SOIL DEPTH AS DETERMINANT OF STRUCTURAL TRENDS IN MEDITERRANEAN GRASSLANDS WITH ABUNDANT SLATY OUTCROPS

Much attention has been given to the spatial organization of Mediterra nean grasslands in Spain. However, in general such descriptions refer to the most typical grasslands and to a large extent those with strong ly oligotrophic characteristics have been overlooked. In this sense, m any surfaces feature rocky outcrops and areas with little top soil. Th e present study addresses slaty areas in which slate is an abundant su rface feature, giving rise to an apparently chaotic landscape since th e soil depth may vary considerably over quite short distances. Soil de pth, however, is not the only variable considered in such areas. An un dulated relief in the form of slopes should theoretically propitiate m ore gradual variations in pedological characteristics, species composi tion and vegetation cover. The application of correspondence analysis to the 113 plot samples permits a partial distinction between both gra dients, even though these do overlap, although soil depth is the princ ipal sequence that seems to dominate the overall ensemble. Additionall y, the presence of outcrops alters certain soil components because suc h outcrops act as zones of retention of nutrients and fine fractions. In particular phosphorus is strongly affected by this type of configur ation of the terrain. The relationships between soil depth and vegetat ion cover are direct and progressive. This is not the case of diversit y, which is maximum at intermediate depths, centred around 16-20 cm. T his coincides with earlier hypotheses suggesting that the greatest div ersity occurs in intermediate-low conditions of resource availability. Thus, apart from aspects common to many types of grasslands there are other peculiar properties, including a certain trend towards bimodali ty in soil depth and vegetation diversity relationships. This trend is due to the presence of a second peak of high diversity in more eutrop hic circumstances where the slopes give way to true river plain zones. These areas feature contact zones which possibly depend on the fluctu ations in the water table.