A Repeat Survey of Vegetation Plots Established to Evaluate Grazing Effects at Saguaro National Park, 1976 and 2007

Swann, Don

Domestic livestock grazing has occurred in southern Arizona for several centuries, and in what is now the Rincon Mountain District of Saguaro National Park for most of the period from the 1870s to the 1970s. In 1976, shortly after the rescission of the grazing permit in the Tanque Verde allotment, NPS ecologist Warren Steenbergh initiated a long-term study, with both treatment (recently grazed) and control (historically grazed) plots, to examine the long-term implications of grazing on vegetative communities within this protected area. Initial results of this study were never published, but photos and data from the project were re-discovered at Saguaro National Park in 2005. In 2007 park staff relocated the original points and repeated the study. This report details an analysis from surveys of ten plots in 1976 and 2007. Our results indicate that during the past three decades, vegetation change has been significant on both the treatment and control plots. Overall, most plant lifeforms (grasses, sub-shrubs, vines, succulents including cacti) have increased in cover and density, but trees have decreased. The results suggest that the effect of removing cattle from the park has not been as great on vegetation as the effect of other environmental changes such as increased precipitation and warming temperatures during the 30-year period between 1976 and 2007.