The effects of riparian grazing exclosures on adjacent riverine ecosystems
Bunn, Victoria Jeanne
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In the western U.S., riparian ecosystems cover 1% of land area while supporting 70-80% of native species. 70% of this land area is available as range for livestock, who use riparian areas preferentially. Ecological concerns have led to numerous studies of the effects grazing has on these ecologically important, easily damaged ecosystems. Exclosure-based research has thoroughly examined the effects of livestock on riparian ecosystem health and function, but failed to investigate the potential for exclosures to intensify adverse effects of use at their boundaries, which could lead to overestimation of their benefits and impair their efficacy in management. This study attempts to supplement existing research by characterizing potential impacts, making exclosures a more informed and effective management strategy. Study exclosures were located on grazed public lands in southwest Montana. Riparian vegetative cover and channel morphology response variables were measured inside the exclosure and in two grazed reaches, one placed 0 to 20 m and one >50 m from the exclosure to capture differences in the spatial extent and severity of any impacts due to differences in livestock behavior caused by the exclosure's influence. Findings were that (1) results were consistent with previous exclosure studies using the same response variables to compare grazed and exclosed areas, (2) significant differences in herbaceous cover, bare ground cover, channel width, and bank angle between the two grazed subreaches were greater than corresponding grazed/exclosed comparisons, indicating that impacts to the subreach adjacent to the exclosure that are greater than corresponding improvements within, (3) spatially inconsistent impact zones within and among sites made it impossible to describe an overall impact zone adjacent to exclosures, but impacts to channel morphology, herbaceous cover, and bare ground occurred within 8 m of the exclosure, and within 2 m for bank angle, (4) data didn't support predicted relationships between impact severity and exclosure duration, size, or stocking rates, most likely due to the many other influencing factors that were not measured. Evidence supporting existence of exclosure-caused impacts should inform exclosure use and make it a more effective management tool, especially when considered in the context of how these impacts might encumber specific management goals.