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 treecrowns and in the open spaces. They differ clearly in structure andfunction. The locations under trees are less variable, tending togreater similarity in species diversity, homogeneity, plant biomass,percentage of biomass of constituent families, and plant tissue nutrientcontent (N, P, K, Na, Ca and Mg), thus showing a certain independence inrelation to the topographic situation. By contrast, the open spaces arevery much influenced by the position they occupy in the slope gradients.It seems that homogenizing factors interact with others which tend tointensify heterogeneity. The less variable structures (under trees)could be interpreted as agents pumping fertility from the subsoil andpresenting it for the use of herbaceous species. The most influencedstructures are undoubtedly those situated under the tree crowns, but thewhole system is influenced in this way. Consequently, topography andtrees create a large number of habitats for herbaceous species,especially due to the overlapping of both factors. The harmony of thecombination of the two factors gives stability to the system and haspermitted its survival, without degradation, for many centuries.