Savannas are known as ecosystems with tree cover below their climate-defined equilibria.  However, a predictive framework for understanding constraints on tree cover with respect to climate is lacking. We present a spatially extensive analysis of tree cover, rainfall, and fire, demonstrating that savanna and forest may be alternative states with respect to rainfall in Africa, with implications for understanding savanna distributions.

Results/Conclusions

Tree cover does not increase continuously with rainfall, but rather is constrained to low (<50%, “savanna”) or high tree cover  (>75%, “forest”).  Intermediate tree cover rarely occurs.  The pattern is evident especially in areas with rainfall between 1000mm and 2000mm, where fire—which prevents trees from establishing—differentiates high and low tree cover. This pattern suggests that complex interactions between climate and disturbance produce emergent alternative states in tree cover. Integrating these types of alternative stable state dynamics into models of biome distributions may have major impacts on our ability to predict changes in biome distributions and in carbon storage under climate and global change scenarios.