The dynamics of vegetation-driven spatial heterogeneity (VDSH) and its function in structuring runoff and sediment fluxes have received increased attention from both geomorphological and ecological perspectives, particularly in arid regions with sparse vegetation cover. This paper reviews the recent findings in this area obtained from field evidence and numerical simulation experiments, and outlines their implications for soil erosion assessment. VDSH is often observed at two scales, individual plant clumps and stands of clumps.At the patch scale, the local outcomes of vegetated patches on soil erodibility and hydraulic soil properties are well established. They involve greater water storage capacity as well as increased organic carbon and nutrient inputs. These effects operate together with an enhanced capacity for the interception of water and windborne resources, and an increased biological activity that accelerates breakdown of plant litter and nutrient turnover rates. This suite of relationships, which often involve positive feedback mechanisms, creates vegetated patches that are increasingly different from nearby bare ground areas. By this way a mosaic builds up with bare ground and vegetated patches coupled together, respectively, as sources and sinks of water, sediments and nutrients.At the stand scale within-storm temporal variability of rainfall intensity controls reinfiltration of overland flow and its decay with slope length. At moderate rainfall intensity, this factor interacts with the spatial structure of VDSH and the mechanism of overland flow generation. Reinfiltration is greater in small-grained VDSH and topsoil saturation excess overland flow. Available information shows that VDSH structures of sources and sinks of water and sediments evolve dynamically with hillslope fluxes and tune their spatial configurations to them.Rainfall simulation experiments in large plots show that coarsening VDSH leads to significantly greater erosion rates even under heavy rainfall intensity because of the flow concentration and its velocity increase.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology. Abstract. A major role of facilitation between higher plant species, particularly in stressful environments, has recently been reported in several papers that suggest that beneficial interactions could be more important in the dynamics of plant communities than has been recognized to date. In a semiarid region in southeastern Spain we determined the effect of a leguminous shrub, Retama sphaerocarpa, on its environment, testing the hypothesis that facilitation by the shrub of one species of its understory, Marrubium vulgare, is reciprocal and that the shrub benefits from sheltering herbs beneath its canopy.Marrubium plants under Retama had greater specific leaf area, leaf mass, shoot mass, leaf area, more flowers, a higher nitrogen (N) concentration in leaf tissue, and more N per plant than isolated plants, suggesting a facilitative effect of Retama on Marrubium including increased availability of resources. Biomass of 1-yr-old cladodes, total biomass, total nitrogen content of 3-yr-old branches, and shoot water potential at midday were higher for Retama shrubs with Marrubium beneath them than in shrubs without Marrubium.Retama strongly improved its own environment, facilitated the growth of Marrubium and other species underneath its canopy, and at the same time obtained benefits from sheltering herbs underneath. The interaction between these two species was indirect, associated with differences in soil properties and with improved nutrient availability under shrubs compared with plants growing on their own.We propose that the mutual benefit of the association between Retama and Marrubium is best termed a facultative mutualism. Each partner benefits from greater availability of resources in the "island of fertility" that results from their association. If beneficial plantplant interactions as described here are widespread, positive relationships may have a major role in determining the pattern and structure of plant communities.

No abstract

No abstract

No abstract

No abstract

No abstract

Summary 1.A dense understorey of annual and perennial herbs grow under the canopy of Retama sphaerocarpa shrubs in semiarid environments of south-east Spain, influencing plant productivity and diversity at a regional scale. We investigated the facilitation by the shrub on its understorey in field and laboratory experiments with Barley designed to explore the mechanisms of interaction between both vegetation layers and their spatial variation. 2.There was a gradient of spatial heterogeneity in soil chemical fertility under the shrub canopy, with organic matter and soil nitrogen contents higher at the centre than at the edge of the canopy. Dry mass production of Barley was also higher in soils from intermediate positions, and lower in soils from both the centre and edge of the canopy. 3. In the field, pots sown with Barley placed near the centre, at an intermediate position and at the edge of the canopy of Retama shrubs showed significant differences in productivity, suggesting a mulching effect of the canopy that also affects seedling establishment. 4. Micro-climatic measurements showed significant differences in total radiation reaching the soil, mean air and soil temperatures and maximum temperature among different positions in the understorey, increasing radially from the centre to the edge of the canopy. 5. These results and field observations suggest that the optimal association of climatic factors under the canopy would combine with a high soil fertility mediated by litter decomposition to increase biomass production mid-way between the centre and the edge of the canopy. Overstorey and understorey thus interact to increase nutrient retention locally, which benefits both the shrub and the herb layer.