The role of bison (Bison bison) herbivory in the function of semi-arid grasslands of Arizona
The American Plains Bison ( Bison bison ) is recognized as a grassland keystone species; however, their effect on grassland ecosystem function can vary widely among different plant communities, ranging from degradation to enhancement. Grazing dynamics, environmental conditions, and the capacity for plants to compensate for losses due to herbivory largely govern the degree and magnitude of herbivory-induced ecosystem change, making it challenging to manage grazing ecosystems. Here, we examine how different grazing intensities and environmental conditions affect aboveground herbaceous production and herbaceous nitrogen yield within high-elevation, semi-arid grasslands of the Grand Canyon ecosystem in northern Arizona, United States. We conducted a replicate herbivore exclusion experiment in grassland meadows using both long-term exclosures (0.40 ha) and temporary grazing exclusion cages (1 m 2 ) to quantify herbaceous production and nitrogen yield in sites with high bison density (Grand Canyon National Park) and low bison density (Kaibab National Forest). Our goal was to assess the influence of bison grazing on aboveground herbaceous production, the relationship between herbaceous consumption (offtake) and production, and evaluate potential differences in herbaceous nitrogen yield. We found that bison grazing enhanced herbaceous production 1.32-fold and nitrogen yield 1.61-fold, regardless of grazing intensity, availability of soil nutrients, or spatiotemporal variation in climate during our study. Although we expected herbaceous production to decline at the highest levels of herbaceous offtake, we observed a linear positive relationship between offtake and production in Grand Canyon. Over the 2-year study, bison grazing was the primary influential factor explaining variation in grassland production compared to other environmental variables (i.e., temperature, precipitation, and soil nutrients). Our results show no evidence of degradation in aboveground plant productivity, which is an important metric of ecosystem function, from the current dynamics of bison herbivory in the grasslands of the North Rim of Grand Canyon National Park.