Wednesday, August 6, 2008 - 8:20 AMOOS 13-2: Savanna vegetation dynamics in north Australia
Caroline Lehmann1, Lynda Prior1, and David Bowman2. (1) Charles Darwin University, (2) University of Tasmania
Background/Question/Methods
Tropical savannas are characterised by a grassy understorey and a discontinuous overstorey of trees. Tropical savannas occur in areas of intense seasonality in rainfall and productivity. An understanding of the patterns of tree biomass in mesic savannas has remained an ecological conundrum, largely due to the complexity and high spatial and temporal heterogeneity of these environments. However, understanding the dynamics of these systems is of substantial applied and theoretical significance. We therefore aimed to answer the question: What determines the spatial pattern of tree biomass in Australian mesic savannas? We focussed on a savanna system dominated by Eucalyptus tetrodonta, which occupies 450, 000 km2 of north Australia. We combined use of remote sensing, landscape ecology, data syntheses and statistical modelling. We investigated this question at three spatial scales: sub-continental (1000’s km); regional (100’s km) and landscape (10’s km). Investigating this problem at multiple scales enabled us to attempt to quantify the roles of rainfall, fire and mega-herbivory in determining patterns of savanna tree biomass.
Results/Conclusions
At the sub-continental scale, and from 700 -1700 mm mean annual rainfall (MAR), we found that tree biomass increased with MAR; MAR limited potential savanna tree biomass yet, this potential was not often realised. Further, we found evidence consistent with the role of disturbance in generating variability in savanna biomass. As example, areas subjected to experimental fire treatments showed that tree biomass could change by +/- 50% over 23 years under a variety of fire regimes. At the regional scale, across a geographically restricted rainfall gradient (1200 -1600 mm MAR), and over a 40 year period of observation, the greater variability in fire activity and the inherently higher tree cover in the more mesic areas resulted in a greater dynamism of tree cover compared to the drier end of the rainfall gradient. At this scale fire was important in determining deviations below a MAR determined potential. At the landscape scale, and over a 40 year period of observation we showed that frequent fire led to declines in tree cover and biomass. In contrast, large-mammal herbivory promoted increases in tree cover likely via the altered spatial and temporal patterning of fire. Across these scales tree cover and biomass in E. tetrodonta dominated savannas is highly spatially and temporally variable, primarily in response to rainfall, fire and their interaction. The historical and spatial perspective provided by this study demonstrates the importance of fire in maintaining this savanna system.
Tropical savannas are characterised by a grassy understorey and a discontinuous overstorey of trees. Tropical savannas occur in areas of intense seasonality in rainfall and productivity. An understanding of the patterns of tree biomass in mesic savannas has remained an ecological conundrum, largely due to the complexity and high spatial and temporal heterogeneity of these environments. However, understanding the dynamics of these systems is of substantial applied and theoretical significance. We therefore aimed to answer the question: What determines the spatial pattern of tree biomass in Australian mesic savannas? We focussed on a savanna system dominated by Eucalyptus tetrodonta, which occupies 450, 000 km2 of north Australia. We combined use of remote sensing, landscape ecology, data syntheses and statistical modelling. We investigated this question at three spatial scales: sub-continental (1000’s km); regional (100’s km) and landscape (10’s km). Investigating this problem at multiple scales enabled us to attempt to quantify the roles of rainfall, fire and mega-herbivory in determining patterns of savanna tree biomass.
Results/Conclusions
At the sub-continental scale, and from 700 -1700 mm mean annual rainfall (MAR), we found that tree biomass increased with MAR; MAR limited potential savanna tree biomass yet, this potential was not often realised. Further, we found evidence consistent with the role of disturbance in generating variability in savanna biomass. As example, areas subjected to experimental fire treatments showed that tree biomass could change by +/- 50% over 23 years under a variety of fire regimes. At the regional scale, across a geographically restricted rainfall gradient (1200 -1600 mm MAR), and over a 40 year period of observation, the greater variability in fire activity and the inherently higher tree cover in the more mesic areas resulted in a greater dynamism of tree cover compared to the drier end of the rainfall gradient. At this scale fire was important in determining deviations below a MAR determined potential. At the landscape scale, and over a 40 year period of observation we showed that frequent fire led to declines in tree cover and biomass. In contrast, large-mammal herbivory promoted increases in tree cover likely via the altered spatial and temporal patterning of fire. Across these scales tree cover and biomass in E. tetrodonta dominated savannas is highly spatially and temporally variable, primarily in response to rainfall, fire and their interaction. The historical and spatial perspective provided by this study demonstrates the importance of fire in maintaining this savanna system.