The Global Extent and Determinants of Savanna and Forest as Alternative Biome States

TLDR

Fire–tree cover feedbacks can sustain savanna and forest as alternative stable states. The study aims to quantify the global extent of fire‑driven tree cover discontinuities, accounting for seasonality and soils. The authors analyze global tree cover, climate, fire, and soil datasets to demonstrate tree cover discontinuity. Globally, tree cover is bimodal at intermediate rainfall and mild seasonality, with fire distinguishing savanna from forest; these alternative states span large regions such as Amazonia and the Congo, and shifts in biome distribution will be abrupt and hard to reverse.

Abstract

Theoretically, fire-tree cover feedbacks can maintain savanna and forest as alternative stable states. However, the global extent of fire-driven discontinuities in tree cover is unknown, especially accounting for seasonality and soils. We use tree cover, climate, fire, and soils data sets to show that tree cover is globally discontinuous. Climate influences tree cover globally but, at intermediate rainfall (1000 to 2500 millimeters) with mild seasonality (less than 7 months), tree cover is bimodal, and only fire differentiates between savanna and forest. These may be alternative states over large areas, including parts of Amazonia and the Congo. Changes in biome distributions, whether at the cost of savanna (due to fragmentation) or forest (due to climate), will be neither smooth nor easily reversible.