Structure, Function and Drought Resilience of Northern Prairie Communities, 50 Years After Grazing Disturbance

Abstract

With climate change threatening the function of grassland ecosystems, conservation and restoration strategies are shifting from comparisons of species compositions with baseline conditions, to assessments of ecosystem functions and resilience. Here, I present research from Riding Mountain National Park, Manitoba, to illustrate the links between plant community composition, leaf traits of dominant plants, and grassland community function. I also discuss applications for the management of grassland ecosystems. I use plant community data, collected in 1973, 2010 and 2020, to understand the long-term effects of grazing on the function and resilience of northern fescue prairies. I test whether legacies of historic grazing continue to affect the structure, diversity, and composition of grassland communities, and whether historic grazing predicted community leaf trait composition in fescue grassland ecosystems. I also explore how nutrient and carbon cycling may be influenced by leaf traits of dominant plants, including their leaf carbon and nitrogen concentrations. Fifty years after grazing, heavily grazed prairies continued to have lower plant diversity. However, prairies with light grazing had lower spatial variation in plant composition. By 2020, community leaf trait composition could not be predicted by historic grazing, and instead, plant trait composition was driven by exotic species invasions. Similarity in traits between Poa pratensis and Festuca hallii resulted in a functional redundancy between lightly and heavily grazed grasslands. Invasions of Poa pratensis increased the values of leaf density, and leaf C:N over the years while Bromus inermis increased the value of specific leaf area (SLA), illustrating that changes in grassland composition correlate with changes in the traits of dominant plants that have the potential to affect the resilience of grasslands to drought as well as their function. This study describes how community trait composition can impact grassland drought tolerance and ecosystem functions, and the management implications of those consequences.