Abstract
Recent studies have interpreted patterns of remotely sensed tree cover as evidence that forest with intermediate tree cover might be unstable in the tropics, as it will tip into either a closed forest or a more open savanna state. Here we show that across all continents the frequency of wildfires rises sharply as tree cover falls below ~40%. Using a simple empirical model, we hypothesize that the steepness of this pattern causes intermediate tree cover (30‒60%) to be unstable for a broad range of assumptions on tree growth and fire-driven mortality. We show that across all continents, observed frequency distributions of tropical tree cover are consistent with this hypothesis. We argue that percolation of fire through an open landscape may explain the remarkably universal rise of fire frequency around a critical tree cover, but we show that simple percolation models cannot predict the actual threshold quantitatively. The fire-driven instability of intermediate states implies that tree cover will not change smoothly with climate or other stressors and shifts between closed forest and a state of low tree cover will likely tend to be relatively sharp and difficult to reverse.
Highlights
The emerging idea that tropical forest and savanna may be alternative stable states over a range of climatic conditions [1,2,3,4,5] has profound implications for predicting and managing change in these biomes
The shape of this relationship remains rather constant across a range of classes of mean annual precipitation (MAP) (S2 Fig)
In line with previous work [20, 21], we find that fire frequency peaks at intermediate MAP (S3 and S1C Figs), but this effect is rather independent from the effect of tree cover
Summary
The emerging idea that tropical forest and savanna may be alternative stable states over a range of climatic conditions [1,2,3,4,5] has profound implications for predicting and managing change in these biomes. Proving the existence of such alternative ecosystem states is notoriously difficult [6], especially in systems such as the tropical rainforest where the relevant spatial and temporal scales make replicated experimentation challenging [7,8,9,10,11]. The central hypothesis proposed to explain bistability of savanna and forest states is the existence of a strong feedback between tree cover and fire risk [4, 12,13,14,15,16,17]. The idea is that if tree cover becomes sufficiently dense, it precludes the growth of grasses
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