Overgrazing erodes the sink role of vegetated mounds in semiarid hillslopes

Abstract

Feedback mechanisms between vegetation and surface water and sediment flows are crucial for semiarid ecosystem dynamics. Vegetation in semiarid regions forms in patches, relying on runoff from bare-soil interpatches, but increased hydrological connectivity from vegetation loss can disrupt this balance. Microtopography influences these dynamics by shaping water and sediment flow patterns. However, limited empirical knowledge exists on the effects of microtopographic changes on vegetation performance. We examined the relations between the size of vegetated soil mounds, the soil organic carbon (SOC) in mounds and inter-mound areas, and the stress of the plants on the mounds for two grazing pressures in semiarid shrublands in Cyprus. We found that mound size increased with plant size, but plant size decreased with grazing pressure due to goat browsing. We thus normalized the mound size by the plant canopy size to study the variation in mound size related to the potential for sediment redistribution. Soil fertility and plant stress varied with the normalized mound size but grazing pressure altered these relations. In the lower-grazed site, larger mounds (relative to plant size) increased SOC in mounds, whereas in higher-grazed sites, they led to decreased SOC in both mound and inter-mound areas, as well as higher plant stress. Combined, our results suggest that larger mounds in low-grazed sites enhanced resource retention while larger mounds in high-grazed sites reduced resource availability and vegetation performance. These findings highlight a dual role of microtopography in dryland ecosystem functioning, especially under conditions of high erosion and grazing pressure. The role of mound development on plant performance in degrading hillslopes has been so far overlooked and deserves further investigation.