Acidic deposition, plant pests, and the fate of forest ecosystems

We present and analyze a nonlinear dynamical system modelling forest-pests interactions and the way they are affected by acidic deposition. The model includes mechanisms of carbon and nitrogen exchange between soil and vegeta tion, biomass decomposition and microbial mineralization, and defoliation b y pest grazers, which are partially controlled by avian or mammalian predat ors. Acidic deposition is assumed to directly damage vegetation, to decreas e soil pH, which in turn damages roots and inhibits microbial activity, and to predispose trees to increased pest attack. All the model parameters are set to realistic values except the inflow of protons to soil and the preda tion mortality inflicted to the pest which are allowed to vary inside reaso nable ranges. A numerical bifurcation analysis with respect to these two pa rameters is carried out. Five functioning modes are uncovered: (i) pest-fre e equilibrium; (ll) pest persisting at endemic equilibrium; (iii) forest-pe st permanent oscillations; (iv) bistable behavior with the system convergin g either to pest-free equilibrium or endemic pest presence in accordance wi th initial conditions; (v) bistable behavior with convergence to endemic pe st presence or permanent oscillations depending on initial conditions. Cata strophic bifurcations between the different behavior modes ape possible, pr ovided the abundance of predators is not too small. Numerical simulation sh ows that increasing acidic load can lead the forest to collapse in a short time period without important warning signals. (C) 1998 Academic Press.