Habitat effect on vegetation ecology and occurrence on urban masonry walls

Cities contain a diverse range of habitats that support plant establishment and persistence. This study focuses on a particular vertical artificial habitat: masonry retaining walls in Hong Kong. We explored the diversity and co-existence of different plant growth forms, synoptic assessment of habitat conditions, and relationship between habitat factors and vegetation occurrence. Some 270 walls with notable plant colonization in old districts were studied. We surveyed intrinsic wall fabric, extrinsic site condition, tree species and abundance, and other types of plant cover. The data were evaluated with the help of principal component and multiple regression analyses. A wide assemblage of species and growth forms have established spontaneously on walls. The tree flora is dominated by Moraceae (Mulberry family) members, genus Ficus (figs or banyans), and particularly Ficus microcarpa. Trees with strangler characteristics pre-adapted to grow on the vertical habitat are strongly favoured, followed by ruderals and garden escapees. Natives outnumber exotics by a large margin. Multiple wall attributes could be condensed into four factors, classified as water-nutrient supply, habitat connectivity, structure-maintenance, and habitat size. The action of habitat factors on vegetation occurrence hinges on plant growth form and dimension. The occurrence of diminutive lichen-moss is related to the fundamental sustenance water-nutrient factor. The bigger mature trees are more dependent on the larger-scale habitat size factor. The medium-sized plants, including herbs, shrubs and tree seedlings, are contingent upon the dual influence of water-nutrient and habitat connectivity. Spatial contiguity with natural ecosystem can secure continual supplies of seeds, water, nutrient, genial microclimate, and clean air to foster wall vegetation growth. The conservation of walls and their companion flora could avoid degrading or reducing these critical enabling factors. The urban ecological heritage deserves to be protected from unnecessary, misinformed and harmful impacts.