COS 212-4 - Functional Redundancy in Mixed-Grass Prairie Rangeland Plant Communities Experiencing Multi-Year Droughts

Abstract: Grasslands have evolved with the occasional occurrence of drought but in future years, rainfall variability, multiyear droughts, and temperatures are projected to increase in the Great Plains. As a result of climate-induced changes in species distribution, plant traits, and therefore community-weighted traits (distribution of plant functional traits across a community), many other ecological changes will result, such as changes in nutrient cycling, soil formation, species interactions, primary productivity, and decomposition. When assessing rangeland health, droughts can detrimentally impact grasslands used for cattle production, especially when heavily grazed during a drought. Longer-term droughts coupled with heavy grazing can lead to a decrease in water use efficiency of plant species; potentially reducing rangeland resilience to drought and increasing degradation of the ecosystem. However, there is limited evidence outlining how multi-year droughts interact with grazing management regimes to alter the community-weighted traits and functional diversity. Therefore, the objective here was to assess how plant communities, community-weighted traits, and functional diversity are altered by a multi-year drought under varying cattle grazing regimes.

To address this goal, plant species composition was measured for four years under five drought treatments spanning from 0% to 99% rainfall reduction in heavy, moderate, and destock cattle grazing management regimes at two working rangelands in mixed-grass prairie. Drought was implemented from the beginning of the growing season in year one, to the end of the growing season in year two and years three and four were drought recovery years. To determine community-weighted trait changes, I measured relevant plant traits of the top 90% of plant species based on species composition data. Using these data, I calculated the community-weighted mean of each trait for each plot. We found that although cattle grazing and drought treatments altered the plant community, there was no significant difference in most functional metrics or community-weighted traits. Therefore, this study shows that there is functional redundancy in mixed-grass prairie rangeland plant communities that have undergone drought, potentially resulting in resilience of the plant community to drought and grazing.