TREE CANOPY EFFECTS ON COMMUNITY STRUCTURE IN SLOPES OF MEDITERRANEAN RANGELANDS

In the dehesa systems there are considerable differences in community structure between grassland situated under trees and in the open spaces between them. This difference is superimposed by another one based on a trophic gradient in the form of sectors which develop along slopes. We studied these differences and our results show that: 1. Community diversity decreased and spatial heterogeneity increased from the higher to the lower sectors of the slope gradient. 2. These trends were significantly smaller under trees. 3. In the open spaces there was a contrast in community structure; fine grain in high sectors and coarse grain in the lower ones. 4. Community heterogeneity in pares of consecutive sectors of the slope gradient was lower under trees than in the open spaces. That means that trees act as a homogenizing factor on slopes, maintaining a basic herbaceous structure which tends to be similar in various sectors. Steady conditions are thus created, which influence both the same sector of different slopes as well as various sectors of each of the slopes. The alternation of one relatively uniform structure (under trees) with another much more variable one (open spaces) is considered as essential for the functioning of the system; just as it has developed following the traditional land-use practices in the area.