Controls on plant functional surface cover types along a precipitation gradient in the Negev Desert of Israel

We studied the controls on functional surface cover types in four catchments along a semi-arid to arid precipitation gradient in the northern Negev Desert of Israel. First, we selected four functional types, based on their unique water use and redistribution functionality: shrubs, Asphodelus ramosus, other herbaceous plants and surface crusts. We estimated percentage of surface covered by these functional types and by bedrock outcrops and loose surface stones. Additionally data was collected on soil depth, relative elevation, insolation, slope, curvature, and overlain with surface cover maps. Relations between functional types and landscape structure variables were analyzed with descriptive statistics. The landscape structure variables bedrock, relative elevation, soil depth and surface stones explained most of the cover variance in the catchments. In catchments with many bedrock outcrops, functional types were best explained by the landscape structure variables. In catchments with homogeneous soils reaching beyond the root zone, the biological interactions between functional types were more important. Along the precipitation gradient the explanatory power of the biological variables decreased with decreasing precipitation, while the explanatory power of landscape structure variables appeared unrelated. Only in homogeneous semi-arid catchments regular vegetation patterns can develop, in arid and heterogeneous catchments irregular patterns dominate.