Impacts of Feral Horse Use on Herbaceous Riparian Vegetation Within a Sagebrush Steppe Ecosystem

Abstract

Feral horses inhabit rangeland ecosystems around the world, and their impacts on riparian ecosystems are poorly understood. We characterized impacts of a free-ranging horse population on the structure and composition of riparian plant communities in the sagebrush steppe ecosystem in the western United States. We used a randomized block design with single 25 × 50 m exclosures and grazed plots on four study sites within Sheldon National Wildlife Refuge in northwestern Nevada. Exclosures were constructed in 2008. Herbaceous plant utilization was measured from 2009 to 2013 by clipping within excluded and grazed plots. Herbaceous production and vertical structure were measured in 2013, and plant functional group and ground cover components were estimated in 2012–2013. Herbaceous utilization ranged from 27% to 84%, and herbaceous production did not differ by grazing treatment (P = 0.472). Grazed plots had seven-fold higher bare ground cover (P < 0.001), 60% less litter cover (P < 0.001), and the basal cover index was 65% higher. Grazing increased rush density by 50% (P = 0.041) but did not affect sedge density (P = 0.514). Grazing decreased herbaceous stubble height up to 80% and visual obstruction by about 70% (P < 0.05). Deep-rooted hydrophytic plant species did not increase with grazing exclusion, but greater vertical structure in excluded plots could improve hiding and nesting habitat for some riparian-associated wildlife species. Additionally, decreased bare ground with grazing exclusion could reduce erosion potential and susceptibility to invasive plant species.