OLD STONE WALLS AS AN ECOLOGICAL HABITAT FOR URBAN TREES IN HONG-KONG

Urban growth in Hong Kong is constrained by rugged topography resultin g in grave shortage of developable land. Besides forming new land by r eclamation from the sea, hillsides have been extensively cut into terr aces to accommodate densely-packed roads and buildings. To maximize us eable area and to provide geotechnical stability, stone retaining wall s were widely built between platforms. Such vertical habitats constitu te a unique opportunity for spontaneous colonization by a diversified humid-tropical flora, including large trees up to 20 m tall. The walls -cum-vegetation, many exceeding 100 years old, furnish a precious natu ral-cum-cultural heritage and decorate some otherwise drab neighborhoo ds. Recent city redevelopment unfortunately has damaged beautiful wall s and their living companions. A city-wide survey was conducted to est ablish a microcomputer database to assess wall and tree characteristic s and to identify candidates for conservation. Some 505 walls with 127 5 trees (> 1 m tall) were found mainly in residential areas. A broad r ange of stone types, wall dimensions, construction methods and wall ag e were recorded. The 30 tree species, largely native, are dominated by Moraceae (Mulberry family), eight of which contribute 88% of the popu lation. About 10% of the trees are >9 m tall, providing conspicuous an d pleasant landscape elements. Some tree attributes are associated wit h wall characteristics. Many trees had been heavily pruned to meet veh icular clearance needs and perceived safety concerns. The absence of a n official policy to preserve champion-caliber trees and walls need to be urgently rectified to prevent further loss of an irreplaceable com munity asset. (C) 1998 Elsevier Science B.V. All rights reserved.