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

Authors: Porensky, Lauren; MCGEE, RACHEL - Us Forest Service (FS); PELLATZ, DAVID - Thunder Basin Grasslands Prairie Ecological Association

Submitted to: Global Ecology and Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/2020
Publication Date: 9/21/2020
Publication URL: https://handle.nal.usda.gov/10113/7109072
Citation: Porensky, L.M., McGee, R.A., Pellatz, D.W. 2020. Long-term grazing removal increased invasion and reduced native plant abundance and diversity in a sagebrush grassland. Global Ecology and Conservation. 24. Article e01267. https://doi.org/10.1016/j.gecco.2020.e01267.
DOI: https://doi.org/10.1016/j.gecco.2020.e01267

Interpretive Summary: Effects of long-term grazing removal on vegetation are highly variable across ecosystems. The effects of grazing removal on invasive species remain poorly understood, despite their importance for conservation and management. The grasslands of North America’s western Great Plains can tolerate large herbivore grazing and resist invasion, while shrublands of the Intermountain West are more sensitive to grazing and at higher risk of invasion. These two ecosystems intersect in northeast Wyoming, where a diverse plant community supports a wide array of sensitive wildlife. In 2016, we measured effects of long-term grazing removal on vegetation and soil properties in this ecosystem. Compared to adjacent grazed sites, exclosures had lower abundances of drought-resistant, warm-season native perennial grasses, fewer native species, and nearly three times as much cover of invasive annual grasses. Long-term livestock removal did not affect live sagebrush shrub abundance, but led to taller herbaceous vegetation structure than grazed sites. In the western Great Plains, and apparently also in the boundary zone separating this system from the sagebrush steppe, plant communities appear well-adapted to grazing by large herbivores. The lack of this 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 levels of plant community variability, which in turn will help conserve high biodiversity and ecosystem function.

Technical 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. Effects of grazing removal on invasive species remain poorly understood, despite their importance for conservation and management. Grasslands in North America’s western Great Plains can tolerate large herbivore grazing and resist invasion, while shrublands of the Intermountain West are more invasible and sensitive to grazing. These two ecosystems intersect in northeast Wyoming, where a diverse ecotonal plant community supports a wide array of sensitive wildlife. 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 native perennial grasses, fewer native species, and nearly three times as much cover of invasive annual grasses. Long-term livestock removal did not affect live sagebrush shrub cover, density or size, but led to taller herbaceous vegetation structure than grazed sites. Our 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.