COVER OF PERENNIAL GRASSES IN SOUTHEASTERN ARIZONA IN RELATION TO LIVESTOCK GRAZING

Tolerance of particular grasslands to the activities of domestic lives tock may depend on their historic association with native grazing anim als. Southwestern grama (Bouteloua) grasslands are floristically allie d to the North American Central Plains but lie outside the historic ra nge of the plains' principal ungulate grazer, Bison bison. We compared perennial grassland cover and species composition on eight sites tran sected by the boundary fence of a 3160-ba 22-year-old livestock exclos ure in a grama grassland in southeastern Arizona Total grass canopy co ver was greatest on the ungrazed portion of each of the eight sites. T wo short stoloniferous species (Hilaria belangeri and Bouteloua eriopo da) were the only taxa substantially more abundant on grazed quadrats overall. Among these and eight taller bunch grasses there was a strong positive correlation between potential height and response to release from grazing, with the three tallest species showing the greatest inc reases on ungrazed treatments (Bouteloua curtipendula, Bothriochloa ba rbinodis, and Eragrostis intermedia). Bouteloua gracilis, the most abu ndant grass in the region, showed an intermediate response to livestoc k exclusion. Grama grasslands at the Arizona site have changed more an d in different ways following livestock exclusion than those on the Ce ntral Plains of Colorado. Contributing factors may include: (1) greate r annual precipitation at the Arizona site, (2) the much larger size o f the Arizona livestock exclosure, and (3) the absence of extensive gr azing by native ungulates in the Southwest since the Pleistocene Lives tock grazing appears to be an exotic ecological force in these southwe stern grasslands, and one destructive of certain components of the nat ive flora and fauna.