Reinterpreting historical data for evidence-based shrubland management

Authors: WILLIAMSON, JEB - New Mexico State University; Burkett, Laura; Bestelmeyer, Brandon; SKAGGS, RHONDA - New Mexico State University; Havstad, Kris

Submitted to: Natural Resources and Environmental Issue Series, Utah State University
Publication Type: Proceedings
Publication Acceptance Date: 12/1/2010
Publication Date: 1/15/2011
Citation: Williamson, J.C., Burkett, L.M., Bestelmeyer, B.T., Skaggs, R., Havstad, K.M. 2011. Reinterpreting historical data for evidence-based shrubland management. Natural Resources and Environmental Issue Series, Utah State University. Volume 17 (1): Article 18.

Interpretive Summary: We compared vegetation across the public lands of the northern Chihuahuan Desert at two points in time, the 1930s and 2005, using vegetation maps produced during the period of public lands grazing adjudication (1930s) and a recent map produced by the Jornada Experimental Range (2005). We asked if changes in vegetation state varied in interpretable ways among ecological sites. We found that (1) the bulk of the region was relatively stable during this period, (2) approximately the same amount of area experienced increased grass dominance as experienced increased shrub dominance, and (3) dynamics are strongly influenced by the properties of specific ecological sites. Major vegetation state changes, involving either increased grass dominance or increased shrub dominance, were common in 11 of 18 ecological sites within this study area. More important to management, significant increases in shrubs occurred within only four ecological sites. These sites were sandy, deep sand, shallow sandy, and gravelly sand. All other ecological sites within this region were relatively stable over the ~70 year period between observations. The results indicate that management and monitoring are usefully stratified by ecological site.

Technical Abstract: Long-term vegetation dynamics in the Chihuahuan Desert of southern New Mexico have been intensively studied for over a century, and interpretations of the broad scale drivers of these dynamics are numerous. We now understand that interpretation of spatially heterogeneous change requires a more nuanced, contextualized, and detailed understanding of edaphic features and landscape characteristics. Recently, state and transition models (STMs) have been employed to represent landscape-specific dynamics for each ecological site within a Major Land Resource Area (MLRA). We re-examined data characterizing vegetation across the public lands of the northern Chihuahuan Desert at two points in time, the 1930s and 2005. In this study, our objectives were to (1) develop geospatial data layers of historical and current vegetation states, (2) compare vegetation states between the 1930s and 2005 where the two data layers overlap, and (3) interpret any major vegetation state changes over this ~70 year period within the context of specific ecological sites. It was our hypothesis that ecological dynamics would vary in interpretable ways among ecological sites. Three primary observations are drawn from our results: (1) the bulk of the region was relatively stable during this period, (2) approximately the same amount of area experienced increased grass dominance as experienced increased shrub dominance, and (3) dynamics are strongly influenced by the properties of specific ecological sites. Major vegetation state changes, involving either increased grass dominance or increased shrub dominance, only occurred to any extent in 11 of 18 ecological sites within this study area. More important to management, significant increases in shrubs occurred within only four ecological sites. These sites were sandy, deep sand, shallow sandy, and gravelly sand. All other ecological sites within this region were relatively stable over the ~70 year period between observations. The obvious management implication is the importance of stratifying by ecological site prior to application of shrub control treatments.