COS 32-6 - Legacy effects of multi-year drought in mixed grass prairie are determined by grazing management decisions during the drought

Abstract: Long-term sustainability of rangeland function is vital for providing food security, soil retention and productivity, nutrient cycling, and carbon sequestration, as well as maintaining the economic well-being of ranchers and rural societies. However, the health of U.S. rangelands is currently under threat by changing climatic conditions. Extreme climatic events, such as drought, are projected to increase in severity and frequency in the coming decades, with substantial effects on ecosystem function and services. In 2019 and 2020, we implemented a two-year experimental drought 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). We then monitored recovery from this drought for two additional years (2021 and 2022). Throughout all 4 years, we crossed these drought and recovery treatments with three different cattle grazing treatments meant to simulate potential management scenarios (destock, stable, and heavy grazing). We aimed to understand (1) how grassland plant communities and their function responded to varying magnitudes of multi-year drought and how they recovered? And (2) how grazing management choices influenced the effect of drought or recovery from drought?

Across multiple metrics (e.g., soil moisture, ANPP, species composition, invasion abundance), we saw drought effects that increased with increasing drought magnitude. For example, at the Montana site, ANPP was negatively correlated with drought magnitude. Effects began in the first year and continued through the second. At the Wyoming site, drought effects only became significant in the second year. Importantly, both sites showed legacy effects of drought, such that even after the drought shelters were removed and all plots received the same rainfall, previous drought significantly impacted recovery of ANPP. Cattle management scenarios had little effect on grassland metrics during the two years of drought; however, cattle management scenarios interacted with drought recovery. For example, in Wyoming, the destock grazing scenario showed full recovery of ANPP in the first year post drought, while the other two more severe grazing strategies still showed persistent legacy effects of drought. Our results suggest that a multiyear drought plan for rangeland management will be critical for ensuring quick recovery from drought.