Determinants and dynamics of the global distribution of the savanna and forest biomes

Abstract

A critical need to predict biosphere responses to climate change has made understanding the drivers and dynamics of biome distributions a fundamental issue in modern ecology. Plant physiology provides a mechanistic basis for prediction, but the boundary between savanna and forest challenges physiological models. This thesis examines the distributions of savanna and forest and their implications for dynamics that characterize savanna systems. I take two main approaches: (i) Chapters 2 through 4 use a combination of empirical and theoretical analyses to identify factors and interactions that differentiate savanna from forest at continental scales, and (ii) Chapter 5 examines tree cover within savannas with respect to the same drivers at a finer scale.

Chapter 2 presents (i) an analysis of tree cover in sub-Saharan Africa, showing that an interaction between climate and fire constrains tree cover, and (ii) a model demonstrating that the threshold response of fire spread to tree cover means that, depending on rainfall, savanna and forest may be alternative stable states. Chapter 3 expands that empirical analysis to South America and Australia, examining the relationship of tree cover with respect not only to mean annual rainfall and fire but also to rainfall seasonality and soils; discontinuities in tree cover driven by fire are apparently global in extent, depending on climatic context. Chapter 4 expands on the model from Chapter 2 to introduce a theoretical integration of rainfall and fire effects on tree cover, showing that bimodalities in tree cover at intermediate rainfall may be universal.

Chapter 5 presents an analysis of variation in vegetation structure and fire patterns with relation to rainfall and soils using long-term datasets from Kruger National Park, in South Africa. Grass biomass and fire responded predictably to abiotic variability, but tree density did not, suggesting that snapshot analyses, even those based on long-term datasets, do not reflect processes that produce variability in tree cover. The last chapter synthesizes general findings to answer the question of what mechanisms give rise to the emergent stability of savanna, despite variability in vegetation structure within the biome.