Soil depth as determinant of stuctural trends in Mediterranean grassslands with abundant slaty outcrops

Much attention has been given to the spatial organisation of Mediterranean grasslands in Spain. However, in general such descriptions refer to the most typical grasslands and to a large extent those with strongly oligotrophic characteristics have been overlooked. Many surfaces feature rocky outcrops and areas with little top soil. The present study addresses areas in which slate is an abundant surface feature, giving rise to an apparently chaotic landscape since the soil depth may vary considerably over quite short distances. Soil depth, however, is not the only variable considered in such areas. An undulated relief in the form of slopes should theoretically propitiate more gradual variations in pedological characteristics, species composition and vegetation cover. The application of correspondence analysis to the 113 plot samples permits a partial distinction between both gradients, even though these do overlap, although soil depth is the principal sequence that seems to dominate the overall ensemble. Additionally, the presence of outcrops alters certain soil components because such outcrops act as zones of retention of nutrients and fine fractions. In particular phosphorous is strongly affected by this type of terrain configuration. The relationships between soil depth and vegetation cover are direct and progressive. Diversity in contrast is maximum at intermediate depths, centred around 16-20 cm. This coincides with earlier hypotheses suggesting that the greatest diversity 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 bimodality in soil depth and vegetation diversity relationships. This trend is due to the presence of a second peak of high diversity in more eutrophic conditions where the slopes give way to true river plain zones. These areas feature contact zones which possibly depend on the fluctuations in the water table