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

Abstract

In the Mediterranean region, semi-natural shrubland communities (named 'matorral') often present a discontinuous cover, where isolated perennial plants alternate with bare inter-plant areas. In such ecosystems, the patchy distribution of the vegetation is usually associated with microtopographic sequences of mounds that develop under isolated plants and break the overall slope continuity. In this study, the influence of three representative species of the Mediterranean matorral (Rosmarinus officinalis, Stipa tenacissima and Anthyllis cytisoides) on slope microtopography is determined and the processes that take part in the development of microtopographic structures beneath the plant canopy are identified. The influence of slope gradient, plant species and plant parameters on the shape and height of microtopographic structures is also studied. The shape of the microtopographic structures is described by using a two-dimensional microprofilemeter and mound height is determined by measuring in the field a 'mound height index' defined as the distance from the top to the 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 structures generally develop under Anthyllis shrubs, microtopographic forms associated with Rosmarinus and Stipa plants vary with slope gradient. The almost symmetric mound-type structures that develop under these two species on gentle slopes change into terrace-type structures as slope gradient increases. Moreover, statistically significant differences exist between the three species with regard to mound height. Mean values of mound height are 19.4, 14.6 and 4.3 cm under the canopy of Stipa, Rosmarinus and Anthyllis respectively. Plant parameters, essentially roughness, and slope gradient have a significant influence on mound height index. Four main processes were identified as affecting mound development in the studied field site: Sedimentation, differential interrill erosion, differential splash erosion and bioturbation. Plant species interact in different ways with these processes according to their morphologies. Since Stipa and Rosmarinus plants are more efficient than Anthyllis shrubs in controlling water erosion, in retaining sediments and in modifying soil properties under their respective canopies, they give rise to higher microtopographic structures that facilitate water and nutrient storage by plants on slopes.