EFFECTS OF GRAZING ON THE ABUNDANCE AND DIVERSITY OF ANNUAL PLANTS INCHIHUAHUAN DESERT SCRUB HABITAT

We assess the impact of release from cattle grazing on the abundance a nd diversity of both winter and summer annual plant communities at an upper Chihuahuan Desert scrub site in south-eastern Arizona. In contra st to previous studies, we found that removal of herbivores (cattle) h ad little impact on ephemeral plant assemblages at our site. The total number of summer annual individuals per quadrat did not differ signif icantly, but there were significantly more winter annual plants on ung razed quadrats. The number of species per quadrat, however, did not di ffer significantly between sites exposed to, or protected from, grazin g in either season. Of 79 annual species recorded (34 in winter, 45 in summer), only 2 species, 1 in each season, responded significantly to the removal of cattle: Stephanomeria exigua and Polygala tweedyi were more abundant on ungrazed plots. Three additional species, Eriastrum diffusum and Cryptantha micrantha in winter, and Mollugo cerviana (sum mer), approached statistical significance. Differences in the effect o f cattle grazing on annual plants between our results and those at oth er sites in the arid southwest most likely reflect differences in the speed of response by annuals in different areas. Comparisons of this w ith other studies underscores recent calls for studies at broader spat ial and greater temporal scales.