Extreme droughts in oligotrophic mountain grasslands cause substantial species abundance changes and amplify community filtering

Questions Mountain grasslands can be strongly affected by extreme droughts such as those related to climate change. What are the impacts of extreme droughts on community composition, diversity, Ellenberg indicator scores and species groups in oligotrophic montane Nardus grasslands, and what are the associated mechanisms of vegetation change? Location Rhön Mountains, Germany. Methods Over three consecutive years, we investigated the effects of yearly droughts (April–August) in an experimental setup with rainout shelters. Owing to the coincidence of ambient extreme dry conditions in those years and our artificial rainfall reduction, we evaluated the contribution to community change of ambient drought conditions and the treatments. We analysed changes in community composition by applying redundancy analysis to species differences in comparison with the pretreatment year, and used mixed-effects models to test for changes in community-weighted means of Ellenberg indicator scores, sociological and functional groups. Results We found significant changes in species abundance and community structures in response to drought. Evenness increased, but species richness remained rather stable over time. Ellenberg indicator scores for temperature and nitrogen increased, whereas the score for moisture decreased. Simultaneously, dominant species declined and subdominants increased. Changes occurred with a time lag and were driven largely by the high ambient drought level and less by the artificial treatments. Conclusions Our results show that drought-related changes in community composition in Nardus grasslands occur across community structures, characteristic species, and species groups. The post-drought recovery of the community is shaped by community filters, which in particular allow subdominants to take advantage of newly available niches in the matrix, even if they lack strong drought tolerance. Our findings indicate a certain resilience of the community to droughts related to climate change, which suggests that the observed changes should not lead to an accelerated short-term decline in these grasslands, but that this cannot be excluded in the long term.