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ARS Home » Pacific West Area » Burns, Oregon » Range and Meadow Forage Management Research » Research » Publications at this Location » Publication #316513
Title: Impacts of feral horse use on herbaceous riparian vegetation within a sagebrush steppe ecosystem

Author
item Boyd, Chad
item Davies, Kirk
item COLLINS, GAIL - Us Fish And Wildlife Service

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/9/2017
Publication Date: 7/1/2017
Citation: Boyd, C.S., Davies, K.W., Collins, G.H. 2017. Impacts of feral horse use on herbaceous riparian vegetation within a sagebrush steppe ecosystem. Rangeland Ecology and Management. 70(4):411-417. https://doi.org/10.1016/j.rama.2017.02.001.
DOI: https://doi.org/10.1016/j.rama.2017.02.001

Interpretive Summary: Feral horses inhabit rangeland ecosystems around the world, but their impacts on riparian ecosystems and associated vegetation are poorly understood. We conducted a five year field study using a randomized block design with grazing exclosures to characterize the impacts of a free-ranging horse population on the structure and composition of riparian plant communities in the sagebrush ecosystem of the western US. Results indicate that grazed plots had higher bare ground and density of rushes (Agrostis sp.), sedge (Carex sp.) density did not differ from ungrazed plots, and that shrub (Rosa woodsii (Lindl.)) density and vertical foliar structure were less in grazed plots. Our results suggest that density of deep-rooted herbaceous species important to riparian function did not increase with grazing exclusion, but greater vertical structure in excluded plots could strongly improve habitat value of these plant communities for riparian-associated wildlife species.

Technical Abstract: Feral horses inhabit rangeland ecosystems around the world, but 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 US. We used a randomized block design with 25 x 50 m exclosures and grazed plots on four study sites. Exclosures were constructed in 2008. Herbaceous plant utilization was measured from 2009 – 2013 by clipping within excluded and grazed plots. Herbaceous production and vertical structure were measured in 2013 and plant species abundance and ground cover components were estimated in 2012 – 2013. Herbaceous utilization ranged 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 65% more plant basal cover. Grazing increased rush density by 50% (p = 0.041) but did not affect sedge density (p = 0.514). Limited data suggest potential for increased shrub abundance with grazing exclusion. Grazing decreased herbaceous stubble height up to 80% and visual obstruction by about 70% (p < 0.05). At utilization levels encountered in this study, deep-rooted herbaceous species important to riparian function did not increase with grazing exclusion, but greater vertical structure in excluded plots could strongly improve habitat value of these plant communities for riparian-associated wildlife species. Additionally, decreased bare ground with grazing exclusion could reduce erosion potential and susceptibility to invasive plant species.