Long-term grazing removal increased invasion and reduced native plant abundance and diversity in a sagebrush grassland

Abstract

Effects of long-term grazing removal on vegetation are highly variable across ecosystems and grazing contexts. In some cases, long-term exclosures can be used to enhance vegetation production, plant diversity, or wildlife habitat. In other situations, exclosures can become more invaded by undesirable species, less drought-tolerant, or less diverse than grazed sites. Grasslands in North America’s semi-arid Great Plains prairies can tolerate large herbivore grazing and resist invasion, while cold desert shrublands of the Intermountain West are more invisible and sensitive to grazing. These major ecoregions intersect in northeast Wyoming, where a diverse ecotonal plant community supports a wide array of sensitive wildlife. The effects of grazing removal on vegetation and invasive species in this ecotone are poorly understood, despite clear implications for conservation and management. In 2016, we measured effects of long-term (>49 year-old) livestock exclosures on vegetation and soil properties in the ecotone. Compared to adjacent grazed sites, exclosures had 72% less foliar cover of drought-resistant, warm-season perennial grasses, fewer native species, and nearly three times as much invasive annual grass cover. Long-term livestock removal did not affect live sagebrush shrub cover, density or size, but led to taller herbaceous vegetation structure. Findings suggest that although long-term grazing removal may be useful for achieving taller structure, light to moderate levels of large herbivore grazing may be necessary to resist invasion and maintain ecosystem integrity in this ecotone. In the context of ongoing global change, it will be critical to maintain natural disturbance regimes such as grazing in ecosystems that evolved with disturbance. In these systems, a total lack of natural disturbance does not represent a reasonable target or reference state. Instead, management for a range of disturbance frequencies and intensities across the landscape will likely support high vegetation heterogeneity, which in turn will help conserve high biodiversity and ecosystem function.