Microclimate shapes vegetation response to drought in calcareous grasslands

Questions Semi‐natural grasslands have been shown to be relatively resilient to drought events, but there seems to be no uniform response of different grassland types or across species. The typical vegetation of calcareous grasslands, which are biodiversity hotspots in Europe, reportedly even profits from droughts. What was the response of their vegetation, considering different species groups, to the recent periods of intense summer droughts? How did differences in microclimate, such as between steep, south‐facing slopes (SSF) and flat or north‐facing slopes (FNF), affect species composition and drought response? Location Semi‐arid calcareous grasslands (Mesobromion) in northwest Germany. Methods In 2019, we resurveyed 112 vegetation plots from 25 sites first established in 2008, after 11 exceptionally dry years. Species identities and cover were assessed in plots of 1 m2. We used linear mixed models to determine the effect of microclimate on species composition and drought response. Vegetation changes were visualised by non‐metric multidimensional scaling (NMDS). Results In the initial survey in 2008, the species composition was significantly influenced by the microclimate, with positive effects of dry and warm conditions on SSF on character species. Phosphorus availability played an equally important, albeit contrasting, role. Vegetation change between 2008 and 2019 showed strong signs of the severe droughts. On SSF slopes, total species richness decreased, with negative trends for all species groups except ruderal species. Remarkably, this also affected the character species of dry grasslands. By contrast, dry conditions on FNF favoured an increase in total species richness and character species. However, eutrophication indicators increased here as well. Conclusions We conclude that sites with a dry and warm microclimate have to date had a positive, stabilising effect on the typical vegetation of calcareous grasslands. However, increasing droughts due to climate change pose a potential threat to the characteristic species composition, especially at sites with hot and dry microclimates. The characteristic species of calcareous grasslands are commonly thought to profit from droughts. Our results confirmed that a drier microclimate had promoted them in the past. However, after a decade of intense drought, steep/south‐facing grassland sites lost species from all groups except ruderal species, whereas species numbers increased on flat/north‐facing sites, in