Urban growth and topographical factors shape patterns of spontaneous plant community diversity in a mountainous city in southwest China

Urban ecosystems can host a considerable amount of biodiversity because of the varying habitats created during urban development processes. Spontaneous vegetation is a neglected element of urban biodiversity, and the roles of a city's natural floristic resources, geomorphic features, and topographic characteristics in shaping spontaneous plant diversity patterns have not been fully clarified. We explored the impact of urban growth and elevation and slope on the diversity patterns of urban spontaneous plant communities in a mountainous city in southwest China. We recorded 279 species belonging to 210 genera and 70 families, with native species accounting for about 85% of the urban spontaneous plants. The magnitude of urban growth and elevation were the most important drivers and they were positively correlated with the γ-diversity patterns in Chongqing. Thus, both urban forms and the city's natural conditions are important in affecting the urban biodiversity patterns. Furthermore, urban spontaneous plant communities in Chongqing metropolis were mainly dominated by a few widespread, cosmopolitan species, with nearly 50% of the 279 species we recorded being narrowly distributed and rarely occurring species. The distribution of the rare, spontaneous species may be highly dependent on chance dispersal, the legacies of earlier land-use, and the availability of specific habitats. We propose that urban construction and landscape design should maintain appropriate configurations for the habitat type to preserve rarely occurring plant species to increase biodiversity in urban landscapes.