TREES AS A HOMOGENIZING FACTOR OF THE HERBACEOUS COMMUNITY STRUCTURE AND NUTRIENT CONTENT IN MEDITERRANEAN GRASSLANDS

The dehesa system consists of two subsystems, situated under the tree crowns and in the open spaces. They differ clearly in structure and fu nction. The locations under trees are less variable, tending to greate r similarity in species diversity, homogeneity, plant biomass, percent age of biomass of constituent families, and plant tissue nutrient cont ent (N, P, K, Na, Ca and Mg), thus showing a certain independence in r elation to the topographic situation. By contrast, the open spaces are very much influenced by the position they occupy in the slope gradien ts. It seems that homogenizing factors interact with others which tend to intensify heterogeneity. The less variable structures (under trees ) could be interpreted as agents pumping fertility from the subsoil an d presenting it for the use of herbaceous species. The most influenced structures are undoubtedly those situated under the tree crowns, but the whole system is influenced in this way. Consequently, topography a nd trees create a large number of habitats for herbaceous species, esp ecially due to the overlapping of both factors. The harmony of the com bination of the two factors gives stability to the system and has perm itted its survival, without degradation, for many centuries.