Diversity increases but community specialists decline over three decades in dry grassland communities of Central Germany

Abstract

Aim: Dry grasslands are vulnerable to climate and land-use change. Increasing temperatures, drought, grazing cessation and nitrogen deposition can all result in shifts in grasslands' taxonomic and functional composition. We tested the hypotheses that both species richness and diversity are decreasing at the scales of both the regional species pool of dry grasslands and for individual communities, with functional composition shifting towards more competitive traits and shortened live spans, and that these responses depend on species' functional characteristics and on drought stress levels to which communities are exposed. Location: Semi-natural dry grasslands north-west of Halle (Saale), Central Germany. Methods: In 2021/2022, a resurvey of vegetation was conducted in 131 relevés surveyed in 1992/1993 on 51 rocky hills, on which our focus was on six common grassland communities found along a gradient of increasing drought stress from harsh to more favourable conditions. Drought stress levels were quantified using slope, aspect, soil depth and soil texture, resulting in a sequence of communities on sun-exposed shallow soils to moister and deeper soils. Changes in taxonomic and functional composition were analysed and explored with paired t-tests, linear models, principal component analysis and (distance-based) redundancy analysis. Results: The size of the regional species pool of the dry grasslands did not decrease over the last 30 years. Its functional turnover points to declining precipitation, longer growing seasons and an increasing frequency of drought events. In the resurveyed pool, a lower number of species flowering in mid summer and a higher number of early flowering species suggest a trend to avoidance of summer droughts and a response to warmer and moister spring conditions. At the community scale, species richness and alpha diversity increased, in spite of a decreasing plant cover over the investigation period. These increases were mainly caused by an increased abundance of annual plant species, as a response to cleared space through the decreasing cover of perennial plant species, pointing to drought avoidance as a successful survival strategy. The studied community types varied in the magnitude of diversity changes as well as in their species and functional responses, where changes in trait composition increased with increasing heat load on two of the community types. Conclusions: Dry grassland communities undergo significant shifts in both species and functional composition, in favour of generalist species with faster life cycles, disadvantaging dry grassland specialist species. Given the predicted higher frequency of drought events in the future and the uncertainty of grazing resumption in the region, these changes can be expected to continue over the next decades. This underlines the importance of continuing locally adapted management practices for landscape-scale biodiversity conservation to compensate for climatic change.