Elevated CO₂ and enriched nitrogen proportionally decrease species richness most at small spatial scales in a grassland experiment

Global changes such as nitrogen (N) enrichment and elevated carbon dioxide (CO₂) are known to exacerbate biodiversity loss in grassland ecosystems. They do so by modifying processes whose strength may vary at different spatial scales. Yet, whether and how global changes impact plant diversity at different spatial scales remains elusive. We collected data on species presence and cover at a high resolution in the third decade of a long-term temperate grassland biodiversity—global change experiment. Based on the data, we constructed species—area relationships across three spatial orders of magnitude (from 0.01 to 3.24 m²) and compared them for the different global change treatments. We found that N enrichment, both under ambient and elevated CO₂ levels, decreased species richness across almost all spatial scales, with proportional decreases being largest at the smallest spatial scales. Elevated CO₂ also reduced richness at both ambient and enriched N supply rates but did so proportionally across all spatial scales. Suppression of diversity was stronger at all scales for diversity indices that include relative abundances than for species richness. Taken together, these results suggest that CO₂ and N are re-organizing this grassland system by increasingly favouring, at fine scales, a small subset of dominant species. Synthesis: Our results highlight the role of spatial scales in influencing biodiversity loss, especially when it is driven by anthropogenic resource changes that might influence species interactions differently across spatial scales.