Plant Community Composition After 75 Yr of Sustained Grazing Intensity Treatments in Shortgrass Steppe

Abstract

Plant community responses to livestock grazing lack conformity across studies, even those conducted within similar ecosystems. Variability in outcomes can often be traced back to short-term or mid-term weather patterns, differences in grazing timing or intensity, or interactive effects of management and weather. Long-term experimental data are needed to determine how grazing intensity affects plant community composition in semiarid ecosystems where precipitation is low and highly variable. 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 yr 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 yr of continuous, season-long (May to October) grazing treatments, and heavy grazing did not extirpate cool-season perennial graminoids. These findings support the long-term sustainability of livestock grazing in the shortgrass steppe, which has high resistance to season-long heavy grazing. Conversely, ungrazed and lightly grazed pastures experienced relatively large shifts in plant community composition, especially in the past 25 yr. Light or no grazing was associated with increased abundance of cool-season perennial graminoids, as well as several weedy and invasive species. Moreover, across most grazing treatments, several aspects of plant community composition have been shifting directionally during the past 25 yr, which recent experiments in this grassland suggest may be a response to increasing atmospheric (CO2). The shortgrass steppe is not only tolerant of fairly high grazing intensities but also likely requires some level of grazing to resist invasion by weedy annuals and to maintain cover of blue grama, a highly drought-tolerant species.