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Several hypotheses have been made to explain the forest decline due to acidic deposition. One of the most credited is the mobilization of toxic aluminium ions when soil pH falls below 4.2. A simple model is presented here that couples soil chemistry with tree biomass dynamics in order to investigate the influence of different proton loads on the existence, stability, and bifurcations of ecosystem equilibria. It is shown that, owing to the intrinsic nonlinear nature of the vegetation response to acid deposition, the equilibrium manifold can have, under certain conditions, the structure of a fold catastrophe. Increasing acidic load can thus drive the forest through a catastrophic transition from a viable equilibrium to extinction. Simulations using realistic ranges for model parameters and acidic input show that forests may indeed meet the conditions for a catastrophic collapse resulting from accumulation of acidic stress in the soil.Copyright 1993, 1999 Academic Press