Plant community composition after 75 years of sustained grazing intensity treatments in shortgrass steppe

Authors: Porensky, Lauren; Derner, Justin; Augustine, David; MILCHUNAS, DANIEL - Colorado State University

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/25/2018
Publication Date: 6/5/2017
Citation: Porensky, L.M., Derner, J.D., Augustine, D.J., Milchunas, D. 2017. Plant community composition after 75 years of sustained grazing intensity treatments in shortgrass steppe. Rangeland Ecology and Management. 70:456-464. doi:10.1016/j.rama.2016.12/001.

Interpretive Summary: Despite decades of research, ecologists are still struggling to understand how livestock grazing affects plant communities, especially in ecosystems with low and variable precipitation. Weather, grazing management details, and combinations of these factors can lead to conflicting results across studies, even those conducted within similar ecosystems. Long-term experimental data are needed to sort out the impacts of grazing while minimizing or accounting for weather-related variation. We capitalized on one of the longest-term grazing studies, with 75 years of sustained stocking rate treatments (none, light, moderate and heavy), to identify long-term effects of livestock grazing on plant community composition in northeastern Colorado. We found that after 75 years of season-long grazing, plant community composition was similar between moderately and heavily grazed pastures. Conversely, ungrazed and lightly grazed pastures experienced relatively large shifts in plant community composition, especially during the most recent 25 years. Light or no grazing was associated with increased abundance of cool-season grasses, as well as several weedy and invasive species. These findings support the long-term sustainability of livestock grazing in the shortgrass steppe. Not only is the shortgrass steppe tolerant of fairly high grazing intensities, it likely requires some level of grazing to resist invasion and maintain key ecosystem functions. Finally, some aspects of the plant community have shifted directionally across most grazing treatments during the past 25 years, and these shifts are consistent with the predicted effects of rising atmospheric carbon dioxide.

Technical Abstract: Plant community responses to livestock grazing lack conformity across studies, even those conducted within similar ecosystems. Variability in outcomes is often related to the strong influences of short-term weather patterns, mid-term climatic cycles, differences in the timing and intensity of grazing, or interactive effects of management, weather, and climate. For semiarid ecosystems characterized by low and highly variable precipitation, long-term experimental data are needed to determine the sensitivity of rangeland plant community composition to sustained grazing intensity treatments while minimizing or accounting for weather- and climate-related variation. However, long-term grazing intensity experiments, particularly experiments with more than two grazing intensity treatment levels, are quite rare. We capitalized on one of the longest-term grazing studies, with 75 years of sustained stocking rate treatments (none, light, moderate and heavy), to identify long-term effects of livestock grazing on plant community composition in shortgrass steppe. Plant community composition was similar between moderately and heavily grazed pastures after 75 years of continuous, season-long (May to October) grazing treatments, and heavy grazing did not extirpate cool-season perennial graminoids. Conversely, ungrazed and lightly grazed pastures experienced relatively large shifts in plant community composition, especially in the last 25 years. Light or no grazing was associated with increased abundance of cool-season perennial graminoids, as well as several weedy and invasive species. These findings support the long-term sustainability of livestock grazing in the shortgrass steppe, which has high resistance to season-long heavy grazing. Not only is the shortgrass steppe tolerant of fairly high grazing intensities, it likely requires some level of grazing to resist invasion and maintain cover of blue grama, a highly drought-tolerant species. Across most grazing treatments, several aspects of plant community composition have been shifting directionally during the past 25 years, which recent experiments in this grassland suggest may be a response to increasing atmospheric [CO2].