Habitat effect on vegetation ecology and occurrence on urban masonry walls C.Y. Jim n , Wendy Y. Chen Department of Geography, The University of Hong Kong, Pokfulam Road, Hong Kong article info Keywords: Factor analysis Habitat connectivity Masonry wall Spontaneous vegetation Urban ecology Urban tree flora abstract 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. & 2010 Elsevier GmbH. All rights reserved. Introduction Urban areas cover approximately 2% of the Earth terrestrial surface (Grimm et al., 2000), representing the most drastically altered ecosystems (Francis and Hoggart, 2009; Williams et al., 2009). With extensive degradation of natural habitats and attendant biota, urban areas are often characterized by low biodiversity, introduction of non-native species, simplified species composition, and community structure (Grimm et al., 2000; McKinney, 2002; Lundholm and Marlin, 2006). Despite the massive and pervasive human perturbation, urban ecosystems can provide a variety of substrata for colonization by vegetation and wildlife. The remnant natural habitats could be enriched by ruderal, heavily managed artificial sites such as urban parks, and assorted habitats associated with human structures and surfaces (Forman and Godron, 1986; Niemel ¨ a, 1999; McKinney, 2002). Amongst the diverse urban habitats, the vertical surfaces of walls and building fac - ades denote a rather novel abode for spontaneous or synanthropic plant life (Segal, 1969; Woodell, 1979). Composed of brick, stone, mortar or concrete, they are widely distributed in the urban ecosystem. Darlington (1981) estimated 1 ha of walls for every 10 ha of urban land. The masonry members represent a specific environment analogous to nature’s rocky cliffs (Cooper, 1997). They share common features such as bare stone surfaces and associated fissures, with scarcity of soil, humus, moisture, and available living space (Segal, 1969; Woodell, 1979; Larson et al., 2004; Lundholm and Marlin, 2006). Walls differ from natural rocky sites due to strong and frequent anthropogenic disturbance (Aslan and Atamov, 2006). Natural and human factors jointly shape wall characteristics, including wall dimensions, stone type, stone size and shape, aspect (exposure to solar radiation, wind, and precipitation), inclination, construction style, age, moisture, and management (maintenance and repair). Half of the world’s population is dwelling in urbanized areas. The urban population is expected to surpass 70% by 2050 (United Nations, 2007). Understanding human effects on urban ecosys- tems is imperative for the welfare of a substantial segment of ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.de/ufug Urban Forestry & Urban Greening 1618-8667/$ - see front matter & 2010 Elsevier GmbH. All rights reserved. doi:10.1016/j.ufug.2010.02.004 n Corresponding author. Tel.: + 852 2859 7020. E-mail address: hragjcy@hkucc.hku.hk (C.Y. Jim). Urban Forestry & Urban Greening 9 (2010) 169–178