Grazing Impacts in Vegetated Dune Fields: Predictions From Spatial Pattern Analysis

Abstract

This study deals with the changes induced by grazing on soil erosion processes in vegetated dune fields of Península Valdés, in the Patagonia region of Argentina. We performed a spatial analysis to assess erosion features’ patterns. Blowouts, used as main indicators of aeolian erosion processes, as well as dune crests, which are susceptible to erosion, were mapped on aerial photographs and images from Landsat 7 Enhanced Thematic Mapper Plus, in eight paddocks under two grazing conditions: lightly (0.4 sheep ha-1) and heavily grazed (0.8 sheep ha-1). From the mapped locations of water points, crests, and blowouts we calculated a spatial statistic (O-ring statistic), which gives the expected intensity of blowouts within the area covered by crests as function of distance away from water points. Additionally, to explore if the density of crests around water points influences the density of blowouts, we estimated the intensity of dune crests in the neighborhood of water points and compared the densities of blowouts among water points with low, medium, and high densities of crests. For the heavy grazing treatment we found highly significant (P < 0.05) aggregation of blowouts around water points with peak densities threefold higher than expected at random occurring between 90 and 210 m. However, the aggregation was only weakly significant for the light grazing treatment and occurred only at distances of about 30 m away from the water point. We found that the impact of grazing on soil stability contrasted among sites with different sensitivity to accelerated erosion. In sites with a high density of dune crests close to water points, wind erosion becomes more intense and the density of blowouts increases. A more sustainable management of these rangelands depends primarily on the conservation of the soils. Therefore, the location of water points in sites not susceptible to accelerated soil erosion would represent a better management strategy of the dune fields of Península Valdés.