Abstract
Moist savannas and tropical forests share the same climatic conditions and occur side by side. Experimental evidences show that the tree cover of these ecosystems exhibits a bimodal frequency distribution. This is considered as a proof of savanna–forest bistability, predicted by dynamic vegetation models based on non-linear differential equations. Here, we propose a change of perspective about the bimodality of tree cover distribution. We show, using a simple matrix model of tree dynamics, how the bimodality of tree cover can emerge from the switching between two linear dynamics of trees, one in presence and one in absence of fire, with a feedback between fire and trees. As consequence, we find that the transitions between moist savannas and tropical forests, if sharp, are not necessarily catastrophic.
Highlights
The tree cover distribution of moist savannas and tropical forests has been recently investigated extensively over Africa, Australia and South America [1,2]. [1] have shown that these biomes co-exist over a very large range of annual rainfall (650– 2500 mm/yr) and that at intermediate rainfall (1000–2500 mm/ yr) tree cover is bimodal, exhibiting one peak for low woody cover and one peak for high woody cover.The bimodality of tree cover distribution has been used as a proof that savanna and forest are alternative stable states [1,2,3]
We have started our analysis from the observed bimodal frequency distribution of tree cover in moist savannas and tropical forests, which is considered a proof of savanna/forest bistability [1,2]
We have presented an alternative explanation to the bimodal frequency distribution of tree, which does not require alternative stable states and corresponding catastrophic transitions
Summary
The bimodality of tree cover distribution has been used as a proof that savanna and forest are alternative stable states [1,2,3]. Many simple models ( denominated minimal) exhibit savanna–forest bistability [4,5,6]. The main advantage of minimal models consists in the possibility of identifying interactions and causal nexi between drivers and state variables [7,8]. These models can be treated analytically and investigated through the bifurcation analysis. What is widely accepted is that fire is responsible for bistability: in savanna and forest the tree cover cannot be adequately explained without explicitly considering the dynamics of fire [9,10,11,12], and feedbacks fire–trees have been used in literature to argue for bistability (see [13] and references therein)
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