Using historic data to assess effectiveness of shrub removal in southern New Mexico

In the late 1930s, the presence of a highly organized labor force, the Civilian Conservation Corps (CCC), in the Jornada Basin of southern New Mexico provided the capability for rangeland scientists to conduct experiments to determine the effectiveness of various techniques for remediating or reversing the encroachment of shrubs into grasslands. Unfortunately, soon after the treatments were performed, the CCC disbanded and most records of the treatments were lost. Despite sketchy documentation, some rangeland treatments left legacies on the landscape, and effects on water retention, erosion, and vegetation dynamics remained long after the CCC work ended. The discovery of historical documents from long-closed files and aerial photography in widely scattered archives allowed some of the experiments to be located and reexamined. Two research areas established in the mid-1930s were of particular interest, namely a tarbush (Flourensia cernua DC.) site where shrubs were grubbed and quadrats established and a creosote (Larrea tridentata [Sesse & Moc. ex DC.] Coville) site where the creosote and tarbush shrubs were grubbed. Here we outline how these sites were rediscovered, how historical measurements were repeated for the first time since the late l930s, and conclusions drawn regarding specific rangeland remediation strategies and vegetation dynamics. Our results show that shrub populations recovered from a radical removal treatment in less than 65 years. Remediation of these sites so that grass will recover to pre-shrub-dominated amounts will require measures additional to just removal of shrubs in order to restore hydrologic function. The fact that we were able to relocate, revisit, and resample these treatment areas provided unique opportunities to understand the long-term vegetation dynamics of these arid ecosystems. It is evident that woody plant populations have a high degree of resilience, that density dependence or interference appears to limit plant size in arid shrub communities, and that shrub populations had not reached any stable equilibrium state at the time of treatment in the 1930s. These insights would have been impossible to gain from short-term studies and without long-term studies initiated in the 1930s combined with recent discoveries of original documentation and historical aerial photography.