Encroachment of the Upright brome (Bromus erectus) in calcareous grasslands - Assessment of the drivers and effects on plant species assemblages

Calcareous grasslands have an outstanding value for biodiversity conservation. However, they are threatened across Europe. The Upright brome (Bromus erectus) is a tall perennial tussock grass, which has increasingly become dominant in calcareous grasslands. Today, its expansion is a major challenge for nature conservation. Here, we studied the drivers and effects of B. erectus encroachment on plant species assemblages across a productivity gradient in semi-dry calcareous grasslands in Germany. Our study revealed, that despite intensified conservation management, the cover of B. erectus increased from 1978-'81 to 2020 in the two most productive types of grasslands (Onobrychido-Brometum typicum and primuletosum) by the factor 1.6 and 2.0, respectively. Across the environmental gradient, B. erectus had its highest cover (mean: 53 %) in plots of the OnobrychidoBrometum typicum, which exhibited an intermediate productivity. Overall, the encroachment of B. erectus has led to a structural and floristic homogenisation. We explain the expansion during the last 40 years particularly by climate change (i.e., milder winters and drier summers) but also nitrogen deposition. The capacity of B. erectus to exploit its habitats even during cool periods of the year is a strong advantage against competitors. This is especially true for deep soils that are enriched in nitrogen due to atmospheric deposition. Summer drought also favours the species. Overall, a higher management frequency is needed to counteract the encroachment of B. erectus. Particularly, grazing in winter and early spring, but also mowing and prescribed burning during that period, are suitable tools.