The Experts below are selected from a list of 219 Experts worldwide ranked by ideXlab platform
Ian Gaigher - One of the best experts on this subject based on the ideXlab platform.
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feedbacks between fire and patches of woody vegetation in Tropical Grasslands and savannas abstract powerpoint
2016Co-Authors: F Van Langevelde, Thomas A Groen, Ignas M A Heitkonig, N Govender, Ian GaigherAbstract:In Tropical Grasslands and savannas, fire is used to reduce woody vegetation expansion. Woody vegetation in these biomes is often patchily distributed with unknown consequences for fire effects. We studied two feedbacks between fire and patches of woody vegetation: effects of fire on tree clustering and effects of tree clusters on fire effects. The first feedback was tested by measuring the extent of tree clustering in Kruger National Park. We found that fire frequency positively affects the clustering of tree species that are not very abundant. We suggest that less abundant species are less resistant to fire and adopt a mechanism of clustering to exclude grass fires under their canopy. The second feedback was experimentally tested. We planted tree seedlings around wooded patches in a grassland and burnt these plots. We found that fire had lower temperature that prolonged for a shorter time period at the leeward side of wooded patches than at the windward side. Also, we found that the seedlings were less damaged at the leeward side. We conclude that fire can result in clustering of trees and that these patches of woody vegetation can have a large effect on the role of fire in Tropical Grasslands and savannas. These two feedbacks may lead to a ‘safe zone’ for seedlings around patches of woody vegetation, which consequently promotes woody vegetation expansion. We modelled these findings and illustrate the results using changing frequency distributions of patch sizes. Our study contributes to understanding of savanna functioning by showing which processes are relevant in the distribution of savanna trees.
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effect of patches of woody vegetation on the role of fire in Tropical Grasslands and savannas
International Journal of Wildland Fire, 2014Co-Authors: F Van Langevelde, C P G M De Groot, Thomas A Groen, Ignas M A Heitkonig, Ian GaigherAbstract:In Tropical Grasslands and savannas, fire is used to reduce woody vegetation expansion. Woody vegetation in these biomes is often patchily distributed, and micro-climatic conditions can largely vary locally with unknown consequences for fire effects. We hypothesised that (1) fire has higher temperature and maintains high temperatures for a longer period at the windward side than at the leeward side of wooded patches, (2) this difference increases with patch size, (3) fire has a larger effect on woody vegetation at the windward side than at the leeward side of wooded patches and (4) this effect increases with patch size. We planted tree seedlings around wooded patches in a grassland and burnt these plots. We found that fire had a lower temperature and had an elevated temperature for a shorter time period at the leeward side of wooded patches than at the windward side. Also, we found smaller effect of fire on the seedlings at the leeward side. We conclude that patches of woody vegetation can have a large effect on the role of fire in Tropical Grasslands and savannas. This effect suggests a ‘safe zone’ for seedlings at the leeward side, which consequently promotes woody vegetation expansion. This paper contributes to understanding of the effect of patchiness of woody vegetation on the role of fire in Tropical Grasslands and savannas in reducing woody vegetation expansion.
Philippe Choler - One of the best experts on this subject based on the ideXlab platform.
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A Benchmark Test for Ecohydrological Models of Interannual Variability of NDVI in Semi-arid Tropical Grasslands
Ecosystems, 2011Co-Authors: Philippe Choler, William Sea, Ray LeuningAbstract:Pulses of aboveground net primary productivity (ANPP) in response to discrete precipitation events are an integral feature of ecosystem functioning in arid and semi-arid lands. Yet, the usefulness of nonlinear, ecohydrological pulse response functions to predict regional-scale patterns of annual ANPP at decadal scales remains unclear. Here, we assessed how different pulse response (PR) models compete with simple linear statistical models to capture variability in yearly integrated values of Normalized Difference Vegetation Index (NDVI_int), a remotely sensed proxy of annual ANPP. We examined 24-year-long time series of NDVI_int calculated from Advanced Very High Resolution Radiometer (AVHRR) NDVI for 350,000 km^2 of Tropical Grasslands in northern Australia. Based on goodness-of-fit statistics, PR models clearly outperformed statistical models when parameters were optimized for each site but all models showed the same error magnitude when all sites were combined in ensemble simulations or when the models were evaluated outside the calibration period. PR models were less biased and their performance did not deteriorate in the driest areas compared to linear models. Increasing the complexity of PR models to provide a better representation of soil water balance and its feedback with plant growth did not improve model performance in ensemble simulations. When error magnitude, bias, and sensitivity to parameter uncertainty were all considered, we concluded that a low-dimensional PR model was the most robust to capture NDVI_int variability. This study shows the potential of long time series of AVHRR NDVI to benchmark process-oriented models of interannual variability of NDVI_int in water-controlled ecosystems. This opens new avenues to examine at the global scale and over several decades the causal relationships between climate and leaf dynamics in the grassland biome.
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a simple ecohydrological model captures essentials of seasonal leaf dynamics in semi arid Tropical Grasslands
Biogeosciences, 2010Co-Authors: Philippe Choler, William Sea, Peter R Briggs, Michael R Raupach, R LeuningAbstract:Abstract. Modelling leaf phenology in water-controlled ecosystems remains a difficult task because of high spatial and temporal variability in the interaction of plant growth and soil moisture. Here, we move beyond widely used linear models to examine the performance of low-dimensional, nonlinear ecohydrological models that couple the dynamics of plant cover and soil moisture. The study area encompasses 400 000 km2 of semi-arid perennial Tropical Grasslands, dominated by C4 grasses, in the Northern Territory and Queensland (Australia). We prepared 8-year time series (2001–2008) of climatic variables and estimates of fractional vegetation cover derived from MODIS Normalized Difference Vegetation Index (NDVI) for 400 randomly chosen sites, of which 25% were used for model calibration and 75% for model validation. We found that the mean absolute error of linear and nonlinear models did not markedly differ. However, nonlinear models presented key advantages: (1) they exhibited far less systematic error than their linear counterparts; (2) their error magnitude was consistent throughout a precipitation gradient while the performance of linear models deteriorated at the driest sites, and (3) they better captured the sharp transitions in leaf cover that are observed under high seasonality of precipitation. Our results showed that low-dimensional models including feedbacks between soil water balance and plant growth adequately predict leaf dynamics in semi-arid perennial Grasslands. Because these models attempt to capture fundamental ecohydrological processes, they should be the favoured approach for prognostic models of phenology.
F Van Langevelde - One of the best experts on this subject based on the ideXlab platform.
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feedbacks between fire and patches of woody vegetation in Tropical Grasslands and savannas abstract powerpoint
2016Co-Authors: F Van Langevelde, Thomas A Groen, Ignas M A Heitkonig, N Govender, Ian GaigherAbstract:In Tropical Grasslands and savannas, fire is used to reduce woody vegetation expansion. Woody vegetation in these biomes is often patchily distributed with unknown consequences for fire effects. We studied two feedbacks between fire and patches of woody vegetation: effects of fire on tree clustering and effects of tree clusters on fire effects. The first feedback was tested by measuring the extent of tree clustering in Kruger National Park. We found that fire frequency positively affects the clustering of tree species that are not very abundant. We suggest that less abundant species are less resistant to fire and adopt a mechanism of clustering to exclude grass fires under their canopy. The second feedback was experimentally tested. We planted tree seedlings around wooded patches in a grassland and burnt these plots. We found that fire had lower temperature that prolonged for a shorter time period at the leeward side of wooded patches than at the windward side. Also, we found that the seedlings were less damaged at the leeward side. We conclude that fire can result in clustering of trees and that these patches of woody vegetation can have a large effect on the role of fire in Tropical Grasslands and savannas. These two feedbacks may lead to a ‘safe zone’ for seedlings around patches of woody vegetation, which consequently promotes woody vegetation expansion. We modelled these findings and illustrate the results using changing frequency distributions of patch sizes. Our study contributes to understanding of savanna functioning by showing which processes are relevant in the distribution of savanna trees.
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effect of patches of woody vegetation on the role of fire in Tropical Grasslands and savannas
International Journal of Wildland Fire, 2014Co-Authors: F Van Langevelde, C P G M De Groot, Thomas A Groen, Ignas M A Heitkonig, Ian GaigherAbstract:In Tropical Grasslands and savannas, fire is used to reduce woody vegetation expansion. Woody vegetation in these biomes is often patchily distributed, and micro-climatic conditions can largely vary locally with unknown consequences for fire effects. We hypothesised that (1) fire has higher temperature and maintains high temperatures for a longer period at the windward side than at the leeward side of wooded patches, (2) this difference increases with patch size, (3) fire has a larger effect on woody vegetation at the windward side than at the leeward side of wooded patches and (4) this effect increases with patch size. We planted tree seedlings around wooded patches in a grassland and burnt these plots. We found that fire had a lower temperature and had an elevated temperature for a shorter time period at the leeward side of wooded patches than at the windward side. Also, we found smaller effect of fire on the seedlings at the leeward side. We conclude that patches of woody vegetation can have a large effect on the role of fire in Tropical Grasslands and savannas. This effect suggests a ‘safe zone’ for seedlings at the leeward side, which consequently promotes woody vegetation expansion. This paper contributes to understanding of the effect of patchiness of woody vegetation on the role of fire in Tropical Grasslands and savannas in reducing woody vegetation expansion.
R C Dalal - One of the best experts on this subject based on the ideXlab platform.
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the effect of pasture utilization rate on stocks of soil organic carbon and total nitrogen in a semi arid Tropical grassland
Agriculture Ecosystems & Environment, 2014Co-Authors: M J Pringle, D E Allen, Steven Bray, D. Phelps, Thomas G. Orton, R C DalalAbstract:The influence of grazing management on total soil organic carbon (SOC) and soil total nitrogen (TN) in Tropical Grasslands is an issue of considerable ecological and economic interest. Here we have used linear mixed models to investigate the effect of grazing management on stocks of SOC and TN in the top 0.5 m of the soil profile. The study site was a long-term pasture utilization experiment, 26 years after the experiment was established for sheep grazing on native Mitchell grass (Astrebla spp.) pasture in northern Australia. The pasture utilization rates were between 0% (exclosure) and 80%, assessed visually. We found that a significant amount of TN had been lost from the top 0.1 m of the soil profile as a result of grazing, with 80% pasture utilization resulting in a loss of 84 kg ha−1 over the 26-year period. There was no significant effect of pasture utilization rate on TN when greater soil depths were considered. There was no significant effect of pasture utilization rate on stocks of SOC and soil particulate organic carbon (POC), or the C:N ratio at any depth; however, visual trends in the data suggested some agreement with the literature, whereby increased grazing pressure appeared to: (i) decrease SOC and POC stocks; and, (ii) increase the C:N ratio. Overall, the statistical power of the study was limited, and future research would benefit from a more comprehensive sampling scheme. Previous studies at the site have found that a pasture utilization rate of 30% is sustainable for grazing production on Mitchell grass; however, given our results, we conclude that N inputs (possibly through management of native N2-fixing pasture legumes) should be made for long-term maintenance of soil health, and pasture productivity, within this ecosystem.
Ignas M A Heitkonig - One of the best experts on this subject based on the ideXlab platform.
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feedbacks between fire and patches of woody vegetation in Tropical Grasslands and savannas abstract powerpoint
2016Co-Authors: F Van Langevelde, Thomas A Groen, Ignas M A Heitkonig, N Govender, Ian GaigherAbstract:In Tropical Grasslands and savannas, fire is used to reduce woody vegetation expansion. Woody vegetation in these biomes is often patchily distributed with unknown consequences for fire effects. We studied two feedbacks between fire and patches of woody vegetation: effects of fire on tree clustering and effects of tree clusters on fire effects. The first feedback was tested by measuring the extent of tree clustering in Kruger National Park. We found that fire frequency positively affects the clustering of tree species that are not very abundant. We suggest that less abundant species are less resistant to fire and adopt a mechanism of clustering to exclude grass fires under their canopy. The second feedback was experimentally tested. We planted tree seedlings around wooded patches in a grassland and burnt these plots. We found that fire had lower temperature that prolonged for a shorter time period at the leeward side of wooded patches than at the windward side. Also, we found that the seedlings were less damaged at the leeward side. We conclude that fire can result in clustering of trees and that these patches of woody vegetation can have a large effect on the role of fire in Tropical Grasslands and savannas. These two feedbacks may lead to a ‘safe zone’ for seedlings around patches of woody vegetation, which consequently promotes woody vegetation expansion. We modelled these findings and illustrate the results using changing frequency distributions of patch sizes. Our study contributes to understanding of savanna functioning by showing which processes are relevant in the distribution of savanna trees.
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effect of patches of woody vegetation on the role of fire in Tropical Grasslands and savannas
International Journal of Wildland Fire, 2014Co-Authors: F Van Langevelde, C P G M De Groot, Thomas A Groen, Ignas M A Heitkonig, Ian GaigherAbstract:In Tropical Grasslands and savannas, fire is used to reduce woody vegetation expansion. Woody vegetation in these biomes is often patchily distributed, and micro-climatic conditions can largely vary locally with unknown consequences for fire effects. We hypothesised that (1) fire has higher temperature and maintains high temperatures for a longer period at the windward side than at the leeward side of wooded patches, (2) this difference increases with patch size, (3) fire has a larger effect on woody vegetation at the windward side than at the leeward side of wooded patches and (4) this effect increases with patch size. We planted tree seedlings around wooded patches in a grassland and burnt these plots. We found that fire had a lower temperature and had an elevated temperature for a shorter time period at the leeward side of wooded patches than at the windward side. Also, we found smaller effect of fire on the seedlings at the leeward side. We conclude that patches of woody vegetation can have a large effect on the role of fire in Tropical Grasslands and savannas. This effect suggests a ‘safe zone’ for seedlings at the leeward side, which consequently promotes woody vegetation expansion. This paper contributes to understanding of the effect of patchiness of woody vegetation on the role of fire in Tropical Grasslands and savannas in reducing woody vegetation expansion.
