Compounding effects of extreme drought and extreme disturbance on ecosystem function and services in US grasslands

Kevin Wilcox, Department of Ecosystem Science and Management, University of Wyoming, Laramie, WY, Sally E. Koerner, Department of Biology, University of North Carolina Greensboro, Greensboro, NC, Kimberly Komatsu and Kimberly Komatsu, Smithsonian Environmental Research Center, Edgewater, MD, Lauren M. Porensky, Rangeland Resources and Systems Research Unit, USDA-ARS, Fort Collins, CO, Kurt O. Reinhart, USDA-ARS, Miles City, MT, Megan Van Emon, Montana State University, Jill H. Baty, Plant Sciences, U C Davis, Davis, CA, Kailey Todoroff, Allison Stewart, Amanda Williams and Mark Shepard, USDA-ARS, John D. Dietrich, USDA-ARS, Fort Collins, CO

Background/Question/Methods
Multi-year droughts are likely to become more frequent and more extreme in the future. Additionally, droughts are also likely to concur with other extreme events, such as disturbance, due to human activities. These drivers will likely have substantial effects on ecosystem function and services, and these effects may be exacerbated when occurring in conjunction. In 2019 and 2020, we implemented a fully factorial field experiment in two northern mixed grass prairies in Wyoming and Montana. At each site, we imposed five levels of drought (0, 25, 50, 75, and 99% precipitation removal) crossed with three levels of cattle grazing to simulate a gradient of disturbance (light, moderate, and heavy grazing). In this talk, we will address three major questions: (1) how do ecosystem function (aboveground net primary productivity (ANPP)) and ecosystem service provision (forage quality) respond to various levels of extreme drought and disturbance? (2) Do the effects of disturbance and drought become more extreme in year two versus year one? (3) Does the combination of drought and disturbance cause more extreme effects on ecosystem function and service provision compared with drought and disturbance effects alone?
Results/Conclusions
Drought treatments at both sites caused declines in soil moisture, ANPP, and forage quality. At the Montana site, ANPP declined with drought magnitude similarly in both years. At the Wyoming site, the effects of drought became more extreme in the second year, potentially due to greater abundance of more drought tolerant, warm season plant species at the Wyoming site versus less drought tolerant, C3 species at the Montana site. It may also be due to differences in how the treatments affected soil moisture among the sites. Additionally, we found that extreme drought caused dominant species to senesce about one month earlier than ambient treatments, indicating a reduction in forage quality, which is an important ecosystem service in this region. Additionally, when drought treatments were combined with heavy disturbance, the effect of extreme drought was intensified, causing senescence another month earlier than non-droughted, low disturbance plots. In summary, we found that these semi-arid prairies were more resistant to single-year than multi-year extreme drought. Similarly, ecosystems resistant to one type of extreme event (e.g., heavy grazing or extreme drought) may be vulnerable to combinations of multiple extreme disturbances. Finally, ecosystem resistance may also depend on site context (vegetation or soils).