Masonry walls as sieve of urban plant assemblages and refugia of native species in Chongqing, China

Urbanization imposes notable threats to global biodiversity. Masonry retaining and city walls offer unusual ruderal habitats for spontaneous establishment of unique plant communities which may flourish despite urban disturbance and intensification. We comprehensively surveyed the wall-plant communities in Chongqing in southwest China to investigate their special urban ecology and their role in sustaining urban biodiversity. From 289 masonry walls, we recorded 205 plant species under 164 genera and 78 families. Perennial and annual herbs were the most common growth forms by species number. Despite accommodating 27 tree species and 6538 tree individuals, three dominant ones, respectively two Moraceae members Ficus virens which is a strangler fig and Broussonetia papyrifera, and Robinia pseudoacacia (Leguminosae), constituted 94% of the wall-tree stock. Detailed analysis of tree height distribution shed light on species recruitment dynamics. The dispersal agents differentiated sharply by growth form, with 73% herbaceous species wind-dispersed in comparison with 70.2% woody species bird-dispersed. The endemism and floristic characteristics of wall-plant communities were influenced by the regional urban flora, particularly for dominant wall trees; similar traits were found in the wall flora of Nanjing and Hong Kong. The low similarity in species composition between walls and nearby ground plant communities suggested the critical role played by walls in protecting the disturbance-intolerant native plants, including many Pteridophytes. The conservation values and biodiversity contributions of masonry walls in Chongqing are worthy of special attention. The findings could provide the basis to maintain urban ecosystem services and for nature-based ecological urban designs.