Grazing and browsing dampen drought effects on Artemisia tridentata in an eastern Wyoming sagebrush steppe

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Ashley M. Dupuis and Kevin R. Wilcox, Ecosystem and Science Management, University of Wyoming, Laramie, WY

Background/Question/Methods
Extreme climate fluctuations, such as drought, are likely to increase throughout the 21st century, and may have strong impacts on the persistence of A. tridentata (sagebrush), a critical component of many ecosystems in the western US. Although sagebrush may be resistant to single year and moderate drought, it may be less resistant to prolonged severe drought. Additionally, disturbance events may exacerbate effects of drought on sagebrush due to consumption by grazers and browsers. In 2019, we established a fully factorial drought x grazing experiment in a sagebrush ecosystem within the Thunder Basin Ecoregion (Bill, Wyoming). In this experiment, light, moderate, and heavy grazing treatments were crossed with five levels of rainfall: ambient, -25%, -50%, -75%, and -99% and a browsing treatment of heavy (50%) or un-browsed. Within this experiment, we assessed performance and growth of sagebrush in these plots to address the following questions: 1) How do A. tridentata populations resist drought? 2) Are A. tridentata individuals more susceptible to drought when combined with grazing and browsing?
Results/Conclusions
Two sagebrush individuals (one 50% browsing, one ambient browsing) were measured within each of 54 plots (2x2 m each, n=3/treatment). Measurements included: pre/midday water potential, soil moisture, leader growth and crown volume of adult sagebrush, and grass height. We found that sagebrush growth was strongly linked with drought effects on water potentials under ambient browsing and light grazing. Interestingly, grazing and browsing treatments decoupled these tight linkages such that water potential was not a strong driver of sagebrush growth under heavy grazing and browsing. This may be due to (1) grazing reducing grass cover and grass uptake of shallow soil water, and (2) browsing reducing total sagebrush transpiration through defoliation. These findings highlight the interactive effects of herbivory and drought for ecosystem function and services in sagebrush ecosystems. As such, we advise that understanding the impacts of multiple simultaneous drivers will be critical for informing land management decisions under future climatic scenarios.