Effect of climate extremes and grazing on functional traits of a grassland community: insights from a 20-year experiment

Background and Aims: Climate change, particularly the increased frequency of extreme climatic events, poses significant challenges to the biodiversity and functionality of semi-natural grasslands. However, the response of plant functional traits of grassland communities to climate extremes is still an unresolved issue. Using data from a long-term experiment, we aimed to characterize the functional response of a grassland community to simultaneous long-term effects of grazing and climate extremes. Methods: For over a 20-year period, we monitored the species composition of grazed and ungrazed grassland plots. We measured functional traits defining the Leaf Economics Spectrum (LES) and the Hydraulic Safety-Efficiency (HSE) trade-offs, and we identified the temporal dynamics of single traits at the community level as well as the changes in functional strategies among grazed and ungrazed communities. Then, we assessed the role of climatic extremes in driving the changes in functional composition. Key Results: Grazed plots, in the first few years, were dominated by fast-growing species with more acquisitive strategies compared to ungrazed plots. However, both communities showed a reorganization in the functional structure over time, pointing towards a selection of trait combinations favoring more conservative, stress-tolerant strategies. The joint effect of grazing and climate extremes significantly altered the functional composition of the grazed community, leading to a shift from species with grazing-tolerant traits to species with grazing-avoidant, and drought-tolerant, traits. Conclusions: We found that grazing pressure generally promoted functional diversity but led to rapid shifts in community composition when combined with prolonged drought events. In contrast, the ungrazed community, dominated by species with conservative resource-use strategies, showed more stable functional richness and divergence, as well as a reduced sensitivity to climatic extremes. These results underscore the importance of carefully evaluating grazing in the context of climate change, particularly to guide restoration and conservation efforts.