Sheep grazing and plant cover dynamics of a shadscale community.

Abstract

Despite extensive coverage and long-term use, the extent to which shadscale [Atriplex confertifolia (Torr. &Frem.) Wats.] community dynamics are driven by grazing rather than by climate and inherent plant characteristics is unresolved. We analyze a 59-year data set from the Desert Experimental Range, southwestern Utah, with the objective of discriminating between grazing and non-grazing effects on cover dynamics. Canopy cover of (5 x 20 ft) 9.3 m2 plots were estimated in 1935, 1958, 1969, 1975, and 1994. Treatments were time (5 dates), grazing (ungrazed versus grazed), and season (spring versus winter). Time was significant; total cover initially increased following release from uncontrolled grazing and improvement of climate (1935-58, P < 0.001), remained unchanged over the following dry interval (1958-69, P = 1.000), increased over a drier interval (1969-75, P < 0.001), and then decreased dramatically over the final wet period (1975-94, P < 0.001). Grazing was also significant (P < 0.001), but cover of ungrazed plots exceeded that of grazed plots only in 1975 and 1994 (P less than or equal to 0.033). The 6 dominant species showed varying responses to grazing and time, with some responding primarily to grazing (e.g., budsage, Artemisia spinescens D.C. Eaton in Wats.) and others responding primarily to time (climate, longevity, etc., e.g., Indian ricegrass, Oryzopsis hymenoides R. &S.). Similarly, seasonal effects were not universal. Shrub cover initially increased and then declined dramatically while grass cover monotonically increased. Results suggest that release from uncontrolled grazing coupled with improving climatic conditions were responsible for initial recovery of the community, but that over time, climate and inherent plant traits (e.g., longevity, establishment ecology, etc.) became relatively more important. For total cover and for many individual species, continued grazing affected the rate more than the direction of vegetation change.