EDAPHIC VARIABILITY AND FLORISTIC STRUCTURE ON MEDITERRANEAN GRASSLAND SLOPES

One of the most important characteristics of grassland communities is their strong dynamic nature. This is accentuated in the case of slopes and allows distinctions among well-differentiated enclaves as regards structure and organization. In relation to slopes, this study first a ttempts to establish whether the edaphic variables governing the veget ation structure of uplands (exportation sectors) are the same as those governing the vegetation structure on lowlands (deposition sectors). A second aim is to determine whether the apparent similarity between t he floristic structure of uplands on granites and on slates, and that of lowlands on granites and on slates, is real. Finally, the issue of which other factors, besides the edaphic variables, might affect the s tructure of the vegetation is addressed, taking into account the lives tock-raising regime that these communities support. With these objecti ves, 30 typical Mediterranean slope grasslands (province of Salamanca, Spain) were chosen. Half of these grasslands were situated on granite s, and the other half on slates. Floristic sampling and edaphic analys es were carried out on the uplands and the lowlands of each slope. A b alanced representation (15 sites in each case) of upland on granites, upland on slates, lowland on granites, and lowland on slates was found . The results show that differences exist between upland and lowland s ites as to which edaphic variables are of greater influence, since in the former the granulometric composition of the soil predominates, whe reas in the latter organic matter, N and P, are more important. Within a particular topographic position, the sites on granites are always p oorer than those situated on slates, which implies greater local diver sity (alpha diversity) and less heterogeneity over large spaces. Lastl y, despite the great importance of the soil in governing vegetation, g razing, competition for light, and the availability of water may be ot her factors that determine floristic composition and structure. The di fference between the structures of the vegetation on uplands and lowla nds basically depends on soil variables. The same is found on comparin g communities developed on granites and slates corresponding to the sa me topographic position, the overall situation being mediated by other environmental factors.