woody plants have increased in grasslands and savannas worldwide. Woody encroachment may significantly alter ecosystem functioning including fire regimes, herbivore carrying capacity, biodiversity and carbon storage capacity. Traditionally, increases in woody cover and density have been ascribed to changes in the disturbance regime (fire and herbivores) or rainfall. Increased atmospheric CO2 concentrations may also contribute, by increasing growth rates of trees relative to grasses. This hypothesis is still heavily debated because usually potential CO2 effects are confounded by changes in land use (disturbance regime). Here we analyse changes in woody density in fire experiments at three sites in South African savannas where the disturbance regime (fire and herbivores) was kept constant for 30 and 50 years. If global drivers had significant effects on woody plants, we would expect significant increases in tree densities and biomass over time under the constant disturbance regime. Woody density remained constant in a semiarid savanna but tripled in a mesic savanna between the 1970s and 1990s. At the third site, a semiarid savanna near the southern limits of the biome, tree density doubled from the mid 1990s to 2010. Interpretation of the causes is confounded by population recovery after clearing, but aerial photograph analysis on adjacent non-cleared areas showed an accompanying 48% increase in woody cover. Increased CO2 concentrations are consistent with increased woody density while other global drivers (rainfall) remained constant over the duration of the experiments. The absence of a response in one semiarid savanna could be explained by a smaller carbon sink capacity of the dominant species, which would therefore benefit less from increased CO2. Understanding how savannas and grasslands respond to increased CO2 and identifying the causes of woody encroachment are essential for the successful management of these systems.
Aim This study aimed to quantify changes in fire severity resulting from the invasion of Australia’s tropical savannas by the African grass Andropogon gayanus Kunth. (gamba grass).
Location Mesic savannas of the Northern Territory, Australia.
Method Byram’s fire‐line intensity (I f ), fuel load and architecture, and two post‐fire indicators of fire intensity – scorch height (SH) and char height (CH) of woody vegetation – were determined for fires in native grass savanna and A. gayanus invaded savanna. Leaf scorch is the height at which the fire’s radiant heat browns leaf tissue, and leaf char is the height that radiant heat blackens or consumes leaf tissue and provides an indirect measure of flame height. These data, and 5 years of similar data collected from the Kapalga Fire Project in Kakadu National Park, were used to develop empirical relationships between I f and the post‐fire indices of fire intensity.
Results A relationship between A. gayanus I f and SH could not be developed because complete canopy scorch occurred in most A. gayanus fires, even at low I f . In contrast, A. gayanus I f was strongly correlated with CH. This empirical relationship was substantially different from that for native grass fires. For a given I f , there was a significantly greater CH in invaded sites. This increase in radiant heat is attributable to the increased biomass (mean 3.6 t ha ⁻¹ in native grasses compared to 11.6 t ha ⁻¹ in A. gayanus ) and height (approximately 0.5 m in native grasses compared to 4 m in A. gayanus ) of the standing fine fuel.
Main conclusion Andropogon gayanus invasion resulted in substantial changes in fire behaviour. This has important regional implications owing to the current (10,000–15,000 km ² ) and predicted (380,000 km ² ) area of invasion and the negative consequences for the native savanna biota that has evolved with frequent but relatively low‐intensity fire.
Tropical savannas are typically highly productive yet fire-prone ecosystems, and it has been suggested that reducing fire frequency in savannas could substantially increase the size of the global carbon sink. However, the long-term demographic consequences of modifying fire regimes in savannas are difficult to predict, with the effects of fire on many parameters, such as tree growth rates, poorly understood. Over 10 years, we examined the effects of fire frequency on the growth rates (annual increment of diameter at breast height) of 3075 tagged trees, at 137 locations throughout the mesic savannas of Kakadu, Nitmiluk and Litchfield National Parks, in northern Australia. Frequent fires substantially reduced tree growth rates, with the magnitude of the effect markedly increasing with fire severity. The highest observed frequencies of mild, moderate and severe fires (1.0, 0.8 and 0.4 fires yr−1, respectively) reduced tree growth by 24%, 40% and 66% respectively, relative to unburnt areas. These reductions in tree growth imply reductions in the net primary productivity of trees by between 0.19 t C ha−1 yr−1, in the case of mild fires, and 0.51 t C ha−1 yr−1, in the case of severe fires. Such reductions are relatively large, given that net biome productivity (carbon sequestration potential) of these savannas is estimated to be just 1–2 t C ha−1 yr−1. Our results suggest that current models of savanna tree demography, that do not account for a relationship between severe fire frequency and tree growth rate, are likely to underestimate the long-term negative effects of frequent severe fires on tree populations. Additionally, the negative impact of frequent severe fires on carbon sequestration rates may have been underestimated; reducing fire frequencies in savannas may increase carbon sequestration to a greater extent than previously thought.
Aim Anecdotal historical and photographic evidence suggests that woody vegetation is increasing dramatically in some northern Australian savanna habitats. Vegetation change in savannas has important implications for pastoral land‐use, conservation management, and landscape‐scale carbon storage, and informs theoretical debates about ecosystem function. This study seeks to determine the nature, extent and cause(s) of woody vegetation change in a seasonally flooded alluvial savanna habitat.
Location The study area is located within the seasonally inundated alluvial zone of the tidal portion of the Victoria River, Northern Territory, Australia. The study area has been grazed by domestic stock since c. 1900, prior to which the area was inhabited and more likely regularly burnt by Aboriginal people for thousands of years.
Methods Digital georeferenced aerial photographic coverages were used to examine and quantify woody vegetation change between 1948 and 1993. Transect surveys of woody and herbaceous vegetation were carried out to ground‐truth air‐photo results and determine the nature and causes of observed vegetation changes.
Results There has been a dramatic increase in woody vegetation cover throughout the study area. Vegetation change patterns are roughly uniform across the full range of edaphic habitat variation and are unrelated to the depositional age of fluvial sediments. Two woody species, Eucalyptus microtheca and Excoecaria parvifolia , are predominantly responsible for observed increases. Demographic analyses reveal that woody invasions have been episodic and indicate that in most locations peak woody species establishment occurred in the mid‐1970s. Grasses are almost absent in a majority of habitats within the study area. Instead, large areas are covered by scalded soil, dense invasive weed populations, and unpalatable forbs and sedges. What grasses do occur are predominantly of very low value for grazing. The condition of the herbaceous layer renders most of the study area almost completely non‐flammable; what fires do burn are small and of low intensity.
Main conclusions Multiple working hypotheses explaining observed patterns of woody vegetation increase were considered and rejected in turn. The only hypothesis consistent with the evidence is as follows: (1) observed changes are a direct consequence of extreme overgrazing by cattle, most likely when stocking rates peaked in the mid‐1970s; (2) prolonged heavy grazing effected the complete transformation of much of the herbaceous vegetation to a new state that is not flammable; and (3) in the absence of regular fire mortality, woody vegetation increased rapidly. The relatively treeless system that existed in 1948 was apparently stable and resilient to moderate grazing levels, and perhaps also to episodic heavy grazing events. However, grazing intensity in excess of a sustainable threshold has forced a transition that is irreversible in the foreseeable future. Stable‐state transitions such as this one inform debates at the heart of ecological theory, such as the nature of stability, resilience, equilibrium and carrying capacity in dynamic savanna ecosystems.
Spatio-temporal variation in tropical savanna tree cover remains poorly understood. We aimed to quantify the drivers of tree cover in tropical mesic savannas in Kakadu National Park by relating changes in tree cover over 40 years to: mean annual rainfall, fire activity, initial tree cover and prior changes in tree cover. Aerial photography, acquired in 1964, 1984 and 2004, was obtained for fifty sites in Kakadu that spanned a rainfall gradient from approximately 1200 to 1600 mm. The remotely sensed estimates of tree cover were validated via field survey. Linear mixed effects modelling and multi-model inference were used to assess the strength and form of the relationships between tree cover and predictor variables. Over the 40 years, tree cover across these savannas increased on average by 4.94 ± 0.88%, but was spatio-temporally variable. Tree cover showed a positive albeit weak trend across the rainfall gradient. The strength of this positive relationship varied over the three measurement times, and this suggests that other factors are important in controlling tree cover. Tree cover was positively related to prior tree cover, and negatively correlated with fire activity. Over 20 years tree cover was more likely to increase if (i) tree cover was initially low or (ii) had decreased in the previous 20-year interval or (iii) there had been fewer fires. Across the examined rainfall gradient, the greater variability in fire activity and inherently higher average tree cover at the wetter latitudes resulted in greater dynamism of tree cover compared with the drier latitudes. This is consistent with savanna tree cover being determined by interactions between mean annual rainfall, tree competition and frequent fire in these tropical mesic savannas.
Abstract LANDSAT Multi-Spectral Scanner imagery was used to determine aspects of the fire regimes of Kakadu National Park (in the wet-dry tropics of Australia) for the period 1980–1995. Three landscape types recognized in this Park were Plateau, Lowlands and Floodplain. Areas burned in early and late dry seasons each year were documented using a Geographical Information System. Regression analyses were used to examine time trends in the areas burned each year and the interrelationships between early and late dry season burning. The proportions of landscapes having different stand ages (years since fire), and the proportions having had different fire intervals, were compared with results expected from the simplest random model (i.e. one in which the probability of ignition at a point [PIP] burning annually was constant). Using overlays of successive stand-age maps, PIP could be calculated as a function of stand age. The Lowlands burned extensively each year; the areas burned by late dry season fires adding to those burned in the early dry season such that around 50–60% of the total area burned annually. Early dry season fires have lower intensities than late dry season fires, on average. Using a theoretically constant PIP and the mean proportion burned per year as the only input, predictions of areas burned as a function of stand age and fire interval were reasonable when compared with the empirical data, but best for the Lowlands landscape. PIP functions for Lowlands and Floodplains had negative slopes, an unexpected result. The nature of these PIP functions may reflect heterogeneity in fire proneness of the various vegetation types within landscapes. The scale of measurement, the scale of variation in vegetation types within a landscape, and the accuracy of the determination of burned areas, are constraints on the accuracy of fire-interval and seasonally determination perceived from an analysis of satellite data.
Abstract Georeferenced digital aerial photographs were used to assess changes in overstorey vegetation cover since 1948 in the Victoria River District, Northern Territory, Australia, across a range of lowland tropical savanna habitats and with explicit consideration of known and variable site-specific grazing and fire management histories. Vegetation surveys at corresponding locations on the ground identified five distinct woody vegetation communities defined primarily by water drainage and secondarily by soil characteristics. Air-photo analyses revealed that, contrary to popular perceptions and in contrast to results from other habitats, there has been no generalized net increase in overstorey woody vegetation cover across the full range of lowland savanna habitats. Rather, different habitats exhibited distinctly different vegetation change mechanisms: low-lying seasonally inundated ‘wet’ habitats have experienced woody vegetation increase since 1948, whereas well-drained ‘dry’ habitats have experienced overstorey vegetation stability or loss. In almost every instance woody vegetation increase could be attributed to the invasion or proliferation of a single species, Melaleuca minutifolia F.Muell. The extent of M. minutifolia increase was unrelated to historical grazing/fire regime. Demographic analyses for this species revealed that recruitment was often episodic and that synchronized recruitment events occurred uniformly across the full range of historical management treatments, most likely as a consequence of favourable climatic conditions in years with an extended wet season. Heavy grazing facilitated juvenile survival and/or recruitment, most likely by reducing grassy fuel loads and eliminating landscape fire. We conclude that while there has been no generalized net increase in overstorey woody vegetation cover in lowland environments, savanna dynamics are complex, and multiple change mechanisms have occurred simultaneously in different habitats, some of which have been significantly transformed since 1948. Where net woody vegetation increase has occurred it is primarily a natural consequence of episodic M. minutifolia establishment in climatically favourable years, but the extent and magnitude of this effect is likely mediated by fire/grazing regime.
Recent IPCC projections suggest that Africa will be subject to particularly severe changes in atmospheric conditions. How the vegetation of Africa and particularly the grassland–savanna–forest complex will respond to these changes has rarely been investigated. Most studies on global carbon cycles use vegetation models that do not adequately account for the complexity of the interactions that shape the distribution of tropical grasslands, savannas and forests. This casts doubt on their ability to reliably simulate the future vegetation of Africa. We present a new vegetation model, the adaptive dynamic global vegetation model (aDGVM) that was specifically developed for tropical vegetation. The aDGVM combines established components from existing DGVMs with novel process-based and adaptive modules for phenology, carbon allocation and fire within an individual-based framework. Thus, the model allows vegetation to adapt phenology, allocation and physiology to changing environmental conditions and disturbances in a way not possible in models based on fixed functional types. We used the model to simulate the current vegetation patterns of Africa and found good agreement between model projections and vegetation maps. We simulated vegetation in absence of fire and found that fire suppression strongly influences tree dominance at the regional scale while at a continental scale fire suppression increases biomass in vegetation by a more modest 13%. Simulations under elevated temperature and atmospheric CO2 concentrations predicted longer growing periods, higher allocation to roots, higher fecundity, more biomass and a dramatic shift toward tree dominated biomes. Our analyses suggest that the CO2 fertilization effect is not saturated at ambient CO2 levels and will strongly increase in response to further increases in CO2 levels. The model provides a general and flexible framework for describing vegetation response to the interactive effects of climate and disturbances.
Aim To study changes in woody vegetation in both floodplains and eucalypt savanna over a 40‐year period using multi‐temporal spatial analysis of variation in density of a large introduced herbivore, the Asian water buffalo ( Bubalus bubalis ). Feral buffalo built up to high densities in the study area until c. 1985, after which a control programme almost eliminated the animals. From 1990, low densities of managed buffalo were maintained inside an enclosure. We compared trends in woody vegetation when buffalo were high‐density feral, low‐density managed or absent.
Location The study area was located in and around a 116‐km ² buffalo enclosure inside Kakadu National Park, in monsoonal northern Australia.
Methods We analysed sequences of digitized and geo‐rectified aerial photographs, acquired in 1964, 1975, 1984, 1991 and 2004, to chart changes in woody cover on the floodplain and in the savanna. On the floodplain we assessed whether trees were present at these times at 14,568 points, and buffalo density was estimated from the density of animal tracks. In the savanna we estimated woody cover at pre‐selected sites. Generalized linear modelling was used to analyse changes in woody vegetation, using elevation and presence of woody vegetation in neighbouring points on the floodplain, and buffalo regime and initial woody cover in the savanna .
Results Changes in animal track density reflected park‐wide historical estimates of buffalo numbers. Tree cover increased in both floodplain and savanna, but this was only weakly related to buffalo density. The best predictor of whether a floodplain cell converted from treeless to woody, or the converse, was the woodiness of neighbouring vegetation. There was slightly less thickening with high buffalo densities. In savanna, low densities of managed buffalo were weakly associated with increases in tree cover relative to either high densities of feral buffalo or no buffalo.
Main conclusions Our study indicates that buffalo are not a major driver of floodplain and eucalypt savanna dynamics. Rather, the observed increase in woody cover in both savanna and flood plains concords with regional trends and may be related to increased atmospheric CO 2 , increasing rainfall and changing fire regimes during the study period.
It is possible that anthropogenic climate change will drive the Earth system into a qualitatively different state. Although different types of uncertainty limit our capacity to assess this risk, Earth system scientists are particularly concerned about tipping elements, large-scale components of the Earth system that can be switched into qualitatively different states by small perturbations. Despite growing evidence that tipping elements exist in the climate system, whether large-scale vegetation systems can tip into alternative states is poorly understood. Here we show that tropical grassland, savanna and forest ecosystems, areas large enough to have powerful impacts on the Earth system, are likely to shift to alternative states. Specifically, we show that increasing atmospheric CO2 concentration will force transitions to vegetation states characterized by higher biomass and/or woody-plant dominance. The timing of these critical transitions varies as a result of between-site variance in the rate of temperature increase, as well as a dependence on stochastic variation in fire severity and rainfall. We further show that the locations of bistable vegetation zones (zones where alternative vegetation states can exist) will shift as climate changes. We conclude that even though large-scale directional regime shifts in terrestrial ecosystems are likely, asynchrony in the timing of these shifts may serve to dampen, but not nullify, the shock that these changes may represent to the Earth system.
Dry season fires are a feature of the tropical savannas of northern Australia. As part of a landscape-scale fire experiment, we examined the effects of fire regimes on tree survival in a tropical savanna in Kakadu National Park, northern Australia. The fire regimes were annual early dry season (June) fires, annual late dry season (September) fires, and, no fire (control). Prescriptive, experimental fires were lit annually, between 1990 and 1994, in replicate compartments, each 15–20 km2. In addition to the prescribed fires, however, one of the control compartments, which had been unburnt for seven years, was burnt by an unplanned, high intensity fire (~ 20 000 kW m−1) in September 1994. This provided an opportunity to compare the impacts on the tree stratum of frequent, prescribed burning at various intensities, and a single unplanned fire. In all fire regimes, stem survival was substantially lower than whole-plant survival, and decreased linearly with increasing fire intensity. Significantly, stem death following the single, high intensity 20 000 kWm−1 fire (75%) was comparable to that of a regime of annual late dry season burning for five years, at an average intensity of c. 8000 kWm−1. In the high intensity unplanned fire, stem survival showed a non-linear response to stem size, being least in the small (< 10 cm DBH) and large (> 40 cm DBH) size classes, and highest in the intermediate size classes. Stem survival was also species-dependent, being higher in the dominant Eucalyptus miniata than in the subdominant, broad-leaf deciduous trees. In the absence of fire for 5–10 years, the structure and composition of the tree stratum of these savannas tends to become more complex than in sites burnt more frequently, especially by high intensity fire. Such a long-term absence of fire may be a conservation objective for some areas of savanna. However, build-up of fuel to near maximal levels can occur in 2–4 years without fire. This may predispose the savannas to high-intensity, late dry season fires. Whatever the fire-management goal within a given patch of savanna, whether it be the prescribed use of fire on a biennial basis, or the exclusion of fire at a semidecadal scale, careful attention still needs to be given to the consequences of fuel build-up in fire-excluded sites.
Increasing densities of woody plants in savannas has been attributed to both elevated atmospheric CO2 and reduced burning with grazing management, such that the biome could represent a substantial carbon sink. However, we show that extreme droughts (less than two-thirds expected rainfall over 3 years) occur in the drier half of the savanna biome and can cause substantial tree death. An Australian case study reveals that a net increase in tree cover over five decades of above-average rainfall was offset by sudden tree death during drought. The relationship between woody cover change and rainfall is moderated by competition with growth being facilitated by low woody cover and drought-induced death more likely as the woody component of savanna increases. The results are not supportive of a sustained increase in the woody component of xeric savannas resulting from CO2 fertilization or land management. Extensive tree death in savanna regions will become a stark consequence of climate change if predictions of increasing severity and frequency of drought are realized.
Contrary to expectations, measurements of pan evaporation show decreases in many parts of the Northern Hemisphere over the last 50 years. When combined with rainfall measurements, these data show that much of the Northern Hemisphere's terrestrial surface has become less arid over the last 50 years. However, whether the decrease in pan evaporation is a phenomenon limited to the Northern Hemisphere has until now been unknown because there have been no reports from the Southern Hemisphere. Here, we report a decrease in pan evaporation rate over the last 30 years across Australia of the same magnitude as the Northern Hemisphere trends (approximately −4 mm a−2). The results show that the terrestrial surface in Australia has, on average, become less arid over the recent past, just like much of the Northern Hemisphere. Copyright © 2004 Royal Meteorological Society.
Modification of fire regimes in tropical savannas can have significant impacts on the global carbon (C) cycle, and therefore,
on the climate system. In Australian tropical savannas, there has been recent, large-scale implementation of fire management
that aims to decrease Kyoto-compliant non-CO2 greenhouse gas emissions by reducing late dry season intense fires through strategic early dry season burning. However, there
is no accounting for changes to soil C stocks resulting from changes to savanna fire management, although impacts on these
pools may be considerable. We present a hypothesis that soil C storage is greatest under low intensity fires with an intermediate
fire return interval. Simulations using the CENTURY Soil Organic Matter Model confirmed this hypothesis with greatest soil
C storage under a fire regime of one low intensity fire every 5years. Key areas of uncertainty for CENTURY model simulations
include fine root dynamics, charcoal production and nitrogen (N) cycling, and better understanding of these processes could
improve model predictions. Soil C stocks measured in the field after 5years of annual, 3year and unburned fire treatments
were not significantly different (range 41–58tha−1), but further CENTURY modelling suggests that changes in fire management will take up to 100years to have a detectable impact
(+4tha−1) on soil C stocks. However, implementation of fire management that reduces fire frequency and intensity within the large
area of intact savanna landscapes in northern Australia could result in emissions savings of 0.17tCO2-eha−1y−1, four times greater than reductions of non-CO2 emissions.
Key wordsfire frequency–carbon sequestration–CENTURY soil organic matter model–soil nitrogen–fire return interval–fire intensity–greenhouse gas abatement
The strength of carbon sink and stock was assessed in a protected savanna of the Orinoco Llanos by the harvesting plant phytomass and using allometric relationships between the dry mass and the censuses of plant height. Thus, changes in the carbon stock and the proportion in the tree/grass proportion were evaluated throughout age states. Results indicate that the carbon stock in the vegetation increased from 207 to 9215 g C m−2 whereas in the soil, it varied 6680 to 12 196 g C m−2. The carbon stock accumulation was mainly related to increases in the woody layer from 36 to 9215 g C m−2 (255-fold) and in the soil from 1341 to 12 196 g C m−2 (nine-fold), respectively. The estimated pool of carbon sequestered in the Orinoco Llanos by the restored forest in 51 years was 5.69 Pg C. The expansion and conservation of this carbon pool might remove CO2 from the atmosphere to help compensate for CO2 liberation associated with other land uses or industrial practices.
To estimate forest biomass in a large area, it is necessary to clarify the relationship between attributes of stand structure obtained by remote sensing and woodland biomass. We examined stand structure and estimated woodland biomass in an arid region of Western Australia. The research site was near Leonora located 600 km from Perth in Western Australia. The annual rainfall is approximately 200 mm. The dominant woody vegetation species is Acacia aneura. The spatial characteristics of the woodland in this region are that woodland canopies are not closed and the tree crown silhouette is relatively clear-cut.We established 35 plots (main size, 50 m × 50 m) and the diameters at 1.3 and 0.3 m heights, tree height and canopy silhouette area of all trees in the plots were determined. Each tree biomass was calculated by allometric equations using a destructive sampling method. Results of regression analysis indicated that the appropriate stand structural attributes for estimation of woodland biomass were stand basal area (SBA), canopy coverage (CC) and leaf area index (LAI). SBA had the highest estimation accuracy of woodland biomass but SBA was not suitable for estimation by satellite imagery. The woodland biomass estimation accuracy by CC (R2 > 0.94, P < 0.0001) or LAI (R2 > 0.92, P < 0.0001) was lower than that by SBA (R2 > 0.99, P < 0.0001) but CC and LAI are considered to be powerful indicators for woodland biomass estimation by satellite imagery, where open forest is distributed. Particularly, in the case of arid land, the correlation between CC and woodland biomass was reported in the Salhel region of Africa and in Queensland, Australia; therefore, the forest biomass estimation method by CC is considered to be applicable to other arid- and semiarid-area open forests and woodlands.
The tropical savanna landscape of northern Australia is perhaps the most extensive and flammable ecosystem in the world. Like savannas worldwide, there still remains uncertainty as to the functioning of much of this system and the mechanisms maintaining a state between open grassland and woodland. We use the development of a process-based model to examine the processes operating on the eucalypt dominated tree component of the northern Australian savannas, focussing on the interaction of water availability and fire.A process-based, tree population dynamics model (Flames) has been developed to understand the interactive effects of fire and rainfall variability on trees in Australian savanna ecosystems where there has been limited modelling to date. This paper shows that the model is capable of simulating the range of tree populations found in northern Australia by considering soil properties (texture and depth) and rainfall distribution. The model indicates that seasonal variability and inter-annual variability in rainfall is critical in defining the structure and density of tree stands in northern Australia. We also show that fire plays an important role in defining tree structure by removing the most sensitive (the very young and old) trees and that frequent fronting fires will lead to ongoing reductions in tree populations. While fire is an important factor in northern Australia, the model indicates its effects on tree populations at a sub-continental scale are secondary to the impact of rainfall variability.