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Comparative water use of wattle thickets and indigenous plant communities at riparian sites in the Western Cape and KwaZulu-Natal
Abstract
Large-scale funding by both the Government and the private sector continues in support of the Working-for-Water Programme, which is active in many regions of the country. One justification for this programme of alien tree removal is the streamflow enhancement that is believed to follow the replacement of dense stands of invasive trees by indigenous, largely herbaceous or shrub dominated plant communities. Often the densest stands of invader trees occur within riparian zones, where removal of trees in close proximity to stream channels is believed to strongly enhance streamflow. Few data are available, however, to support this assumption. Results from a number of research catchments have consistently shown that afforestation significantly decreases streamflow where the pre-afforestation vegetation was seasonally dormant mountain grassland or fynbos (Versfeld, 1994). The net difference in evapotranspiration (ET) between riparian thickets of alien trees and riparian fynbos may be quite different, due to the yearlong availability of soil water and enhanced plant growth in riparian zones. The water use of alien invasive trees in South Africa remains largely unknown, adding further uncertainty to the effect of alien removal on streamflow. This paper describes the results of a comparative study of annual ET between indigenous riparian plant communities and riparian wattle thickets (Acacia mearnsii() at four sites in the Western Cape and KwaZulu-Natal. The Bowen ratio energy balance (BREB) technique was used to record a 12-month record of 20 min evaporation rates from a fynbos riparian plant community in the Jonkershoek valley (Western Cape), and a grassland riparian community on the property Gilboa in the KwaZulu-Natal midlands. Closed-canopy, mature stands of self-established (A. mearnsii( in the Wellington and Groot Drakenstein areas of the Western Cape were selected to provide comparative transpiration data. The heat pulse velocity (HPV) technique was used to record hourly sap-flow rates in six sample trees representing the range of tree sizes at both wattle sites. Total daily sap flow in all sample trees experiencing adequate soil water availability was found to be very closely correlated to tree size and an index defined as the product of mean daily vapour pressure deficit (VPD) of the air and the number of daylight hours. These relationships were used to predict the water use of wattle thickets at Jonkershoek and Gilboa, using VPD and day-length data recorded at these sites. Published estimates of canopy rainfall interception were added to the sap flow (transpiration) component to yield a combined annual ET to compare to the BREB ET data. Table 1 summarises the annual evapotranspiration at each site. (table***) We conclude that the removal of riparian wattle and its replacement by indigenous herbaceous plants may indeed result in significant reductions in annual ET, and could very likely lead to streamflow enhancement. However, this study has clearly shown that annual ET varies considerably in different riparian plant communities, and that one must consider the structural and physiological characteristics of both the pre-clearing and post-clearing vegetation in order to predict the net change in ET. This conclusion supports an earlier view (Versfeld et al., 1998) that an improved methodology of general applicability is required to enhance the accuracy of water use predictions for a wide range of alien and indigenous plant communities. Such predictions are important to prioritise clearing operations in areas invaded by alien trees.
WaterSA Vol.27(4) 2001: 529-538
... Ecotones in general, and riparian zones in particular, are known to be sensitive to environmental change (Naiman & Decamps 1987) and particularly prone to invasion by alien plant species (Tickner, Angold et al. 2001). This vulnerability of riparian zones to invasion by non-native plants is particularly pronounced in semi-arid areas (Dye et al. 2001), at least partly due to the dispersal of water-borne alien seed by floods (Knight 1985), the availability of water in otherwise dry areas, and the dynamic, frequently disturbed nature of riparian systems that creates patches of open sediment ideal for germination (Versveld et al. 1998). Invasion of alien plants into riparian zones in arid regions has important implications for our already scarce water resources, since these species have free access to water and consume more of it than do indigenous riparian species (Versveld et al. 1998). ...
... Biological invasions are the second largest threat (after habitat destruction) to global biodiversity (Richardson & van Wilgen 2004). Semi-arid riparian zones are particularly prone to invasion by alien plant species (Dye et al. 2001), to a large degree because of the availability of water in otherwise dry areas (Versveld et al. 1998). They are frequently disturbed by floods (Richardson et al. in press), which uproot plants and create new habitat patches (Versveld et al. 1998) through fluvial deposition and erosion of sediments. ...
... One of the most frequently cited impacts of woody alien invasion of riparian zones is the excessive use of surface and ground water by the aliens compared to the volumes used by indigenous riparian vegetation (Dye & Poulter 1995, Versveld et al. 1998Calder & Dye 2001). In fact, the recovery of streamflow following clearance of alien-infested catchments, was one of the primary motivating factors for the Working for Water programme (van Wilgen et al. 1998) based on the premise that indigenous forest or scrub uses less water (Scott & Lesch 1996;Dye et al. 2001). Woody alien species have additional impacts, such as fundamentally altering the vegetation-mediated processes caused by changes in canopy structure and in the abundance and variety of species and growth forms present. ...
... Evaporation from palmiet wetlands was measured using infrared scintillometry and also using the remote-sensing-based SEBAL model of Bastiaanssen et al. 1998(for results, see Rebelo 2012. Measurements of evaporation from riparian invasions of A. mearnsii are available from a number of studies in South Africa (Dye et al. 2001;Dye & Jarmain 2004;Clulow et al. 2011). The model initially did not provide acceptable estimates of the evaporation for these HRUs (according to available literature, sensu Table 2), but by improving soil depth input and plant characteristics, we were able to improve the model outputs. ...
... The fact that the modeled evaporation values were still lower than the measured ones does indicate that the ACRU riparian routines, which allow for increased water availability within the rooting zone, need to be improved. Measured evaporation of A. mearnsii is 265-600 mm higher than that modeled in this study (Dye et al. 2001;Dye & Jarmain 2004;Clulow et al. 2011). By potentially underestimating groundwater availability and, thus, the water-use of both A. mearnsii and P. serratum in the Kromme River alluvium, the ACRU-modeled riverflow may underestimate the actual flow reductions. ...
... Comparison of the modeled evaporation for key hydrological response units (vegetation types) from the upper Kromme (Eastern Cape) with water-use measurements from other studies in South Africa(Dye et al. 2001;Dye & Jarmain 2004; Clulow et al. 2011). Annual evaporation outputs from the ACRU model were averaged over 50 years(from 1950 to 2000). ...
Valley-bottom wetlands are valuable assets as they provide many ecosystem services to mankind. Despite their value, valley-bottom wetlands are often exploited and land-use/land-cover (LULC) change results in trade-offs in ecosystem services. We coupled physically based hydrological modeling and spatial analysis to examine the effects of LULC change on water-related ecosystem services in the Kromme catchment: an important water-providing catchment for the city of Port Elizabeth. LULC scenarios were constructed to match 5 different decades in the last 50 years to explore the potential effects of restoring the catchment to different historic benchmarks. In the Kromme catchment, valley-bottom wetlands have declined by 84%, driven by key LULC changes: an increase in irrigated land (307 ha) and invasion by alien trees (336 ha). If the wetlands were restored to the relatively pristine extent and condition of the 1950s, riverflow could increase by approximately 1.13 million m3/a, about 6% of the current supply to Port Elizabeth. Wetland restoration would also significantly improve the catchment's ability to absorb extreme rainfall events, decreasing flood damage. We conclude that in the face of the water scarcity in this region, all ecosystem services, particularly those related to water flow regulation, should be taken into account by decision makers in charge of land zonation. Zonation decisions should not continue to be made on the basis of provisioning ecosystem services alone (i.e. food provision or dam yield). We recommend prioritization of the preservation and restoration of valley-bottom wetlands providing water-related ecosystem services to settlements downstream.
... For example, the conservation of biodiversity in riparian areas strongly depends on adequate water allocations to meet environmental flows. Accurate information on water use by the riparian vegetation is crucial to ensure 'safe' abstraction levels from the region's rivers and groundwater sources (Dye et al., 2001;. In studies to quantify the hydrological impacts of alien invasive plants, information on the water use by the indigenous vegetation is critical for determining the incremental water gains that can be achieved by clearing the alien plants (Doody et al., 2011;Dzikiti et al., 2013a). ...
... In studies to quantify the hydrological impacts of alien invasive plants, information on the water use by the indigenous vegetation is critical for determining the incremental water gains that can be achieved by clearing the alien plants (Doody et al., 2011;Dzikiti et al., 2013a). Potential water savings depend on the water use characteristics of the indigenous vegetation that replace the cleared alien plants (Bosch et al., 1986;Dye et al., 2001;Dzikiti et al., 2013b;Le Maitre et al., 1996). In a recent Water Research Commission study (Jovanovic et al., 2012), uncertainties in groundwater recharge estimates were reduced significantly by including accurate data on the water use by indigenous vegetation in the hydrological models. ...
... The vegetation types include; Sand Plain, Sandstone, and Swartland Fynbos. There has been one other study which directly measured ET at these time scales from a riparian fynbos wetland using the Bowen ratio measurement technique at Jonkershoek, which found that the annual ET was quite high, being 1 332 mm (Dye et al., 2001). We calibrated and validated a dualsource ET model and used it to scale up the measured water use data from selected periods to annual time scales. ...
Many studies have investigated the water relations of indigenous plants in the fynbos shrublands of the Cape, South Africa. These have mainly focused on understanding the mechanisms by which individual plant species respond to droughts, the frequency and severity of which is expected to increase due to climate change. However, comparatively little information exists on the dynamics of water use by indigenous plants in the region, and, in particular, how water use varies seasonally and between sites. In this study we determined water use by 3 fynbos vegetation types growing at 4 different sites, namely: (i) lowland Atlantis Sand Plain fynbos growing on deep sandy soils, (ii) Kogelberg Sandstone fynbos growing in a riparian zone on deep alluvial soils, (iii) dryland Kogelberg Sandstone fynbos growing on shallow sandy soils at a montane site, and (iv) alluvial Swartland fynbos growing in clayey soils. Evapotranspiration (ET) was quantified at each site during specific periods using a boundary layer scintillometer and energy balance system. A simple dual source model in which the stand ET was calculated as the algebraic sum of outputs from soil evaporation and transpiration sub-models was used to scale up the ET measurements to annual values. The data showed large differences in ET depending on site characteristics and on plant attributes. Dense stands of riparian Sandstone Fynbos had an annual ET of 1 460 mm which exceeded the reference ET of 1 346 mm. Dryland Sandstone Fynbos used only 551 mm of water per year while the Sand Plain Fynbos’ annual ET was 1 031 mm, which was similar to the reference ET of 1 059 mm. We conclude that some indigenous plant species use large volumes of water which should be accounted for in, e.g., groundwater recharge estimates, and calculations of incremental water gains after clearing alien invasive plants, among other applications.
... For example, the conservation of biodiversity in riparian areas strongly depends on adequate water allocations to meet environmental flows. Accurate information on water use by the riparian vegetation is crucial to ensure 'safe' abstraction levels from the region's rivers and groundwater sources (Dye et al., 2001;). In studies to quantify the hydrological impacts of alien invasive plants, information on the water use by the indigenous vegetation is critical for determining the incremental water gains that can be achieved by clearing the alien plants (Doody et al., 2011; Dzikiti et al., 2013a). ...
... In studies to quantify the hydrological impacts of alien invasive plants, information on the water use by the indigenous vegetation is critical for determining the incremental water gains that can be achieved by clearing the alien plants (Doody et al., 2011; Dzikiti et al., 2013a). Potential water savings depend on the water use characteristics of the indigenous vegetation that replace the cleared alien plants (Bosch et al., 1986; Dye et al., 2001; Dzikiti et al., 2013b; Le Maitre et al., 1996). In a recent Water Research Commission study (Jovanovic et al., 2012), uncertainties in groundwater recharge estimates were reduced significantly by including accurate data on the water use by indigenous vegetation in the hydrological models. ...
... The vegetation types include; Sand Plain, Sandstone, and Swartland Fynbos. There has been one other study which directly measured ET at these time scales from a riparian fynbos wetland using the Bowen ratio measurement technique at Jonkershoek, which found that the annual ET was quite high, being 1 332 mm (Dye et al., 2001 ). We calibrated and validated a dualsource ET model and used it to scale up the measured water use data from selected periods to annual time scales. ...
ABSTRACT
Many studies have investigated the water relations of indigenous plants in the fynbos shrublands of the Cape, South Africa. These have mainly focused on understanding the mechanisms by which individual plant species respond to droughts, the frequency and severity of which is expected to increase due to climate change. However, comparatively little information exists on the dynamics of water use by indigenous plants in the region, and, in particular, how water use varies seasonally and between sites. In this study we determined water use by 3 fynbos vegetation types growing at 4 different sites, namely: (i) lowland Atlantis Sand Plain fynbos growing on deep sandy soils, (ii) Kogelberg Sandstone fynbos growing in a riparian zone on deep alluvial soils, (iii) dryland Kogelberg Sandstone fynbos growing on shallow sandy soils at a montane site, and (iv) alluvial Swartland fynbos growing in clayey soils. Evapotranspiration (ET) was quantified at each site during specific periods using a boundary layer scintillometer and energy balance system. A simple dual source model in which the stand ET was calculated as the algebraic sum of outputs from soil evaporation and transpiration sub-models was used to scale up the ET measurements to annual values. The data showed large differences in ET depending on site characteristics and on plant attributes. Dense stands of riparian Sandstone Fynbos had an annual ET of 1 460 mm which exceeded the reference ET of
1 346 mm. Dryland Sandstone Fynbos used only 551 mm of water per year while the Sand Plain Fynbos’ annual ET was 1 031 mm, which was similar to the reference ET of 1 059 mm. We conclude that some indigenous plant species use large volumes of water which should be accounted for in, e.g., groundwater recharge estimates, and calculations of incremental water gains after clearing alien invasive plants, among other applications.
Keywords: Evapotranspiration, fynbos, scintillometer, Western Cape
... Riparian stands of invasive alien plants have been shown to have higher rates of evapotranspiration compared to native riparian communities (e.g. Dye et al., 2001;Dye and Jarmain, 2004). ...
... mearnsii, Dye et al. (2001) found that evapotranspiration and sapflow is highest in summer, but again, this declines later when the seasonal drought progresses. Acacia species were found to be more profligate water users, and this may be at the core of its ability to outcompete native species in the fynbos. ...
Riparian zones exist at the nexus of high resource availability, and high disturbance within landscapes, however, are also centrally involved in delivering several hydrological ecosystem services. As riparian ecotones usually exist at the lowest topographical position within catchments, and they are thus receivers of material, propagules, genes and information, they are often also the first to show significant alterations that affect the catchments. As such, they are useful to monitor overall ecosystem health within
catchments. In addition to natural disturbances such as floods, fires and droughts, several anthropogenic stressors can affect riparian zones. In the fynbos biome, these stressors can range from impoundments, habitat destruction, urban development, agricultural activities and alien invasive species. In the Western Cape, and also other parts of South Africa, the Australian tree Acacia mearnsii has displaced many native riparian plants in some upper, but many middle and lower reaches, establishing populations with high
stand density, which regenerate quickly following a natural disturbance such as fire. Where stands of Acacia mearnsii have established, evapotranspiration has been shown to be higher than where native riparian scrub and forests are found, prompting a major restoration initiative, the Working for Water programme, which aims to clear the invasive species from riparian ecotones. Thus far little information is available on the impact of invasive Acacia species in fynbos riparian ecotones on ecosystem function as it pertains to nutrient cycling and soil processes, some of which are central to riparian ecosystem services. Furthermore, it is unknown whether soil chemistry and processes return to pre-invasion levels after removal of the alien invasive Acacia species from fynbos riparian ecotones. One of the objectives of the Working for Water Programme is to clear riparian zones of invasive alien trees like Acacia species, which takes place with the expectation that both vegetation structure and ecosystem function will recover following clearing. However, while vegetation structure recover in some instances, it remains unclear whether ecosystem function also recovers. The main objective of this project, therefore, was to investigate the impact of the invasive Acacia mearnsii (occasionally co-occurring with A. longifolia in the Western Cape and Acacia dealbata in the southern and eastern Cape) on nitrogen, carbon and phosphorus stocks and cycling, as well as its impact on the soil bacterial and fungal community structure within fynbos riparian ecotones and nearby upland areas. Our sites were located mostly in the upper reaches of catchments, which was dictated by the availability of reference sites. We also went further and investigated the relationship between riparian soil processes and soil and plant diversity in “pristine” (reference; natural), invaded and cleared ecotones in order to improve the strategies for riparian repair after removal of alien invasive Acacia species. Finally, we compared aspects of nitrogen, carbon and phosphorus cycling in soils at our main sites in the Western Cape with measurements at sites in the southern and eastern Cape.
... Stratigraphy of the study area (Toerien, 1971(Toerien, -1972Toerien, 1972;Toerien, 1986;Thamm and Johnson, 2006 and Toerien, n.d.). Table 5.12 Annual ET estimates for Prionium serratum, Phragmites communis (common wetland reed), Acacia mearnsii and Fynbos derived using various methods including the Agricultural Catchments Research Unit (ACRU) model, Pitman model, MODIS, Bowen ratio energy balance (BREB) and eddy covariance technique (Dye et al., 2001;Dye et al., 2008;Rebelo, 2012;Rebelo et al., 2015 and Tanner, pers. comm 1 Wetlands are considered to be one of the most vital ecosystems on the Earth. ...
... of riparian vegetation and upland alien vegetation areas were measured in ArcGIS. ET rates for alien vegetation were estimated using rates inDye et al. (2001) andMeijninger and Jarmain (2014). ...
... These woody species tend to be taller in stature than the native fynbos shrubs, causing an increase in above-ground biomass. Replacement of native fynbos by invasive species has been associated with reductions in river flow due to increased transpiration rates (Scott, 1999;Le Maitre et al., 2000;Dye et al., 2001). Estimates of river flow reductions have generally been made using empirical relationships between catchment biomass and flow reductions (e.g., Le Maitre et al., 2000), small catchment experiments where the fynbos shrubs have been replaced by tree plantations (e.g., Scott 1999), and measurements of river flow or evapotranspiration over short periods following the removal of invasive vegetation in specific locations (e.g. ...
... This distinction was also recognised by Le Maitre et al. (2000) in a modelling study to predict the impact of invasive tree species on river flows in the Western Cape region. The distinction is related largely to the differences in general availability of water in the root zone (Le Maitre et al., 2000) and plant growth (Dye et al., 2001). The contrast between upland and lowland evapotranspiration (ET) losses in a catchment may be particularly evident in semi-arid regions where ET in upland areas is often constrained by water availability during dry periods (Goodrich et al., 2000). ...
Mountain fynbos catchments in the Western Cape region of South Africa are prone to substantial changes in land cover due to invasion by exotic tree species (and their clearing), fires, and vegetation response to inter-annual variations in rainfall. While small catchment experiments and modelling studies have pointed to reductions in river flow as catchment biomass increases, there is little empirical evidence of land cover change affecting river flow in large catchments that are important sources of water for the region. Monitoring changes in above-ground green biomass in multiple large catchments is challenging, but may be accomplished using a remotely sensed spectral vegetation index. It was hypothesised in this study that annual river yield (river flow as a fraction of rainfall) in the Molenaars catchment near Paarl, South Africa co-varies with an index of green vegetation cover derived from satellite data (the normalised difference vegetation index, NDVI). The catchment was parti- tioned into 'upland' and 'lowland' zones and the relationship between annual river yield and summer NDVI was determined for each zone over an 18-year period. There was a statistically significant negative linear relationship between annual river yield and the NDVI of the lowland zone when three outliers were excluded from the analysis. These outliers corresponded to periods with prolonged drought conditions when river yield appeared to be decoupled from vegetation water use in the lowland zone. There was no relationship between river yield and changes in the NDVI in the upland zone where plants were unlikely to have sustained access to adequate soil water for transpiration. The importance of considering the location of land cover changes in a catchment, and inadequacies in high-elevation measurements of rainfall in this mountainous region, were highlighted in the study.
... En general, las acacias forman doseles vegetales densos que aumentan la pérdida del agua en el suelo debido a las altas tasas de transpiración. Estudios en distintos ecosistemas invadidos en Sudáfrica (fynbos) y en Portugal (pinares en dunas costeras) han detectado que las acacias presentan una mayor evapotransporación que la vegetación nativa (Dye et al. 2001;Rascher et al. 2011). Esta mayor evapotranspiración puede reducir el agua disponible para las especies nativas limitando su crecimiento . ...
... Cuando las acacias invaden zonas de ribera estas elevadas tasas de evapotransporación provocan una reducción de la cantidad de agua en las cuencas hidrográficas (Chamier et al. 2012). Al mismo tiempo, la invasión reduce la calidad del agua de ríos y riachuelos por la eutrofización derivada de una entrada muy elevada de nutrientes y materia orgánica provenientes de la gran cantidad de hojarasca producida por las acacias (Dye et al. 2001). Por otra parte, la invasión por acacias puede aumentar el riesgo de inundaciones incluso a niveles moderados de precipitación (Le Maitre et al. 2014). ...
La invasión por acacias australianas altera profundamente tanto las características abióticas del suelo como la estructura de las comunidades microbianas edáficas, modificando los procesos y servicios de los ecosistemas invadidos. En general, las acacias invasoras conllevan un gran aumento del contenido de hojarasca, carbono y nitrógeno, así como modificaciones en los ciclos biogeoquímicos de estos elementos y una disminución de la disponibilidad hídrica en los ecosistemas invadidos. Producen también modificaciones en la diversidad funcional y genética de las comunidades de bacterias y hongos del suelo. Durante la invasión por algunas especies de acacias australianas se ha comprobado además que se produce una co-invasión de bacterias fijadoras de nitrógeno exóticas asociadas a las acacias. Estas bacterias exóticas pueden asociarse a leguminosas nativas que existen en el ecosistema invadido lo que conlleva una disrupción de la simbiosis entre leguminosas y rizobios nativos. Todos estos cambios en las propiedades abióticas y bióticas del suelo pueden tener consecuencias negativas para las especies vegetales nativas, reduciendo su crecimiento o impidiendo su establecimiento. Además, los cambios introducidos por las acacias refuerzan la invasión ya que favorecen la germinación, crecimiento y capacidad competidora de estas especies. Los estudios realizados sugieren que estos procesos de retroalimentación positivos entre las acacias y los suelos que invaden aumentan con el tiempo de invasión y dificultan la recuperación natural de los ecosistemas.
... In 1995, the South African government instigated the Working for Water programme (Van Wilgen et al., 1998) to clear watersheds of invasive trees to enhance stream flow (Le Maitre et al., 1996;Dye et al., 2001;Dye and Jarmain, 2004). Labour intensive approaches are taken with dense infestations (as was the case for our study site in 1998 (Prins, 2003)). ...
... The vegetation type of the catchment is Boland Granite Fynbos growing in soils derived from deeplyweathered granites with the upper layers containing colluvial sandstone material (Rebelo et al., 2006). A small, alien-invaded stream, that was subject to some studies in the past (Dye et al., 2001;Dye and Jarmain, 2004), was chosen for the study. Acacia mearnsii De Wild. ...
... Native species physiology was contrasted with the evergreen invasive woody species A. mearnsii DeWild (L.) (Fabaceae), which poses a major threat to functioning of riparian zones in the south-western Cape (Dye and Jarmain 2004). Although A. mearnsii can also grow in upland areas under low water availability (for example, it is a forestry tree in some parts of South Africa), it is highly competitive in riparian areas where it characteristically uses large amounts of water (Dye et al. 2001). Research has shown that intra-specific variation in drought-tolerance across wet and dry habitats does exist, where one species that showed high drought-tolerance in the dry areas did not show the same degree of drought-tolerance in wet areas (Mencuccini and Comstock 1997; Sparks and Black 1999). ...
... Swift et al. (2008) suggested that species that operate near their hydraulic limit, such as M. angustifolia might employ branch sacrifice as an adaptation against drought conditions, as anecdotal field observations suggest. In contrast, A. mearnsii had low isotopic values, which confirms that it is a profligate user of water where it has access to large amounts, as suggested by Dye et al. (2001). There is strong evidence that small changes in environmental conditions induce selection of improved plant adaptation strategies with regards to safe and efficient hydraulic transport (Ponton et al. 2001 ). ...
Invasive alien plants (IAPs) have successfully invaded many riparian zones in South Africa, especially Australian Acacia spp. which are prevalent along riverbanks in the south-western Cape of South Africa. This Mediterranean-type climate region is predicted to endure severe future water shortages under likely scenarios of increased population growth and climate change, and IAPs aggravate this problem due to their profligate water use. Acacia mearnsii competes aggressively with native species, however, it remains unclear what physiological advantage the species has over co-occurring native species under the predicted reduced streamflow scenarios. A mechanistic approach was used to investigate how key native fynbos riparian woody tree species compare in vulnerability to drought-induced cavitation against A. mearnsii by comparing findings from three Mediterranean-type fynbos river systems that differ in streamflow. A. mearnsii showed lower water potential at 50% hydraulic conductivity loss (P 50 values) compared to native species at certain sites, an indication of drought-tolerance. This suggests it is likely to persist under future drier conditions and it therefore remains a top priority for control. The native Brabejum stellatifolium had consistently higher water potentials across all sites than the other studied species, and is a potentially valuable species for restoration of south-western Cape riparian zones. Consistency in the shapes of species vulnerability curves across sites illustrated a species-specific hydraulic response to different water availability, strengthening the argument that this approach to distinguish site-level drought-tolerance between trees is a practical technique, with great application in understanding future geographic distribution under climate change, and potential for use in restoration research. Additionally, streamflow was an inaccurate predictor of species drought-tolerance along these riparian systems.
... In 1995, the South African government instigated the Working for Water programme (Van Wilgen et al., 1998) to clear watersheds of invasive trees to enhance stream flow (Le Maitre et al., 1996;Dye et al., 2001;Dye and Jarmain, 2004). Labour intensive approaches are taken with dense infestations (as was the case for our study site in 1998 (Prins, 2003)). ...
... The vegetation type of the catchment is Boland Granite Fynbos growing in soils derived from deeplyweathered granites with the upper layers containing colluvial sandstone material (Rebelo et al., 2006). A small, alien-invaded stream, that was subject to some studies in the past (Dye et al., 2001;Dye and Jarmain, 2004), was chosen for the study. Acacia mearnsii De Wild. ...
In 1998, a restoration field trial was initiated in a catchment near Wellington (Western Cape, South Africa) to determine whether fynbos riparian scrub vegetation cleared of woody invasive alien trees require post-clearance restoration actions to accelerate indigenous vegetation recovery. The aim was to assess the relative effectiveness of three sowing treatments for restoring indigenous vegetation cover after the widely used “Fell & Burn” method of clearing invasive alien trees. Sowing treatments included non-invasive alien grasses to determine whether they have a negative effect on recovering native vegetation. A summer fire, eight years after trial initiation, provided an opportunity to determine how resilient restoration treatments are to alien re-invasion and fire. Restoring the site after alien clearing by sowing indigenous seeds increased both diversity, by improving species presence and abundance. However, a census done 8 years later (in 2006) revealed that seedlings of woody invasive alien plants dominated all plots, and had also survived the burn by resprouting, indicating the importance of follow-up control to justify initial clearing and restoration costs. Indigenous grass density was significantly reduced in plots where alien grasses were sown, while in the control and fynbos sowing treatment, indigenous grass density increased. By 2006, alien grass density was negligible in all treatments, indicating that the two grass species sown are not persistent or invasive. Active restoration of riparian areas after alien plant clearing has potential to facilitate vegetation recovery, but must be coupled with a long-term plan for adequate follow-up removal of post-clearance and post-fire alien recruits.
... It has been established that the removal of riparian wattle leads to significant reductions in annual evapotranspiration (ET) which in turn will enhance streamflow (Dye et al., 2001). This has been further elaborated recently in comprehensive reviews by Everson et al. (2011) andLe Maitre et al. (2020), which provided support for clearing efforts. ...
Ecosystem services in the South African grassland biome are being impacted by the presence of invasive alien plants (IAPs), particularly from the Australian genus Acacia. IAPs have elevated landscape water use and determining water fluxes is crucial to defining suitable interventions. This study evaluates three models of water flux over IAP-invaded grassland against evapotranspiration (ET) measured by a large-aperture scintillometer (LAS). ET was modelled using an energy balance model (MEDRUSH), a biophysical model (Penman-Monteith-Leuning (PML)) and a remotely sensed product (MOD16), and their results compared with ET measured by the LAS. ETLAS was measured during an abbreviated field campaign in November 2019 over a dense silver wattle (A. dealbata) stand associated with hillslope seeps. Of the three models tested against the LAS campaign, MEDRUSH performed well-enough to be used to model ET over the continuous scientific-grade micro-meteorological record (315 days). We used MEDRUSH to model ET over the invaded hillslope seeps and compared this with the expected ET that would occur over the adjacent, un-invaded grassland. Total ET over the IAP-invaded area was approximately 536 mm (60% of reference ET) as compared with the grassland (202 mm), showing that woody encroachment significantly increases landscape water use. To estimate the local impact of this, we used earth observation to determine the area of woody invasion in a 2 664 km2 regional window. Approximately 10% (274 km2) of this window was invaded by IAPs and, assuming the geographic window was representative of all the areas invaded by wattle, this represents ~ 89 000 ML of water used annually by the IAPs. Removal of wattle, followed by suitable restoration of landscape functionality, would greatly enhance rangeland productivity and water production from hillslope seeps.
... Water availability in many areas of South Africa has become a significant concern, and efforts to conserve water are increasingly shifting towards demand management (Turpie et al. 2008;Egoh et al. 2012;Nel et al. 2017;Shackleton et al. 2019f). Invasive alien trees use significantly more water than indigenous grasslands (Dye et al. 2001;Dye and Jarmain 2004;Everson et al. 2011;Le Maitre et al. 2015), thereby reducing surface run-off, streamflow, and subsequently water supply and security (Le Maitre et al. 1996Dzikiti et al. 2013;Preston et al. 2018). Awareness of the link between IAPs and water resources has increased considerably since the establishment of the Working for Water programme in the 1990s (Le Maitre et al. 1996Turpie et al. 2008;van Wilgen et al. 2012). ...
Many alien tree species were introduced into grassland ecosystems in South Africa by the commercial forestry industry for paper and timber for furniture. Over decades some of these introduced species escaped into neighbouring farms and community land. Adult trees from these alien species now provide other ecosystem services, notably fuelwood. Depending on the spatio-temporal context, many of these species can also negatively affect ecosystem services. We collected interview data from commercial and communal farmers in the upper Umzimvubu catchment in South Africa to compare farmers’ knowledge and perceptions of invasive wattle species invasion and their associated ecosystem services and disservices. Fuelwood and fencing poles were the most common uses of wattle by commercial (83%; 67%) and communal (99%; 49%) farmers. On the other hand, the reduction of grass cover and loss of grazing land were the most commonly mentioned negative impacts of wattles by commercial (83%; 75%) and communal (92%; 80%) farmers. Although both groups recognise the importance of wattles in providing ecosystem services, most communal farmers perceived wattles to have more negative effects than benefits. The findings demonstrate that both farmer groups highly depend on ecosystem services and are affected by disservices of wattles. However, while large-scale commercial households favour the presence of wattles in the landscape, communal households prefer complete removal of the wattles from the landscape. This may be due to lack of locally available alternative options or inability to replace or purchase ecosystem services affected by wattles from other sources or markets like commercial farmers.
... Both leaves and phyllodes of Acacia have high water-use efficiency compared with other C3 plants (Brodribb and Hill 1993), but being capable of high water-use efficiency in times of water stress does not necessarily imply high water-use efficiency at other times. For example, in South Africa, the bipinnate Acacia mearnsii has higher water use than does woody and grassland vegetation in riparian sites where it is invasive (Dye et al. 2001). It is possible that this higher water use supports higher relative growth rates. ...
In Acacia, 90% of species have drought-tolerant phyllodes as their adult foliage, the remaining species have bipinnate leaves. We conducted tests for relationships between foliage type and 35 bioclimatic variables at the continental scale and found significant correlations of both ‘moisture seasonality’ and ‘radiation in the coldest quarter’ with foliage type. Bipinnate species have lower species mean values of each variable, growing in stable soil moisture and generally darker environments (longer nights and lower incident radiation), on average. Evolutionary transformations between bipinnate and phyllodinous adult foliage exhibit asymmetry across the Acacia phylogeny, with transformations from bipinnate leaves to phyllodes occurring times faster than the reverse. At least three (and up to seven) transitions from phyllode to bipinnate adult foliage were inferred. Foliage type in the most recent common ancestor of extant Acacia is unresolved, some analyses favour a phyllodinous ancestor, others a bipinnate ancestor. Most ancestral nodes inferred as having bipinnate adult foliage had median age estimates of less than 5 million years (Ma), half having ages between 3 and 1.5 Ma. Acacia lineages with bipinnate adult foliage diversified during the Pliocene, perhaps in response to wetter climatic conditions experienced by the continental margin during this period.
... This is likely due to relatively dry soils, a result of higher summer evapotranspiration (ET) rates, a common pattern in semi-arid regions [28]. Moreover, the pine plantations in Bosboukloof have been observed to cause additional reductions in streamflow given their high ET rates compared to the indigenous fynbos vegetation which dominates Langrivier [20,29]. ...
The stable isotopes of water (¹⁸O, ²H) and other hydrochemical properties were used to assess seasonal changes between sources of stream water and flow pathways in baseflow conditions for two headwater sub-catchments (∼3 km²) of the Jonkershoek, a mountainous catchment in the Western Cape of South Africa. The sub-catchments differ in land cover, one is dominated by indigenous fynbos vegetation and the other by pine plantation. Stream water, higher elevation springs, and lower elevation groundwater samples were collected monthly from January 2018 to January 2019, and were analysed for stable isotopes, electrical conductivity (EC) and pH. The stream water isotopic values resembled those of groundwater during the dry and wet seasons. Results indicated a steady contribution of spring discharge to streams during the dry season, with rainfall contribution less evident. Wet season flows were attributed to interflow including perennial and ephemeral springs. Spatial variations in EC between the sub-catchments were indicative of a greater proportional input from subsurface water, more evaporation and/or more mineral dissolution in the pine-dominated sub-catchment. The spatiotemporal variability in EC was significantly larger than for the stable isotopes. These findings enabled conceptualization of streamflow generation processes that can support strategic water resource management practices in this region.
... It was noted that annual ET for dune and spekboom thicket in the Western Cape ranges between 515 and 660 mm respectively while valley thicket in KwaZulu-Natal is 755 mm. Dye et al. (2001) noted that removal of riparian wattle, an invasive alien tree will lead to significant reductions in annual ET which in turn will enhance streamflow. So far, studies on ET over AT include: measurement of ET using an EC system (Gwate et al., 2016); development of ET predictive model to be applied at biome scale (Gwate et al., 2018); validation of improved simple single layer ET model (Penman-Monteith-Palmer (PMP)) using large aperture scintillometer (LAS) and EC system data (2015)(2016) (Gwate et al., 2019). ...
The Albany Thicket (AT) biome contains outstanding global biodiversity as well as the potential to achieve carbon credits associated with water-efficient Crasslucean acid metabolism (CAM). Understanding the water fluxes in the AT is crucial to determining carbon (C) sequestration rates and water-use efficiency. Despite large variation in water fluxes across the AT, only a few studies have been conducted in this region with their results validated against short periods of observed data. This study aims to evaluate three models of water fluxes over AT against data from an eddy covariance (EC) system active from October 2015 to May 2018. ET was modelled using the BioGeoChemistry Management (BGC-MAN) model, a biophysical model (Penman-Monteith-Leuning (PML)) and a remotely-sensed product (MOD16), and their results compared with that from the EC system. More than three decades of rainfall data from Climate Hazards Group InfraRed Precipitation with Station Data (CHIRPS) was used to assess some rainfall characteristics of the region. The mean annual rainfall is 404 mm and mean monthly rainfall ranges from 16.0–50.7 mm, with minima likely to occur in winter period (between May and July) and monthly maxima in the summer period (between October and March). Among the three hydrological years in this study, total ET for 2016-2017 exceeded rainfall received by about 7% which shows that AT is likely to be supported by groundwater at some point but this requires further investigations. Generally, the three models applied in this study performed reasonably well when compared with the measured ET. The cumulative ET from BGC-MAN was slightly higher than that from EC by 16% and 8% in 2015-2016 and 2017-2018 hydrological years respectively while PML was slightly lower by 3% and 17% in 2016-2017 and 2017-2018; additionally, MODIS was slightly lower by 14% and 7% in 2016-2017 and 2017-2018, respectively. However, the correlation between the ET from EC and simulated ET from the three models was significant at p < 0.01.
... Negative impacts of Acacia invasion on biodiversity and ecosystems ultimately affect human well-being (Le Maitre et al., 2011). For example, stands of A. mearnsii invasion in South Africa have been shown to utilize more water compared to fynbos biome native vegetation (Dye et al., 2001;Dye and Jarmain, 2004), this resulting in water reduction for agriculture, industry, recreation, conservation and domestic use (Görgens and van Wilgen, 2004). Also, the high biomass of Acacia species has been known to increase fire severity (van Wilgen and Richardson, 1985), thus not only affecting re-sprouting native plants but also housing properties. ...
Short term monitoring of soil and vegetation recovery following alien plant removal is required to reveal how ecological restoration is progressing. This study examined the recovery of soil physical properties and vegetation following Acacia mearnsii removal at Zvakanaka farm in Limpopo Province, South Africa. Soil and vegetation measurements were conducted in paired cleared, invaded and natural sites on 10 x 10 m plots. Results of the study show significantly (P < 0.001) higher soil moisture content in invaded and natural compared to cleared sites. Soil penetration was significantly (P < 0.001) higher in cleared than invaded and natural sites. Both infiltration rate and hydraulic conductivity showed no significant (P > 0.05) difference between the three sites. Strongly repellent soils were recorded in cleared sites only. Results showed a significant (P < 0.05) increase in measured diversity indices (species richness, Shannon-Wiener, Simpson’s and evenness index) in cleared and natural than in invaded sites. However, most secondary woody invasive alien plants were recorded in cleared sites. The study concludes that A. mearnsii clearing triggers varying changes in soil physical properties. Although native plants are present in cleared sites, recovery may be hampered by the growth of secondary woody invasive alien plants.
... Besides outcompeting endemic species, wattle forms dense impenetrateable stands with a high evergreen leaf area throughout the year that forms extensive canopies. Previous work indicates that wattle trees utilise a significant amount of water to retain their extensive evergreen foliage (Binns, Illgner, and Nel 2001;Dye et al. 2001). Binns, Illgner, and Nel (2001) reported that about seven percent of river water, equivalent to 3300 million cubic meters, is utilised by wattle trees in South Africa. ...
This study assessed the strength of Sentinel-2 multispectral instrument (MSI) derived Red Edge (RE) bands in estimating Leaf Area Index (LAI) and mapping canopy storage capacity (CSC) for hydrological applications in wattle infested ecosystems. To accomplish this objective, this study compared the estimation strength of models derived, using standard bands (all bands excluding the RE band) with those including RE bands, as well as different vegetation indices. Sparse Partial Least Squares (SPLSR) and Partial Least Squares Regression (PLSR) ensembles were used in this study. Results showed that the RE spectrum covered by the Sentinel-2 MSI satellite reduced the estimation error by a magnitude of 0.125 based on simple ratio (RE SR) vegetation indices from 0.157 m²· m⁻² based on standard bands, and by 0.078 m²· m⁻² based on red edge normalised difference vegetation (NDVI-RE). The optimal models for estimating LAI to map CSC were obtained based on the RE bands centered at 705 nm (Band 5), 740 nm (Band 6), 783 nm (Band 7) as well as 865 nm (Band 8a). A root mean square error of prediction (RMSEP) of 0.507 m²· m⁻² a relative root mean square error of prediction (RRMSEP) of 11.3% and R² of 0.91 for LAI and a RMSEP of 0.246 m²/m² (RRMSEP = 7.9%) and R² of 0.91 for CSC were obtained. Overall, the findings of this study underscore the relevance of the new copernicus satellite product in rapid monitoring of ecosystems that are invaded by alien invasive species.
... Allen et al. (1998) and Pereira et al. (2015) established that transpiration is influenced by climatic variables, physiological characteristics and soil water retention properties. In South Africa, sap flow rates of representative sample trees have been used to estimate stand transpiration rates (Dye and Olbrich 1992;Dye et al. 2001) as well as the response of transpiration rates to soil water deficits (Dye 1996). Process-based models have proved to be useful in simulating predictions of tree growth and water use (Dye et al. 2004). ...
In a water-scarce country such as South Africa, timber and fibre production often stands in conflict with catchment water yields. The optimal provision of both is sought. Forest hydrological experiments improve our understanding of the soil–plant–atmosphere continuum and facilitate the development of rapid estimation techniques through extrapolation. To quantify and characterise the influence of local environmental conditions on single-tree water use, whole-tree daily sap flow for three indigenous afrotemperate tree species (Podocarpus latifolius, Ilex mitis and Ocotea bullata) and one introduced and intensively managed tree species (Pinus radiata) was measured. Hourly sap flux, soil water content and weather data were collected for a year in an indigenous forest and a plantation stand in the southern Cape afrotemperate region. Correlation analyses for whole-tree daily sap flow and environmental variables related to water and energy availability were done. Cross-validation was then employed to test combinations of non-covarying independent parameters for the prediction of whole-tree sap flow responses to environmental variables. Generalised linear models were developed for each species. For the Pinus radiata plantation species, separate models were developed for tree specimens in the subdominant, average and dominant diameter classes. The dominant diameter class model had the best R2 value of 0.80 (p < 0.00). For the indigenous tree specimens, the Ilex mitis model had the highest R 2 (0.65, p < 0.00). Validation of the models with independent data indicated positive and significant Pearson correlation coefficients for the observed versus predicted daily sap flow values, with the dominant diameter Pinus radiata specimens showing the highest correlation value (r = 0.91, p < 0.00), whereas those for the indigenous species were r = 0.63, p < 0.00; r = 0.78, p < 0.00; and r = 0.66, p < 0.00 for Podocarpus latifolius, Ilex mitis and Ocotea bullata, respectively. The results indicate that variable-specific linear models can be employed to estimate whole-tree sap flow and water use as a function of environmental factors with an acceptable degree of accuracy for both introduced and indigenous tree species.
Keywords: generalised linear modelling, indigenous tree species, introduced tree species, sap flow
... This is particularly relevant in areas that present very low surface runoff, as in coastal arid regions. Novel A. mearnsii populations presented higher water losses compared to natives (Dye et al. 2001), whereas A. longifolia reduced the water flow on average by 26% in pine forests of coastal dunes in Portugal (Rascher et al. 2011b). At the same time, changes in hydrologic dynamics produced by A. longifolia were also associated with decreased C fixation rates of native trees (Rascher et al. 2011b). ...
The above- and belowground impacts due to Acacia invasions have been described in detail over the last 25 years. Future research should focus on the earlyd etection and prevention of new Acacia introductions and on a cost-effective and sustainable management of the novel ecosystems resulting from invasions.
... Sufficient equipment was available for eight probe sets per species. A sampling intensity of 5-12 sap flow measurements is common in sap flow studies within homogeneous, monospecific stands of even aged trees and regular spacing (Granier, 1987;Olbrich et al., 1993;Vertessy et al., 1997;Dye et al., 2001). As the Searsia trees approach maturity, however, one can expect a higher proportion of larger, measurable stems as well as greater competition among trees, leading to a wider range of measurable stem size, and greater variation in such environmental constraints such as canopy exposure to sun, tree water status and leaf area. ...
The Witwatersrand Basin Goldfields (WBG) have seen over a century of continuous mining that has generated extensive tailings storage facilities (TSF), together with “footprints” remaining after the residue has been removed for reprocessing or consolidation into larger TSFs. These are now believed to number several hundred and cover a total area of 400–500 km². Acid mine drainage (AMD) from these structures is widespread and has resulted in contamination of soils, groundwater and surface water systems. Sustainable and long-term control measures are required to limit environmental contamination. The Mine Woodlands Project, initiated by the University of the Witwatersrand and AngloGold Ashanti Ltd, aims to investigate the use of trees for hydraulic control of mine seepage, as well as contaminant immobilization. A variety of exotic and indigenous tree species was planted in high density stands within site species trials located close to TSFs in the Orkney and Carltonville districts. The aim is to evaluate their survival and growth, as well as water use and contaminant uptake or immobilization.
... In South Africa, invasive acacias, eucalypts and pines have substantial negative impacts on biodiversity (Richardson and Van Wilgen 2004;Le Maitre et al., 2011), stream flow from water catchments (Bosch and Hewlett 1982;Le Maitre et al., 1996;Dye et al., 2001;Dye and Jarmain 2004), and water quality (Chamier et al., 2012;Tye and Drake 2012). This represents a dilemma as plantation forestry is a part of many South African agricultural landscapes with significant socio-economic benefits, critically important for a developing nation (Van Wilgen and Richardson 2012). ...
Non-native trees have become dominant components of many landscapes, including urban ecosystems, commercial forestry plantations, fruit orchards, and as invasives in natural ecosystems. Often, these trees have been separated from their natural enemies (i.e. insects and pathogens) leading to ecological disequilibrium, that is, the immediate breakdown of historically co-evolved interactions once introduced into novel environments. Long-established, non-native tree plantations provide useful experiments to explore the dimensions of such ecological disequilibria. We quantify the status quo of non-native insect pests and pathogens catching up with their tree hosts (planted Acacia, Eucalyptus and Pinus species) in South Africa, and examine which native South African enemy species utilise these trees as hosts. Interestingly, pines, with no confamilial relatives in South Africa and the longest residence time (almost two centuries), have acquired only one highly polyphagous native pathogen. This is in contrast to acacias and eucalypts, both with many native and confamilial relatives in South Africa that have acquired more native pathogens. These patterns support the known role of phylogenetic relatedness of non-native and native floras in influencing the likelihood of pathogen shifts between them. This relationship, however, does not seem to hold for native insects. Native insects appear far more likely to expand their feeding habits onto non-native tree hosts than are native pathogens, although they are generally less damaging. The ecological disequilibrium conditions of non-native trees are deeply rooted in the eco-evolutionary experience of the host plant, co-evolved natural enemies, and native organisms from the introduced range. We should expect considerable spatial and temporal variation in ecological disequilibrium conditions among non-native taxa, which can be significantly influenced by biosecurity and management practices.
... Few studies have quantified the incremental water use by invasions in South Africa above that used by the indigenous vegetation. For example, Dye et al. (2001) compared the water use of wattle thickets (A. mearnsii) relative to indigenous plant communities on riparian sites in the Western Cape and Kwa-Zulu Natal Provinces. ...
Groundwater is a vital source of water for many communities in South Africa and elsewhere. Besides the changing climate, rapidly spreading invasive alien plants with deep roots e.g. Prosopis spp pose a serious threat to this water source. Dense impenetrable thickets of Prosopis occur in the drier parts of the country mainly along river channels in the Northern, Eastern and Western Cape Provinces. Few studies have quantified the actual water use by this species outside of the USA where it is native. Consequently the impacts of Prosopis invasions on groundwater resources are not well documented in South Africa. The aim of this study was to quantify the actual volumes of water used by Prosopis invasions and to establish the effects on groundwater. Because deep rooted indigenous trees that normally replace Prosopis once it has been cleared also use groundwater, we sought to quantify the incremental water use by Prosopis over and above that used by indigenous trees in order to determine the true impacts on groundwater. The study was conducted at a site densely invaded by Prosopis at Brandkop farm near the groundwater dependent town of Nieuwoudtville in the Northern Cape. One in seven trees at the site is the Vachellia karroo (formerly Acacia karroo) which is the dominant deep rooted indigenous tree species. Actual transpiration rates by Prosopis and V. karroo trees were measured using the heat pulse velocity (HPV) sap flow technique. Groundwater levels were monitored using solinst level loggers while sources of water used by the trees were determined using O/H stable isotopes. For similar size trees, V. karroo had higher transpiration rates than Prosopis because of the larger sapwood to heartwood ratio in V. karroo than in Prosopis. However, at the stand level Prosopis consumed significantly larger amounts of water than V. karroo. This is because Prosopis invasions had a much higher tree density than V. karroo. From August 2013 to August 2014, annual stand transpiration for Prosopis (~ 353 mm) was more than 4 times higher than that of V. karroo (~ 137 mm). Tree water uptake was correlated to changes in groundwater levels (R 2 ~ 0.42) with groundwater abstractions of ~ 2 600 m 3 /ha/y by Prosopis compared to only ~ 610 m 3 /ha/y for V. karroo. In addition, Prosopis showed evidence of hydraulic redistribution of groundwater while the co-occurrring V. karroo did not.
... Everson (2001) reported a total catchment ET of 57% of MAP for Cathedral Peak VI (MAP = 1299 mm). The high ET/P ratio obtained for the marsh was also similar to values reported by Dye et al. (2001) for two riparian grasslands in South Africa, namely 96% at Gilboa (MAP = 867 mm) and 101% at Jonkershoek near Stellenbosch in the winter rainfall area (MAP = 1324 mm). The high ET at the marsh indicates the impact marshes have on reducing catchment water yield. ...
Evapotranspiration (ET) is a major component of the soil water balance and therefore requires accurate determination to quantify catchment water yield. Six-year daily ET was determined at 300 mm depth intervals in the root zone of 28 soil profiles in the Weatherley catchment in South Africa. Daily ET was calculated based on the soil water balance equation and interpretation of the physical and morphological properties of the soils. Water losses by ET were studied at various soil water contents. This established the ET/ET0 for veld grass and therefore enabled the calculation of daily ET from daily ET0. Resulting annual mean daily ET ranged from 1.4 mm d-1 (50% of rainfall) at the driest hillslope site to 2.6 mm d-1 (90% of rainfall) at the marsh. Results indicated that the proposed procedure enabled fairly accurate determination of ET for the grassland catchment.
... Dit word gesien as 'n sterk negatiewe invloed op stroomvloei asook die watervloei uit stroomgebiede omdat dit so vinnig langs oewersones versprei en geneig is om vinnig digte ruigtes te vorm wat deur 'n hoë biomassa en blaararea gekenmerk word. 59 Daar is meer fondse deur die WVW-program vir die beheer van die swartwattel as vir alle ander indringerplante tesame toegewys. 60 Om die probleme te bekamp wat met die indringing van uitheemse bome gepaardgaan, het die regering verskeie indringerplantbeheermaatreëls van stapel gestuur. ...
UITTREKSEL Daar is reeds baie geskryf oor uitheemse indringerplante, veral Acacia mearnsii De Wild, die swartwattelboom en die skade wat hierdie plante aan die plaaslike ekologie aanrig. In hierdie artikel word aandag gegee aan die ontstaan van die probleem, asook aan die inbring en verspreiding van swartwattelbome in die Republiek van Suid-Afrika (Suid-Afrika). Die probleme wat deur die onbeheerde verspreiding van wattelbome veroorsaak is, het die regering verplig om op te tree ten einde hierdie probleme te bekamp. Voor 1936 was daar aanvanklik geen formele beleid met betrekking tot die invoer, kweek en beheer van hierdie bome nie. Dit het daartoe gelei dat die regering in die wattelbedryf ingegryp het om 'n sekere mate van beheer te vestig en om probleme met betrekking tot uitheemse indringerplante in die algemeen – en die swartwattelboom in die besonder – die hoof te bied. Voorts sal daar ook gelet word op die rol van swartwattel in volhoubare ontwikkeling deur te fokus op die omgewings-/ekologiese probleem van impak op die biodiversiteit en die waterbronne en ook op die voordele wat dit het om woudherstel aan te help. Die artikel bespreek sosiale voordele wat die boom het vir inwoners van landelike gebiede as voorsiener in bou-en konstruksiemateriaal en vuurmaakhout, wat andersins uit die boomveld en/of inheemse woude verkry moet word. Die boom voorsien ook in die houtbehoeftes van die industrie in die vorm van byvoorbeeld pale, pulp en ekstrak. Laastens volg 'n bespreking van die Werk-vir-Water-program. ABSTRACT Black wattle trees: blessing or curse to South Africa Much has been written about alien invasive plants, especially Acacia mearnsii De Wild, the black wattle tree and the damage done by these plants to the local ecology. In this article cognisance is taken of how the problem was created, the introduction into and the distribution of black wattle trees in the Republic of South Africa (South Africa). Problems experienced by the uncontrolled spreading of wattle trees forced the government to act, as there was no formal policy before 1936 on the importation, cultivation and control thereof. This led to intervention by government in the wattle industry to establish some form of control and to address the problems related to alien invasive plants in general, and the black wattle tree in particular. Furthermore, cognisance will be taken of the role the black wattle plays in sustainable development by considering the environmental/ecological problem of its impact on bio-diversity and the water sources as well as the advantages it has in the process of forest recovery. Secondly notice will be taken of the social advantages the tree has for the inhabitants of rural areas as a provider of building and construction material and firewood, which would otherwise come from the woodlands and/or indigenous forests. The tree also provides in the timber requirements of industry in the form of for instance poles, pulp and extract. Lastly, the article explores the Work-for-Water Programme.
... The invasion of indigenous vegetation by alien trees such as Acacia mearnsii has been recognised as a serious threat to South Africa's water resources and biodiversity (Dye et al., 2001; Galatowitsch and Richardson, 2005). Fynbos, a schlerophylous vegetation type typical of Mediterranean climates, dominates the natural vegetation of the Cape Fold Mountains of South Africa (Rebelo et al., 2006). ...
Concerns exist over increasing sediment yields in areas of South Africa with a Mediterranean climate that result from the clearing of invasive woody vegetation from steep slopes, as part of the Working for Water Programme. Erosion rates were monitored with erosion pins over a 26‐month period. Measurements were made on slopes covered with indigenous fynbos vegetation, intact Acacia mearnsii stands (the invasive species) and on slopes from which A. mearnsii had been cleared either within the last 2 years or 11 years ago. Net erosion was observed at all sites following the largest storm during the monitoring period, whereas smaller storms were often accompanied by net deposition. Cleared locations showed the most variable response through time and sites with native fynbos vegetation the least. A stepwise multiple regression showed that vegetation cover, slope gradient and stone cover were the main variables affecting soil loss, but the relationship varied between sites and measuring periods. Soil loss was negatively related to vegetation cover and stone cover, as could be expected; the negative relationship with slope was more surprising. Stone cover, which increases with slope gradient, is considered to be an important control on soil loss. The findings of this study suggest that the rehabilitation of hillslopes invaded by alien woody vegetation needs to take into account the slow recovery of vegetation after initial clearing as, 11 years since clearing, significant erosion still occurs during severe storms. Copyright © 2012 John Wiley & Sons, Ltd.
... Whereas the utility of pan coefficients combined with an open-water model has been demonstrated for Salix stands in riparian areas across Australia, a similar approach to predict and monitor evaporative losses of various tree species across the globe seems possible. Introduced invasive wattle stands (Acacia mearnsii) in South Africa (Dye et al., 2001) and paperbark stands (Melaleuca quinquenervia) in the Florida Everglades (Chin, 1998) have permanent access to water. Past research to elucidate ET and potential water salvage illustrates that development and application of a pan coefficients model could be used to inform broad-scale water resource monitoring and management decision making. ...
The long-term ‘Millennium Drought’ has put significant pressure on water resources across Australia. In southeastern Australia and in particular the Murray-Darling Basin, removal of exotic, high-water-use Salix trees may provide a means to return water to the environment. This paper describes a simple model to estimate evapotranspiration of two introduced Salix species under nonwater-limited conditions across seven biogeoclimatic zones in Australia. In this study, Salix evapotranspiration was calculated using the Penman–Monteith model. Field measurements of leaf area index and stomatal conductance for Salix babylonica and Salix fragilis were used to parameterize the models. Each model was validated using extensive field estimates of evapotranspiration from a semi-arid (S. babylonica, r2 = 0.88) and cool temperate (S. fragilis, r2 = 0.99) region. Modelled mean annual evapotranspiration showed strong agreement with field measurements, being within 32 and 2mmyear�1 for S. babylonica and S. fragilis, respectively. Monthly pan coefficients (the ratio of mean evapotranspiration to mean pan evaporation) were
developed from 30 years of meteorological data, for 30 key reference sites across Australia for both species using the validated Penman–Monteith models. Open-water evaporation was estimated from field measurements and was used to develop a simple linear regression model for open-water evaporation across the 30 reference sites. Differences between modelled evapotranspiration and open-water evaporation at each site provide an indication of the amount of water that might be returned to the environment from removal of in-stream Salix species. The monthly pan coefficient method reported has application across riparian environments worldwide where measured evapotranspiration is available for model validation.
... The invasion of indigenous vegetation by alien trees such as Acacia mearnsii has been recognised as a serious threat to South Africa's water resources and biodiversity (Dye et al., 2001;Galatowitsch and Richardson, 2005). Fynbos, a schlerophylous vegetation type typical of Mediterranean climates, dominates the natural vegetation of the Cape Fold Mountains of South Africa (Rebelo et al., 2006). ...
Concerns exist over increasing sediment yields in areas of South Africa with a Mediterranean climate that result from the clearing of invasive woody vegetation from steep slopes, as part of the Working for Water Programme. Erosion rates were monitored with erosion pins over a 26-month period. Measurements were made on slopes covered with indigenous fynbos vegetation, intact Acacia mearnsii stands (the invasive species) and on slopes from which A. mearnsii had been cleared either within the last 2 years or 11 years ago. Net erosion was observed at all sites following the largest storm during the monitoring period, whereas smaller storms were often accompanied by net deposition. Cleared locations showed the most variable response through time and sites with native fynbos vegetation the least. A stepwise multiple regression showed that vegetation cover, slope gradient and stone cover were the main variables affecting soil loss, but the relationship varied between sites and measuring periods. Soil loss was negatively related to vegetation cover and stone cover, as could be expected; the negative relationship with slope was more surprising. Stone cover, which increases with slope gradient, is considered to be an important control on soil loss. The findings of this study suggest that the rehabilitation of hillslopes invaded by alien woody vegetation needs to take into account the slow recovery of vegetation after initial clearing as, 11 years since clearing, significant erosion still occurs during severe storms. Copyright © 2012 John Wiley & Sons, Ltd.
... Fynbos riparian zones in the southwestern Cape of South Africa are predicted to endure severe future drought conditions (New 2002;Steynor et al. 2009). The invasive alien plant (IAP), Acacia mearnsii, an important forestry species in South Africa and known invader of especially riparian areas, aggravates this problem as it is a profligate water user (Dye et al. 2001;Dye and Jarmain 2004). An important question has been raised concerning the future invasion success of alien plants under global climate change-would IAPs become more invasive, and thus be even more competitive, or would their invasion success be reduced (Dukes and Mooney 1999). ...
Riparian ecotones in the fynbos biome of South Africa are heavily invaded by woody invasive alien species, which are known to reduce water supply to downstream environments. To explore whether variation in species-specific functional traits pertaining to drought-tolerance exist, we investigated wood anatomical traits of key native riparian species and the invasive Acacia mearnsii across different water availability proxies. Wood density, vessel resistance against implosion, vessel lumen diameter and vessel wall thickness were measured. Wood density varied significantly between species, with A. mearnsii having denser wood at sites in rivers with high discharge. As higher wood density is indicative of increased drought tolerance and typical of drier sites, this counter-intuitive finding suggests that increased wood density was more closely related to midday water stress, than streamflow quantity per se. Wood density was positively correlated with vessel resistance against implosion. Higher wood density may also be evidence that A. mearnsii is more resistant against drought-induced cavitation than the studied native species. The observed plastic response of A. mearnsii anatomical traits to variable water availability indicates the ability of this species to persist under various environmental conditions. A possible non-causal relationship between wood anatomy and drought tolerance in these riparian systems is discussed.
... While these earlier researchers assumed a constant rate of groundwater recharge into the riparian aquifer, Loheide (2008) noted that the hydraulic gradient of groundwater flow into the riparian zone will actually vary throughout each cycle. This gradient is altered by forest water use lowering the water table and reduces the transfer rate of subsurface water to the stream (Rowe and Pearce, 1994;Dye et al., 2001). While the White method gives sub-daily estimates of ET rate, another technique developed by Reigner (1966) yields daily estimates of total ET derived from groundwater in riparian zone. ...
Diel cycles in stream discharge during baseflow periods in a headwater stream in La Selva Biological Station, Costa Rica, a tropical wet forest site, appear to be associated with groundwater withdrawal by the forest for evapotranspiration (ET). Analysis of the cycles indicates a strong correlation of stage change with ET demand, similar to the variation found in riparian water table elevation by previous researchers. Links between daily forest ET demand cycles and stream discharge cycles have been reported in temperate humid and semi-arid regions, but the frequent flood hydrographs of the wet tropics tend to obscure this daily signal. This study modifies and combines two established empirical methods for analyzing the diel ET signal in streamflow which lead to estimates of riparian ET derived from groundwater (ETG) at hourly time scales and spatial extent of the riparian area. The model has a direct dependence on the estimate of specific yield, a difficult to constrain parameter, which we estimate from previously published soil analyses. For the six baseflow periods analyzed, the model estimates groundwater ET losses ranging from 1.8 to 3.9 mm/day within the riparian area. These estimates are 52–81% of the total ET estimated with the Penman–Monteith equation (ETPM). The signal of ETG in the stream lags ETPM by 1.5–3 h, with apparent peak decay and signal duration lengthening during propagation. Model results indicate that the area of the riparian zone that influences streamflow by means of ET withdrawal increases with stream stage and ranges from 2.5% to 6.6% of the total basin area. Variations in the rate of change of nightly stream stage recovery suggest possible variations in the relative importance of subsurface hydraulic properties. At high stages, the rate of stream stage recovery from ET losses decreases throughout the night, whereas at low stages the rate of stream stage recovery increases throughout the night. Future work with numerical models could explore mechanistic controls on these empirically-derived recovery functions.
... However, one study from South Africa indicates that if the replacement vegetation has substantially different growth habit, for example, herbaceous vegetation rather than trees, ET might be significantly altered. Based on ET determined from Bowen ratio and sap flow measurements, Dye et al. (2001) suggested that, in South African riparian areas, replacing dense, non-native wattle stands (Acacia mearnsii) with native herbaceous plants may result in a substantial decrease in ET and increased stream flow. This is consistent with paired catchment studies showing that, in general, for a given rainfall, forest generates less stream flow than grass or herbaceous vegetation (Bosch and Hewlett, 1982; Zhang et al., 2001). ...
Globally, expansion of non-native woody vegetation across floodplains has raised concern of increased evapotranspiration (ET) water loss with consequent reduced river flows and groundwater supplies. Water salvage programs, established to meet water supply demands by removing introduced species, show little documented evidence of program effectiveness. We use two case studies in the USA and Australia to illustrate factors that contribute to water salvage feasibility for a given ecological setting. In the USA, saltcedar (Tamarix spp.) has become widespread on western rivers, with water salvage programs attempted over a 50-year period. Some studies document riparian transpiration or ET reduction after saltcedar removal, but detectable increases in river base flow are not conclusively shown. Furthermore, measurements of riparian vegetation ET in natural settings show saltcedar ET overlaps the range measured for native riparian species, thereby constraining the possibility of water salvage by replacing saltcedar with native vegetation. In Australia, introduced willows (Salix spp.) have become widespread in riparian systems in the Murray–Darling Basin. Although large-scale removal projects have been undertaken, no attempts have been made to quantify increases in base flows. Recent studies of ET indicate that willows growing in permanently inundated stream beds have high transpiration rates, indicating water savings could be achieved from removal. In contrast, native Eucalyptus trees and willows growing on stream banks show similar ET rates with no net water salvage from replacing willows with native trees. We conclude that water salvage feasibility is highly dependent on the ecohydrological setting in which the non-native trees occur. We provide an overview of conditions favorable to water salvage. Copyright © 2011 John Wiley & Sons, Ltd.
... Native species that depend on timed, seasonal flooding for germination show reduced recruitment and, as a result, are replaced by more opportunistic species. Interacting problems of human disturbance and invasive species have been documented for rivers in arid and semiarid areas in South Africa (Dye et al. 2001 ), Australia (Cowie & Werner 1993), Asia (Dudgeon 1992), and North America (Stromberg & Chew 2002a, 2002b Katz & Shafroth 2003). In the southwestern United States, the native riparian forests of cottonwood (Populus spp.) and willow (Salix spp.) have declined on most major rivers over the last 100 years, whereas an invasive, exotic shrub, saltcedar (Tamarix ramosissima Lebed.), has spread at rates of up to 20 km/year along some river systems (Cleverly et al. 1997; DiTomosa 1998; Everitt 1998 ). ...
Many riparian zones in the Sonoran Desert have been altered by elimination of the normal flood regime; such changes to the flow regime have contributed to the spread of saltcedar (Tamarix ramosissma Ledeb.), an exotic, salt-tolerant shrub. It has been proposed that reestablishment of a natural flow regime on these rivers might permit passive restoration of native trees, without the need for aggressive saltcedar clearing programs. We tested this proposition in the Colorado River delta in Mexico, which has received a series of large-volume water releases from U.S. dams over the past 20 years. We mapped the vegetation of the delta riparian corridor through ground and aerial surveys (1999–2002) and satellite imagery (1992–2002) and related vegetation changes to river flood flows and fire events. Although saltcedar is still the dominant plant in the delta, native cottonwood ( Populus fremontii S. Wats.) and willow (Salix gooddingii C. Ball) trees have regenerated multiple times because of frequent flood releases from U.S. dams since 1981. Tree populations are young and dynamic (ages 5–10 years). The primary cause of tree mortality between floods is fire. Biomass in the floodplain, as measured by the normalized difference vegetation index on satellite images, responds positively even to low-volume (but long-duration) flood events. Our results support the hypothesis that restoration of a pulse flood regime will regenerate native riparian vegetation despite the presence of a dominant invasive species, but fire management will be necessary to allow mature tree stands to develop.
... There are many examples of impacts on ecosystem services and the benefits they provide (see Table S1). Riparian stands of Acacia mearnsii in South Africa use more water than adjacent dryland invasions by the same species or the native vegetation the invaders replaced (Dye et al., 2001;Dye & Jarmain, 2004). The high biomass in dryland Acacia stands is directly related to their transpiration, and thus, the extent of reductions in river flows from invaded watersheds relative to the natural vegetation (Le Maitre et al., 1996Le Maitre, 2004). ...
Aim The biophysical impacts of invasive Australian acacias and their effects on ecosystem services are explored and used to develop a framework for improved restoration practices.
Location South Africa, Portugal and Chile.
Methods A conceptual model of ecosystem responses to the increasing severity (density and duration) of invasions was developed from the literature and our knowledge of how these impacts affect options for restoration. Case studies are used to identify similarities and differences between three regions severely affected by invasions of Australian acacias: Acacia dealbata in Chile, Acacia longifolia in Portugal and Acacia saligna in South Africa.
Results Australian acacias have a wide range of impacts on ecosystems that increase with time and disturbance, transform ecosystems and alter and reduce ecosystem service delivery. A shared trait is the accumulation of massive seed banks, which enables them to become dominant after disturbances. Ecosystem trajectories and recovery potential suggest that there are important thresholds in ecosystem state and resilience. When these are crossed, options for restoration are radically altered; in many cases, autogenic (self-driven and self-sustaining) recovery to a pre-invasion condition is inhibited, necessitating active intervention to restore composition and function.
Main conclusions The conceptual model demonstrates the degree, nature and reversibility of ecosystem degradation and identifies key actions needed to restore ecosystems to desired states. Control and restoration operations, particularly active restoration, require substantial short- to medium-term investments, which can reduce losses of biodiversity and ecosystem services, and the costs to society in the long term. Increasing restoration effectiveness will require further research into linkages between impacts and restoration. This research should involve scientists, practitioners and managers engaged in invasive plant control and restoration programmes, together with society as both the investors in, and beneficiaries of, more effective restoration.
... The invasion of the South African fynbos biome by the Australian Black Wattle tree (Acacia mearnsii de Wild) has various negative impacts on the fynbos ecosystem. Reduced species richness (Galatowitsch and Richardson, 2005), increased water use (Dye et al., 2001), increased nutrients (Musil and Midgley, 1990) and increased nitrogen cycling rates (Yelenik et al., 2004) are all negative impacts experienced under similar invasion conditions. ...
The invasion of Acacia mearnsii in the Kouga catchment, Eastern Cape, South Africa, has various negative impacts on the ecosystem. These impacts include: reduced species richness, increased water use, increased nutrients and increased N cycling rates. The native shrubby fynbos vegetation has adapted to the acidic nutrient poor soils and Mediterranean climate of the Kouga Mountains. Fynbos, however, is currently being out competed by the much taller Acacia mearnsii trees, due to their competitive nature and ability to fix nitrogen, thereby enriching the soil. The invaded sections of the valley bottoms and lower hill slopes are characterised by an almost complete monoculture of Acacia mearnsii, with very few fynbos species still present. The Department of Water and Environmental Affairs sponsored Working for Water programme started clearing Acacia mearnsii in 1996 in the Kouga Mountains. Cleared sites have remained bare for long periods, indicating that soil properties are not favourable for indigenous propagule re-establishment. The aim of this research was to assess how A. mearnsii invasion and clearing affect fynbos recovery through its impact on soils. This was done by characterising vegetation and soil properties on fynbos, infested and cleared slopes. Vegetation cover for various growth forms was determined and a species list was compiled for each plot. The slope angle, surface hardness, litter cover, bare ground cover and soil depth were measured in the field, whereas water repellency, particle size and the chemical composition were measured in the laboratory. Furthermore, the plant establishment capacity of soils from fynbos, infested and cleared slopes was calculated. This was done by germinating fynbos seeds and growing fynbos plants in soils from the various slopes. The effect that invasion and clearing has on soil erosion was quantified using erosion plots on fynbos, infested and cleared slopes. The invasion and clearing of Acacia mearnsii led to an increase in soil nutrients, especially nitrogen, phosphorus, potassium, carbon and manganese. Furthermore, soils became more acidic, with increased water repellency and reduced surface hardness. The vegetation changed to a tree-dominated structure, replacing the native species. Native plant germination was relatively unaffected by invasion and clearing, with an increase in germination just after clearing. Plant growth of a native grass, Themeda triandra, and herb, Helichrysum umbraculigerum, has increased on soils from cleared slopes. This study showed that soil movement increased on slopes which are invaded and cleared of Acacia mearnsii, with erosion rates doubling on invaded slopes.
Invasive trees in catchments around the world are a source of concern due to their hydrological and ecological impacts. A large number of studies have focused on their impact on dry-season stream discharge due to enhanced transpiration. The impact of invasive trees on stream discharge and flood risk during extreme rain events, which are becoming more frequent due to climate change, has not been addressed adequately. We examined the influence of land-cover, canopy cover, and other catchment morphological characteristics on stream discharges in hilly catchments during light, heavy, and extreme rain events. Three years of rainfall-runoff observations, between January 2014 and December 2016, were collected in eleven neighbouring mountainous catchments in Nilgiris, South India. Each catchment had a distinct land cover, namely shola forests, montane grasslands, and catchments invaded by wattle (Acacia mearnsii). Hourly rain intensities between the percentiles 25-90, 90-95 and over 95 were categorised as light, heavy and extreme respectively, and were used to study hourly peak stream discharge responses. We compared discharge between native grassland catchments and grassland catchments invaded by wattle. Discharge rates were corrected for the catchment area. We found that land cover and canopy-cover influenced the hydrologic response to extreme rain events. Regression models with flood event as the response variable suggested an increase in rainfall-runoff response with an increase in canopy cover. The maximum hourly discharge observed in the reference grassland catchment (0.00098 mm s-1) was lower than the maximum discharge observed in low density (0.00529 mm s-1) and high density (0.00497 mm s-1) wattle catchments. At higher discharges, the flood magnitude in wattle invaded catchments was much greater than that in the grassland catchment for a given flood frequency - indicating a higher risk of severe flooding in wattle-dominated catchments. We attribute the enhanced flood discharges to the effects of spreading wattle roots on the enhancement of rapid, shallow sub-surface flows in invaded catchments. Catchments dominated by shallow sub-surface flows are known to have reduced water retention times in response to high rain intensities. Antecedent moisture conditions and drainage density were other factors that influenced the local rainfall-runoff relationships. In the Western Ghats and other parts of India grasslands have been planted with exotic woody trees, and some have become invasive. Our study indicates that replacing grasslands with woody plants at landscape scales, could affect hydrological processes and could increase flood risks during the monsoon, especially in mountainous regions. Management of invaded catchments by regulation of wattle and restoration of semi-natural. grassland, combined with the installation of early warning systems in these impacted areas will help reduce flood risk during extreme rain under climate change.
Hydrological and vegetation models, with the help of various micro-meteorological methods, are widely used to calculate water fluxes. Correctly estimating these fluxes requires accurate transpiration measurements. Tree water use is known to be influenced by many factors but there is scant literature on the factors influencing tree water use in tropical forest areas. This study presents the first systematic review of research into tree water use in the tropics. The aim of the study was to understand the research trends and the influence of tree functional traits on water use. The study found a clear bias in research focus on geographic area and species group selection. The results indicate that water use increases in tropical tree species with increased tree size, sapwood area, leaf area index, and seed mass. On the other hand, wood density is negatively correlated with tree water use. Season is highly significant in explaining variations in tree water use, as was leaf phenology. Tropical trees’ water use significantly increases during the dry season. Native trees use more water than exotic trees species. Brevi-deciduous trees uptake significantly more water than evergreen and deciduous species during the dry season. These tree traits and their relationship with water use can provide a tool for better understanding ecohydrological processes, which can support improved plantation management and conservation of water resources. This systematic review identified research gaps that, if addressed, could inform the development of hydrological and vegetation models for the efficient management of water resources.
Palmiet, Prionium serratum, is an endemic wetland plant which dominates oligotrophic wetlands throughout the Cape Floristic Region, South Africa. Palmiet is often perceived as undesirable by landowners, in part because it is thought to have high water-use, although little is known about the water-use of this important wetland species. We estimated the water-use dynamics of Palmiet at the leaf scale, using stomatal conductance measurements, and at the wetland scale, by modelling evapotranspiration using remote sensing and an energy-balance model. Factors that influenced Palmiet water-use were also considered, and seasonal variations were analysed. The aim was to estimate Palmiet wetland water-use, and to develop a set of crop factors (Kc) for use in hydrological modelling of catchments containing Palmiet wetlands. Results show that Palmiet has a comparatively low stomatal conductance (11–152 mmol∙m-2∙s-1), which was lower in summer than winter, and moderate evapotranspiration for a riparian species (1 220 mm∙a-1 compared to a local reference evapotranspiration of 1 302 mm∙a-1 and A-Pan evaporation of 2 809 mm∙a-1), which was higher in summer (more energy to drive evapotranspiration and higher vapour pressure deficits). Morphological and physiological adaptations to nutrient poverty or periodic drought are suggested theories which may explain the controls on transpiration for Palmiet.
Considerable advances have been made since the first estimates of the impacts of invasive alien plants on water resources in the early 1990s. A large body of evidence shows that invasive alien plants can increase transpiration and evaporation losses and thus reduce river flows and mean annual runoff. Riparian invasions, and those in areas where groundwater is accessible, have 1.2–2 times the impact of invasions in dryland areas. The magnitude of the impacts is directly related to differences between the invading species and the dominant native species in size, rooting depth and leaf phenology. Information on the impacts has been successfully used to compare the water use of invasive plants and different land cover classes, to quantify the water resource benefits of control measures, and to prioritise areas for control operations. Nationally, the impacts of invasive alien plants on surface water runoff are estimated at 1.44–2.44 billion m³ per year. The most affected primary catchments (>5% reduction in mean annual runoff) are located in the Western and Eastern Cape, and KwaZulu-Natal. If no remedial action is taken, reductions in surface water runoff could increase to 2.59–3.15 billion m³ per year, about 50% higher than current reductions. This review illustrates the importance of measuring water-use over as wide a range of species as possible, and combining this with information from remote sensing to extrapolate the results to landscapes and catchments. These methods will soon provide much more robust estimates of water use by alien plants at appropriate spatial and temporal scales. The results of these studies can be used in water supply system studies to estimate the impacts on the assured yields. This information can also be used by catchment water resource managers to guide decision-makers when prioritising areas for clearing and rehabilitation, and for targeting species for control measures.
In South Africa, the development of a plantation tree industry using fast-growing introduced species was accelerated by the limited extent of indigenous forests. However, concerns about the impacts of plantations on the country’s limited water resources has initiated forest hydrology research and subsequent regulation of the industry since 1972. The forestry industry’s continued efforts to sustainably meet fibre and timber demands for the country’s growing economy have prompted questions whether indigenous tree species can provide an additional low water-use form of forestry. Single-tree water use and water-use efficiencies of three indigenous species (Ilex mitis, Ocotea bullata and Podocarpus latifolius) and one introduced species (Pinus radiata) in the Southern Cape region of South Africa were quantified. The heat-pulse velocity method was used to collect hourly sap flow data over a 12-month period. Hourly weather and soil-water data were concurrently recorded and tree growth rates were determined for the year. Biophysical water-use efficiency was calculated as the ratio of utilisable biomass gained per volume of water transpired. Patterns of water use through the year were different for the different species. Pinus radiata had higher transpiration volumes and water-use efficiency levels than the indigenous species. The most transpiring Pinus radiata tree had a transpiration volume that was 4.7 times that of the most transpiring indigenous tree. Indigenous species’ relatively lower water-use efficiencies were more a consequence of slow growth rates and not high water-use rates, which could be attributed to competition for resources in the dense indigenous forest. Potential implications for further hydrological research on the development of water-use-efficient tree production systems are discussed.
The negative effects of alien invasive plants on
habitats have been well-documented. However, the
exchange of organisms between these and native taxa has
been far less researched. Here we assess the exchanges of
arthropod associates of a native (Virgilia divaricata) and an
invasive (Acacia mearnsii) legume tree within the ecotone
between forest and fynbos vegetation within the Cape
Floristic Region of South Africa. Arthropod species richness,
abundance, species assemblage composition and
measures of beta-diversity were assessed between these
two legume species where they grow sympatrically. Except
for spiders and ants, arthropod species richness did not
differ significantly between the two tree taxa. The overall
abundance of arthropods was, however, significantly higher
on the native tree species. This pattern was strongly driven
by herbivores, as is consistent with predictions of the
Enemy Release Hypothesis. When excluding rare taxa,
over 75 % of all arthropod species collected in this study
were associated with both host trees. However, arthropod
community composition differed significantly between the
two host plant taxa, largely due to differences between
their herbivore communities. Arthropod beta diversity was
high on the native host, with arthropod communities on the
invasive host being much more homogenous across the
sampling range. These results indicate that there are
numerous exchanges of arthropods between these native
and invasive plants. The invasive plant may provide
arthropods with a pathway to other habitats between previously
isolated native populations. This will have significant
implications for biodiversity conservation at the
habitat, species and population level.
In this study we assessed the impact that invasive alien plant species (IAPs), and the clearing thereof by the Working for Water (WFW) programme, have on total evaporation (ET) and the availability of water resources in two highly-invaded provinces of South Africa. The Surface Energy Balance Algorithm for Land (SEBAL) model, using MODIS satellite imagery, was used to estimate the annual total ET at 250 m pixel resolution. ET was estimated for 3 climatically different years for the Western Cape and KwaZulu-Natal. The average annual ET from areas under IAPs, native vegetation, exotic plantation forestry species and control (clearing) areas were compared. The ET of the 5 dominant IAPs (Acacia mearnsii, Acacia saligna, Eucalyptus spp., Hakea spp. and Pinus spp.) in the Western Cape province was 895 mm, which was significantly higher than the ET of most of the native vegetation (thicket 575 mm and fynbos 520 mm), but similar to the ET of dominant exotic plantation forestry species (805 mm). On average, the ET was reduced by 13% to 780 mm, following clearing. In KwaZulu-Natal Province, the ET of the 5 dominant IAPs (Acacia mearnsii, Chromoleana odorata, Eucalyptus spp., Lantana camara and Solanum mauritanium) was 875 mm, which was also higher than the ET of the native vegetation (thicket 755 mm, savanna 685 mm and grassland 640 mm). Following IAP control the ET was decreased by 6%, to 825 mm. This study has demonstrated that spatial ET data with GIS-information on land use can be used to assess the impact of IAPs, and clearing thereof, on water resources. We confirmed results from previous studies, which showed that ET from invaded areas exceeded that from native vegetation. The ET data needs further validation as validation appeared to be impossible. Our results are likely conservative since the majority of invaded areas considered in this analysis represent non-riparian areas. The impact of WFW control of densely-invaded riparian areas is likely more pronounced. We concluded that the clearing of IAPs by the WFW programme has a positive effect on the availability of water resources through a reduction in ET.
This paper identifies the historical precedents and recent advances in descriptive and analytical aspects of invasive plant ecology. The paper takes a global perspective that focuses primarily on natural and semi-natural systems. The dynamics of plant invasions depend on the unique combination of species and recipient environments in light of short-lived, but highly influential, stochastic events. Spreading from the original point of establishment can be virtually instantaneous or follow a prolonged timelag. Range extension proceeds according to a variety of patterns dependent on the interplay between dispersal modes and landscape characteristics. The impacts of plant invasions are all-encompassing: biodiversity loss, economic impacts and aesthetic impacts occasioned by the loss of traditional cultural or natural landscapes. From the conservation perspective, costs are incalculable, but undoubtedly high. The impacts of invasive plants on natural ecosystems occur across all levels of biotic organization and, in the worst case, result in global extinctions and modification of fundamental ecosystem properties that make restoration practically impossible. Plant invasions occur across all habitat types and have spawned complementary theories, which are briefly presented within particular contexts.
Aim We explored morphological and ecophysiological traits that enable invasive Australian acacias to compete with native species for resources (light, water and nutrients) necessary to support the substantial growth associated with successful invasions.
Location Global.
Results Invasive Australian acacias grow large and seed prolifically in invaded regions. The greater capacity for vegetative growth is underpinned by their ability to acquire and efficiently use resources in non-native habitats. Key biological traits that enhance acquisition include (1) rapid and substantial allocation to root mass (up to 6-fold more than co-occurring native species) directed towards deep roots (at least 50% longer than those of natives) and to extensive shallow root networks; (2) heteroblasty, in most species, conferring high relative growth rates as bipinnate seedlings but long-lived, nutrient-conserving phyllodes as adults and (3) strong N2-fixation abilities.
Main conclusions The ecophysiological traits that govern the competitive interaction of invasive Australian acacias with native species are an important component of the recognized suite of factors including introduction history, human use and enemy release that combine to produce successful invasions. Traits interact to give Australian acacias competitive advantage over many native species. One such interaction is that of N2 fixation, which when coupled with slow decomposition of sclerophyllous phyllodes results in alteration of soil nutrient cycling. The lasting legacy of soil N-enrichment hinders the competitive ability of native species and further enhances invasions. The importance of edaphic factors and competitive interactions in determining invasive success should be considered in predictive modelling of species distributions.
While the first edition focused on ecosystems and landscapes in Europe, this new edition covers biomes and contexts all over the world. Several new chapters deal with broad issues such as biological invasions, climate change, and agricultural land abandonment as they relate to restoration science and ecological restoration.
This is an accessible textbook for senior undergraduate and graduate level students, and early career scientists. The book also provides a solid background for managers, volunteers, and mid-career professionals involved in the practic of ecological restoration.
The invasion of natural ecosystems by alien plants is a serious environmental problem that threatens the sustainable use of benefits derived from such ecosytems. Most past studies in this field have focussed on the history, ecology and management of invasive alien species, and little work has been done on the economic aspects and consequences of invasions. This paper reviews what is known of the economic consequences of alien plant invasions in South Africa. These economic arguments have been used to successfully launch the largest environmental management programme in Africa.
Ten million hectares of South Africa has been invaded by 180 alien species, but their impacts are not fully understood, although they are undoubtedly significant. The indications are that the total costs of these impacts are substantial. Selected studies show that invasions have reduced the value of fynbos ecosystems by over US$ 11.75 billion; that the total cost of invasion would be about US$ 3.2 billion on the Agulhas Plain alone; that the net present cost of invasion by black wattles amounts to US$ 1.4 billion; that invasions by red water fern have cost US$ 58 million; and that the cost to clear the alien plant invasions in South Africa is around US$ 1.2 billion. These few examples indicate that the economic consequences of invasions are huge.
One of the unique aspects of invasive plant control programmes in South Africa has been the ability to leverage further benefits (mainly through employment) for the expensive control programmes from the government's poverty relief budget. This has made it possible to allocate substantial funding to a programme that would otherwise have struggled to obtain significant support. Biological control of invasive species also offers considerable benefits, but is often the subject of debate. We believe that, at least in the case of many invasive alien plant species in South Africa, biological control offers one of the best, and most cost-effective, interventions for addressing the problem.
The water resources of the Western Cape province, and the catchment areas that produce them, need to be carefully managed if future demands for water are to be met. Good catchment management, in the form of programmes to control alien invasive plants, provides an additional means for preventing losses of water, but needs to be justified financially. Previous programmes aimed at the control of alien plants have been curtailed due to a lack of funds in the recent past, as they were conducted for reasons of nature, rather than water, conservation. However, recent studies have highlighted the impacts of alien plants on water resources, and have forced planners to take these impacts into account. In this paper, we review the history of alien plant control programmes, and assess the costs (financial) and benefits (in the form of reduced losses of water from invaded catchments) that would arise from such control. We use the standard practice of discounting future costs and benefits arising from planned and existing water supply schemes to estimate the relative efficiency of the schemes. By modelling the spread and effects of alien plants on streamflow, we show that alien plant control is effective and efficient. In the case of the proposed Skuifraam scheme (the preferred next option to supply water to Cape Town), water can be delivered at a cost of 57 and 59 c kl-1, respectively, with and without the management of alien plants, for example, indicating that such management is cost-effective. Clearing invasive plants from the existing Theewaterskloof catchment would deliver additional water at only 13.6% of the cost of delivery from the new Skuifraam scheme (unit reference values of 8 and 59 c kl-1 respectively). The analyses also show that an early investment in clearing is more cost effective than a later investment, as costs increase and yields decrease the longer the catchment is left to become more invaded. Many other benefits, including avoiding serious impacts on biodiversity, catchment stability and fire management, and enhancing social upliftment, have not been included in this assessment, and add to the argument for implementation of the programme for clearing invasive alien plants.
The use of potential transpiration models to simulate transpiration rates in areas prone to soil water deficits leads to overestimates of water use as the soil dries. Therefore, I carried out studies on Eucalyptus grandis W. Hill ex Maiden trees subjected to soil drying at two field sites in the Mpumalanga province of South Africa to determine the relation between transpiration rate and soil water availability. I hypothesized that, with this relationship defined, simple modeling of the soil water balance could be used to predict what fraction of potential transpiration was taking place at a given time.
Site 1 supported a stand of 3-year-old E. grandis trees, whereas 9-year-old trees were growing on Site 2, situated 2 km away. At each site, plastic sheeting was laid over the ground to prevent soil water recharge and thereby allow the roots in the soil to induce a continuous progressive depletion of soil water. Measurements of predawn xylem pressure potential, leaf area index, growth and sap flow rates revealed that prevention of soil water recharge resulted in only moderate drought stress. At Site 1, the trees abstracted water down to 8 m below the surface, whereas trees at Site 2 obtained most of their water from depths below 8 m. I found that modeling the water balance of deep rooting zones is impractical for the purpose of simulating nonpotential transpiration rates because of uncertainties about the depth of the root system, the soil water recharge mechanism and the water retention characteristics of the deep subsoil strata. I conclude that predicting the occurrence and severity of soil water deficits from the soil water balance is not feasible at these sites.
The impacts of the widespread invasions by alien plants in South Africa are increasingly recognised. Most of the past concern has been about the impacts on conservation areas, other areas of natural vegetation and on agricultural productivity. The potential impact of invading alien woody plants on water resources was known to be serious but there has been no information available to evaluate the significance of these water losses across the whole country. This paper reports on the results of a preliminary survey aimed at obtaining an overview of the extent, impacts and implications of alien plant invasions at a national and regional level for South Africa and Lesotho. Data on the extent and location of the invaded areas were obtained from a variety of sources including detailed field mapping, mainly at a 1:250 000 scale with some at 1:50 000 and 1:10 000, and generalised information on species and densities. The density class of each species in each polygon was mapped and used to derive the condensed areas (the equivalent area with a canopy cover of 100%). Each of the invading species was classified as a tall shrub, medium tree or tall tree - based on growth form and likely water use - and its biomass was estimated from a function based on vegetation age. The incremental water use (i.e. the additional water use compared with the natural vegetation) was calculated using the following equation: Water use (mm) = 0.0238 x biomass (g/m (2)) which was derived from catchment studies. Alien plants, mainly trees and woody shrubs, have invaded an estimated 10.1 million ha of South Africa and Lesotho, an area larger than the province of KwaZulu-Natal. The equivalent condensed area is 1.7 million ha which is greater than the area of Gauteng Province. The Western Cape is the most heavily invaded at about a third of the total area, followed by Mpumalanga, KwaZulu-Natal and Northern Province. The catchments of the Berg and Breede Rivers are the most heavily invaded followed by the George-Tsitsikamma region, Port Elizabeth coastal region and the Drakensberg escarpment in Mpumalanga. The total incremental water use of invading alien plants is estimated at 3 300 million m(3); of water per year, equivalent to about 75% of the virgin MAR of the Vaal River system. About a third of the estimated total water use, by volume, is accounted for by alien invaders in the Western Cape, followed by KwaZulu-Natal (17%), the Eastern Cape (17%) and Mpumalanga (14%). The greatest reduction as a percentage of MAR was found in the arid Northern Cape (17%), followed by the Western Cape (15%) and Gauteng (10%). For primary catchments, the greatest percentage reductions were in the Namaqualand coast (catchment F, 91%) followed by the Eastern Cape Coast (P, 42%) and the south-western Cape (G, 31%). The extent and density of the invasions and thus the impact on water resources could increase significantly in the next 5 to 10 years, resulting in the loss of much, or possibly even all, of the available water in certain catchment areas. Alien plant control is expensive but it has been shown that control programmes are cost-effective compared with alternative water supply schemes. This preliminary assessment needs to be interpreted with caution because the results are based on a data set that contains some important uncertainties. The water-use estimates also involve some critical assumptions. Nevertheless, the scale of the invasions, the magnitudes of the impacts and the rapid expansion we are observing are such that a national control programme is essential if the country's water resources are to be protected.
1. The invasion of fynbos shrublands by woody weed species can reduce the water yield from catchment areas dramatically. We modelled the consequences of uncontrolled invasion on water yield using a geographical information system (Arc/Info). 2. Five important processes were recognized: the occurrence of fire; the spread and establishment of alien plants after fire; rainfall-to-run-off ratios; growth and changes in biomass between fires; and effects of these changes on streamflow. 3. The simulations of water yield were modelled with the Arc/Info GRID module using a 200 x 200-m grid. It was assumed that the interval between fires was 15 years and that proliferation and dispersal of alien plants took place only after fires. 4. Between fires, the model simulated the growth of the vegetation and its effects on streamflow, using relationships between rainfall and run-off, and run-off and above-ground biomass. 5. Results for the Kogelberg area in the Western Cape Province showed that alien plants invaded about 40% of the grid cells within 50 years. Cover of alien plants increased from an initial estimate of 2.4% to 62.4% after 100 years. 6. Invasion of catchment areas would result in an average decrease of 347 m3 of water per hectare per year over 100 years, resulting in average losses of more than 30% of the water supply to the city of Cape Town. In individual years, where large areas would be covered by mature trees, losses would be much greater. 7. In addition, invasion of fynbos by alien plants will cause the extinction of many plant species, increase the intensity of fires, destabilize catchment areas with resultant erosion and diminished water quality, and decrease the aesthetic appeal of mountain areas. 8. Control of alien weed species is necessary to avert the above impacts, and the costs of control operations could be justified by the savings achieved in maintaining adequate water run-off from stable catchments in the long term.
It is necessary to regulate afforestation in South Africa because of the high demand exotic forest plantations make on the scarce water resources of the country. The present procedure and information base for the regulation of afforestation with exotic plantations in South Africa are useful, but rely on a large degree of subjectivity and exclude recent research information from the procedure.The problem of regulating afforestation can also be dealt with as a simple linear programming model. Solution of the linear programming model requires quantitative and qualitative information obtained by using different methods. The use of the Analytic Hierarchy Procedure (Saaty, 1983) as a qualitative method to determine a preference rating for the water demand of five user categories, i.e. terrestrial ecosystems, river ecosystems, industry, irrigation and dam storage, is demonstrated. Quantitative relationships between afforestation and annual water yield are updated. We suggest the use of this new procedure in preference to the present one.Suggestions are made for incorporating hydrological aspects such as spatial variation of evapotranspiration in catchments, critical low flow constraints and the influence of rotation age on water use.The need for accurate determination of evapotranspiration, not only of exotic plantation, but also of natural vegetation being replaced by the plantations, is emphasised.
The heat pulse velocity technique was verified on a diffuse porous hardwood, Eucalyptusgrandis Hill ex. Maiden, using the cut-tree method. The heat pulse velocity apparatus accurately reflected sap flow in the stems of 3-year-old trees and of a 16-year-old tree. Measured sap flow was found to be particularly sensitive to errors in the estimation of the wound size and probe separation. Four probes, each providing a point estimate of heat pulse velocity, were sufficient to estimate sap flow in young trees, but a minimum of eight was needed for trees larger than about 20 cm in diameter. For the successful field application of the heat pulse velocity technique on standing E. grandis trees, it is necessary to wait 5 days after implanting the probes to enable tyloses to form, which facilitates wound size measurement. This technique is recommended for the accurate and rapid measurement of water use by E. grandis and other similar plantation tree species.
Two portable weirs were used to quantify changes in streamflow following clearfelling of a dense stand of self-sown Pinus patula and Acacia mearnsii along a riparian zone on Kalmoesfontein, a SAPPI forest plantation south-east of Lydenburg, Eastern Transvaal. The weirs were set up at positions 500 m apart on the same stream, and used to monitor streamflow levels before and after the clearing of all trees between the weirs to an average distance of 25 m from the stream. Analysis of streamflow data revealed that the clearing operation resulted in a 120% increase in streamflow at the lower weir, equivalent to 30,5 m3 of water per day. Streamflow at the lower weir was less than at the upper weir before clearfelling, but equalled that at the upper weir after clearfelling. This demonstrates that the intervening trees were responsible for the initial difference in streamflow. Two further lines of evidence point to the adjacent trees exerting a strong influence on streamflow. Firstly, a clear daily fluctuation in streamflow was evident at both weirs. This is a consequence of transpiration by trees taking place only during hours of daylight. Secondly, the occurrence of cloudy, rain-free weather led to an increase in streamflow. Such weather reduces the evaporative demand of the air, causing transpiration rates to decline as well.We conclude that invasive exotic trees should be removed from riparian zones to promote significant streamflow increases from afforested catchments. There is a need for a broader body of information to assess the effects of species, density and age distribution, as well as catchment characteristics, on streamflow responses to clearing invasive trees. This study has demonstrated the suitability of the portable weir technique in capturing such information.
The changes in streamflow following the removal of invasive wattle (Acacia mearnsii and A. longifolia) from riparian zones during the dry summer months in three small catchments in the Western Cape Province are described. Portable steel box weirs, with a 30° compound V-notch and equipped with Belfort water level recorders, were used to measure streamflow. Each of the three streams had a control catchment against which it was correlated during a pre-treatment period. The riparian zones of the treatment catchments were cleared after this period of calibration, and the response of stream-flow after treatment was recorded.
In the three catchments there was a marked increase in streamflow after clearing of the riparian invasive vegetation. The streamflow increases in the three catchments were 8,8, 10,4 and 12 m³/day per ha cleared. The responses measured in these experiments are the result of changing from tall vegetation to minimal cover and represent a maximum response. Streamflow is expected to decrease again as vegetation regrows, but not to the levels that characterised the invaded site. The riparian areas should be kept under short indigenous vegetation, such as grass or fynbos, to sustain the long-term increases in streamflow.
Daily patterns of stomatal conductance (gs), xylem pressure potential (P) and canopy microclimatic variables were recorded on 11 sample days as part of a one-year study of the water use of Eucalyptus grandis Hill ex Maiden in the eastern Transvaal, South Africa. Measured gs was found to be largely controlled by quantum flux density (Q) and ambient vapour pressure deficit (D). Canopy conductance (gc) was determined for hourly intervals using gs measurements and leaf areas in four different canopy levels. A simple model was constructed to allow the prediction of gc and transpiration from Q, D and season of year. The model was used to estimate transpiration rates from 10 trees in a later study of similarly-aged E. grandis trees, in which sap flow in each tree was measured using the heat pulse velocity (HPV) technique. Five of the trees were monitored on a summer day and five on a winter day. Correspondence between HPV sap flow and modelled transpiration was good for the summertime comparisons, but measured winter-time sap flow rates were underestimated by the model, especially under conditions of high sap flow. The discrepancy is believed to result from having insufficient data from the conductance study to describe the response of gs to relatively high D in winter. Marked variation in transpiration per unit leaf area indicates that a relatively large number of trees must be sampled for the HPV technique to be used to obtain a mean rate for an entire stand in winter.
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