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When is a ‘forest’ a savanna, and why does it matter?
Abstract
Savannas are defined based on vegetation structure, the central concept being a discontinuous tree cover in a continuous grass understorey. However, at the high-rainfall end of the tropical savanna biome, where heavily wooded mesic savannas begin to structurally resemble forests, or where tropical forests are degraded such that they open out to structurally resemble savannas, vegetation structure alone may be inadequate to distinguish mesic savanna from forest. Additional knowledge of the functional differences between these ecosystems which contrast sharply in their evolutionary and ecological history is required. Specifically, we suggest that tropical mesic savannas are predominantly mixed tree–C4 grass systems defined by fire tolerance and shade intolerance of their species, while forests, from which C4 grasses are largely absent, have species that are mostly fire intolerant and shade tolerant. Using this framework, we identify a suite of morphological, physiological and life-history traits that are likely to differ between tropical mesic savanna and forest species. We suggest that these traits can be used to distinguish between these ecosystems and thereby aid their appropriate management and conservation. We also suggest that many areas in South Asia classified as tropical dry forests, but characterized by fire-resistant tree species in a C4 grass-dominated understorey, would be better classified as mesic savannas requiring fire and light to maintain the unique mix of species that characterize them.
... The popularity of rangeland afforestation is supported by the myth that rangelands represent degraded forests, rather than natural biomes supported by specific climatic conditions and natural disturbance regimes (Davis and Robbins 2018;Bond 2019). This erroneous, but widely held, interpretation is believed to have originated with Western European scholars in the 18th century and then became more widespread during colonial expansion (Ratnam et al. 2011;Joshi et al. 2018). The institutional dominance of forestry services during this period promoted forested landscapes as environmentally beneficial and rangelands as degraded-a perception that remains to this day. ...
... Insufficient distinctions between savannas and forests also contribute to the justification of rangeland afforestation (Ratnam et al. 2011;Veldman et al. 2015). The UN Food and Agricultural Organization defines a forest as any area greater than 0.5 ha in size with trees exceeding 5 m in height and a tree canopy that extends over more than 10% of the ground area (FAO 2020). ...
... Consequently, many of the proposed tree planting targets have been designated in areas that historically were savannas or only sparsely wooded (Veldman et al. 2015;Veldman 2016). However, forests and savannas have fundamentally different ecological dynamics, in addition to ecosystem structures, that are reflected in distinct species assemblages with different functional traits and responses to fire (Ratnam et al. 2011;Veldman et al. 2015). Tree planting in historically non-forested areas therefore represents afforestation rather than reforestation (Veldman 2016). ...
Large‐scale tree planting on global rangelands is promoted as a natural climate solution (NCS), but there is little scientific evidence to support this narrative. The presumed benefits of rangeland afforestation originate from five major misconceptions: (1) conflation between reforestation and afforestation, (2) overestimation of carbon (C) sequestration potential, (3) insufficient recognition of rangeland ecosystem services, (4) potential for adverse ecological outcomes, and (5) neocolonial tendencies of afforestation programs. Rangeland afforestation possesses minimal potential for additional C storage, but it has high potential to reduce vital rangeland ecosystem services that benefit rangeland residents and non‐residents alike. Conservation of existing C—most of which is stored belowground, where it is less vulnerable to loss—may prove to be the most appropriate NCS for extensively managed rangelands. Stewardship strategies promoting rangeland multifunctionality will not only contribute to climate‐change mitigation but also support biodiversity conservation and sustainable production of high‐protein foods for marginalized populations.
... Tropical savannas can be more explicitly defined as mixed tree-C 4 grass systems, with an understory predominantly comprised of warm season grasses that utilize the C 4 photosynthetic pathway, while the trees utilize the C 3 pathway (Huntley, 1982;Scholes and Archer, 1997;Sage, 2004;Beerling and Osborne, 2006;Ratnam et al., 2011). C 3 photosynthesis, in which CO 2 is first fixed to produce a three-carbon compound phosphoglyceric acid (PGA), is the more ancestral form, having dominated the process of photosynthetic CO 2 fixation for over 99% of the time since it evolved B2.7 billion years ago . ...
... The limits to its distribution and the nature of the biome boundaries at both the arid and the mesic end are highly variable across the globe, and the dynamics underlying these patterns remain poorly understood. Many landscapes in the mesic end of the savanna biome, for example southern peninsular India, mainland Southeast Asia from Myanmar through northern Thailand, and the Guinean and Congolian savanna-forest regions in Africa are characterized by a dynamic mosaic of vegetation with open savannas, woodland savannas, and tropical dry forests occurring in a patchwork within the same climatic envelope (White, 1983;Stott, 1991;Ratnam et al., 2011). Climate alone thus appears insufficient to explain the distribution of the savanna biome (Bond, 2008;Hirota et al., 2011;Staver et al., 2011). ...
... The forest-savanna mosaics, as their name suggests, occur at the mesic limits of the savanna biome where savannas transition into closed-canopy forests. The savannas at these regions are densely wooded and structurally begin to approach the appearance of slightly open forests, but grasses are still dominant in the understorey (Bond and Parr, 2010;Ratnam et al., 2011). ...
... Savanas possuem numerosas e diferenciadas acepções, conceitos e definições mundo afora, com uma vastíssima literatura -uma revisão abrangente sobre isso é encontrada em Walter et al. (2008) componentes arbóreo e arbustivo-herbáceo, dão bastante destaque à necessária presença de árvores C3 e, especialmente, de gramíneas C 4 (p.ex. Mistry 2000a, Sankaran et al. 2004, Beerling & Osborne 2006, Accatino et al. 2010, Romero-Ruiz et al. 2010, Ratnam et al. 2011, Baudena et al. 2015, Nakanyala et al. 2017, ou até gramíneas C 3 , no caso do Cerrado (p.ex. Lloyd et al. 2008). ...
... Não se justifica este emprego abusivo do termo savana, mas isso remonta ao clássico sistemata e fitogeógrafo prussiano Adolf Engler, que dentro de suas formações xerófilas, indicou "florestas das savanas" entre as fitofisionomias dos países tropicais e subtropicais. Ratnam et al. (2011), por sua vez, discutiram diretamente o sombreamento de conceitos entre savana e floresta, e sugeriram algumas características morfológicas, fisiológicas e de história de vida para separá-los, pois essa diferenciação afeta práticas de manejo e de conservação. ...
... As savanas sul-americanas estão distribuídas em solos que variam de rasos e arenosos, a profundos e argilosos. Apesar da imensa variação quanto distribuição e características abióticas, em geral tais savanas são caracterizadas pela combinação, em diferentes níveis de proporção, de um componente herbáceo usualmente contínuo, com um componente arbustivo-arbóreo de variada densidade (Ratnam et al. 2011). Eventualmente, este componente se torna tão denso que o componente herbáceo se reduz bastante, ou mesmo desaparece, dando espaço às formações florestais (Sankaran et al. 2004). ...
As savanas da América do Sul representam entre 8 e 10% da cobertura das savanas globais. De forma dispersa, elas cobrem pouco mais de 2,29 milhões de quilômetros quadrados no continente, e representam o espectro mais úmido e mais biodiverso das savanas globais. As savanas sul-americanas apresentam considerável grau de endemismo e suas características climáticas e composição em espécies são bastante influenciadas por biomas florestais vizinhos, como a Amazônia e a Mata Atlântica. Savanas sul-americanas ocorrem sobre solos que variam de arenosos a argilosos, em altitudes desde cerca de 150 metros até em torno de 2.000 metros acima do nível do mar, e em enclaves tão pequenos quanto 50 km2 até extensões imensas como aquelas ocorrentes no planalto central brasileiro. Muitas destas savanas são sujeitas a alagamento periódico o que as tornam únicas na sua dinâmica. Este artigo apresenta a revisão de um conjunto de informações sobre as savanas ocorrentes no continente sul-americano, analisando conceitos e abordando desde suas principais características climáticas, até particularidades da vegetação e da flora, espécies mais comuns e atributos funcionais. Muitas destas informações estão organizadas em tabelas, facilitando o acesso a dados pontuais. Assuntos correlatos, como a toxicidade imposta pelo alumínio presente nos solos de muitas delas e a presença de formações florestais e campestres ocorrendo na paisagem junto às formações savânicas são apresentados de forma complementar. Este artigo cobre uma lacuna acerca das formações savânicas na América do Sul e vem contribuir para a divulgação do conhecimento deste importante, complexo, mal compreendido e ameaçado ecossistema.
... The total tree species counts for the both forest sites were observed to be relatively higher than those for both savanna sites (Table 2). This situation agrees with the findings of Adejuwon and Adesina (1988) and Ratnam et al (2011). The relatively lower species richness values observed in the exploited forest supports other forest research results highlighting tropical forests as possessing lower specie densities, higher diversities while also manifesting very little degree of trees clumping (Ratnam et al, 2011;Keay, 1953;Emuh & Gbadegesin, 2009). ...
... This situation agrees with the findings of Adejuwon and Adesina (1988) and Ratnam et al (2011). The relatively lower species richness values observed in the exploited forest supports other forest research results highlighting tropical forests as possessing lower specie densities, higher diversities while also manifesting very little degree of trees clumping (Ratnam et al, 2011;Keay, 1953;Emuh & Gbadegesin, 2009). The exploited and control forests exhibited slightly different patterns of specie composition. ...
With population on the rise, efficient management of vegetation landscapes in developing countries has become increasingly challenging, owing to fuelwood usage as an affordable source of fuel for daily use. Proper understanding of fuelwood exploitation effect on varying vegetation landscapes for the purpose of effective management in Southern Nigeria, is limited. This study, therefore, examines the effect of fuelwood extraction on forest and savanna landscapes in Oyo State, South Western Nigeria. Vegetation measurements were taken from twelve (20m by 20m) quadrats respectively within the forest and savanna exploited sites and their associated control sites, totaling 48 quadrats. These quadrats were randomly selected along a 400m transect. The findings of the study reveal that mean dendrometric values were generally lower in exploited sites. The mean values for the exploited/control savanna and the exploited/control forests respectively include: Girth (cm): 25.3/48.5 and 53.0/128.8, Height (m): 4.84/6.02 and 10.04/20.74, Basal Area (msq)-0.05/0.28 and 0.32/2.32, Tree density (m/Ha): 6.08/13.25 and 11.66/12.33, Tree Diversity-0.79/0.863 and 0.85/ 0.97. Funtumia elastica, Triplochiton scleroxylon, Albizia zygia and Vitellaria paradoxa, Anogeissus leiocarpus and Azadiracta indica were dominant in the forest and savanna plots respectively. Student 't' test analysis result showed significant variation in mean dendrometric parameters and diversity values at p<0.05. Provision of affordable environmentally friendly energy alternatives for the local people is being recommended.
... Ecological definitions of savanna vs. seasonally dry tropical forest in Asia are complex and debated (30). On the one hand, more open canopy units (termed "dry deciduous forest") (31) within the naturally mosaicked complex of seasonally dry tropical forest could be classified as savanna given that they host firetolerant, mixed tree-C 4 grass systems (e.g. ...
... On the one hand, more open canopy units (termed "dry deciduous forest") (31) within the naturally mosaicked complex of seasonally dry tropical forest could be classified as savanna given that they host firetolerant, mixed tree-C 4 grass systems (e.g. ref. 30). The other argument (e.g., ref. 32) considers tropical savanna an alternative stable state to tropical forest, and that transitions between forest and savanna resulting in abrupt losses of tree cover are positively reinforced by climate and fire, and are difficult to reverse (33,34). ...
The dominant paradigm is that large tracts of Southeast Asia’s lowland rainforests were replaced with a “savanna corridor” during the cooler, more seasonal climates of the Last Glacial Maximum (LGM) (23,000 to 19,000 y ago). This interpretation has implications for understanding the resilience of Asia’s tropical forests to projected climate change, implying a vulnerability to “savannization”. A savanna corridor is also an important foundation for archaeological interpretations of how humans moved through and settled insular Southeast Asia and Australia. Yet an up-to-date, multiproxy, and empirical examination of the palaeoecological evidence for this corridor is lacking. We conducted qualitative and statistical analyses of 59 palaeoecological records across Southeast Asia to test the evidence for LGM savannization and clarify the relationships between methods, biogeography, and ecological change in the region from the start of Late Glacial Period (119,000 y ago) to the present. The pollen records typically show montane forest persistence during the LGM, while δ13C biomarker proxies indicate the expansion of C4-rich grasslands. We reconcile this discrepancy by hypothesizing the expansion of montane forest in the uplands and replacement of rainforest with seasonally dry tropical forest in the lowlands. We also find that smooth forest transitions between 34,000 and 2,000 y
ago point to the capacity of Southeast Asia’s ecosystems both to resist and recover from climate stressors, suggesting resilience to savannization. Finally, the timing of ecological change observed in our combined datasets indicates an ‘early’ onset of the LGM in Southeast Asia from ~30,000 y ago.
... Subtropical monsoon grasslands tend to shift to forests as high precipitation (where mean annual precipitation exceeds 1200 mm) favours tree establishment in grasslands (Sankaran et al., 2005;Staver et al., 2011;van Langevelde et al., 2003). The coexistence of grasses and trees in the high-rainfall region of subtropical Asia is maintained by frequent fire, herbivory, and human activities (Banerjee et al., 2023;Ratnam et al., 2016Ratnam et al., , 2011Sankaran, 2016;Sankaran et al., 2005;van Langevelde et al., 2003). A change in the fire regime, herbivory or human activities might increase the risk of the establishment of forests in grasslands (Banerjee et al., 2023;Gross et al., 2013;Kumar et al., 2021Kumar et al., , 2020, as observed in high-rainfall regions of Australia, Africa, and South America (Sankaran et al., 2005;Staver et al., 2011). ...
... While fire plays an important role in maintaining the coexistence of grasses and trees (Ratnam et al., 2016(Ratnam et al., , 2011Sankaran, 2016;Staver et al., 2011;van Langevelde et al., 2003), it can disrupt the positive interaction between grazing and grazing lawns (Archibald et al., 2005). Fire generally occurs at the landscape scale during the dry season, and after a burn, the entire burned area consists of nutritious re-sprouts (Donaldson et al., 2018;Moe and Wegge, 1997;Van de Vijver et al., 1999). ...
Subtropical monsoon grasslands in Asia are commonly in a fire-dominated state with tall grasses > 2 m) that provide poor-quality forage for mammalian herbivores. In contrast, small patches of
grazing lawns with short, nutritious grasses are sparsely distributed within these subtropical monsoon grasslands. Despite the importance of grazing lawns in providing high-quality forage for mammalian herbivores, the process of formation and maintenance of grazing lawns has not been studied in the monsoon grasslands of subtropical Asia. We conducted a large-scale multi-year experiment in subtropical monsoon grasslands within a protected area in Nepal to examine
whether mowing tall grasses at different frequencies and spatial scales, along with the application of chemical fertilisers (urea or single superphosphate), could change tall grasslands into grazing
lawns. We found that nitrogen and phosphorus concentrations in grasses increased to levels exceeding the minimum maintenance requirements for ruminants in 3600 m2 plots that were
mown four times and fertilised. The concentrations of nitrogen and phosphorus remained below the minimum maintenance level for ruminants in unfertilised and unmown plots and in small
plots that were four-time mown and fertilised. The frequently mown plots exhibited a significant increase in the proportion of ground cover of lawn grass, from 1% to 3%, and a corresponding
decrease in the proportion of ground cover of tall grasses, from 24% to 1%. These results supported our hypothesis that frequent mowing at a larger spatial scale with the addition of chemical
fertilisers and grazing leads to the inception of grazing lawns that are favourable for wild herbivores. However, the persistence of the grazing lawns presents challenges due to the highly
productive monsoon season and the functional extinction of large bulk grazers. To our knowledge, this is the first empirical study conducted in the subtropical monsoon grasslands of Asia to
experimentally demonstrate the importance of active management for the initiation of grazing lawns. Our findings have important implications for the management of subtropical monsoon grasslands.
... It is important to manage these Cymbopogon-dominated tropical mesic savanna ecosystems (FES, 2009;Kumar, 2014;Ratnam et al., 2011) in the Eastern Ghats toward a state that more resembles the recent past and offers ecosystem services such as biodiversity and palatable herbaceous plant biomass. Because ...
... Cymbopogon grasses can rapidly resprout and recover quickly from fire (Sankaran, 2005), it is possible that reducing the frequency of fire by creating firebreaks will be adequate to allow the recovery (Bachinger et al., 2016) of less fire-tolerant species, some of which will be palatable species (Archibald et al., 2019). Alternatively, an additional treatment in which unpalatable and fire-tolerant Cymbopogon are manually removed might be necessary to significantly increase the abundance and diversity of palatable herbaceous Our study area is classified as tropical mesic savannas (Ratnam et al., 2011(Ratnam et al., , 2016 and occurs between 379 and 556 m above sea level. This biome has open tree canopies and C 4 grasses in the understory (Ratnam et al., 2019). ...
Human activities in mesic savanna ecosystems have resulted in plant communities that are heavily dominated by fire‐tolerant grass species, are less diverse, and offer fewer ecosystem services such as palatable plant biomass. Experimental studies manipulating fire and grass presence have mostly been conducted in ecosystems invaded by exotic grass species. However, these experiments are also relevant to ecosystems that have become dominated by native grass species due to changes in human activities. Our study compared three different management treatments in the Eastern Ghats of India, where mesic savanna ecosystems are highly dominated by the unpalatable native Cymbopogon grasses, specifically: (1) control (both Cymbopogon grasses and fire are present), (2) fire exclusion, and (3) manual removal of Cymbopogon grasses and fire exclusion. We found that both grass removal and fire exclusion were necessary to significantly increase palatable herbaceous plant biomass and species diversity, and that species diversity responses were only significant at larger spatial grains of investigation. High site‐to‐site variation in the grass removal and fire exclusion treatment prevented us from detecting significant differences in species composition across treatments, even though particular palatable grass species benefited from the treatment. Our study is in line with research from other mesic savanna systems showing that fire management alone is not sufficient to transition ecosystems to a desired or historical state. We demonstrate how normally costly management practices, such as manual removal of dominant grasses, can be reasonably achieved through collaboration between scientists, community governed village institutions, and government and nongovernment agencies in socioecological systems.
... Typical pioneer TRF species can also become established in open degraded areas and are fast growers, initiating rapid Pennington and Sarukhan, 2005). Once pioneer species colonize an empty land, later successional phase TRF tree species, which are more shade tolerant, can easily become established (Ratnam et al. 2011;Hoffmann et al. 2012). Furthermore, forest species reduce the likelihood of fire ocurrence and spread of fires as their large canopies cast more shade, reducing herbaceous growth, increasing humidity, and reducing wind speed, which reduces fire occurrence (Ratnam et al. 2011;Hoffmann et al. 2012;Parr et al. 2014). ...
... Once pioneer species colonize an empty land, later successional phase TRF tree species, which are more shade tolerant, can easily become established (Ratnam et al. 2011;Hoffmann et al. 2012). Furthermore, forest species reduce the likelihood of fire ocurrence and spread of fires as their large canopies cast more shade, reducing herbaceous growth, increasing humidity, and reducing wind speed, which reduces fire occurrence (Ratnam et al. 2011;Hoffmann et al. 2012;Parr et al. 2014). However, most TRF (Aide et al. 2000;Ashton et al. 2014;Holl 1999;Lwanga 2003). ...
The factors governing the dynamics of tropical rainforest (TRF) and humid native savannas are little understood. In Mexico, both biomes are critically endangered owing to land-use change. Tropical pine plantations established in such sites might alter the savanna-TRF dynamics helping to elucidate the underlying factors of the vegetation dynamics in lowland humid tropics. This study aims to investigate the role of Pinus caribaea plantations on the savanna-TRF dynamics in the Gulf of Mexico coastal plain in areas where both biomes are close to one another. Such plantations were established in areas previously occupied by savannas and close to TRF and savanna fragments. Due to unusual circumstances, the plantations remained virtually unmanaged after being planted. We found that after 28–33 years of growth, the understory of pine plantations was similar to that of the TRF in tree species composition, species richness, stem density, and basal area. Pine trees cannot become established under the canopy of TRF, savanna trees, and pine plantations. The understory of savanna trees is unsuitable for establishing most tree species, including savanna trees. Pinus caribaea can alter the TRF-savanna dynamics by facilitating the establishment of TRF species in their understory in areas occupied by savannas, probably due to a combination of properties of both savanna (e.g., fre tolerance) and pioneer TRF (e.g., fast-growing rates) tree species. Thus, our results suggest that exotic pine plantations could be promising tools for TRF regeneration and suppressing savanna regeneration, thus creating a dilemma on what to preserve and restore.
... The canopy cover can be higher than 10%, up to 60% (Hirota et al. 2011, Kutsch et al. 2011. The understorey, the layer of vegetation beneath the main canopy of a forest, is characterised by C4 grasses (warm season grasses) (Putz and Redford 2010, Ratnam et al. 2011, Oliveras and Malhi 2016. The C4 photosynthetic pathway makes these grasses tolerant to higher temperatures and drought but less tolerant to shade compared to C3 grasses (cool season grasses) (Ratnam et al. 2011, Oliveras andMalhi 2016). ...
... The C4 photosynthetic pathway makes these grasses tolerant to higher temperatures and drought but less tolerant to shade compared to C3 grasses (cool season grasses) (Ratnam et al. 2011, Oliveras andMalhi 2016). This woodland definition overlaps with that of savanna in the more heavily 'treed' versions of the spectrum (Ratnam et al. 2011). The characterising presence of C4 grasses in woodlands, and savannas in general, is, however, difficult to monitor as they are not always present during long droughts, after fires, or because of overgrazing by livestock or game. ...
An overview of Namibia's forests and the current status of sustainable forest management and research, concluded with a SWOT analysis.
... The canopy cover can be higher than 10%, up to 60% (Hirota et al. 2011, Kutsch et al. 2011. The understorey, the layer of vegetation beneath the main canopy of a forest, is characterised by C4 grasses (Putz and Redford 2010, Ratnam et al. 2011, Oliveras and Malhi 2016. The C4 photosynthetic pathway makes these grasses tolerant to higher temperatures and drought but less tolerant to shade compared to C3 grasses (Ratnam et al. 2011, Oliveras andMalhi 2016). ...
... The C4 photosynthetic pathway makes these grasses tolerant to higher temperatures and drought but less tolerant to shade compared to C3 grasses (Ratnam et al. 2011, Oliveras andMalhi 2016). This woodland definition overlaps with that of savanna in the more heavily "treed" versions of the spectrum (Ratnam et al. 2011). The characterising presence of C4 grasses in woodlands, and savannas in general, is however difficult to monitor as they are not always present during long droughts, after fires, or because of overgrazing by livestock or game. ...
This baseline study on sustainable forest management in Namibia was published as part of the ‘Promoting Sustainable Forest Management in the Kavango-Zamebzi Region in Namibia’ project, an initiative implemented by the Hanns Seidel Foundation (HSF) Namibia together with the Desert Research Foundation of Namibia (DRFN). The report is also available on the Think Namibia website: https://www.thinknamibia.org.na/images/projects/forest/pdf/Baseline_Study_final-komprimiert_2x_compressed.pdf
... In Cambodia, forest area decreased by 11.01% over the 10-year period, transitioning to other land cover types such as grasslands (6.24%) and savannas (2.58%). These changes, characterized by sparse tree coverage and prevalent grasses, are often the consequence of deforestation for agricultural expansion (Ratnam et al., 2011). The area of cropland and small-scale cropland mixed with natural vegetation increased by 2.24%, indicating that Cambodia experienced the most significant agricultural development compared to the other three SEA countries. ...
Foreign direct investment (FDI) can reshape landscapes in developing countries, but its impact remains unclear. This study examines how China’s Belt and Road Initiative (BRI) FDI impacts land-cover and land-use change in Southeast Asia, a key trade partner receiving significant Chinese infrastructure investments. Focusing on areas with BRI investments from 2008 to 2018, we utilize satellite data to analyze land-use changes across Southeast Asia (Cambodia, Laos, Myanmar, Vietnam), particularly urban growth and deforestation. We find that districts that received BRI investment experienced nearly a 5% greater rate of deforestation than the regional average, with a 001% and 0.007% in tree cover associated with a 1% increase in Chinese investment. Districts receiving investments also showed greater increases in cropland and grasslands. While statistical tests indicate the immediate land-cover changes are modest, our findings suggest potential future environmental consequences in Southeast Asia, particularly with ongoing economic development.
... Pastures cover approximately 1.5 million km 2 , an area equivalent to the size of the Cerrado, the second largest biome in Brazil (Souza Jr et al., 2020). After the decades of poor pasture management, these areas tend to become a low-diversity, savanna-like vegetation (Barlow & Peres, 2008;Cava et al., 2018;Ratnam et al., 2011). Such changes may affect the feeding ecology of animal communities, altering the energy distribution along food chains, potentially affecting ecosystem functions. ...
Since consumers reflect the isotopic composition of an assimilated diet, stable isotopes can be a useful tool to address the feeding ecology of tropical snakes. This is the first study reporting carbon and nitrogen stable isotopic composition of Bothrops atrox (Linnaeus, 1758) living in different landscapes located in the lower Amazon river, encompassing four main natural landscapes of the Amazon: old‐growth forests, várzeas (flooded forests), savannas, and pastures. Our null hypothesis is that the δ ¹³ C of forest specimens of B.atrox is more negative because forests are dominated by C 3 plants, while C 4 plants are common in the other landscapes. On the other hand, δ ¹⁵ N of forest specimens should be more positive, since the δ ¹⁵ N of old‐growth forests are higher than plants of savanna, várzea , and pastures. Confirming our hypothesis, the δ ¹³ C of B. atrox scales of the Tapajós National Forest was approximate −25‰ to −24‰, increased to approximately −23.5‰ to −23.0‰ in the savanna and pasture, and to −21‰ in the várzea , showing an increased contribution of C 4 ‐derived carbon. Some specimens of B. atrox had δ ¹⁵ N as high as 18‰, which is much higher than the average δ ¹⁵ N of the snake's prey (7‰), confirming the apex position of B. atrox in the Amazon region. The δ ¹⁵ N values of the forest specimens were 5‰ higher than the savanna specimens, and this difference decreased to 3‰ between the forest and the pasture, and the v árzea specimens. Finally, there were not large differences between δ ¹⁵ N values of livers and scales in any of the landscapes, suggesting a constant diet through time, and reinforcing the possibility of the use of snake's scale as a less invasive and non‐lethal tissue to analyze.
Abstract in Portuguese is available with online material.
... For the African continent, we follow a contribution made in the Yangambi agreements, where vegetation formations were synthesized, labelled and defined in both French and English languages (Aubréville, 1957). The fundamental characteristic of savannas is their propensity to burn and to host large herbivores, specifically mammalian grazers and browsers (Charles-Dominique et al., 2016), and the associated importance of C4 grasses in the understorey (Aubréville, 1957;Dexter et al., 2015;Ratnam et al., 2011). ...
Aim
In tropical Africa, savannas cover huge areas, have high plant species richness and are considered as a major natural resource for most countries. There is, however, little information available on their floristics and biogeography at the continental scale, despite the importance of such information for our understanding of the drivers of species diversity at various scales and for effective conservation and management. Here, we collated and analysed floristic data from across the continent in order to propose a biogeographical regionalization for African savannas.
Location
We collated floristic information (specifically woody species lists) for 298 samples of savanna vegetation across Africa, extending from 18° N to 33° S and from 17° W to 48° E.
Taxa
We focused on native woody species.
Methods
We used ordination and clustering to identify the floristic discontinuities and gradual transitions across African savannas. Floristic relationships, specificity and turnover, within and between floristic clusters, were analysed using a (dis‐)similarity‐based approach.
Results
We identified eight floristic clusters across African savannas which in turn were grouped into two larger macro‐units. Ordinations at species and genus levels showed a clear differentiation in woody species composition between the North/West macro‐unit and the South/East macro‐unit. This floristic discontinuity matches to the High (i.e. N&W) and Low (S&E) division of Africa previously proposed by White (1983) and which tracks climatic and topographical variation. In the N&W savannas, the floristic gradient determined by rainfall was partitioned into the Sudanian (drier) and Guinean (wetter) clusters. Within the highly heterogeneous S&E savannas and woodlands, six clusters were identified: Ugandan, Ethiopian, Mozambican, Zambezian, Namibian and South African.
Main conclusions
The proposed pan‐African classification of savannas and woodlands might assist the development of coordinated management and conservation policies.
... The C 4 grasses are the functionally dominant element in the savanna ground layer, abundant in fire-intensive environments, characterized by fire-grass positive feedbacks (D'Antonio and Vitousek, 1992), with consistent evidence of a decline in C 4 grass richness under fire exclusion (Peterson and Reich, 2008;Bowles and Jones, 2013;Diaz-Toribio et al., 2020). The C 3 grasses are uncommon in tropical savannas but can be common in shaded sites and in sites of fire exclusion reflecting a transition from open savanna to closed forest (Ratnam et al., 2011). Nevertheless, C 3 grass species richness might still increase with high fire frequency in some environments (e.g. ...
Background and Aims
Little is known about the response of ground layer plant communities to fire in Miombo ecosystems, which is a global blind spot of ecological understanding. We aimed: (1) to assess the impact of three experimentally imposed fire treatments on ground layer species composition and compare it with patterns observed for trees; and (2) to analyse the effect of fire treatments on species richness to assess how responses differ among plant functional groups.
Methods
At a 60-year-long fire experiment in Zambia, we quantified the richness and diversity of ground layer plants in terms of taxa and functional groups across three experimental fire treatments of late dry-season fire, early dry-season fire and fire exclusion. Data were collected in five repeat surveys from the onset of the wet season to the early dry season.
Key Results
Of the 140 ground layer species recorded across the three treatments, fire-maintained treatments contributed most of the richness and diversity, with the least number of unique species found in the no-fire treatment. The early-fire treatment was more similar in composition to the no-fire treatment than to the late-fire treatment. C4 grass and geoxyle richness were highest in the late-fire treatment, and there were no shared sedge species between the late-fire and other treatments. At a plot level, the average richness in the late-fire treatment was twice that of the fire exclusion treatment.
Conclusions
Heterogeneity in fire seasonality and intensity supports diversity of a unique flora by providing a diversity of local environments. African ecosystems face rapid expansion of land- and fire-management schemes for carbon offsetting and sequestration. We demonstrate that analyses of the impacts of such schemes predicated on the tree flora alone are highly likely to underestimate impacts on biodiversity. A research priority must be a new understanding of the Miombo ground layer flora integrated into policy and land management.
... Trees may also be more able to escape competition on nutrient-rich soils (Silva et al., 2013;Van Der Waal et al., 2009), indicating that trees may compete more effectively in higher nutrient environments by investing more biomass above-ground due to decreased belowground competition (Hoffmann et al., 2012;Pellegrini, 2016). While savannas are not considered classically light-limited (Hoffmann & Franco, 2003;Ratnam et al., 2011), tree-grass competition for light may be substantial (Ludwig et al., 2001;Moustakas et al., 2013;Vadigi & Ward, 2013) and trees also accumulate above-ground biomass as a strategy to escape the effects of fire and herbivory (Hoffman et al., 2011;Staver et al., 2009;Wakeling et al., 2011). Under this second hypothesis, we expect (1) that grasses will experience little below-ground competition from trees (February, Higgins, et al., 2013;Moustakas et al., 2013;Tjoelker et al., 2005), invest biomass in roots for more aggressive nutrient acquisition, aggressively compete for both N and P in low nutrient settings, and increase their phosphatase activity (Craine, 2005;Craine et al., 2008), whereas (2) trees will experience substantial competition from grasses (Riginos, 2009) and invest above-ground when nutrients are readily available, as tree biomass in savannas should theoretically increase with higher nutrient availability (Hoffmann et al., 2012, Pellegrini, 2016 to avoid shading or fire disturbance. ...
Plant essential macronutrients like nitrogen (N) and phosphorus (P) can limit savanna tree growth and are important determinants of savanna vegetation dynamics, along with rainfall, fire and herbivory. How nitrogen and phosphorus shape tree‐grass competition and their coexistence remain unclear, hindering our ability to predict how savannas may respond to altered nutrient cycling.
Here, we evaluate (1) if trees and grasses respond differently to N versus P availability, or (2) if grasses are more competitive in low nutrient environments while trees are more competitive in high nutrient environments. To do this, we grew saplings of 6 tree and 1 grass species from the Kruger National Park, South Africa, for 16 weeks under fully factorial nutrient and competition treatments (with/without competitors, low/high rate of N supply and low/high rate of P supply) under a watering regime designed to mimic wet season rainfall in a mesic savanna.
Trees and grasses foraged most aggressively for nitrogen and allocated biomass differently depending on nitrogen availability. Overall, tree growth decreased in competition with grass, even in high nutrient environments where they grew faster. Grasses were always better below‐ground competitors, utilising aggressive nutrient foraging strategies, including high root phosphatase activity in response to nitrogen and large root biomass allocation.
Synthesis . In low nutrient environments (e.g. on nutrient‐poor sandy soils), nutrients may limit tree growth. Nutrient rich environments enable tree growth, but grasses continue to compete effectively with trees. Understanding what this means for ecosystem responses to nutrient availability is not trivial, especially in the context of fire and herbivory. However, it is clear that soil nutrients likely affect tree and grass growth and competition in savannas, which suggests that future changes in nutrient cycling, such as N deposition, may have important effects on savanna vegetation.
... Burned soil biota reduced negative feedback effects on S. scoparium and promoted S. scoparium growth in S. scoparium trained soil. This trend matches observations that fire benefits C4 grass reseeding and dominance in fire frequented grassland and savanna ecosystems (Robinson et al. 1979;Tix and Charvat 2005;Ratnam et al. 2011;Ripley et al. 2015;Simpson et al. 2020). The observed fire driven reduction of negative PSFs on S. scoparium suggest that fire may remove harmful pathogens known to build-up in grass trained soil (Bauer et al. 2017). ...
Background and aims
Plant soil feedbacks (PSF) are reciprocal mechanisms through which interactions between plants and soil biota and affect future plant growth. When scaled up to the community level, PSFs are important determinants of above- and belowground community dynamics that influence long-term successional trajectories. Despite over three decades of ecological PSF research, we have a poor understanding of how common environmental processes like fire influence the strength and direction of PSFs. The aim of this study was to evaluate fire effects on PSFs between two common grassland species: Schizachyrium scoparium and Rudbeckia hirta.
Methods
In this work we evaluated how fire effects on S. scoparium and R. hirta associated soil biota influenced feedbacks on plant growth using a two phase experiment. We tested this by first growing S. scoparium and R. hirta with the same soil inocula, and then simulating low intensity, grassland fires in a controlled greenhouse pot experiment (soil training). We then evaluated plant growth responses to burned and unburned inter- and intraspecific soil biota treatments (response phase).
Results
Fire effects on inocula neutralized negative feedbacks in S. scoparium, and caused negative feedbacks in R. hirta. This shows that environmental disturbance like fire can alter the strength and direction of PSFs in ways that modify plant growth and potentially influence plant fuel loads and community dynamics.
Conclusion
That fire can alter the strength and direction of PSFs on plant growth suggests that fire effects on soil related processes may influence plant community dynamics and fire-fuel dynamics in fire recurrent grassland ecosystems. Further, this study shows that fire effects on PSFs vary between plant species.
... Although there is increased focus on reducing overstory tree density to increase light availability, historical evidence suggests that a closed canopy with little to no midstory may have been the prominent stand structure across the Ozarks and would still have allowed enough light to sustain a diverse herbaceous understory (Fralish and McArdle 2009;Hanberry, Jones-Farrand, et al. 2014). In fact, some savanna tree species can have lower speci c leaf areas and less dense canopies that are more lightpermeable than closely related forest tree species (Hoffmann et al. 2005;Ratnam et al. 2011). Therefore, overstory tree density may not be as critical to light availability as the midstory. ...
Background
Decades of fire suppression have caused drastic changes to community structure and composition across ecosystems, including in Ozark woodlands in the central Midwestern United States. Reintroducing fire can restore ground flora by reducing midstory tree density, increasing ground layer light, and reducing leaf litter accumulation, but we lack a clear understanding of how these effects vary across time and space. We investigated the effects of repeated prescribed fire on ground flora species richness, floristic quality, abundance, community composition, and stand structure over 20 years in a landscape matrix of glades, dry woodlands, and dry mesic woodlands using data collected from the Ozark National Scenic Riverways Fire Effects Monitoring program in the Current River Watershed in the Missouri Ozarks.
Results
We found that fire plays a key role in driving community structure and dynamics across community types, although with varying levels of intensity. Herbaceous species richness, abundance, and floristic quality index increased across all community types, while mean coefficient of conservatism decreased. Effects were stronger in drier sites. Community composition changed with successive burns, resulting in several indicator species for post burn treatments. The density of midstory trees declined across community types with repeated fire. The number of burns significantly affected overstory tree density overall, but overstory tree density only declined in dry woodlands and glades and not in dry mesic woodlands.
Conclusions
Our results suggest that landscape fire can shape plant community structure and dynamics. Specifically, these findings show that fire effects vary among community types, and suggest that land managers should consider landscape heterogeneity. Understanding repeated fire effects over several decades across multiple community types is critical to informing fire-driven woodland restoration across landscape scales.
... Open and closed habitats are distinct categories of terrestrial environments (Bond, 2019), each with unique ecology and differing in ground layer vegetation strongly constrained by available light (Ratnam et al., 2011;Bond, 2022). Open habitats, comprising nearly 60% of land area (Dinerstein et al., 2017), include treetops that provide suitable habitats for epiphytes, areas too extreme to support trees (e.g. ...
Poales are one of the most species‐rich, ecologically and economically important orders of plants and often characterise open habitats, enabled by unique suites of traits. We test six hypotheses regarding the evolution and assembly of Poales in open and closed habitats throughout the world, and examine whether diversification patterns demonstrate parallel evolution.
We sampled 42% of Poales species and obtained taxonomic and biogeographic data from the World Checklist of Vascular Plants database, which was combined with open/closed habitat data scored by taxonomic experts. A dated supertree of Poales was constructed. We integrated spatial phylogenetics with regionalisation analyses, historical biogeography and ancestral state estimations.
Diversification in Poales and assembly of open and closed habitats result from dynamic evolutionary processes that vary across lineages, time and space, most prominently in tropical and southern latitudes. Our results reveal parallel and recurrent patterns of habitat and trait transitions in the species‐rich families Poaceae and Cyperaceae. Smaller families display unique and often divergent evolutionary trajectories.
The Poales have achieved global dominance via parallel evolution in open habitats, with notable, spatially and phylogenetically restricted divergences into strictly closed habitats.
... Savannas are characterized by a continuous grassy cover and varying degrees of woody plant cover. However, savannas may often climatically support forests or dense woodlands without a grassy understory [16]. Fire regimes and megafauna browsing and grazing naturally generate and maintain an open or semi-open savanna state, which additionally may be promoted by human activities such as livestock grazing and tree cutting [17][18][19][20][21]. ...
Climate change will cause substantial vegetation shifts across the world. Africa may face varying dynamics such as tree decline, savannization, and woody encroachment due to rising temperatures and rainfall changes. This study examines the potential effects of climate change on Kenyan vegetation and vegetation shifts for 2050 and 2100, employing a statistical model to predict vegetation state as driven by environmental variables, including temperature, soil moisture, livestock density, and topography. We evaluate the model by hindcasting it from 2020 to 2000 and then project future vegetation states for 2050 and 2100 under SSP 2–4.5 and SSP 5–8.5. In response to moderate emissions, a notable increase in arid-associated shrubland vegetation (53–58%) is forecasted, leading to the expansion of drylands at the expense of savannas, grasslands, and forests. Under high-emission scenarios, savannas are forecasted to expand (52–65%) at the expense of forested areas. Overall, dense forest cover declines across scenarios, affecting protected areas by promoting increased savanna cover and reducing forest area (40% to 50%). These projected shifts in major vegetation types would likely alter ecosystem functioning and associated services, impacting pastoralists and wildlife and raising biodiversity concerns. Protected areas in Kenya could lose 50% of their forests, highlighting the urgency of climate change mitigation. These findings offer a crucial foundation for future research and action on Kenya’s vegetation.
... All savanna landscapes are shaped by fire; they are a result, at least in part, of past and present fire regimes [1][2][3]50]. There is a palpable resistance to switch from being a locale dominated by grassland, susceptible to frequent surface fires, followed by the eager regrowth of the culms, to a forest vegetation type with multiple strata, and accumulating leaf litter. ...
Savanna landscapes are shaped by the interactions of disturbances with land use goals. Elephant hunting in a site in Botswana, and its consequences for wildlife, people, and landscapes, are described and discussed in order to make broader generalizations about the dynamics of savanna landscapes. Change comes from alterations in tree-grass interactions, fire regimes, predator-prey relations, livestock raising, and conservation goals. Some of these implications are specific to African landscapes, but others may be apt in global contexts.
... recently argued that among undervalued and under threat grassy biomes, mesic savannas in particular are misperceived. Whereas arid to semiarid grasslands and savannas are well recognized as natural habitats, mesic savannas are often viewed as degraded forest because their tree densities are highly variable (Ratnam et al., 2011;Strömberg & Staver, 2022). This makes them vulnerable to misclassification as forest and to ecologically inappropriate tree planting as a climate change mitigation strategy (Griffith et al., 2017;Veldman et al., 2019). ...
Biased understanding of savanna biogeography
Grasslands and savannas exist across a wide range of climates. Mesic savannas, with highly variable tree densities, are particularly misunderstood and understudied in comparison to arid and semi‐arid savannas. North America contains historically extensive mesic savannas dominated by longleaf pine. Longleaf pine savannas may have once been the largest savanna type on North America, yet these ecosystems have been overlooked in global syntheses. Excluding these “Forgotten Ecosystems” from global syntheses biases our understanding of savanna biogeography and distribution.
Evolutionary history and distinct climate of longleaf savannas
We assessed the evolutionary history and biogeography of longleaf pine savannas. We then harmonize plot data from longleaf savannas with plot data from valuable existing global synthesis of savannas on other continents. We show that longleaf pine savannas occur in a strikingly distinct climate space compared to savannas on Africa, Australia, and South America, and are unique in having wide ranging tree basal areas.
Future directions
Grass‐dominated ecosystems are increasingly recognized as being ancient and biologically diverse, yet threatened and undervalued. A new synthesis of savanna ecosystems considering their full range of distributions is needed to understand their ecology and conservation status. Interestingly, the closest analogues to North American savannas and their relatives in Mesoamerica and the Caribbean may be Asian savannas, which also contain mesic fire‐driven pine savannas and have been similarly neglected in existing global syntheses.
... Rooted in eco-evolutionary feedbacks (Pausas and Bond, 2022), distributions of underground tree taxa are likely filtered by abiotic and biotic interactions (Fig. 1Ai) that shape vegetation assemblages across heterogeneous landscapes (Fig. 1Aii). Grassy ecosystems are characterized by an open canopy with a continuous shade-intolerant but disturbance-tolerant C 4 grass ground layer (Bond, 2019), ranging from arid grasslands at boundaries with deserts to mesic savannas at boundaries with closed-canopy forests (Scholes and Archer, 1997;Ratnam et al., 2011;Pennington et al., 2018). Underground trees have been recorded across vast environmental gradients, inhabiting a range of open ecosystem settings from suffrutex grasslands on high-elevation plateaus (Zigelski et al., 2019a) Environmental drivers and processes interact at varying spatial scales, influencing and feeding back over ecological and evolutionary time: (i) across a region (10-10 000 km 2 ), climatic, edaphic and some broad-scale disturbances control vegetation distributions [map data for biomes from Pennington et al. (2018) and vegetation cover from GlobCover 2009: http://due.esrin.esa.int/page_globcover.php]; (ii) within a landscape (1-10 km 2 ), plant communities are primarily determined by environmental stresses that govern vegetation structures, compositions and functions; (iii) locally (<1 km 2 ), the specific environmental context, topographic positioning and habitat ecology will influence plant habits, strategies and traits for survival in a particular niche space. ...
Background and aims:
Geoxyles, a distinctive feature of Afrotropical savannas and grasslands, survive recurrent disturbances by resprouting subshrub branches from large belowground woody structures. Underground trees are a type of geoxyle that independently evolved within woody genera of at least 40 plant families in Africa. The environmental limits and determinants of underground tree biogeography are poorly understood with the relative influence of frost and fire debated in particular. We aim to quantify variability in the niche of underground tree species relative to their taller, woody tree/shrub congeners.
Methods:
Using occurrence records of four Afrotropical genera, Parinari (Chrysobalanaceae), Ozoroa (Anacardiaceae), Syzygium (Myrtaceae) and Lannea (Anacardiaceae), and environmental data of nine climate and disturbance variables, the biogeography and niche of underground trees are compared with their open and closed ecosystem congeners.
Key results:
Along multiple environmental gradients and in a multidimensional environmental space, underground trees inhabit significantly distinct and extreme environments relative to open and closed ecosystem congeners. Niche overlap is low among underground trees and their congeners, and also among underground trees of the four genera. Of the study taxa, Parinari underground trees inhabit hotter, drier and more seasonal environments where herbivory pressure is greatest. Ozoroa underground trees occupy relatively more fire prone environments, while Syzygium underground trees sustain the highest frost frequency and occur in relatively wetter conditions with seasonal waterlogging. Lannea underground trees are associated with the lowest temperatures, highest precipitation, and varying exposure to disturbance.
Conclusions:
While underground trees exhibit repeated convergent evolution, distinct environments shape the ecology and biogeography of this iconic plant functional group. The multiplicity of extreme environments related to fire, frost, herbivory and waterlogging that different underground tree taxa occupy, and the distinctiveness of these environments, should be recognised in the management of African grassy ecosystems.
... We propose that the ubiquity of unimodal responses reflects vegetation structure and tradeoffs between forage quantity and quality. With respect to vegetation structure, the unimodal responses indicate high probabilities of presence in savanna ecosystems (e.g., wooded grassland, woodland, bushland; Fig. 1), which are characterized by a continuous grass layer interspersed with varying amounts of trees and shrubs (Owen-Smith, 2021;Ratnam et al., 2011;Reed, 1997). This provides a mixture of C 3 and C 4 foods suitable for grazers, mixed feeders, and browsers. ...
... For example, high levels of volatile organic compounds in the foliage of some plant species can increase combustibility. Also, the low leaf area index of many trees species in savannas can support grasses and other ne fuels that aid re (Ratnam et al., 2011;Beckage et al., 2019). ...
Tropical savannas are biomes with grass-dominated herbaceous cover interspersed with scanty trees in regions with alternating wet and dry conditions. The herbaceous cover in this biome shows seasonality and acts as a driver for fire invasion. Climate change impacts are increasing fire impacts by imposing hot, dry conditions (via drought, strong winds, and low relative humidity) which are increasing periods of fire occurrence and areas exposed to fire disturbance. This chapter highlights the nature and frequencies of fires, fire impacts on plant communities, soil, and microclimates, as well as strategies for adaptation tropical savannas flora to fire disturbance. Highlighted are various strategies adopted by plants in tropical savannas after fire events combine phenological alternatives and anatomical and physiological adaptations, which include the mechanisms for resisting drought imposed by the heat from the fire. The chapter concludes that climate change imposed conditions such as droughts and heat waves may induce modifications in plants and alter post-fire survival, growth, and regeneration potentials among plant populations, communities, and ecosystems while raising serious concerns for the diversity and conservation of flora in savannas.
... India currently retains some of the most biodiversity-rich savanna-grassland habitats within the subcontinent, characterized predominantly by C4 grasses and a woody layer of deciduous/ evergreen trees (Ratnam et al. 2011). Covering c. 24% of the total landmass of India (Rawat & Adhikari 2015), these grasslands have been considered as wastelands since the colonial period, resulting in systematic management lapses (Vanak 2019, Lahiri et al. 2022. ...
Grassland habitats currently face severe anthropogenic exploitation, thereby affecting the survival of grassland-dependent biodiversity globally. The biodiversity-rich grasslands of India lack quantitative spatiotemporal information on their status. We evaluated the status of upper Gangetic Plains grasslands in 2015 and compared it with those from 1985, 1995 and 2005. On-ground mapping and visual classifications revealed a 57% decline in these grasslands between 1985 (418 km 2) and 2015 (178 km 2), mostly driven by habitat conversion (74% contribution by cropland). Limited radiotelemetry data from endemic swamp deer indicated a possible grassland-dominated average home range size of 1.02 km 2 , and these patches were highly preferred (average Ivlev's index = 0.85) over other land-use classes at both spatial and temporal scales. Camera-trapping within the core habitats suggests the critical use of these patches as fawning/breeding grounds. Habitat suitability analysis indicates only c. 17% of the area along the Ganges is suitable as swamp deer habitat. We recommend the protection of these critical grassland patches to maintain 'dynamic corridors', with restoration and other management approaches involving multiple stakeholders to ensure the survival of this critical ecosystem.
... In contrast, C 3 grasses may have responded weakly to fire and increased with fire over longer experiment durations but did not depend on biome or climate. This finding is consistent with past work showing that, while C 3 grasses may be more prevalent in the forest understorey (Ratnam et al., 2011), C 4 grasses are particularly well adapted to open, fire-prone systems because of their high resource use efficiency and their flexibility in biomass allocation (Simpson et al., 2016). These physiological traits contribute to both rapid resprouting and higher probabilities of survival after fire. ...
Fire-vegetation feedbacks potentially maintain global savanna and forest distributions. Accordingly, vegetation in savanna and forest biomes should have differential responses to fire, but fire response data for herbaceous vegetation has yet to be synthesized across sites. Here, we examined herbaceous vegetation responses to experimental fire at 30 sites spanning four continents. Across a variety of metrics, herbaceous vegetation increased in abundance where fire was applied, with larger responses to fire in wetter and in cooler and/or less seasonal systems. Compared to forests, savannas were associated with a 4.8 (±0.4) times larger difference in burned versus unburned herbaceous vegetation abundance. In particular, grass cover decreased with fire exclusion in savannas, largely via decreases in C4 grass cover, whereas changes in fire frequency had a relatively weak effect on grass cover in forests. These differential responses underscore the importance of fire for maintaining the vegetation structure of savannas and forests.
... Within each climate zone, plant characteristics can help to explain the occurrence of different fire regimes in different biomes (Archibald et al. 2013;Pausas and Ribeiro 2013). In tropical ecosystems, for instance, fast-growing and drying savanna grasses encourage frequent low-intensity fires, thus preventing the growth of forest trees that are poorly adapted to fires (Beckage et al. 2011;Ratnam et al. 2011). Such vegetation-fire feedback has been suggested to preserve savannas in areas where a closed humid forest might be expected on the basis of climatic conditions alone (Van Langevelde et al. 2003;Bond 2008;Dantas et al. 2016;D'Onofrio et al. 2018). ...
Across plant communities worldwide, fire regimes reflect a combination of climatic factors and plant characteristics. To shed new light on the complex relationships between plant characteristics and fire regimes, we developed a new conceptual, mechanistic model that includes plant competition, stochastic fires, and fire-vegetation feedback. Considering a single standing plant functional type, we observed that highly flammable and slowly colonizing plants can persist only when they have a strong fire response, while fast colonizing and less flammable plants can display a larger range of fire responses. At the community level, the fire response of the strongest competitor determines the existence of alternative ecological states, i.e. different plant communities, under the same environmental conditions. Specifically, when the strongest competitor had a very strong fire response, such as in Mediterranean forests, only one ecological state could be achieved. Conversely, when the strongest competitor was poorly fire-adapted, alternative ecological states emerged, for example between tropical humid savannas and forests, or between different types of boreal forests. These findings underline the importance of including the plant fire response when modeling fire ecosystems, e.g. to predict the vegetation response to invasive species or to climate change.
... It is often misclassified as savanna and dry woodland, or tropical forest, with which it shares similar key species on a genus or family level (Ni et al 2010). The functional diversity of savannas (fire tolerance, functional ecology shade intolerance) is also not usually well-represented in DGVMs (Ratnam et al 2011, Dallmeyer et al 2019. This biome is unstable and vulnerable to grazing, fire regime, and climate, which transform savannas into forests or grasslands (Franco et al 2014). ...
Climate model simulations are inherently biased. It is a notably difficult problem when dealing with climate impact assessments and model-data integration. This is especially true when looking at derived quantities such as biomes, where not only climate but also vegetation dynamics biases come into play. To overcome such difficulties, we evaluate the performance of an existing methodology to correct climate model outputs, applied here for the first time to long past climate conditions. The proposed methodology relies on the “Cumulative Distribution Function - transform” (CDF-t) technique, which allows to account for climate change within the bias-correction procedure. The results are evaluated in two independent ways: i- using forward modelling, so that model results are directly comparable to reconstructed vegetation distribution; ii- using climatic reconstructions based on an inverse modelling approach. The modelling is performed using the intermediate complexity model iLOVECLIM in the standard global and interactively downscaled over the Europe version. The combined effects of dynamical downscaling and bias correction resulted in significantly stronger agreement between the simulated results and pollen-based biome reconstructions (BIOME6000) for the pre-industrial (0.18 versus 0.44) and mid-Holocene (0.31 versus 0.40). Higher correlation is also observed between statistically modelled global gridded potential natural distribution and modelled biomes (0.36 versus 0.41). Similarly, we find higher correlation between the reconstructed and the modelled temperatures for the mid-Holocene (0.02 versus 0.21). No significant difference is found for the Last Glacial Maximum when using temperature reconstructions, due to the low number of data points available. Our findings show that the application of the CDF-t method on simulated climate variables enables us to simulate palaeoclimate and vegetation distribution in better agreement with independent reconstructions.
... Within each climate zone, plant characteristics can help to explain the occurrence of different fire regimes in different biomes (Archibald et al. 2013;Pausas and Ribeiro 2013). In tropical ecosystems, for instance, fast-growing and drying savanna grasses encourage frequent low-intensity fires, thus preventing the growth of forest trees that are poorly adapted to fires (Beckage et al. 2011;Ratnam et al. 2011). Such vegetation-fire feedback has been suggested to preserve savannas in areas where a closed humid forest might be expected on the basis of climatic conditions alone (Van Langevelde et al. 2003;Bond 2008;Dantas et al. 2016;D'Onofrio et al. 2018). ...
Across plant communities worldwide, fire regimes reflect a combination of climatic factors and plant characteristics. To shed new light on the complex relationships between plant characteristics and fire regimes, we developed a new conceptual mechanistic model that includes plant competition, stochastic fires, and fire-vegetation feedback. Considering a single standing plant functional type, we observed that highly flammable and slowly colonizing plants can persist only when they have a strong fire response, while fast colonizing and less flammable plants can display a larger range of fire responses. At the community level, the fire response of the strongest competitor determines the existence of alternative ecological states (i.e., different plant communities) under the same environmental conditions. Specifically, when the strongest competitor had a very strong fire response, such as in Mediterranean forests, only one ecological state could be achieved. Conversely, when the strongest competitor was poorly fire adapted, alternative ecological states emerged-for example, between tropical humid savannas and forests or between different types of boreal forests. These findings underline the importance of including the plant fire response when modeling fire ecosystems, for example, to predict the vegetation response to invasive species or to climate change.
... For the African continent, we follow a contribution made in the Yangambi agreements, where vegetation formations were synthesized, labelled and defined in both French and English languages (Aubréville, 1957). The fundamental characteristic of savannas is their propensity to burn and to host large herbivores, specifically mammalian grazers and browsers (Charles-Dominique et al., 2016), and the associated importance of C4 grasses in the understorey (Aubréville, 1957;Dexter et al., 2015;Ratnam et al., 2011). ...
... Yet, we cannot generalize findings from other biomes to savanna ecosystems due to differences in dominant plant functional types and the differences in the stem and canopy architectures of the trees. Forest trees, for example, are taller with wider canopy diameter for a given basal area and have higher specific leaf area (Archibald & Bond, 2003;Ratnam et al., 2011). This, therefore, remains a significant gap in savannas, especially African savannas, as it is not well documented how much canopy interception occurs generally and or how this component of the water cycle shifts with woody encroachment (Honda & Durigan, 2016;Savenije, 2004). ...
Woody plant encroachment (WPE) has been found to alter ecosystem functioning and services in savannas. In rain-limited savannas, increasing woody cover can reduce streamflow and groundwater by altering evapotranspiration rates and rainfall partitioning , but the ecological relevance of this impact is not well known. This study quantified the altered partitioning of rainfall by two woody plant structural types (fine-and broad-leaved trees) across a gradient of encroachment in a semi-arid savanna in South Africa. Averaged across both plant functional types, loss of rainfall through canopy interception and subsequent evaporation roughly doubled (from 20.5% to 43.6% of total rainfall) with a roughly 13-fold increase in woody cover (from 2.4 to 31.4 m 2 /ha tree basal cover). Spatial partitioning changes comprised fourfold increases in stem-flow (from 0.8% to 3.9% of total rainfall) and a decline in throughfall proportion of about two-fifths (from 80.2% to 47.3% of total rainfall). Changes in partitioning were dependent on plant functional type; rainfall interception by the fine-leaved multi-stemmed shrub Dichrostachys cinerea was almost double that of the broad-leaved tree Terminalia sericea at the highest levels of woody encroachment (i.e., 49.7% vs. 29.1% of total rainfall intercepted at tree basal area of 31.4 m 2 /ha). Partitioning was also dependent on rainfall characteristics, with the proportion of rainfall intercepted inversely related to rainfall event size and intensity. Therefore, increasing tree cover in African grassy ecosystems reduces the amount of canopy throughfall, especially beneath canopies of fine-leaved species in smaller rainfall events. Rainfall interception traits may thus confer a selective advantage, especially for fine-leaved woody plant species in semi-arid savannas. K E Y W O R D S functional traits, rainfall interception, stemflow, throughfall, woody plant encroachment
... Despite being a nutrientrich environment, it is also hostile, presenting different obstacles to long-term human survival. Several challenges, including the instability of food resources (6,7), low light penetration (8), and high diversity of pathogens (9), probably contribute to strong selective pressures for human survival and reproduction in this ecoregion. ...
Ecological conditions in the Amazon rainforests are historically favorable for the transmission of numerous tropical diseases, especially vector-borne diseases. The high diversity of pathogens likely contributes to the strong selective pressures for human survival and reproduction in this region. However, the genetic basis of human adaptation to this complex ecosystem remains unclear. This study investigates the possible footprints of genetic adaptation to the Amazon rainforest environment by analyzing the genomic data of 19 native populations. The results based on genomic and functional analysis showed an intense signal of natural selection in a set of genes related to Trypanosoma cruzi infection, which is the pathogen responsible for Chagas disease, a neglected tropical parasitic disease native to the Americas that is currently spreading worldwide.
... factors (e.g., climate and soil) which, through the complex role of fire on the overstory and understory components of the community, set it apart from other temperate forested ecosystems in North America (Peet 2007;Ratnam et al., 2011). Dendroecological studies provide more in-depth and long-term approaches to untangling how abiotic variables can influence longleaf pine radial growth across ecosystems and help define the foundational composition, structure, and dynamics of longleaf pine ecosystems. ...
The longleaf pine ( Pinus palustris Mill.) and related ecosystem is an icon of the southeastern United States (US). Once covering an estimated 37 million ha from Texas to Florida to Virginia, the near-extirpation of, and subsequent restoration efforts for, the species has been well-documented over the past ca. 100 years. Although longleaf pine is one of the longest-lived tree species in the southeastern US—with documented ages of over 400 years—its use has not been reviewed in the field of dendrochronology. In this paper, we review the utility of longleaf pine tree-ring data within the applications of four primary, topical research areas: climatology and paleoclimate reconstruction, fire history, ecology, and archeology/cultural studies. Further, we highlight knowledge gaps in these topical areas, for which we introduce the Longleaf Tree-Ring Network (LTRN). The overarching purpose of the LTRN is to coalesce partners and data to expand the scientific use of longleaf pine tree-ring data across the southeastern US. As a first example of LTRN analytics, we show that the development of seasonwood chronologies (earlywood width, latewood width, and total width) enhances the utility of longleaf pine tree-ring data, indicating the value of these seasonwood metrics for future studies. We find that at 21 sites distributed across the species’ range, latewood width chronologies outperform both their earlywood and total width counterparts in mean correlation coefficient (RBAR = 0.55, 0.46, 0.52, respectively). Strategic plans for increasing the utility of longleaf pine dendrochronology in the southeastern US include [1] saving remnant material ( e.g., stumps, logs, and building construction timbers) from decay, extraction, and fire consumption to help extend tree-ring records, and [2] developing new chronologies in LTRN spatial gaps to facilitate broad-scale analyses of longleaf pine ecosystems within the context of the topical groups presented.
Fire is a fundamental part of the Earth system, with impacts on vegetation structure, biomass, and community composition, the latter mediated in part via key fire-tolerance traits, such as bark thickness. Due to anthropogenic climate change and land use pressure, fire regimes are changing across the world, and fire risk has already increased across much of the tropics. Projecting the impacts of these changes at global scales requires that we capture the selective force of fire on vegetation distribution through vegetation functional traits and size structure. We have adapted the fire behavior and effects module, SPITFIRE (SPread and InTensity of FIRE), for use with the Functionally Assembled Terrestrial Ecosystem Simulator (FATES), a size-structured vegetation demographic model. We test how climate, fire regime, and fire-tolerance plant traits interact to determine the biogeography of tropical forests and grasslands. We assign different fire-tolerance strategies based on crown, leaf, and bark characteristics, which are key observed fire-tolerance traits across woody plants. For these simulations, three types of vegetation compete for resources: a fire-vulnerable tree with thin bark, a vulnerable deep crown, and fire-intolerant foliage; a fire-tolerant tree with thick bark, a thin crown, and fire-tolerant foliage; and a fire-promoting C4 grass. We explore the model sensitivity to a critical parameter governing fuel moisture and show that drier fuels promote increased burning, an expansion of area for grass and fire-tolerant trees, and a reduction of area for fire-vulnerable trees. This conversion to lower biomass or grass areas with increased fuel drying results in increased fire-burned area and its effects, which could feed back to local climate variables. Simulated size-based fire mortality for trees less than 20 cm in diameter and those with fire-vulnerable traits is higher than that for larger and/or fire-tolerant trees, in agreement with observations. Fire-disturbed forests demonstrate reasonable productivity and capture observed patterns of aboveground biomass in areas dominated by natural vegetation for the recent historical period but have a large bias in less disturbed areas. Though the model predicts a greater extent of burned fraction than observed in areas with grass dominance, the resulting biogeography of fire-tolerant, thick-bark trees and fire-vulnerable, thin-bark trees corresponds to observations across the tropics. In areas with more than 2500 mm of precipitation, simulated fire frequency and burned area are low, with fire intensities below 150 kW m−1, consistent with observed understory fire behavior across the Amazon. Areas drier than this demonstrate fire intensities consistent with those measured in savannas and grasslands, with high values up to 4000 kW m−1. The results support a positive grass–fire feedback across the region and suggest that forests which have existed without frequent burning may be vulnerable at higher fire intensities, which is of greater concern under intensifying climate and land use pressures. The ability of FATES to capture the connection between fire disturbance and plant fire-tolerance strategies in determining biogeography provides a useful tool for assessing the vulnerability and resilience of these critical carbon storage areas under changing conditions across the tropics.
Abstract
Disturbance-mediated shifts in carbon persistence – the inverse likelihood of experiencing severe losses due to disturbances – within terrestrial ecosystems remain poorly understood despite their critical role in global carbon dynamics. Moreover, huge uncertainties in estimating carbon storage in disturbance-prone dryland ecosystems renders the assessment of their contribution to the global carbon budget difficult. This study investigated the effects of land-use change on carbon storage in an African savanna landscape, focusing on agricultural intensification and wildlife conservation as major land-use change pathways that alter disturbance regimes. We adapted conventional tree inventory and soil sampling methods to suit dryland ecosystems, enabling robust quantification of carbon storage for aboveground and belowground carbon in woody vegetation (AGC and BGC, respectively), and soil organic carbon (SOC) across land-use pathways and two vegetation types (savannas and woodlands). For assessing the effects of environmental drivers on AGC, and whole-ecosystem carbon (Ctotal), Generalized Additive Mixed Models were used. Results indicate different carbon persistence across carbon reservoirs, vegetation types and along land-use change pathways. Shrub AGC always was the least persistent carbon reservoir in savannas. Compared to shrub AGC in low-disturbance sites, it decreased on average by 56% along the conservation pathway and by 90% along the intensification pathway. Tree AGC was the least persistent reservoir along the intensification pathway in woodlands, with decreases of 95%. Elevated SOC stocks, particularly along the intensification pathway, suggest preferential use of naturally carbon-richer soils for agriculture. Strong unimodal impacts of disturbance agents, notably large herbivores and woodcutting, on AGC and Ctotal indicate that intermediate disturbance levels benefit carbon storage. Our findings suggest complex, interactive effects of natural and human disturbances on the carbon persistence of ecosystem compartments and whole-ecosystem carbon, and highlight the substantial role of locally adapted disturbance regimes for carbon sequestration, offering insights crucial for carbon certification programmes in drylands.
The article is available with full open acces at
https://www.sciencedirect.com/science/article/pii/S2950475924000017
The savannah ecosystem is influenced by seasonal fires that shape the landscape, with different fire intensities. The objective of this study is to assess the trend in fire intensity and the influence of the vegetation types. A linear regression model was employed to analyse the trend in fire intensity over the years. The results demonstrate a linear positive evolution of mean fire intensity over the years, with the highest fire intensity observed during the months of November to January. Woodland, shrub savannah and grassland areas exhibited high fire intensity, whereas wetland areas and forests displayed low fire intensity. Overall, remote sensing techniques can facilitate the monitoring of fire events, specifically fire intensity, in the savannah regions of West Africa, thereby aiding in the implementation of appropriate fire management plans.
Savanna systems are among the most sensitive to future climate and land‐use change, yet we lack robust, direct quantifications of savanna carbon cycling. Together with fire, decomposition is the main process by which the carbon and nutrients are recycled and made available again to plants. Decomposition is largely mediated by microbes and soil invertebrates. Using a novel large‐scale termite suppression experiment, we quantify, for the first time, the relative contribution of microbes, termites, and other invertebrates to the decomposition of wood (fresh native and dry non‐native), dry dung, and grass in a mesic savanna. We found that termites were responsible for two thirds of the mass loss from dry wood and a third of the mass loss from fresh native wood, dry dung, and dry grass. Microbes were wholly responsible for the difference as there was no evidence of other invertebrates contributing to decomposition, even with fresh wood. Using multiple substrates in savanna decomposition studies is important where a mixture of contrasting life forms occur because both the rates of decomposition and the dominant agent varied considerably. In addition, including both a dry non‐native and fresh native wood cast light on possible explanatory variables such as wood density, green‐ness and the presence of bark, and the necessity of teasing these variables apart in future studies. Termites stand apart from all other insects in their impact on decomposition within savannas and should be acknowledged alongside microbes and fire as the primary agents of wood, grass, and dry dung turnover in global carbon models.
RESUMO. – Chimpanzés e seres humanos derivaram de um mesmo ancestral comum. Evidências fósseis e moleculares indicam que o ramo ancestral se dividiu em dois há 5-8 milhões de anos. De um lado, prosperou a linhagem que daria origem aos chimpanzés; de outro, a que daria origem aos humanos. As duas linhagens se diferenciaram e se afastaram, dando origem a múltiplas ramificações e a diferentes estilos de vida. Os ancestrais dos chimpanzés continuaram a viver em florestas fechadas, enquanto os nossos ancestrais desceram das árvores e passaram a viver em hábitats mais abertos, dominados por gramíneas e outras plantas herbáceas.
SUMMARY. – (Coming down from the trees: Climate, habitat, niche and the origins of humanity.) Chimpanzees and humans derived from the same common ancestor. Fossil and molecular evidence indicates that the ancestral branch split in two 5-8 million years ago. On the one hand, the lineage that would give rise to chimpanzees prospered; on the other, the lineage that would give rise to humans. The two lineages differentiated and moved apart, giving rise to multiple branches and different lifestyles. Chimpanzee ancestors continued to live in closed forests, while our ancestors descended from the trees and began to live in more open habitats dominated by grasses and other herbaceous plants.
Soil disturbance threatens native perennial grasslands and savannas worldwide, including pine savannas of the North American Coastal Plain. Disk harrows are used in the region to plow linear features for firebreaks to contain prescribed fires, to manage game and other wildlife, and to reduce wildfire hazard to protect forest resources. However, the long‐term response of vegetation to these disturbances has not been well investigated. Our aim was to compare vegetation changes over time (0–9 years) following repeated disturbance by disking and a single disturbance by disking for firebreaks with undisturbed vegetation within a native pine savanna. We hypothesized that (1) a single disking event has multiyear effects on plant species composition and abundance because of the loss of perennial, dispersal‐limited species, but that partial survival of propagules allows the recovery to be more complete than following repeated disturbance, and (2) post‐disturbance changes are determined by species' life‐history characteristics resulting in a successional trajectory toward the undisturbed community. We established 10 plots within a repeated‐disturbance firebreak and a single‐disturbance firebreak, and in undisturbed vegetation ( n = 30). We identified plant species within the plots six times over nine years, categorized plant species by life span, seed bank persistence, and dispersal mechanism, and assessed changes in the plant community using ordination. Changes in species composition in both repeated and single disturbance treatments showed a pattern consistent with succession toward the undisturbed plant community, but vegetation in neither disturbance treatment matched undisturbed treatment conditions within the nine years of study. Repeated‐disturbance plots progressed from a high occurrence of annuals to species with persistent seed banks and wind‐dispersed species. Single‐disturbance plots were more strongly associated with perennials, species lacking a persistent seed bank, and species dispersed by vertebrate consumption, but not to the same degree as undisturbed plots, although differences decreased slightly over time. Our results relating to narrow mechanical soil disturbances in pine savanna vegetation are consistent with studies concluding that similar but larger scale disturbances have long‐term degradational effects on the plant community. Therefore, conservation management plans should consider the possible negative long‐term effects of soil disturbance on native perennial herbaceous plant communities.
The distribution of forest and savanna biomes and the role of resources (climate and soil) and disturbances (fire and herbivory) in determining tree-grass dynamics remains elusive and variable across geographies. This is especially problematic in Indian savannas which have been historically misclassified as degraded forests and are targeted for tree-planting. Here, we examine biome distribution and determinants through the lens of tree cover across India. Our analyses reveal four distinct zones of differing tree cover, with intermediate zones containing savanna vegetation. Rainfall seasonality determines maximum possible tree cover non-linearly. Once rainfall seasonality is factored out, soil sand fraction and topography partially explain residual variation of tree cover. High domestic livestock herbivory and other anthropogenic pressures reduce tree cover. Lastly, lack of detectable fires precludes robust conclusions about the relationship between fire and tree cover. By considering these environmental drivers in restoration planning, we can improve upon simplistic tree planting initiatives that may be detrimental to Indian savannas.
Background and aims
Plant soil feedbacks (PSF) are reciprocal mechanisms through which plants modify soil biota and affect future plant growth. When scaled up to the community level, PSFs are important determinants of above- and belowground community dynamics that influence long-term successional trajectories. Despite over three decades of ecological PSF research, we have a poor understanding of how common environmental processes like fire influence the strength and direction of PSFs.
Methods
In this work we evaluated how fire effects on Schizachyrium scoparium and Rudbeckia hirta trained soil biota influenced feedbacks on plant growth. We tested this by experimentally manipulating fires and evaluating plant growth responses to burned and unburned inter- and intraspecific soil biota treatments.
Results
Fire effects on inocula neutralized negative feedbacks in S. scoparium, and caused negative feedbacks in R. hirta. This shows that environmental disturbance like fire can alter the strength and direction of PSFs in ways that potentially modify plant growth, plant fuel loads, and community dynamics.
Conclusion
That fire can alter the strength and direction of PSFs on plant growth suggests that fire effects on soil related processes contribute to plant community assembly and fire-fuel dynamics in fire recurrent grassland and savanna ecosystems. Further, this study shows that fire effects on PSFs vary between plant species, and may contribute to the dominance of C4 grasses in pyrophilic ecosystems.
Poales are one of the most species-rich, ecologically and economically important orders of plants and often characterise open habitats, enabled by unique suites of traits. We test the hypotheses that Poales species are assembled into distinct phyloregions, with centres of high phylogenetic diversity and endemism clustered in tropical regions, and that cosmopolitan families show parallel transitions into open and closed habitats at different times.
● We sampled 42% of Poales species and obtained taxonomic and biogeographic data from the World Checklist of Vascular Plants database, which was combined with open/closed habitat data scored by taxonomic experts. A dated supertree of Poales was constructed. We integrated spatial phylogenetics with regionalization analyses, historical biogeography, ancestral state estimations, and models of contingent evolution.
● Diversification in Poales and assembly of open and closed habitats result from dynamic evolutionary processes that vary across lineages, time, space, and traits, most prominently in tropical and southern latitudes. Our results reveal parallel and recurrent patterns of habitat and trait transitions in the species-rich families Poaceae and Cyperaceae, yet other smaller families display unique evolutionary trajectories.
● The Poales have achieved global dominance via parallel evolution in open habitats, with notable, spatially and phylogenetically restricted divergences into strictly closed habitats.
Climate change has led to increased fire risk in many tropical regions, with concerning implications for fire sensitive forests. Understanding the comparative fire ecology of co-occurring fire-prone and fire sensitive habitats and the drivers of canopy fires is paramount for recognizing fire risk and for selecting plants that may be planted in green firebreaks that could mitigate wildfire damage to these habitats. However, baseline plant flammability data is needed from both habitats. To accomplish this, we compared three flammability measures (maximum temperature, total burn time and burnt biomass) by burning branches of 35 species in a fire-protected forest and fire-prone savanna in tropical northeast Australia. We also measured a set of leaf functional traits (leaf area, leaf mass per unit area and leaf dry matter content) on these species and examined their relationship with shoot flammability. We found that maximum temperature, burnt biomass, and burn time were significantly higher in savanna compared to forest species. Leaf area and leaf mass per unit area did not influence flammability measures, but species with higher leaf dry matter content had a higher percentage of biomass burnt, burnt for longer and hotter. Underpinning these observations, savanna species had significantly higher leaf dry matter content than forest species. Our results enable us to recommend species with low flammability that could be used in the green firebreaks in the study area to mitigate fire risk in sensitive forest habitats. Future studies should look experimentally at the effectiveness of green firebreaks using low flammability species, and examine their
post fire recovery.
Societal Impact Statement
Madagascar is famous for its unique forests and their fauna. Most of the island is covered by flammable grassy ecosystems long considered to be of human origin and threatening the remaining forests. Yet new studies show that many plants and animals of the grassy systems are unique to Madagascar and restricted to these open habitats. Open grassy ecosystems have markedly different management requirements from forests and bring different contributions to society. We argue that the grassy ecosystems can benefit Madagascar if understood and managed wisely using expanded knowledge bases that also include collaboration with locals.
Summary
Until recently, nearly all research and interests in Madagascar focused on forested habitats. To help place Madagascar's grassy ecosystems in context, we provide a summary of the origin, development, and evolution of open tropical, C4 grassy ecosystems elsewhere, especially those from Africa; we summarize similarities and differences with the distribution of C3 and C4 grasses in the Malagasy landscape, their plant traits, and inferences on the evolutionary legacy of grasses. We also discuss the animal communities that use and have coevolved in these grassy systems; to help resolve controversies over the pre‐settlement extent of grassy ecosystems, we suggest a variety of complementary geochemical, palaeobotanical, and molecular genetic tools that have been effectively used elsewhere to untangle forest/grassy ecosystem mosaics and the ecological and evolutionary processes that influence them. Many of these tools can and should be employed in Madagascar to fully understand the spatio‐temporal dynamics of open, grassy, and closed forest systems across the island; as regards conservation, we discuss the ecosystem services provided by grassy systems, which are too often ignored in general, not only as a biome, vis‐à‐vis forests, but also for their global importance as a carbon sink and role they play in water management and providing goods to local villagers. We conclude by outlining the necessary research to better manage open ecosystems across Madagascar without threatening endangered forest ecosystems.
In tropical Africa, forests and savannas are the two most widespread biomes and potentially represent alternative stable states with a divergent species composition. A classic, but untested, hypothesis posited by White (1983) suggests that the transition zones between forests and savannas contain a floristically impoverished assemblage, with few representatives from each biome. Further, the evolutionary dimension of diversity has received limited attention, despite its importance for understanding the biogeographic history of biomes. Here, we quantified species richness and several measures of evolutionary diversity in 1° grid cells, using c. 300,000 occurrence records of trees and shrubs combined with biome affiliation data for 3203 species. We found that assemblages in transition zones hold fewer woody species than assemblages in forest and savanna zones, as posited by White. However, transition zones hold more phylogenetic diversity than expected given their species richness, whether one considers forest and savanna assemblages separately or together. We also show that the Congo Basin forest has low levels of phylogenetic diversity, given the number of species, and highlight south-eastern African savannas as a centre of savanna woody species richness and phylogenetic diversity. Regions with high phylogenetic diversity, given the number of both forest and savanna species, were centred around the Dahomey Gap and Cameroon, mainly in transition zones. Overall, our study shows that even if floristically impoverished, transition zones hold unexpectedly high evolutionary diversity. This suggests that they are important centres of evolutionary innovation and diversification and/or serve as evolutionary crossroads, where lineages that diversified in contrasting environments coexist within a single area.
One of the foundational premises of ecology is that climate determines ecosystems. This has been challenged by alternative ecosystem state models, which illustrate that internal ecosystem dynamics acting on the initial ecosystem state can overwhelm the influence of climate, and by observations suggesting that climate cannot reliably discriminate forest and savanna ecosystem types. Using a novel phytoclimatic transform, which estimates the ability of climate to support different types of plants, we show that climatic suitability for evergreen trees and C4 grasses are sufficient to discriminate between forest and savanna in Africa. Our findings reassert the dominant influence of climate on ecosystems and suggest that the role of feedbacks causing alternative ecosystem states is less prevalent than has been suggested.
C4 grassland ecosystems expanded across North America between ca. 8 and 3 Ma. Studies of ungulate enamel and environmental indicators from the middle Miocene Barstow Formation of southern California (USA) have demonstrated the presence of C4 vegetation prior to the late Miocene expansion of C4 grasslands. Fire promotes the growth of modern C4 grasslands and may have contributed to the Miocene expansion of C4 vegetation. We analyzed the concentration and accumulation rate (CHAR) of microscopic charred particles from sediment samples spanning the Barstow Formation in order to investigate the relationship between fire activity, canopy cover, and the presence of C4 vegetation. Concentration and CHAR were low throughout the formation then increased dramatically at 13.5 Ma. Enriched values of δ13C from soil organic matter and phytolith counts indicate the presence of C4 grasses and seasonally dry, open-canopy habitats at this time. The spike in concentration coincides with climatic cooling and drying in southern California after the Miocene Climatic Optimum. Increased fire activity may have contributed to habitat opening from forest to woodland and promoted the spread of C4 plants. This is the first charcoal record of fire activity from the middle Miocene of southwestern North America.
"Anthropogenic fires in Indian forests probably date back to the arrival of the first hominids on the Indian subcontinent. However, with our continuing dependence on forests for a variety of resources, but with shrinking forested areas, forests are being subjected to more intensive use than before. As a result, fires are occurring more frequently today than at any time in the past. This altered fire regime is probably qualitatively different from historical fire regimes in its impact on forests at multiple spatial scales. Present-day fires have possibly led to forest degradation, increasing susceptibility to invasion by alien species such as lantana (Lantana camara). We hypothesise that there may be a positive feedback between present-day fires and invasion by lantana, leading to a fire-lantana cycle that can have deleterious compositional and functional consequences for forest ecosystems and the commodities and services that society derives from them. Despite the widespread nature of the problem, we lack good empirical information on the effects of varying fire frequency and severity in Indian dry forests. So also, we lack a sound understanding of the mechanistic underpinnings of lantana's success and barriers to its control in Indian forests. Without such information we have little hope of a way out of the fire-lantana cycle."
The photosynthetic pathway of grasses collected in several vegetation formations in Sao Paulo State, Brazil, was identified by leaf anatomy. Of the 78 species collected, 48 sun species and seven shade species possessed the C4 pathway. Twenty-three species possessed the C3 pathway, of which three were sun species and 20 were shade species. This result indicates that the C4 species are more abundant in open habitats and C3 species in shaded habitats. The data also show the taxonomic distribution of the photosynthetic pathways. All genera of the collected species are exclusively C3 or C4, except Panicum which has both C3 and C4 species.
The evolution of grasses using C₄ photosynthesis and their sudden rise to ecological dominance 3 to 8 million years ago is among the most dramatic examples of biome assembly in the geological record. A growing body of work suggests that the patterns and drivers of C₄ grassland expansion were considerably more complex than originally assumed. Previous research has benefited substantially from dialog between geologists and ecologists, but current research must now integrate fully with phylogenetics. A synthesis of grass evolutionary biology with grassland ecosystem science will further our knowledge of the evolution of traits that promote dominance in grassland systems and will provide a new context in which to evaluate the relative importance of C₄ photosynthesis in transforming ecosystems across large regions of Earth.
We demonstrate a significant relationship between leaf attributes and growth rates of mature trees under natural conditions in northern Australia, a pattern that has not been widely reported before in the literature. Increase in diameter at breast height (DBH) was measured every 3 months for 2 years for 21 tree species from four habitats near Darwin: Eucalyptus open forest, mixed eucalypt woodland, Melaleuca swamp and dry monsoon rainforest. Assimilation rates and foliar chlorophyll, nitrogen and phosphorus concentrations were positively correlated with growth rate and negatively correlated with leaf mass per area. For most species, increases in DBH were confined to the wet-season (summer) period between November and May. Average annual increases in DBH were larger in the dry monsoon rainforest (0.87 cm) and the Melaleuca swamp (0.65 cm) than in the woodland (0.20 cm) and the open forest (0.16 cm), and were larger in non-Myrtaceous species (0.53 cm) than in Myrtaceous species (0.25 cm). These results are discussed in relation to the frequent fire regime prevailing over much of northern Australia which causes the marked contrast between the small pockets of fire-tender closed monsoon rainforest and large expanses of fire-tolerant savanna.
In order to investigate how environmental factors other than light availability affect tree architecture, differences in branching architecture and allometry were analysed in populations of Acacia karroo Hein. from three different environments in South Africa: forests, savannas and arid-shrublands. Factors such as fire and herbivory have a large effect on tree life history in certain environments and are likely to have selected for trees that have different architectures from those of forest trees, whose major limitation is light assimilation.
Significant differences were found in stem architecture and branching architecture between trees in each environment. Compared with forest trees, trees in savannas had an elongated growth form with small canopy and leaf areas, and tall, thin, unbranched trunks. Trees in arid areas showed opposite trends with wider canopies, and increased lateral branching. Savanna trees had significantly smaller spines than trees in other environments, and both forest and savanna trees showed delayed reproduction.
These differences are probably related to a trade-off between an architecture geared towards rapid height-gain and one promoting lateral spread, and can be explained with reference to the different selective pressures in each environment. In forests, vertical and horizontal growth are both important. However, in savannas there is a great pressure for rapid vertical growth to escape fires, while in arid areas a defensive, lateral growth form is selected for.
Savanna trees and arid karoo trees have evolved architectures that are more extreme vertically and laterally than the range of architectures displayed in a forest community.
In the state of Para, Brazil, in the eastern Amazon, the authors studied the potential for sustained fire events within four dominant vegetation cover types (undisturbed rain forest, selectively logged forest, second-growth forest, and open pasture), by measuring fuel availability, microclimate, and rates of fuel moisture loss. They also estimated the potential tree mortality that might result from a wide-scale Amazon forest fire by measuring the thermal properties of bark for all trees in a 5-ha stand of mature forest, followed by measurements of heat flux through bark during simulated fires. In pastures the average midday temperature was almost 10°C greater and the average midday relative humidity was 30% lower than in primary forest. The most five-prone ecosystem was the open pasture followed by selectively logged forest, second growth forest, and undisturbed rain forest in which sustained combustion was not possible even after prolonged rainless periods. Even though the autogenic factors in primary forest of the eastern Amazon create a microclimate that virtually eliminates the probability of fire, they are currently a common event in disturbed areas of Amazonia. As many as 8 à 10ⶠha burned in the Amazon Basin of Brazil in 1987 alone. In terms of current land-use patterns, altered microclimates, and fuel mass, there are also striking similarities between the eastern Amazon and East Kalimantan, Indonesia (the site of recent rain forest wildfires that burned 3.5 à 10ⶠha).
▪ Abstract Savannas occur where trees and grasses interact to create a biome that is neither grassland nor forest. Woody and gramineous plants interact by many mechanisms, some negative (competition) and some positive (facilitation). The strength and sign of the interaction varies in both time and space, allowing a rich array of possible outcomes but no universal predictive model. Simple models of coexistence of trees and grasses, based on separation in rooting depth, are theoretically and experimentally inadequate. Explanation of the widely observed increase in tree biomass following introduction of commercial ranching into savannas requires inclusion of interactions among browsers, grazers, and fires, and their effects on tree recruitment. Prediction of the consequences of manipulating tree biomass through clearing further requires an understanding of how trees modify light, water, and nutrient environments of grasses. Understanding the nature of coexistence between trees and grass, which under other ci...
The ecology of clonal species has rarely been studied in savannas. Dichrostachys cinerea, a common invasive shrub in southern African savannas, forms root suckers. This paper examines the effect of disturbance type and frequency on this form of clonal spread. Small plants were excavated (n = 370) at 11 sites exposed to different fire frequencies and grazing intensities and classified were as either seedlings or root suckers. Most of the plants (55%) were found to be root suckers. There was no significant effect of disturbance type and frequency on 'seedling' establishment from seeds versus root suckers. Even when burnt annually, D. cinerea continued to form root suckers. The combination of establishment from seeds and spread by root suckers makes this species a formidable native invasive woody shrub.
Tree characteristics were recorded from 2829 standing trees in 24 0.25-ha terra
firme forest plots in central Amazonia, 3 y after a surface fire had swept through the study area. Sixteen of the plots were within forest that burnt for the first time at the end of the 1997-98 El Niño (ENSO) event, and the remaining eight plots were within unburnt primary forest. In order to investigate the morphological correlates of tree mortality, we measured tree diameter at breast height (dbh) and bark thickness, and recorded burn height, bark roughness and the presence of latex, resin and buttress roots. Leaf litter depth was also recorded at the base of all trees in the unburnt forest. Using logistic regression models, tree mortality was best explained by the burn height, although dbh and the presence of buttresses were also important. Buttressed trees were associated with deeper leaf litter accumulation at their bases and higher char heights than trees without buttresses. Moreover, trees surviving the fire had significantly thicker bark than living trees in unburnt forest plots, indicating that thin-barked trees are more prone to selective mortality induced by heat stress. Latex did not appear to have had any significant effects on mortality, though resins were less abundant amongst the live trees in the burnt forest than in the unburnt controls. Levels of fire-mediated tree mortality in this study are compared with those in other Amazonian forest regions in light of historical factors affecting tree resistance to fires.
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.
Upland tropical forests have expanded and contracted in response to past climates, but it is not clear whether similar dynamics were exhibited by gallery (riparian) forests within savanna biomes. Because such forests generally have access to ample water, their extent may be buffered against changing climates. We tested the long-term stability of gallery forest boundaries by characterizing the border between gallery forests and savannas and tracing the presence of gallery forest through isotopic analysis of organic carbon in the soil profile. We measured leaf area index, grass vs. shrub or tree coverage, the organic carbon, phosphorus, nitrogen and calcium concentrations in soils and the carbon isotope ratios of soil organic matter in two transitions spanning gallery forests and savanna in a Cerrado ecosystem. Gallery forests without grasses typically show a greater leaf area index in contrast to savannas, which show dense grass coverage. Soils of gallery forests have significantly greater concentrations of organic carbon, phosphorus, nitrogen and calcium than those of savannas. Soil organic carbon of savannas is significantly more enriched in 13C compared with that of gallery forests. This difference in enrichment is in part caused by the presence of C4 grasses in savanna ecosystem and its absence in gallery forests. Using the 13C abundance as a signature for savanna and gallery forest ecosystems in 1 m soil cores, we show that the borders of gallery forests have expanded into the savanna and that this process initiated at least 3000–4000 bp based on 14C analysis. Gallery forests, however, may be still expanding as we found more recent transitions according to 14C activity measurements. We discuss the possible mechanisms of gallery forest expansion and the means by which nutrients required for the expansion of gallery forest might accumulate.
A century of annual burning of the understory of otherwise fire-free decid-uous tropical forest in central India has favored seven tree species that produce sprouts or suckers from root buds (root-sprouters) over 37 species that produce sprouts basally from root crowns (root-crown resprouters). Experiments over two years demonstrated that low-intensity ground fires killed seedlings (1 year old), resulting in a 30% decrease in seedling diversity in burned relative to unburned plots. Overall fire-related mortality of seedlings was 74% for 17 root-crown resprouters, compared to 63% for six root-sprouters. Repeated-measures ANOVA showed that the number of juvenile (1 year old) stems of root-sprouters increased in burned study plots but decreased in plots protected from fire. Annual burning by people favors species that repair damage by root sprouts. Root-sprouting offers a means of occupying new ground with clonal ramets away from the original parental base. Over time, forests may become dominated by clonal root-sprouters, in contrast to historical accounts of forest dominated by root-crown resprouters that do not spread by clonal growth. If this process continues in the Mendha Forest in India, 80% of its tree diversity could be lost within 100–200 years.
There is interest in the global community on how fire regimes are changing as a function of changing demographics and climate. The ground-based data to monitor such trends in fire activity are inadequate at the global scale. Satellite observations provide a basis for such a monitoring system. In this study, a set of metrics were developed from 6 years of MODIS active fire data. The metrics were grouped into eight classes representing three axes of fire activity: density, season duration and interannual variability. These groups were compared with biophysical and human explanatory variables on a global scale. We found that more than 30% of the land surface has a significant fire frequency. The most extensive fire class exhibited high fire density, low duration and high variability and was found in boreal and tropical wet and dry environments. A high association was found between population distribution and fire persistence. Low GDP km−2 was associated with fire classes with high interannual variability and low seasonal duration. In areas with more economic resources, fires tend to be more regular and last longer. High fire duration and low interannual variability were associated with croplands, but often with low fire density. The study was constrained by the limited length of satellite data record but is a first step toward developing a comprehensive global assessment of fire regimes. However, more attention is needed by the global observing systems to provide the underpinning socio-economic observations to better quantify and analyze the human characteristics of fire regimes.
A yearly global fire history is a prerequisite for quantifying the contribution of previous fires to the past and present global carbon budget. Vegetation fires can have both direct (combustion) and long-term indirect effects on the carbon cycle. Every fire influences the ecosystem carbon budget for many years, as a consequence of internal reorganization, decomposition of dead biomass, and regrowth. We used a two-step process to estimate these effects. First we synthesized the available data available for the 1980s or 1990s to produce a global fire map. For regions with no data, we developed estimates based on vegetation type and history. Second, we then worked backwards to reconstruct the fire history. This reconstruction was based on published data when available. Where it was not, we extrapolated from land use practices, qualitative reports and local studies, such as tree ring analysis. The resulting product is intended as a first approximation for questions about consequences of historical changes in fire for the global carbon budget. We estimate that an average of 608 Mha yr(-1) burned (not including agricultural fires) at the end of the 20th century. 86% of this occurred in tropical savannas. Fires in forests with higher carbon stocks consumed 70.7 Mha yr(-1) at the beginning of the century, mostly in the boreal and temperate forests of the Northern Hemisphere. This decreased to 15.2 Mha yr(-1) in the 1960s as a consequence of fire suppression policies and the development of efficient fire fighting equipment. Since then, fires in temperate and boreal forests have decreased to 11.2 Mha yr(-1). At the same time, burned areas increased exponentially in tropical forests, reaching 54 Mha yr(-1) in the 1990s, reflecting the use of fire in deforestation for expansion of agriculture. There is some evidence for an increase in area burned in temperate and boreal forests in the closing years of the 20th century.
In tropical West Africa, distribution patterns of forest islands in savannas are influenced by fires which occur regularly in the grass stratum. Along continuous forest-savanna transects in the Comoe National Park, the change in the amount and composition of non-woody phytomass was investigated from savanna to forest interior. This was correlated with the cover of vegetation strata above, soil depth, and the occurrence of seasonal surface fires. Phytomass mainly consisted of leaf litter in the forests (about 400 g m(-2) at the end of the rainy season, and about 600 g m(-2) at the end of the dry season) and of grasses in the savanna (about 900 g m(-2)). Low grass biomass appeared to be primarily the result of suppression by competing woody species and not of shallow soil. The occurrence of early dry-season fires seemed to be determined mainly by the amount of grass biomass as fuel because fires occurred in almost all savanna plots while forest sites remained unaffected. However, late dry-season fires will encounter higher amounts of leaf litter raising fire probability in forests. Due to the importance of the amount of combustible phytomass, fire probability and intensity might increase with annual precipitation in both savanna and forest.
The incidence and importance of fire in the Amazon have increased substantially during the past decade, but the effects of
this disturbance force are still poorly understood. The forest fire dynamics in two regions of the eastern Amazon were studied.
Accidental fires have affected nearly 50 percent of the remaining forests and have caused more deforestation than has intentional
clearing in recent years. Forest fires create positive feedbacks in future fire susceptibility, fuel loading, and fire intensity.
Unless current land use and fire use practices are changed, fire has the potential to transform large areas of tropical forest
into scrub or savanna.
The ecological significance of fire is reviewed, particularly in relation to the savanna forest of mainland South East Asia, a community, in which dry season burns occur annually or at least every 2-3 years. It is argued that this tropical deciduous formation is basically an edaphic climax which is well adapted physiognomically, physiologically and phenologically to fire and which in consequence has been gradually spread into the more fire-sensitive adjacent associations, such as tropical semi-evergreen rain forest, by cutting and burning. A careful examination is made of the forest fire environment, of the fuels involved, of the life forms and their adaptive capacity, and of the origins and characteristics of the fires themselves. A case is presented for the prescribed application of fire in this type of habitat and suggestions for appropriate future work are given.
The savanna forests and the rarer open savanna grasslands of mainland SE Asia are described biogeographically and ecologically. These distinctive formations, the canopies of which are dominated by 6 leaf-shedding members of the Dipterocarpaceae often comprise the most important single formation over much of the region. The formations are seen to possess edaphic or topographic "cores', from which they have been spread by fire and the axe, but only within the general area of the monsoon forest or savanna forest climate, where "running' fire is characteristic in the dry season. physiognomically, ecophysiologically and phenologically, the key taxa exhibit a wide range of adaptations to the main ecological stresses of such environments. The natural limits of these adaptations are examined. The ecotonal characteristics of savanna forests and savanna are considered in relation to a wide range of parapatric formations. Current ecological stress factors include agricultural extensification and development; war; mismanagement of fire; flood; edaphic and geomorphological pressures; over-exploitation; and faunal changes. -from Author
A nursery experiment revealed that the growth of Eucalyptus tetrodonta seedlings is significantly inhibited by unsterilised monsoon rainforest surface soil, and that this inhibition can not be eliminated by the addition of fertiliser. This result is in marked contrast to similar previously published experiments performed on two rainforest tree seedlings. (Bombax ceiba and Sterculia quadrifida) and Eucalyptus nesophila. The results of this study suggest that the observed high densities of E. tetrodonta seedlings and saplings on a site formerly occupied by monsoon rainforest on Elcho Island could only have established once the rainforest soil biota had been killed. One possible cause of soil sterilisation is wildfire.
The seasonally deciduous tree species Bombax ceiba and Sterculia quadrifida formed 9.1% and 2.3%, respectively, of the total basal area in a coastal dry monsoon rainforest in the Australian seasonally dry tropics. Neither species occurred in a nearby Eucalyptus savanna. Both communities occurred on deep, iron-rich, sandy loam soils. The savanna had lower concentrations of nutrients in the surface soil than the rainforest but experienced greater maximum ground surface air temperatures than the monsoon rainforest when averaged over a year. Annual mean surface soil moisture content was similar in savanna and rainforest, but savanna soils were drier in the dry season. During this time both communities had soil matrix pressure potentials below 1.3 MPa. S. quadrifida established more successfully in the rainforest than in the savanna but B. ceiba establishment was <2% in both communities. The long-term development of a monsoon rainforest-savanna ecotone is critical for creating microclimatic and soil conditions favourable for monsoon-rainforest seedling establishment and growth. Experiments point toward the importance of mycorrhizas, soil fertility and soil moisture in controlling monsoon-rainforest seedling establishment. This study helps explain why so few monsoon-rainforest seedlings have been observed to establish in fire-protected Eucalyptus savannas. -from Authors
Eucalyptus tereticornis seedlings occurring on the edges of grassy balds on the Bunya Mountains were burnt by four separate fires. From the results, a logistic model demonstrated that lignotuber size was positively related and fire temperature negatively related to survivorship. While mortality was high for young seedlings there was no mortality of 5-year old survivors from these trials subject to repeat burning. The model predicted that burning every 2 years will not substantially limit seedling establishment. This prediction was strengthened by results verifying that management fires on the grassy balds are generally of low intensity. Fire intensity is weakly related to a Fire Danger Index, indicating that the timing of burning in relation to weather conditions will not substantially enhance opportunities for more intense fires. Thus, even with biennial burning under optimal conditions eucalypt forest will replace grassy balds where they adjoin. Regular burning by aborigines may have maintained grassy bald-rainforest boundaries, but not boundaries with eucalypt forest. Seed dispersal and migration barriers may have limited the expansion of eucalypt forest. It is concluded that under current conditions the long-term preservation of the grassy balds is only possible where they are entirely surrounded by rainforest and are regularly burnt.
In tropical West Africa, distribution patterns of forest islands in savannas are influenced by fires which occur regularly in the grass stratum. Along continuous forest2 at the end of the rainy season, and about 600 g m2). Low grass biomass appeared to be primarily the result of suppression by competing woody species and not of shallow soil. The occurrence of early dry-season fires seemed to be determined mainly by the amount of grass biomass as fuel because fires occurred in almost all savanna plots while forest sites remained unaffected. However, late dry-season fires will encounter higher amounts of leaf litter raising fire probability in forests. Due to the importance of the amount of combustible phytomass, fire probability and intensity might increase with annual precipitation in both savanna and forest.
Extensive parts of subtropical South America are covered by grassland vegetation, despite climatic conditions that allow for forest development, and fire may have been an important factor in the evolutionary history of these grasslands. In a regularly burned grassland area, situated in a forest–grassland-mosaic near Porto Alegre, RS, Brazil, life form spectrum and plant species’ reaction to fire were examined, allowing for (1) a physiognomic description of the grassland, and (2) a functional classification of grassland species in relation to fire. Grassland sites with different time since the last fire occurred were compared between each other as well as to sites at the forest–grassland border. South Brazilian grassland is dominated by hemicryptophytic caespitose graminoids that resist fires, but contains a large number of geophytic or hemicryptophytic forbs, in general sprouting after fire. Shrubs, mostly sprouting species of the grassland community, were present with high cover values even in recently burned areas. In contrast to Central Brazilian Cerrado, trees were of little importance: most species found were forest pioneer species without the capacity to survive fires unless growing on sites protected from fire or at the forest border where burns stop. Non-sprouting species were of little importance in the community, and only two species found were therophytes. Lack of therophytes in South Brazilian grassland vegetation deserves further attention.
Though the distribution of global vegetation can generally be predicted from climate, grasslands are an exception. C4 grassy biomes cover vast areas that are warm enough and wet enough to support closed forests. The extent of this climate mismatch has been revealed by physiologically based global vegetation simulations and by large empirical data sets. Reasons for the existence of grassy biomes have long been debated, polarized into bottom-up (resources) or top-down (fire, herbivory) arguments. Recent studies indicate that both are important, especially in suppressing woody recruits. Grasses are formidable competitors belowground, create highly flammable fuels, and can support large herbivore densities. The net effect on trees is rare and episodic recruitment of adults in tree-fall gaps. The implication is that ecosystem structure and function depend on demographic transitions. Tree cover is increasing and grass/forest boundaries are changing. These changes can have large feedbacks to the earth-atmosphere system. Though progress has been made, there is still great uncertainty in predicting the future of C4 grassy biomes.
On a worldwide basis epicormic resprouting after intense or crown fire is extremely rare, but is quite common in the eucalypts. Recent research has shown that the eucalypts have a highly modified epicormic structure that provides the bud-forming tissues with excellent protection from heat. A small number of non-eucalypts from the Myrtaceae have also been recorded as post-fire epicormic resprouters and it was considered of interest to determine whether this response was achieved through a similar or different structural adaptation. Leaf axils and epicormic structures of two species of Syncarpia and three species of Tristaniopsis were examined anatomically. Although the leaf axil anatomy of Syncarpia and Tristaniopsis was quite different (and different from that of the eucalypts), the epicormic structure was similar to that of the eucalypts, as the outer region of each epicormic strand possessed several strips of cells of meristematic appearance that were best developed in the innermost bark or even the outermost secondary xylem. As Syncarpia, Tristaniopsis and the eucalypts are only distantly related to each other within the Myrtaceae, it appears that this specialised fire-adapted epicormic structure may have developed multiple times within the family or originated from a common ancestor of the family.
Options for a new definition of, and key for, rainforest in Australia are provided. The definitions take a national perspective, and are based on the ecological characteristics of rainforest species and some structural and floristic characteristics. Rainforest plant species are defined as those adapted to regenerating under low-light conditions experienced under the closed canopy or in localised gaps caused by recurring disturbances which are part of the natural rainforest ecosystem, and are not dependent on fire for successful regeneration. Three definitions are provided which differ in the extent of inclusion of transitional and seral communities. The first definition recognises communities such as mixed forests as transitional to rainforests and therefore as separate communities. The second definition includes a minimal component of emergent non-rainforest species in rainforest in the recognition that the main floristic component and functioning of the communities cannot be distinguished. The third definition includes the late successional stages of transitional and seral communities in rainforest on the presumption that such communities
include non-rainforest species which are close to senescence, and that these communities are essential for the long-term conservation of rainforest in areas where rainforest is vulnerable and subject to major disturbance, particularly by fire. The first definition is concluded to be the least ambiguous and arbitrary, and enables a consistent approach to rainforest management. Recognition of mixed forests as a distinctive and mappable vegetation type should be incorporated in a comprehensive conservation strategy inclusive of all ecosystem developmental stages.
Forests and grassy vegetation (savannas and grasslands) are alternative ecosystem states in many tropical landscapes. Relative to forests the grassy ecosystems are poorly known and poorly conserved, partly because they were thought to be products of forest clearance. However many grasslands have proved to be ancient. Commensurate with their antiquity, grassy biomes have distinct suites of plant and animal species that contribute a large fraction of the diversity of forest–grassland mosaics. Grasslands differ strikingly from forests in their ecology and in the nature of threats to their future. Here we highlight the high biodiversity value of grassy biomes and, in contrast to tropical forests, we illustrate the importance of fire in maintaining these systems. We discuss the major threats to, and consequences for, biodiversity in these regions including land clearance and elevated CO2-driven forest expansion. Finally we focus on the difficulties of grassland restoration. A new approach to understanding and conserving grassy ecosystems, free from cultural prejudices of the past, is long overdue.
Monsoon rainforests in the Darwin area occur as isolated patches ranging in size from 2 to 112 ha. Aerial photographic records over a 45 year period indicate a 60 per cent reduction in the cover of pre‐1945 rainforest. Urban development, cyclone damage, weed invasion and wildfire were identified as the major causes of this contraction. Urban development contributed 40 per cent of the total rainforest loss during this period. Rainforests occurring on dry substrates have been most affected by urban development. Cyclonic storm damage and indirect human affects such as weed invasion and anthropogenic fires continue to disturb the remaining rainforest patches. Although the largest expanses of rainforest presently occur within parks and reserves, providing adequate protection from further urban encroachment, smaller remnants occurring on vacant crown land are vulnerable to changes in land use. As well as the introduction of appropriate legislation, the control of fire and weeds should be given the highest priority by land managers to ensure the stability and long‐term maintenance of this remnant vegetation.
Amazonian forests are being rapidly cleared, and the remaining forest fragments appear unusually vulnerable to fire. This occurs because forest remnants have dry, fire-prone edges, are juxtaposed with frequently burned pastures, and are often degraded by selective logging, which increases forest desiccation and fuel loading. Here we demonstrate that in eastern Amazonia, fires are operating as a large-scale edge effect in the sense that most fires originate outside fragments and penetrate considerable distances into forest interiors. Multi-temporal analyses of satellite imagery from two frontier areas reveal that fire frequency over 12-14-y periods was substantially elevated within at least 2400 m of forest margins. Application of these data with a mathematical core-area model suggests that even large forest remnants (up to several hundred thousand ha in area) could be vulnerable to edge-related fires. The synergistic interactions of forest fragmentation, logging and human-ignited fires pose critical threats to Amazonian forests, particularly in more seasonal areas of the basin.
Abstract Seventy-three per cent of dry rainforest in Forty Mile Scrub National Park and large areas in adjacent savanna woodland have more than 5000 individuals per ha of lantana (Lantana camara L.). Transect studies in dry rainforest and savanna woodland across varying intensities of lantana infestation show a negative correlation between the density of lantana and tree cover in rainforest. The density of pig rooting is very high in areas of the dry rainforest on deep soil that was not heavily infested with lantana. It is suggested that the digging activities of these animals may cause tree death and subsequent increased light penetration, which favours lantana. The species richness of the dry rainforest declines as the density of lantana increases. However, the saplings and seedlings and the soil seed bank of dry rainforest and savanna woodland tree species have comparable densities in heavy and light lantana infestations. The proliferation of lantana results in the build up of heavy fuel loads across the boundary of dry rainforest and savanna woodland. Recent fires have killed the canopy trees in a large area of dry rainforest within the Park. Active management of Forty Mile Scrub National Park is urgent and some initiatives are suggested.
The distribution and abundance of trees can be strongly affected by disturbance such as fire. In mixed tree/grass ecosystems, recurrent grass-fuelled fires can strongly suppress tree saplings and therefore control tree dominance. We propose that changes in atmospheric [CO2] could influence tree cover in such metastable ecosystems by altering their postburn recovery rates relative to flammable herbaceous growth forms such as grasses. Slow sapling recovery rates at low [CO2] would favour the spread of grasses and a reduction of tree cover. To test the possible importance of [CO2]/fire interactions, we first used a Dynamic Global Vegetation Model (DGVM) to simulate biomass in grassy ecosystems in South Africa with and without fire. The results indicate that fire has a major effect under higher rainfall conditions suggesting an important role for fire/[CO2] interactions. We then used a demographic model of the effects of fire on mesic savanna trees to test the importance of grass/tree differences in postburn recovery rates. We adjusted grass and tree growth in the model according to the DGVM output of net primary production at different [CO2] relative to current conditions. The simulations predicted elimination of trees at [CO2] typical of the last glacial period (180 ppm) because tree growth rate is too slow (15 years) to grow to a fire-proof size of ca. 3 m. Simulated grass growth would produce an adequate fuel load for a burn in only 2 years. Simulations of preindustrial [CO2] (270 ppm) predict occurrence of trees but at low densities. The greatest increase in trees occurs from preindustrial to current [CO2] (360 ppm). The simulations are consistent with palaeo-records which indicate that trees disappeared from sites that are currently savannas in South Africa in the last glacial. Savanna trees reappeared in the Holocene. There has also been a large increase in trees over the last 50–100 years. We suggest that slow tree recovery after fire, rather than differential photosynthetic efficiencies in C3 and C4 plants, might have been the significant factor in the Late Tertiary spread of flammable grasslands under low [CO2] because open, high light environments would have been a prerequisite for the spread of C4 grasses. Our simulations suggest further that low [CO2] could have been a significant factor in the reduction of trees during glacial times, because of their slower regrowth after disturbance, with fire favouring the spread of grasses.
1. C4 grasses possess characteristics that potentially advantage growth in fire-prone environments, including high photosynthetic productivity, efficient light and nutrient use and significant allocation to below-ground reserves and reproduction. Such characteristics allow fast regeneration after fire, and may be the consequence of photosynthetic physiology, phylogenetic ancestry or may have been acquired as adaptations to frequently burnt environments.
2. The aim of this study was to examine the role of photosynthetic pathway by comparing fire ecology in the closely related C3 and C4 subspecies of Alloteropsis semialata. Its focus was on above-ground characteristics that would contribute to a fire fuel load, and the re-growth responses of plants subsequent to a controlled experimental burn during the natural winter fire season.
3. Prior to the burn, but after frost, above-ground biomass was entirely dead in the C4 plants, and was more flammable than that of the C3 plants. C3 plants maintained 33% of their canopy alive despite frosts and hence lost a significant proportion of living tissue in the experimental burn.
4. Subsequent to the burn, C3 plants did not entirely recover their above-ground biomass, but canopy area returned to its pre-burn level through the production of leaves with greater specific leaf area. There was little evidence of the remobilization of below-ground reserves, which were less than half the size of C4 reserves. Re-growth of C4 plants was strongly supported by the reallocation of below-ground biomass and was similar for burnt and control plants, although fire had a weak stimulatory effect on re-growth.
5. Synthesis. Differences in the responses of the A. semialata subspecies indicated that the C4 subspecies is better adapted to fire. Not only did it produce a flammable fuel load, but it was not detrimentally affected by the fire and recovered more rapidly than the C3 subspecies, without the requirement for altered growth allometry. Such characteristics may be the direct consequence of photosynthetic pathway or an indirect effect of adaptation in the C3 and C4 subspecies to environments with differing fire regimes.
A yearly global fire history is a prerequisite for quantifying the contribution of previous fires to the past and present global carbon budget. Vegetation fires can have both direct (combustion) and long-term indirect effects on the carbon cycle. Every fire influences the ecosystem carbon budget for many years, as a consequence of internal reorganization, decomposition of dead biomass, and regrowth. We used a two-step process to estimate these effects. First we synthesized the available data available for the 1980s or 1990s to produce a global fire map. For regions with no data, we developed estimates based on vegetation type and history. Second, we then worked backwards to reconstruct the fire history. This reconstruction was based on published data when available. Where it was not, we extrapolated from land use practices, qualitative reports and local studies, such as tree ring analysis. The resulting product is intended as a first approximation for questions about consequences of historical changes in fire for the global carbon budget. We estimate that an average of 608 Mha yr−1 burned (not including agricultural fires) at the end of the 20th century. 86% of this occurred in tropical savannas. Fires in forests with higher carbon stocks consumed 70.7 Mha yr−1 at the beginning of the century, mostly in the boreal and temperate forests of the Northern Hemisphere. This decreased to 15.2 Mha yr−1 in the 1960s as a consequence of fire suppression policies and the development of efficient fire fighting equipment. Since then, fires in temperate and boreal forests have decreased to 11.2 Mha yr−1. At the same time, burned areas increased exponentially in tropical forests, reaching 54 Mha yr−1 in the 1990s, reflecting the use of fire in deforestation for expansion of agriculture. There is some evidence for an increase in area burned in temperate and boreal forests in the closing years of the 20th century.
Eucalypts (Eucalyptus spp. and Corymbia spp.) dominate many communities across Australia, including frequently burnt tropical savannas and temperate forests, which receive less frequent but more intense fires. Understanding the demographic characteristics that allow related trees to persist in tropical savannas and temperate forest ecosystems can provide insight into how savannas and forests function, including grass–tree coexistence. This study reviews differences in critical stages in the life cycle of savanna and temperate forest eucalypts, especially in relation to fire. It adds to the limited data on tropical eucalypts, by evaluating the effect of fire regimes on the population biology of Corymbia clarksoniana, a tree that dominates some tropical savannas of north-eastern Australia. Corymbia clarksoniana displays similar demographic characteristics to other tropical savanna species, except that seedling emergence is enhanced when seed falls onto recently burnt ground during a high rainfall period. In contrast to many temperate forest eucalypts, tropical savanna eucalypts lack canopy-stored seed banks; time annual seed fall to coincide with the onset of predictable wet season rain; have very rare seedling emergence events, including a lack of mass germination after each fire; possess an abundant sapling bank; and every tropical eucalypt species has the ability to maintain canopy structure by epicormically resprouting after all but the most intense fires. The combination of poor seedling recruitment strategies, coupled with characteristics allowing long-term persistence of established plants, indicate tropical savanna eucalypts function through the persistence niche rather than the regeneration niche. The high rainfall-promoted seedling emergence of C. clarksoniana and the reduction of seedling survival and sapling growth by fire, support the predictions that grass–tree coexistence in savannas is governed by rainfall limiting tree seedling recruitment and regular fires limiting the growth of juvenile trees to the canopy.
Aim To explore successional processes associated with rain forest expansion in Eucalyptus-dominated woodland savanna vegetation in north-eastern Australia.
Location Iron Range National Park and environs, northeast Queensland, Australia. This remote region supports probably the largest extent of lowland (< 300 m) rainforest remnant in Australia. Rainfall (c. 1700 mm p.a.) occurs mostly between November and June, with some rain typically occurring even in the driest months July–October.
Methods (1) Sampling of rain forest seedling distributions, and other vegetation structural attributes, in fifteen 10 × 10 m quadrats distributed equi-distantly between mature rain forest margins (range: 70–840 m), at each of 10 sites which were open-canopied vegetation in 1943. (2) Assessment of relationships between rain forest seedling densities and structural characteristics, including distance-to-rain forest-margin, canopy height, stem density. (3) Assessment of lifeform and dispersal spectra for defined vegetation structural types.
Results Rates of rain forest invasion were found to be substrate-mediated. Transects established on hematite schist, diorite, riverine alluvium, and granite developed closed canopies (termed phase III sites) by 1991. The remainder (four transects on poorly drained colluvial/alluvial sediments; one on dune sands) continued to occur either as grassy woodland (phase I), or with developing rain forest understoreys (phase II). Rain forest seedlings were observed at maximum sampled distances from mature rain forest margins at all sites. Lifeform and dispersal spectra data illustrated that: (1) the proportions of woodland trees, shrubs and graminoids declined with successional phase, with concomitant increases in rain forest primary trees and all other lifeform categories save rain forest trees; (2) the proportions of major dispersal syndromes did not vary between successional phases, neither for rain forest nor woodland taxa.
Main conclusions Rain forest seedling distribution data for phases I and II sites illustrate three successional processes: margin extension – seedling density significantly negatively correlated with distance from mature rain forest margins at two sites; nucleation – seedling densities significantly positively correlated with tall trees at two sites; and irruption – seedling densities at two sites neither correlated with distance from mature rain forest margins, nor with measured vegetation structural features. The observation of irruptive rain forest regeneration at these sites, combined with decadal-scale rain forest canopy development at the five remaining sites, illustrates that under conditions conducive to growth (moisture, substrate), low fire disturbance, and maintenance of diverse dispersal processes (high frugivore richness), rain forest can rapidly invade regional landscapes.
A numerical floristic analysis of samples across a monsoon forest-savanna boundary, from an area that had been actively protected from fire for 15 years, at Weipa, northern Australia, revealed three communities: (i) a monsoon forest with a low closed canopy composed mainly of tree species with extra-Australian tropical affinities and a sparse ground layer; (ii) an ecotone with a distinct closed microphyll shrub layer beneath the open canopy of savanna trees; and (iii) a savanna dominated by Eucalyptus tetrodonta. The development of the ecotone has occurred since fire protection and is of limited extent within the fire protected block. The monsoon forest occurred on soils with significantly higher concentrations of bauxitic pisoliths than the other two communities. Soils under the monsoon forest had significantly higher concentrations of total K, S, C, N, exchangeable K and Ca, and higher pH and electrical conductivity than for soils of either of the other communities. A positive relationship between woody basal area and concentrations of surface soil total P, N, C, exchangeable Ca, CEC and gravel was detected across a 20 m transect from the ecotone community into the savanna. The invasion of monsoon forest seedlings was greatest in the ecotone, with few occurring in the savanna. It appears that the expansion of the monsoon forest requires the development of a layer of shrubs. The mechanism of this facilitation is unclear, although the possible role of nutrient enrichment by the shrubs requires further investigation.
Epicormic bud producing structures in the eucalypts, a large group of woody plants of considerable ecological, horticultural and silvicultural importance, are described here.
The outer portion of epicormic strands excised from the bark of large diameter stems of 18 Eucalyptus species, two Angophora species and Lophostemon confertus was examined anatomically in semithin sections.
In the inner bark each eucalypt strand usually possessed 5–12 radially orientated strips of tissue of meristematic appearance. The meristem strips were c . 30–50 µm high, 70–110 µm wide and 2000–10 000 µm long, with a lacuna above the meristem surface. Few buds or bud primordia were associated with the strands and the strands appeared to have a reduced regenerative potential in the outer bark.
In most angiosperm trees dormant epicormic buds are present in the outer bark, a position where they could be killed by fire. By contrast, in eucalypts the greatest epicormic bud initiation potential is at the level of the vascular cambium, which is protected by the maximum bark thickness. This might explain the pronounced ability of eucalypts to produce bole and branch epicormic shoots after moderate to intense fire.
Patterns of leaf attributes were compared at regional and global scales in relation to the seasonal availability of water.
Light‐saturated assimilation rate ( A mass ), leaf mass per area (LMA), leaf density, thickness, life span, saturated water content, chlorophyll, nitrogen and phosphorus content were determined during the wet season for 21 tree species in four contrasting habitats in northern Australia. Rainfall in this area is extremely seasonal.
A mass and foliar chlorophyll, N and P contents were positively correlated with each other, and were all negatively correlated with LMA, leaf thickness, density and life span.
Deciduous species had smaller LMA and leaf life span, and larger foliar N and P contents than did evergreen species.
The eight Myrtaceous species had smaller A mass , foliar chlorophyll, N and P contents, and larger LMA, leaf thickness and leaf life span than did the non‐Myrtaceous species.
Leaves from the closed canopy dry monsoon forest had significantly larger A mass , chlorophyll and P contents than did leaves from the three open canopy habitats (eucalypt open forest, mixed woodland and Melaleuca swamp). This reflected the relatively low proportions of evergreen and Myrtaceous species in the dry monsoon forest. There were also significant intraspecific differences among habitats.
Leaf thickness, density and LMA were lower than predicted from globally derived relationships with temperature and precipitation. Tropical seasonally dry biomes are under‐represented in such global analyses.
In severely disturbed habitats, the onset of resprouting as a persistence strategy might be problematic for tree species which do not accumulate sufficient reserves before the first disturbance event. This is due to the trade‐off between the growth of reserves required to recover after disturbance and that of photosynthetic tissues.
In humid savannas, fire prevents trees from invading the whole landscape and nearby gallery forests have a completely different floristic composition. We test if the variations of survival during the first years of a young tree's life can explain the exclusion of forest species and the dominance pattern within savanna species.
Every six months for four years, we censused all seedlings and resprouts in 1 ha area of an annually burned savanna, to estimate their seasonal survival rates. We used capture–recapture statistical models to control for the probability of missing seedlings in the tall grass.
There were two main distinct patterns of survival among seedlings: ‘fire‐responding’ species showed a 20–80% decrease in survival during the dry season, interpreted as mainly due to fire; ‘drought‐responding’ species showed 20–80% variations in survival positively correlated to early‐growing‐season rainfall.
Yearly averaged survival probabilities of seedlings ranged between 0.10 and 0.63, reaching 0.850–0.996 for > 3‐year‐old resprouts of savanna species. Forest species showed no increase in survival with age.
A 4‐year‐survival‐probability analysis showed that forest species were excluded from the savanna at the seedling stage. No parameter of the early survival curve related to the abundance of savanna species at the adult stage.
Synthesis . Savanna tree species follow two mutually exclusive main patterns of early survival probably related to fire and early‐wet‐season drought. The exclusion of forest species is consistent with a build up of reserves that is too slow due to the growth‐resistance trade‐off. We conclude from these findings that the use of resprouting as a persistence strategy is heavily constrained by disturbance frequency and imposes strong trade‐offs on plant growth strategy.
Fire is important in the dynamics of savanna–forest boundaries, often maintaining a balance between forest advance and retreat.
We performed a comparative ecological study to understand how savanna and forest species differ in traits related to fire tolerance. We compared bark thickness, root and stem carbohydrates, and height of reproductive individuals within 10 congeneric pairs, each containing one savanna and one forest species.
Bark thickness of savanna species averaged nearly three times that of forest species, thereby reducing the risk of stem death during fire. The allometric relationship between bark thickness and stem diameter differed between these two tree types, with forest species tending to have a larger allometric coefficient.
The height of reproductive individuals of forest species averaged twice that of congeneric savanna species. This should increase the time necessary for forest species to reach reproductive size, thereby reducing their capacity to reach maturity in the time between consecutive fires.
There was no difference in total non‐structural carbohydrate content of stems or roots between savanna and forest species, though greater allocation to total root biomass by savanna species probably confers greater capacity to resprout following fire.
These differences in fire‐related traits may largely explain the greater capacity of savanna species to persist in the savanna environment.
1. Burning typically occurs at intervals of 1–3-years in the Brazilian cerrado, a rate that exceeds the precolonization fire regime. To determine if woody plants of the cerrado successfully reproduce within the short span of time between burns, experimental burns were used to quantify the effects of fire on sexual and vegetative reproduction of six species of resprouting trees and shrubs.
2. Four of the six species reproduce vegetatively by producing root suckers. For three of these species, Rourea induta, Myrsine guianensis and Roupala montana, sucker production was seven to 15 times greater in burned plots than in unburned controls.
3. Fire had a negative impact on sexual reproduction. Fire caused an immediate reduction in sexual reproductive success by destroying developing reproductive structures and seeds. Additionally, five of the six study species exhibited overall reductions in seed production in the years following fire. Fire had this effect by reducing the individual size of all species and, for three species, by reducing size-specific reproductive output. Only the tree Piptocarpha rotundifolia exhibited increased seed production following burning.
4. Fire caused substantial mortality to both seedlings and suckers. Suckers were larger than seedlings and experienced lower mortality rates for two of three species. Fire-induced mortality of seedlings varied greatly among species, ranging from 33% to 100%.
5. The results indicate that vegetative reproduction is much more successful than sexual reproduction under the high fire frequency typical of current fire regimes. It is concluded that current fire regimes must be causing a shift in species composition, favouring species capable of vegetative reproduction.
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.
Anthropogenic understory fires affect large areas of tropical forest, particularly during severe droughts. Yet, the mechanisms that control tropical forests' susceptibility to fire remain ambiguous. We tested the widely accepted hypothesis that Amazon forest fires increase susceptibility to further burning by conducting a 150 ha fire experiment in a closed-canopy forest near the southeastern Amazon forest–savanna boundary. Forest flammability and its possible determinants were measured in adjacent 50 ha forest plots that were burned annually for 3 consecutive years (B3), once (B1), and not at all (B0). Contrary to expectation, an annual burning regime led to a decline in forest flammability during the third burn. Microclimate conditions were more favorable compared with the first burn (i.e. vapor pressure deficit increased and litter moisture decreased), yet flame heights declined and burned area halved. A slight decline in fine fuels after the second burn appears to have limited fire spread and intensity. Supporting this conclusion, fire spread rates doubled and burned area increased fivefold in B3 subplots that received fine fuel additions. Slow replacement of surface fine fuels in this forest may be explained by (i) low leaf litter production (4.3 Mg ha−1 yr−1), half that of other Amazon forests; and (ii) low fire-induced tree and liana mortality (5.5±0.5% yr−1, SE, in B3), the lowest measured in closed-canopy Amazonian forests. In this transitional forest, where severe seasonal drought removed moisture constraints on fire propagation, a lack of fine fuels inhibited the intensity and spread of recurrent fire in a negative feedback. This reduction in flammability, however, may be short-lived if delayed tree mortality or treefall increases surface fuels in future years. This study highlights that understanding fuel input rate and timing relative to fire frequency is fundamental to predicting transitional forest flammability – which has important implications for carbon emissions and potential replacement by scrub vegetation.
Fire is a frequent disturbance in the tropical dry forests of Central America, yet very little is known about how native species respond to such events. We conducted an experimental burn in a tropical dry forest of western Nicaragua to evaluate plant responses to fire with respect to survivorship and recruitment. Measurements of woody vegetation of all size classes were carried out prior to the prescribed burn and three successive years post fire. We selected the 15 most abundant species <10 cm DBH to assess percent survivorship and sprouting responses post fire. Changes in seedling densities for these 15 most abundant species and the 15 least abundant species were analyzed using a repeated measure ANOVA. We also assessed changes in seedling densities for three species of international conservation concern. We found three major fire-coping strategies among common dry forests plants: resisters (low fire-induced mortality), resprouters (vigorous sprouting), and recruiters (increased seeding post-fire). While survivorship was generally high relative to tropical moist forest species, those species with lower survivorship used either seeding or sprouting as an alternative strategy for persisting in the forest community. Seed dispersal mechanisms, particularly wind dispersal, appear to be an important factor in recruitment success post-fire. Burn treatment led to a significant increase in the density of seedlings for two species of conservation concern: Guaiacum sanctum and Swietenia humilis. Results of this study suggest that common dry forest species in western Nicaragua are fire tolerant. Further study of individual species and their fire responses is merited.
Aim This study aims to improve the formulation and results of the Brazilian Center for Weather Forecasting and Climate Studies Potential Vegetation Model (CPTEC-PVM) by developing a new parameterization for the long-term occurrence of fire in regions of potential savannas in the tropics. Compared with the relatively slow processes of carbon uptake and growth in vegetation, fast mortality and biomass consumption by fires may favour grasses and reduce tree coverage.
Location The tropics.
Methods For finding large-scale relationships between fires and other environmental factors, we made two main simplifying assumptions. First, lightning is the most important source of ignition for natural fires. Second, over continental areas in the tropics, lightning is mainly related to the zonal flux of moisture transport.
Results The parameterization of fire occurrence was built based on a simple empirical relationship, combining information on mean and intra-annual variance of the zonal wind.
Main conclusions The implementation of this new relationship improved the formulation and the results of the CPTEC-PVM. As a result of this new parameter, the accuracy of the model in allocating the correct vegetation (seasonal forests) instead of savannas for large regions in India and Southeast Asia is now substantially higher than in previous studies.
Abstract Exotic grasses are becoming increasingly abundant in Neotropical savannas, with Melinis minutiflora Beauv. being particularly invasive. To better understand the consequences for the native flora, we performed a field study to test the effect of this species on the establishment, survival and growth of seedlings of seven tree species native to the savannas and forests of the Cerrado region of Brazil. Seeds of the tree species were sown in 40 study plots, of which 20 were sites dominated by M. minutiflora, and 20 were dominated by native grasses. The exotic grass had no discernable effect on initial seedling emergence, as defined by the number of seedlings present at the end of the first growing season. Subsequent seedling survival in plots dominated by M. minutiflora was less than half that of plots dominated by native species. Consequently, at the end of the third growing season, invaded plots had only 44% as many seedlings as plots with native grasses. Above-ground grass biomass of invaded plots was more than twice that of uninvaded plots, while seedling survival was negatively correlated with grass biomass, suggesting that competition for light may explain the low seedling survival where M. minutiflora is dominant. Soils of invaded plots had higher mean Ca, Mg and Zn, but these variables did not account for the higher grass biomass or the lower seedling survival in invaded plots. The results indicate that this exotic grass is having substantial effects on the dynamics of the tree community, with likely consequences for ecosystem structure and function.
Patterns of growth, activity and renewal of stems and branches are primary determinants of ecosystem function and strongly influence net primary productivity, water and energy balance. Here we compare patterns of leaf phenology, stem radial growth and branch growth of co‐occurring savanna and forest trees in the Cerrado region of central Brazil to gain insight into the influence of these parameters in forest–savanna boundary dynamics. We hypothesized that forest species would have higher radial growth rates but later leaf flush than savanna species.
We studied 12 congeneric species pairs, each containing one savanna species and one forest species. All individuals were growing in savanna conditions under full sun. We measured specific leaf area (SLA), light‐saturated photosynthesis and monthly increments in stem circumference, branch length, leaf flush and leaf fall.
Relative to savanna species, forest species had 68% higher diameter growth rates, 38% higher SLA, and displayed a greater crown area for a given basal area. Across species, radial growth was positively correlated with SLA ( r ² = 0·31), but not with CO 2 assimilation.
Peak leaf production of savanna species was in the late dry season, 1 month earlier than for forest species, which suggests a strategy to avoid nutrient losses during leaf expansion due to herbivory or leaching. However, savanna and forest species did not differ in annual branch growth, number of leaves produced per branch, or in timing of leaf fall.
Radial growth was tightly coupled to monthly rainfall in forest species whereas the growth of savanna species ceased before the end of the wet season. The cessation of above‐ground growth at a time of active photosynthesis may reflect a shift in allocation to roots and reserves.
These results contribute to recent studies showing that savanna and forest species represent different functional types and that despite the limiting resources in savanna environments, forest trees that invade the savanna tend to present higher growth rates and larger and denser crowns, which enhance shading and could promote changes in equilibrium of forest–savanna boundaries.
Aim The objective of this study was to document succession from grassland thickets to rain forest, and to provide evidence for their potential as restoration tools.
Location The Linganamakki region (State of Karnataka) of the Central Western Ghats of India.
Method We selected thirty vegetation thickets ranging from 4 to 439 m2 in area in the vicinity of rain forest. The area of each small thicket was estimated as an oval using its maximum length and its maximum width. When the shape was irregular (mostly in large thickets) the limits of the thicket were mapped and the area calculated from the map. Plant species were identified, the number of individuals was estimated and their heights measured.
Results There was a progression in the thickets from early to late successional species. Small thickets were characterized by ecotone species and savanna trees such as Catunaregam dumetorum. Savanna trees served as a nucleus for thicket formation. Colonizing species were mostly bird-dispersed. As succession proceeded in larger thickets, the proportion of evergreen, late-successional rain forest trees increased. The species composition of the large thickets differed depending on the species composition of reproductive adults in the nearby forested areas. The species within small thickets were also found in the large thickets. The nestedness in species composition suggested that species turnover was deterministic based on thicket size. Human disturbance (leaf and wood collection by the local populations) affected the species composition and the species–area relationship of thickets.
Main conclusions Vegetation thickets are nodal centres for rain forest colonization within grasslands. They expand and replace savanna. Early successional bird-dispersed species established around savanna trees followed by late-successional rain forest trees dispersed from the nearby forest by birds. Restoration programmes that reproduce natural successional processes such as those observed in thickets will be more successful and less expensive than the methods currently being employed, where trees are individually planted in grassland. Wood harvesting is the only factor that prevents thicket growth and coalescence and hampers forest expansion.