Site productivity and plant size explain the response of annual species to grazing exclusion in a Mediterranean semi-arid rangeland

1 The response of an annual plant community to protection from grazing as a function of variation in site productivity was studied in a semi-arid Mediterranean rangeland in Israel over 4 years (1996-99). The abundance of species was compared in grazed vs. ungrazed plots (exclosures) in four neighbouring topographic sites (south- and north-facing slopes, hilltop and Wadi shoulders), representing a gradient of resource availability and productivity. 2 Above-ground potential productivity at peak standing crop in spring (i.e. inside exclosures) varied considerably between years and topographic sites. Productivity was similar among the hilltop, south- and north-facing slopes, and was typical of semi-arid ecosystems (10-200 g(-2)). Productivity in the Wadi was consistently greater (up to 700 g(-2)) and reached the range of subhumid grassland ecosystems. 3 The effect of grazing exclusion on the composition of the annual vegetation was productivity-dependent. Lower similarity (Sorenson's quantitative similarity index) between grazed and ungrazed subplots was observed in the productive Wadi compared with the less productive sites. The small-scale variation in grazing impact on species composition, due to differences in productivity, is consistent with models predicting similar trends in perennial grasslands across larger scale gradients. 4 The relationship between plant size (above-ground dry-weight), site productivity and response to fencing was analysed for the 36 most abundant annual species. Large species were more abundant in more productive sites, and small species at lower productivity, although few species were restricted to particular productivity levels. The response of individual species to protection from grazing was productivity dependent, with plant size playing a central role. Larger species tended to increase and small ones to decrease in abundance after fencing, with a mixed response in species with intermediate size. 5 A conceptual model is presented relating the response to protection from grazing along gradients of productivity to species plant size.