Abrupt regime shifts and non-equilibrium dynamics in Mediterranean ecosystems: The interplay of changing environments and fire-vegetation feedbacks

Background/Question/Methods

Biological systems may exhibit non-linear and divergent responses to environmental changes, so understanding system dynamics and feedbacks is critically important. Interactions between fire and vegetation are of particular interest, as fire is a major driver of vegetation change, and vegetation properties (e.g. amount, flammability) alter fire regimes. We developed a minimal modeling framework to evaluate the relative influence of extrinsic and intrinsic factors on disturbance and successional dynamics in semi-arid ecosystems such as Mediterranean-type biomes (MTEs). Our model is conceptualized as a state and transition modeling framework and incorporates three key components of fire ecology and vegetation dynamics in MTEs: (i) the rate of vegetation succession in the absence of disturbance; (ii) the probability of fire and the severity of fire, including flammability feedback effects of vegetation; and (iii) stochasticity in fire return intervals.

Results/Conclusions

Varying fire-vegetation feedbacks in MTEs can lead to different critical points in underlying processes of disturbance and sudden shifts in system states, despite gradual changes in ecosystem drivers as defined by the environment. Vegetation flammability and disturbance stochasticity thus effectively modify system behavior, determining landscape heterogeneity and the existence of alternative stable states. Changing climates and associated alterations in fire activity may promote abrupt changes in landscape composition, but some state shifts could unfold slowly after a tipping point has been exceeded. Our work suggests that plant flammability may be key in determining the response of fire-prone communities to environmental change, which, under some circumstances, may be characterized by non-equilibrium dynamics and slow, unrecognized regime shifts.