Mechanisms maintaining biodiversity in Mediterranean pine-oak forests: insights from a spatial simulation model

Recurrent anthropogenic and natural perturbations, resource limitations and heterogeneous environments contribute to the maintenance of a remarkable biodiversity in Mediterranean plant communities. Yet, the essential mechanisms of community assembly in these systems remain largely unexplored. In the current paper we investigate the coexistence of Aleppo pine (Pinus halepensis Mill.) and Holm oak (Quercus ilex L.), two of the most widely distributed species in the Iberian Peninsula, in relation to gradients in water availability and disturbance. A spatial model of landscape forest dynamics was implemented, calibrated with experimental data, with stands arranged on a heterogeneous lattice and coupled by dispersal. It was found that pine population persistence in the landscape can be explained independently by a competition-colonization tradeoff (in mesic homogeneous environments) and by a tradeoff between shade and drought tolerance (in heterogeneous low disturbed environments). Both mechanisms reinforce mutually to maintain a shifting mosaic of both taxa along disturbance and aridity gradients. This view is consistent with palynological, historical and forest inventory records. In turn, equilibrium theories of vegetation dynamics (phytosociological), that neglect the role of heterogeneity and disturbances may be inadequate for Mediterranean forests, and have been shown to result in mismanagement practices. We claim that biologically informed models of forest dynamics are desperately needed as diagnosis tools for sustainable forest management.