Leveraging Historic Cattle Exclosures to Detect Evidence of State Change in an Arid Rangeland

Abstract

Grazing, a defining use of rangelands, can have immediate and legacy impacts in ecosystems with short evolutionary histories of grazing and where climatic conditions are marginal and variable. In arid rangelands of the Colorado Plateau, domesticated ungulates were introduced in the 1800s at high densities and grazing has been maintained in the region at variable stocking rates for approximately the past 150 yr. Historic grazing practices, combined with a marginal climate, may have caused an irreversible shift in the vegetation community leading to limited recovery with cessation of grazing. Using three exclosures ranging from 12 to 50 yr of age, we evaluated vegetation composition, diversity, and cover inside the exclosures and compared them with directly adjacent, actively grazed areas outside of the exclosures in the fall and spring of 2018 and 2019. We then asked 1) How do exclosures and their adjacent unexclosed sites differ in vegetation diversity, composition, and cover? and 2) Are there significant differences in cover of key forage species in exclosed versus unexclosed areas? Overall, there were significant differences in vegetation communities inside and outside of exclosures in both seasons. Composition, alpha, and beta diversity differed between grazed and exclosed sites, with higher alpha and beta diversity in the 50-yr exclosure in the fall. Composition was significantly different between exclosures and unexclosed sites at all sites and in all seasons. Cover of several important forage grasses (Achnatherum hymenoides, Hilaria jamesii, Elymus elymoides) was higher in exclosed areas, while cover of Scleropogon brevifolius and Bouteloua gracilis was higher in unexclosed areas. Taken together, these results indicate limited recovery of vegetation in the absence of grazing, with exclosures exhibiting higher plant diversity, as well as cover of some desirable forage species, suggesting this area has not undergone irreversible state change but recovery may require > 50 yr of grazing cessation.