干旱区生态环境知识资源中心

Vegetation species cover and photographic data have been collected at multiple grass- and shrub-dominated sites in 1967, 1994, 1999, and 2005 at the U. S. Department of Agriculture Agricultural Research Service Walnut Gulch Experimental Watershed (WGEW) in southeastern Arizona. This study combines these measurements with meteorological and edaphic information, as well as historic repeat photography from the late 1880s onward and recent satellite imagery to assess vegetation change at WGEW. The results of classification and ordination of repeated transect data showed that WGEW had two main vegetation structural types, shrub dominated and grass dominated. Spatial distribution was closely linked to soil type and variations in annual and August precipitation. Other than the recent appearance of Eragrostis lehmanniana (Lehmann lovegrass) at limited sites in WGEW, little recruitment has taken place in either shrub or grass vegetation types. Effects of recent drought on both vegetation types were apparent in both transect data and enhanced vegetation index data derived from satellite imagery. Historic photos and a better understanding of WGEW geology and geomorphology supported the hypothesis that the shift from grass- to shrub-dominated vegetation occurred substantially before 1967, with considerable spatial variability. This work reaffirmed the value of maintaining long-term data sets for use in assessments of vegetation change.