Moving beyond the aggregated models: woody plant size influences on savanna function and dynamics
Date
2008
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Abstract
Historically, models have played important roles in studying aspects of savannas, including tree-grass competition, fire, and plant-herbivore interactions. The models can be categorized as either (1) "aggregated" ones that neglect size structure but have the advantage of mathematical tractability or (2) complicated process-oriented ecosystem models incorporating mechanistic ecophysiology capturing greater ecological realism but constrained to simulation modeling. The aggregated class of models can be further separated into those focusing on resource utilization and tree-grass competition ("resource-based models") and those focusing on demographic impacts of disturbances by fire and herbivory ("demographic bottleneck models"). The resource and demographic models separately consider important aspects of savanna ecology, yet the two approaches have rarely been integrated, resulting in a significant gap in our understanding of savannas. For this study, I investigated the role of woody plant size in savanna ecology. Using extensive datasets along broad resource gradients of annual precipitation in southern Africa, I examined patterns of size-abundance for woody plants in relatively undisturbed savannas to see if relationships for savannas showed similar patterns to theoretical predictions for tropical forests. Contrary to assumptions and predictions made by aggregate savanna models, I found that the percentage of wood biomass subject to fire loss actually decreases in wetter savannas. Since resource limitation and "thinning" have been mentioned as potential factors in savannas, I investigated the suitability of self-thinning in savannas. I developed a simple theoretical model hypothesizing three potential impacts of tree-grass interactions on the self-thinning relationship. Results from the analyses, testing with field data, suggest that tree-grass competition is asymmetric with respect to tree size. For the formal modeling component of my dissertation, I developed a simple savanna model that integrates demographic bottleneck and resource-based approaches. The model is unique in that the woody carrying capacity has both resource and demographic constraints. Model simulations showed that modest amounts of variation in adult mortality during fires and size-asymmetric tree-grass competition lead to very different model outcomes. The work opens up an entirely new class of ecological models for savanna ecology: analytically tractable with enough size structure to capture realistic savanna vegetation-disturbance interactions.
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Subject
Africa
fire
savannas
woody plants
ecology