Urban Grassland Plant Communities Assemble According to Productivity and Disturbance Gradients

Question Urban grasslands host important plant diversity. They comprise numerous open habitats varying from species‐rich semi‐natural grasslands to often species‐poor ‘novel’ grasslands. To foster plant diversity in urban grasslands, we need to understand their community assembly. We asked: how do vascular plant communities and their composition of species attributes assemble according to patch‐ and landscape‐scale filters in urban grasslands? Location The Helsinki Capital region, Finland. Methods We studied 200 urban grasslands, representing a wide variation of grassland types and landscape contexts. We investigated valuable grasslands, landscape grasslands, lawns and grasslands resulting from biodiversity‐oriented actions: lawn grasslands (former lawns with reduced management) and sown grasslands (sown with seed mixtures). We analysed plant communities, the environmental factors driving them (e.g., grassland type, soil and landscape variables) and the relationships between species niches and species attributes (e.g., specific leaf area, life form, seed mass) by using ordination methods and hierarchical modelling. Results We observed numerous statistically supported relationships between species, species attributes and environmental variables. The communities varied according to two main gradients: productivity and disturbance, aligning with Grime's CSR strategies. The productivity gradient arranged sites from rocky to dry and mesic grasslands. The disturbance gradient arranged sites from ruderal to semi‐natural and forest‐influenced grasslands. The highest total species richness and specialised species occurrence were associated with long‐term intermediate disturbance and non‐extreme productivity. Biodiversity‐oriented actions increased species occurrences compared to lawns, but often failed to support many specialised species (e.g., positive grassland indicators). Conclusions Our results suggest that the wide variation in urban grassland plant communities can be understood through their degree of disturbance and productivity. Moreover, novel grasslands may start resembling semi‐natural grasslands under suitable long‐term management if productivity is moderate and if dispersal occurs. Thus, long‐term extensive maintenance, reducing nutrients and facilitating dispersal, could help in developing species‐rich urban grasslands.