Interpreting woody cover data in tropical and subtropical areas: Comparison between the equilibrium and the non-equilibrium assumption
Résumé
We compared two basic assumptions about the woody cover distribution in tropical and subtropical areas: the equilibrium (woody cover always reaches a long-term steady state) vs the non-equilibrium assumption (woody cover fluctuates in response to fire disturbances). We considered two models each one representative of one of the two assumptions: an equilibrium and a non-equilibrium model. The equilibrium model considered fire as an a priori determined parameter, whereas the non-equilibrium model assumed fires as stochastic events whose probability increased with grass density. We compared the results of the models with large datasets containing woody cover values sampled at the continental and at the global scale. In particular, we focused on two evidences shown by data. The first evidence is that woody cover is limited by water scarcity for low rainfall values and by fire for high rainfall values (arid-moist savanna distinction). The second evidence is the bimodality of woody cover data observed for high rainfall values. The equilibrium model gave a static interpretation of the data. The non equilibrium model, instead, gave a more general interpretation of the data. In particular, the non equilibrium model detected the arid-moist savanna distinction as emergent along a rainfall gradient and demonstrated that the bimodality observed in the woody cover data could be obtained in the woody cover values exhibited by a vegetation system in different times. Thus, woody cover data do not necessarily represent steady states. Rather, they could represent snapshots of a vegetation system in certain time instants.