The relative importance of nitrogen deposition and climate change in driving plant diversity decline in roadside grasslands.

Nitrogen deposition and climate change have been identified as major threats to the biodiversity of semi-natural grasslands. Their relative contribution to recent biodiversity loss is however not fully understood, and may depend on local site conditions such as soil type, which hampers efforts to prevent further decline. We used data from >900 permanent plots in semi-natural grasslands in Dutch roadsides to investigate whether trends in plant diversity and community composition (2004-2020) could be explained by: (1) nitrogen deposition (NH_x and NO_y) and climate change (winter degree days and summer drought), (2) the interactive effect of nitrogen deposition and climate change, and (3) the interactive effect of nitrogen deposition and climate change with soil type. Overall we observed a decline in plant diversity and an increased dominance of tall species and grasses. These changes were linked to winter warming, but not to changes in summer drought and nitrogen deposition. The effect of winter warming was more pronounced in areas with higher NO_y deposition, but was consistent across different soil types. Our results suggest that winter warming will become an important driver of plant diversity loss by altering competitive interactions, which could have major repercussions for other trophic levels and ecosystem services. Future conservation and restoration of grassland biodiversity therefore requires management regimes that are adapted to winter warming.