Non-additive effects of alternative stable states on landscape flammability in NW Patagonia: fire history and simulation modelling evidence

Abstract

Understanding the relationship between flammability and time since fire (TSF) is crucial for predicting ecosystem responses to changes in fire regimes. Landscapes composed of alternative stable states displaying positive fire–vegetation feedbacks are especially sensitive to these changes. We derived TSF–flammability functions (Logistic, Olson, Moisture, Weibull) from survival analysis applied to north-west Patagonian landscapes and simulated landscapes composed of different proportions of alternative stable states (shrublands and forest) and fire frequencies. We expected that landscapes dominated by shrublands would show an asymptotic growth (Logistic or Olson) and those dominated by forest would show a hump-shaped growth (Moisture). Additionally, we expected that the landscape-level flammability functions would resemble the pattern of the most abundant community. We found that shrublands tended to dominate the TSF–flammability relationship (Logistic) even when they were less abundant in the landscape (non-additive effects). The flammability function followed a hump-shaped growth (Moisture) only when the forest cover was >80%. Our results highlight that alternative stable states occur not only because of positive fire–vegetation feedbacks, but also thanks to the non-additivity of the flammability of the different states in the landscape. Non-additive effects could have an important role in accelerating landscape transformations towards more flammable states.