Are recent climate change and airborne nitrogen deposition responsible for vegetation changes in a central German dry grassland between 1995 and 2019?

Temperate dry grasslands are adapted to heat and drought and may therefore be resilient to global warming. We investigated vegetation changes in a dry grassland comprising three xeric associations (Carici-Seslerietum, Festuco-Stipetum, Trinio-Caricetum) and one meso-xeric association (GentianoKoelerietum) in the dry region of Central Germany (lower Unstrut valley) between 1995 and 2019 by one-time repetition of 46 permanent 1 m(2) plots distributed along a 244 m line. During this period, the mean summer temperature in the region has increased by 3.1 degrees C, and the frequency and intensity of heat waves and drought events have increased strongly. Because there was also persistent airborne N deposition, we hypothesized both environmental factors as major causes of potential vegetation changes. We found a significant change in vegetation composition indicated by a 50% species turnover based on the presence/absence of species. However, the mean indicator value for nutrients did not increase and the mean indicator value for moisture did not decrease. This result contradicts to our prediction, but can be explained by the fact that both indicator values were strongly intercorrelated, i.e. eutrophication and drought compensated for each other in their indicator values. In addition, a sharp decline in the proportion of meso-xerophilic plant species (as opposed to that of xerophilic species) clearly indicated increasing drought. Another indication of the now drier conditions was the strong increase in winter annuals, which was presumably due to the drought-induced lower competition from perennial plants. In addition, many graminoids and all of the few summer annuals declined sharply, probably due to drought, while sub-ruderal biennials increased, probably due to the combination of eutrophication and drought. Among graminoids, only the xerophilic Stipa capillata and the xerotolerant Bromus erectus increased, probably due to the drier summers (S. capillata) and due to milder winters (B. erectus). These increased Mediterranean climate conditions were also indicated by a decreased continentality indicator value. The positive correlation of the NMDS axis of vegetation change and the mean Ellenberg indicator value for nutrients indicated eutrophication (airborne N deposition) as a reason of vegetation change. However, N deposition without increasing drought would have resulted in denser and more mesophilic grasslands, i.e. the opposite pattern than we observed. Therefore, we assume that climate change (mainly summer drought, but also mild winters) was the main cause for the observed changes. This assessment is also supported by the fact that the grasslands became floristic more heterogeneous (because prevailing eutrophication would have led to homogenisation). With 29 loser species and 9 winner species, and 18.2% decline in species richness and 30% decline in the number of threatened species, we conclude that the conservation value of our grassland has decreased significantly. Overall, we conclude that dry grasslands in the dry area of Central Germany, are already affected by climate change (especially increased drought) in addition to N deposition, and that climate change is very likely the greatest threat to these grasslands.