Landscape species pools and connectivity patterns influence tree species richness in both managed and unmanaged stands

Disturbance intensity and metapopulation dynamics are among the theoretical mechanisms explaining species coexistence at the local and landscape scale. Both mechanisms might interact, so that the consequences of local disturbances might depend on long distance dispersal events. In this study we examined whether the richness of tree species potentially able to colonize a locality from the surroundings was associated with the tree species richness observed in that locality, and/or with the response of that richness to partial harvesting. The study was located in a Mediterranean region in central Spain where partially harvested forests had been found to have more tree species than unmanaged forests. We used a top-down hierarchical modeling structure to account for the effect of other factors such as climate, lithology and amount of forest cover at the landscape scale. Species richness of trees was strongly associated to annual precipitation, and was maximized at intermediate rainfall levels. Under homogeneous climate and lithological conditions, the composition and connectivity of seed sources in the landscape seemed to play a more relevant role explaining tree species richness than the amount of forest habitat in the surroundings. Particularly, higher species richness was observed in forest stands susceptible of receiving a higher diversity of seed fluxes. Patterns in the response of species richness to partial harvesting were less clearly explained by differences in the diversity of potential seed fluxes, but time lags in the responses, or differences in the proportion of shade-tolerant species in the landscape could mediate this interaction. Stronger importance of the amount of forest habitat and diversity of potential seed fluxes may be masked by their correlations with precipitation gradients in the study area. Our results emphasize the need of a wide scale approach to forest planning in order to be able to account for and to influence determinant processes of diversity patterns from the stand to the landscape levels. (C) 2012 Elsevier B.V. All rights reserved.