Towards a functional understanding of rehabilitated urban road verge grasslands: Effects of planting year, site conditions, and landscape factors

Urbanization causes ecosystem degradation and losses of biodiversity. Still, urban landscapes favor organisms, depending on how well they fit the anthropogenic conditions. Creating urban green spaces of high ecological quality, such as pollinator-friendly road verges, promotes biodiversity in cities. We sowed a seed mixture consisting of 26 native plant species with diverse functional traits in 46 road verge patches along four urban roads in Munich (South Germany) in 2019/20 and monitored in 2020/21. Control were pre-existing turfgrass patches with conventional management. We assessed species richness, Shannon diversity, and Pielou's evenness, whereas functional composition was evaluated via functional richness, evenness, and divergence, based on 13 plant traits. To study urbanization effects on species and functional composition, we analyzed soil characteristics and tree shading at the local level, as well as distance to city center, imperviousness, and edge density at the landscape level. We used linear models and ordination analysis to assess treatment, local and landscape effects, and the influence of planting year on taxonomic and functional composition. Rehabilitation treatment explained most differences in species and functional composition. Species richness was lower in the city center and at sites with high soil organic matter; Pielou's evenness increased with soil pH. Trait composition differed between rehabilitated and control patches, and varied according to planting year. Soil bulk density negatively affected functional richness for all traits combined, and edge density reduced functional evenness of establishment traits. We conclude that urban filtering, in combination with planting year, shapes species and trait composition of rehabilitated road verges. Hence, local and landscape-level trait–environment interactions affect the assembly of road verge grasslands. Finally, our findings suggest that establishment traits have a pivotal role in the development of rehabilitated road verges, while the predictability of rehabilitation outcomes may be hindered even when seeding specifically designed seed mixtures.