Overlooked plant diversity in urban streetscapes in Oulu and Yokohama

Streets comprise a major area of urban land use and connect built areas to one another. However, the role of streetscapes for maintaining urban plant diversity remains poorly understood. In this study, urban street vegetation was investigated in Oulu, Finland, and Yokohama, Japan, to explore how species richness and composition of plant communities change among different street habitat types, including road-verge grass beds, asphalted surfaces, stone pavements, curbside gaps, wall bases, street shrubbery, and tree bases. All plant species were recorded separately for different street habitat types along a maximum of 500 m of street sections. The gamma diversity of each habitat type and the contributions of native and exotic species were evaluated based on species accumulation curves. Plant species composition was examined using non-metric multidimensional scaling and Venn diagrams. A total of 145 and 273 species were recorded in Oulu and Yokohama, respectively. In Oulu, native species contributed more to the total diversity across habitat types, whereas in Yokohama, we observed similar contributions of native and exotic species to the total diversity. There was a large overlap in species occurrence among different habitat types in Oulu, where the road-verge grass beds included over 90 % of the total diversity, and all species present in asphalted habitats. Road verge grass beds in Oulu and street shrubbery in Yokohama include a large number of unique species, implying that these species have adapted to the environment in each habitat. Thus, streetscape vegetation includes a mixture of common urban species and some unique species depending on habitat type, which is often overlooked as a part of the spontaneous plant diversity across streetscapes. We propose that diverse street habitats, including both gray and green surfaces, support diverse plant species and may thereby create biodiverse urban streetscapes.