Background on Irreversibility and Bistability in Deforested Ecosystems

In this chapter, we examine the potential situations in which deforestation induces a change in the physical and/or the chemical environment that leads to a loss of environmental conditions necessary to sustain forest vegetation. The reversibility or irreversibility of deforestation is often determined by the absence or presence of positive feedbacks of adequate strength.

Sudden and often irreversible changes in the structure and functioning of ecosystems are typically associated with the existence of multiple stable ecosystem states (e.g., May, 1977; Holling, 1973). We focus on the case of a system that can be stable both with and without forest vegetation (e.g., Box 4.1), although bistable forest dynamics can also emerge in systems with two forested states but with different species compositions (e.g., Pastor and Post, 1988; Ridolfi et al., 2008). The presence of alternative states or “attractors” is commonly associated with positive feedbacks (i.e., a sustained sequence of processes) between forest vegetation and its physical environment, though bistability may emerge in nonlinear dynamics even in the absence of such feedbacks (e.g., Ridolfi et al., 2011; Petraitis, 2013). A change among attractors may be an effect of changes in environmental conditions or disturbance regime that are sustained by changes in forest vegetation (e.g., Wilson and Agnew, 1992). The magnitude of the perturbation required to push the system into the basin of attraction of the stable “deforested state” depends on the resilience of the “forest state,” a property defined as the ability of the system to recover that state after a disturbance (Holling, 1973). The occurrence of shifts between ecosystem states depends both on the magnitude of the external disturbance and on the resilience of the initial state of the system (e.g., Folke et al., 2004). As the resilience of an ecosystem's state declines, it becomes increasingly vulnerable to state shifts such that progressively smaller external events can cause regime shifts (Holling, 1973). When a disturbance imposed on a system causes some critical bifurcation point to be passed, this can produce a shift to an alternative stable state (e.g., a state of low vegetation) (Kuznetsov, 1995; Figures 4.1 and 4.2).