SHEEP GRAZING AND PLANT COVER DYNAMICS OF A SHADSCALE COMMUNITY

Despite extensive coverage and long-term use, the extent to which shad scale [Atriplex confertifolia (Torr. & Frem.) Wats.] community dynamic s are driven by grazing rather than by climate and inherent plant char acteristics is unresolved. We analyze a 59-year data set from the Dese rt Experimental Range, southwestern Utah, with the objective of discri minating between grazing and non-grazing effects on cover dynamics. Ca nopy cover of (5 x 20 ft) 9.3 m(2) plots were estimated in 1935, 1958, 1969, 1975, and 1994. Treatments were time (5 dates), grazing (ungraz ed versus grazed), and season (spring versus winter). Time was signifi cant; total cover initially increased following release from uncontrol led grazing and improvement of climate (1935-58, P < 0.001), remained unchanged over the following dry interval (1958-69, P = 1.000), increa sed over a drier interval (1969-75, P < 0.001), and then decreased dra matically over the final wet period (1975-94, P < 0.001). Grazing was also significant (P < 0.001), but cover of ungrazed plots exceeded tha t 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. Eaten in Wats.) and others responding primarily to tim e (climate, longevity, etc., e.g., Indian ricegrass, Oryzopsis hymenoi des R & S). Similarly, seasonal effects were not universal. Shrub cove r initially increased and then declined dramatically while grass cover monotonically increased. Results suggest that release from uncontroll ed grazing coupled with improving climatic conditions were responsible for initial recovery of the community, but that over time, climate an d inherent plant traits (e.g., longevity, establishment ecology, etc.) became relatively more important. For total cover and for many indivi dual species, continued grazing affected the rate more than the direct ion of vegetation change.