Old masonry walls as ruderal habitats for biodiversity conservation and enhancement in urban Hong Kong

Urban Hong Kong has a unique ruderal habitat in the form of old stoneretaining walls. In 160 years of development, the lack of developable land has required the city authorities to adopt elaborate engineering measures to convert steep slopes into platforms by cut and fill. Hundreds of stone-retaining walls of various dimensions and designs have been constructed to support the unstable slopes. The rough surface of the walls, the joints between the stone blocks, the soil behind the wall and groundwater seepage have permitted spontaneous plant growth, adding a varied vegetationmantle to these artificial 'cliffs'. Many of the retaining walls have been colonized by large trees, mainly Ficus spp. with a strangler habit, accompanied by shrubs, herbs and animals that formed distinctive landscape and ecological features. Recent engineering reinforcement and urban redevelopment, which are unsympathetic to nature, have brought deleterious modifications to or demolition of the retaining walls. Efforts to protect this natural-cum-cultural asset are beset by the inadequate understanding of the intricate association between walls and vegetation. The study was a systematic assessment of the walls, wall trees, and relationship between them, pinpointing mural attributes that facilitate plant growth on the vertical habitat. The findings could inform management and conservation of a valuable and irreplaceable heritage. Copyright © 2010 Blackwell Publishing Ltd.