Effects of Livestock Grazing Exclusion on Perennial Grasses and Woody Species on the San Pedro Riparian National Conservation Area, Arizona

Abstract

The objective of this study was to determine the effects of livestock exclusion on native and non-native perennial grass richness and frequency, and woody species frequency on the San Pedro Riparian National Conservation Area. A significant increase in species richness of native perennial grass species was noted overall and in retired agricultural fields, supporting the hypothesis of increased species richness with grazing exclusion. There was a significant increase in the number of plots that contained an increase in bristlegrass (Setaria macrostachya Kunth and S. leucopila (Scribn. & Merr.) K. Schum.). No significant change in species richness of non-native invasive perennial grass species was observed. Lehmann lovegrass (Eragrostis lehmanniana Nees) had a significant increase in the number of plots that contained the species, supporting the hypothesis that Lehmann lovegrass expansion would occur in previously uninvaded areas even without current livestock grazing. Velvet mesquite (Prosopis velutina Wooton) exhibited a significant increase in canopy frequency, but velvet mesquite basal frequency showed no significant increase or decrease. Results indicate that past grazing was a primary factor leading to the current distribution and density of mesquite on the SPRNCA because the same basal (i.e. rooted stem) frequency existed prior to and after grazing exclusion. Management considerations include continued grazing exclusion, inventory and monitoring of pollinator plants, continued monitoring of pace frequency transects, use of prescribed fire in uplands, monitoring of soil erosion, and implementation of erosion controls.