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Integrating Theoretical Climate and Fire Effects on Savanna and Forest Systems

The role of fire and climate in determining savanna and forest distributions requires comprehensive theoretical reevaluation. Empirical studies show that climate constrains maximum tree cover and that fire feedbacks can reduce tree cover substantially, but neither the stability nor the dynamics of these systems are well understood. A theoretical integration of rainfall effects with fire processes in particular is lacking. We use simple, well-supported assumptions about the percolation dynamics of fire spread and the demographic effects of climate and fire on trees to build a dynamic model examining the stability of tree cover in savannas and forests. Fire results in the potential for one or possibly two stable equilibria, while the effects of increasing rainfall on tree demography result in (discontinuous) increases in tree cover and in forest tree dominance. As rainfall increases, the system moves from (1) stable low tree cover to (2) bistability of low and high tree cover to (3) stable high tree cover. Thus, theory suggests that tree cover uniquely determined by climate at low and high rainfall but determined by fire feedbacks at intermediate rainfall—as empirical studies suggest—may be a universal feature of systems where fire has strong effects on tree demography.