|Holifield Collins, Chandra|
Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/26/2007
Publication Date: 4/4/2008
Citation: King, D.M., Skirvin, S., Holifield Collins, C.D., Moran, M.S., Biedenbender, S., Kidwell, M., Weltz, M.A., Diaz-Gutierrez, A. 2008. Assessing Vegetation Change Temporally and Spatially in Southeastern Arizona. Water Resources Research, Vol. 44, W05S15, doi:10.1029/2006WR005850. Interpretive Summary: Vegetation change has been occurring over the past 100 years in grasslands across the Sonoran and Chihuahan deserts in the Southwestern United States. There is a great deal of controversy about why these changes have been occurring (e.g. grazing, drought, fire, climate). As a result, end users such as cattle growers and producers, physical scientists, land management agencies and extension agents do not have the information needed to make informed decisions to protect the land and its resources. In this study, vegetation data, photography, and satellite imagery were examined to assess vegetation change and some of the factors behind its occurrence within a 150 km2 watershed in southeastern Arizona. This study showed that shrub encroachment did not appear to be occurring in the watershed over the past 100 years. We also found that while shrubs have suffered as a result of drought conditions over the last decade, grass cover has been maintained by fewer but larger, plants. Information of this nature may be beneficial to cattle growers and land management agencies as they develop management plans that help foster rangeland sustainability.
Technical Abstract: Vegetation species, cover, and photographic data have been collected at multiple grass- and shrub-dominated sites in 1967, 1994, 1999 and 2005 at the USDA 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. Drought effects on both vegetation types were apparent in Enhanced Vegetation Index (EVI) 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, probably on a geologic time scale. This work reaffirmed the value of maintaining long-term data sets for use in assessments of vegetation change.