Mound development as an interaction of individual plants with soil, water erosion and sedimentation processes on slopes

in the Mediterranean region, semi-natural shrubland communities (named 'mat orral') often present a discontinuous cover, where isolated perennial plant s alternate with bare inter-plant areas. In such ecosystems, the patchy dis tribution of the vegetation is usually associated with microtopographic seq uences of mounds that develop under isolated plants and break the overall s lope continuity. In this study, the influence of three representative species of the Mediter ranean matorral (Rosmarinus officinalis, Stipa tenacissima and Anthyllis cy tisoides) on slope microtopography is determined and the processes that tak e part in the development of microtopographic structures beneath the plant canopy are identified. The influence of slope gradient, plant species and p lant parameters on the shape and height of microtopographic structures is a lso studied. The shape of the microtopographic structures is described by using a two-di mensional microprofilemeter and mound height is determined by measuring in the field a 'mound height index' defined as the distance from the top to th e bottom of the mound. The results obtained show that plant species play a major role in the shape and height of the microtopographic structures. Whereas terrace-type struct ures generally develop under Anthyllis shrubs, microtopographic forms assoc iated with Rosmarinus and Stipa plants vary with slope gradient. The almost symmetric mound-type structures that develop under these two species on ge ntle slopes change into terrace-type structures as slope gradient increases . Moreover, statistically significant differences exist between the three s pecies with regard to mound height. Mean values of mound height are 19.4, 1 4.6 and 4.3 cm under the canopy of Stipa, Rosmarinus and Anthyllis respecti vely. Plant parameters, essentially roughness, and slope gradient have a si gnificant influence on mound height index. Four main processes were identified as affecting mound development in the s tudied field site: sedimentation, differential interill erosion, differenti al splash erosion and bioturbation. Plant species interact in different way s with these processes according to their morphologies. Since Stipa and Ros marinus plants are more efficient than Anthyllis shrubs in controlling wate r erosion, in retaining sediments and in modifying soil properties under th eir respective canopies, they give rise to higher microtopographic structur es that facilitate water and nutrient storage by plants on slopes. Copyrigh t (C) 2000 John Wiley & Sons, Ltd.