The influence of herbivory on plant cover and species composition in the Pryor Mountain Wild Horse Range, USA

We investigated the effects of short- and long-term ungulate grazing on pla nt species cover and composition in arid lowland and more mesic upland comm unities of the Pryor Mountain Wild Horse Range (PMWHR). Measurements were t aken over two years which differed significantly in growing season precipit ation. Interannual differences in plant cover were significantly greater th an differences between grazed and ungrazed communities. In the arid lowland s total plant cover decreased from 47% in 1993, a relatively wet year, to 2 9% in 1994, a relatively dry year. In the more mesic uplands total plant co ver decreased from 107% in 1993 to 56% in 1994. The magnitude of change in cover was greatest in the grasses, especially for Pseudoroegneria spicata, the most abundant species in the lowland communities, which decreased from 21% cover in 1993 to 11% in 1994. There was not a consistent effect of herb ivory on plant cover across sites, but its effects, particularly on the dom inant perennial grasses, were conspicuous at some sites. For instance, in t he lowlands cover of P. spicata was 3-12% in long-term grazed sites and 9-2 8% in short- to long-term ungrazed sites. Our study indicates that abiotic factors (e.g., precipitation) are more likely than grazing to affect abunda nces of key plant species, and hence ecosystem dynamics, in the PMWHR, and that the effects of herbivory are more localized and more prevalent in the lowland grasses than in the other plant functional groups.