Assessing the rate and reversibility of large herbivore effects on community composition in a semi-arid grassland ecosystem

Authors: Wilmer, Hailey; Augustine, David; Derner, Justin; MILCHUNAS, DANIEL - Colorado State University

Submitted to: Journal of Vegetation Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/29/2020
Publication Date: 10/7/2020
Citation: Wilmer, H.N., Augustine, D.J., Derner, J.D., Milchunas, D. 2020. Assessing the rate and reversibility of large herbivore effects on community composition in a semi-arid grassland ecosystem. Journal of Vegetation Science. 32(1). Article e12934. https://doi.org/10.1111/jvs.12934.
DOI: https://doi.org/10.1111/jvs.12934

Interpretive Summary: In this study, we are interested in understanding how fast, how reversible, and how much plant species change on the shortgrass steppe of Colorado (North American Great Plains) when we add and take away grazing. To study this, we evaluated rates and magnitude of basal cover change in newly ungrazed and newly grazed sites (established in 1991), relative to change in a long-term (grazed and ungrazed) comparison treatments (established in 1939) over 25 years. Our findings documented rapid, reversible and largely symmetric effects of both adding and removing grazing between 1992-2017. This was primarily due to differences in the rate of increase in basal cover of cool season midgrasses, litter, and bare ground. This is important because effective adaptive management will need to consider the ways the effects of grazing can be reversible in a relatively symmetrical pattern and that they appear to be changing more rapidly in the early 21st century than was observed in the mid-late 20th century.

Technical Abstract: Questions: What are the rate, reversibility, and degree of symmetry in plant species compositional change in response to the addition and removal of cattle grazing in the shortgrass steppe? Specifically, how does the imposition and removal of grazing affect the abundance of perennial C4 shortgrasses and C3 midgrasses that are of primary importance for livestock production in the region? Location: Shortgrass steppe, northeastern Colorado, USA, in the North American Great Plains. Methods: We evaluate rates and magnitude of basal cover change in newly ungrazed and newly grazed sites (established in 1991), relative to change in a long-term (grazed and ungrazed) comparison treatments (established in 1939) over 25 years. We also compare shifts in species basal cover in newly implemented treatments relative to baseline community composition measured at the start of the study. Results: Unlike the limited change observed in long-term treatments between 1939-1991, we documented more rapid, reversible and largely symmetric effects of both the imposition and removal of grazing between 1992-2017. This was primarily due to differences in the rate of increase in basal cover of C3 midgrasses, litter, and bare ground. However, the rate and direction of change differed when assessed relative to long-term and (early treatment) baseline cover data. Conclusions: Studies of plant community change in response to large herbivore grazing under varying ecological conditions and management regimes are needed to effectively guide the development and revision of state-and-transition models (STMs) for rangeland managers, and to better detect and plan for dynamic ecological conditions in rangelands. Effective adaptive management and STM development under a changing climate should recognize that effects of grazing on shortgrass steppe can not only be reversible in a relatively symmetrical pattern but appear to be changing more rapidly in the early 21st century than was observed in the mid-late 20th century.