Trampling routes modified size distribution of Sarcopoterium spinosum in semiarid rangelands

•Studying the spatial association of trampling routes and Sarcopoterium spinosum•Shrubs located close to a trampling route were bigger than more distant ones.•The shrubs of the southern aspect had higher values height, area, volume.•Flocks of sheep and goats engineer water conditions.•A strip-like shrub pattern along the routes of big shrubs was manifested.

The presence of livestock creates a “three-phase-mosaic” pattern of dominant surface patches: shrubs, trampling routes, and intershrub areas. These patches differ in soil properties, runoff generation, and sediment production; runoff yield is highest in the routes. In light of knowledge that the productivity of arid and semiarid ecosystems is controlled primarily by water dynamics, the spatial correlation between flock trampling routes and the size of the Sarcopoterium spinosum shrubs was studied on north- and south-facing hillslopes in the northern Negev of Israel. The shrubs were divided into two categories, according to their location relative to the trampling route above them: close to the route (CR), i.e., within 50 cm, and far from the route (FR), i.e., further than 50 cm. More shrubs per unit of area (FR and CR together) were located on the north-facing hillslopes, and they had lower height and canopy area than those on the south-facing ones. Sarcopoterium spinosum shrubs adapt to the mesoclimatic conditions, which are determined by the aspect: on the wet north aspect they grow in much greater numbers but are smaller than those on the dryer south. On the former aspect, the soil water content is high, which accounts for the large number of shrubs; however, their small size possibly results from competition for soil nutrients. On the latter aspect, the more arid conditions promote the activity of water-enriching mechanisms which accounts for their small number and large size. On both south- and north-facing hillslopes most of the shrubs were located close to and downslope from a trampling route, and were bigger than more distant ones. Flocks of sheep and goats engineer water conditions that favor enhancing the growth of shrubs close to and downslope from the route; and this is manifested in a strip-like shrub pattern along the routes with predominantly horizontally oriented lines of bigger shrubs.