Woody cover in African savannas: mapping strategies and ecological insights at regional and continental scales
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Hanan, Niall P.Savanna ecosystems are characterized by the coexistence of woody and herbaceous vegetation. They are recognized as highly heterogeneous, for their diversity of growth forms and woody plant spatial arrangements. The relative fraction of woody versus herbaceous cover is particularly important in determining ecosystem functions such as water and biogeochemical cycles and energy fluxes, availability of graze and browse resources for wild and domestic herbivores, and availability of fuel-wood and other savanna products for human societies. This dissertation research focused on woody cover in tropical African savannas, with two main objectives, i) to map woody cover at regional to continental scales across Africa, and ii) to model its dependence on biotic and abiotic factors, at landscape, regional and continental scales. Among the most important outcomes are the creation of woody cover maps for Kruger National Park (South Africa) and the African continent using combined optical and radar imagery, and the development of ecological models that provided empirical evidence for resource-competition and disturbance mechanisms. The two-scale approach allowed the identification of relationships between woody cover and spectral predictors which can successfully be scaled up to predict the continental distributions of woody vegetation across the full gradient from deserts, through grasslands and savannas, to the dense tropical forests. The ecological models identified mean annual precipitation (MAP) as the main determinant of woody cover at the continental level. Regional variations of this MAP-driven woody cover arose from dynamics dependent on perturbations such as fire frequency, herbivory, and anthropogenic activities combined with soil characteristics.