HISTORIC VEGETATION CHANGE, MESQUITE INCREASES, AND CLIMATE IN SOUTHEASTERN ARIZONA

Long-term directional vegetation changes in the wild landscape of southeastern Arizona since the advent of major Anglo-American settlement in the 1870s are identified and their relation to climate variations is examined. Particular emphasis is placed on verifying the purported changes in the distribution of major vegetation types and the link between velvet mesquite (Prosopis velutina Woot.) increases and precipitation trends since the turn of the century. While there is little doubt that climatic oscillations have resulted in short-term fluctuations in vegetation, precipitation variations do not appear to be connected to any major directional vegetation change since 1870 in southeastern Arizona. In fact, no single trend is evident in regional precipitation during this period. Furthermore, there is no clear evidence for the upward displacement along a xeric-to-mesic gradient of any major vegetation type, and, except possibly for increases in woody xerophytes such as mesquite, all of the identified long-term vegetation changes appear to be of anthropogenic origin. Mesquite increases. however, are irregular, show no clear relation to precipitation variations, and are most likely the result of livestock grazing and/or fire exclusion.