Old stone walls as an ecological habitat for urban trees in Hong Kong

Abstract

Urban growth in Hong Kong is constrained by ragged topography resulting in grave shortage of developable land. Besides forming new land by reclamation from the sea, hillsides have been extensively cut into terraces to accommodate densely-packed roads and buildings. To maximize useable area and to provide geotechnical stability, stone retaining walls were widely built between platforms. Such vertical habitats constitute 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 natural-cum-cultural heritage and decorate some otherwise drab neighborhoods. Recent city redevelopment unfortunately has damaged beautiful walls and their living companions. A city-wide survey was conducted to establish a microcomputer database to assess wall and tree characteristics and to identify candidates for conservation. Some 505 walls with 1275 trees (>1 m tall) were found mainly in residential areas. A broad range of stone types, wall dimensions, construction methods and wall age were recorded. The 30 tree species, largely native, are dominated by Moraceae (Mulberry family), eight of which contribute 88% of the population. About 10% of the trees are >9 m tall, providing conspicuous and pleasant landscape elements. Some tree attributes are associated with wall characteristics. Many trees had been heavily pruned to meet vehicular clearance needs and perceived safety concerns. The absence of an official policy to preserve champion-caliber trees and walls need to be urgently rectified to prevent further loss of an irreplaceable community asset.