Grazing impact on structure and dynamics of bare soil areas in a Patagonian grass-shrub steppe

Abstract

Bare soil is an idiosyncratic and important feature of arid ecosystems that can be affected by human land use such as domestic grazing intensification. Here we focused on the study of individual bare soil areas (BSA) and their long-term dynamics using DINVEG (spatially-explicit simulation model). In Patagonian rangelands we photographed BSA in grazed and ungrazed stands (n = 50). Compared with exclosure, grazing increased the size of the BSAs (17%, p < 0.05) and produced less irregular forms (13%, p < 0.05). The number of plants that delimit BSAs were not different (~9), but under grazing the sum of spaces among plants (BSA opening) was 15% higher than in exclosure (p < 0.05). The difference in wind speed (center-edge of the BSA) increased 70% under grazing. Wind difference was poorly correlated with the BSAs structure variables. Simulations (200 years) with DINVEG indicated that grazing decreased the likelihood of BSAs being colonized by grasses and increased the proportion of bare soil cells. Our study suggests that in these Patagonian steppes domestic grazing promotes structural changes in individual BSA and in the transition to grass covered patches. Changes in wind speed may result from grazing effect at the whole vegetation mosaic since BSA features are not enough to explain differences.