Slow or rapid collapse? Transients between stable states as a source of uncertainty in predicting ecosystem shifts

Session C1: Compexity, Sensitivity, and Uncertainty Issues in Integrated Environmental Models

In this study I explore the rate of change during the collapse of a vegetation-soil system on a hillslope from a vegetated state to an unvegetated, bare-soil, state. From a distributed, stochastic model coupling hydrology, vegetation, weathering and wash erosion, I derive two differential equations describing the interaction between the vegetation and the soil. Two stable states--vegetated and bare--are identified by means of analytical investigation, and it is shown that the change between these two states is a critical transition as indicated by hysteresis. Surprisingly, transitions between these states can either unfold rapidly, over a few years, or gradually, occurring over decennia up to millennia, depending on unforeseen soil parameters. This result emphasizes the considerable uncertainty associated with forecasting critical transitions, which is due to both the timing of the transition and the rate of change after the tipping point has been reached.