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

doi:10.1016/S0169-2046(98)00072-3

Landscape and Urban Planning

Volume 42, Issue 1, 24 July 1998, Pages 29-43

https://doi.org/10.1016/S0169-2046(98)00072-3 Get rights and content

Abstract

Urban growth in Hong Kong is constrained by rugged 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.

Section snippets

Introduction and preamble

A continual quest for developable land underscored 150 years of urban-expansion in Hong Kong. The predominantly rugged terrain is dominated by steep slopes which constrain city growth in the old circum-harbor core. Rapid population and economic growth demand land which has to be created without respite by reclamation from the sea and cutting the hillslopes to form platforms. These two modes of site formation over the years have molded the direction and pace of urban development which must be

Study area and methods

The location of walls were initially pinpointed on 1:1000 large-scale maps. A reconnaissance of 100 walls in different districts provided basic knowledge about walls and trees to design two detailed field record forms to facilitate systematic data collection. The wall form assembles data on location, material, stone dimensions, surface smoothness, surface moisture, weathering status, joint type and condition, wall aspect, inclination and exposure, wall environs, vegetation cover, integrity and

Stone walls as vegetation habitat

The enumerated 505 stone walls are found in the older hilly neighborhoods built on Hong Kong Island. Two districts have a significant clustering of walls, taking up 40% of the total number. There is a marked concentration in several winding two-lane roads that run largely along the contour where most large wall trees (<6 m) are attached. The old urban-sprawl mode tended to terrace hillslopes into small pockets separated by stone retaining walls; recent developments formed large tracts bounded by

Characteristics of wall trees

The surveyed walls contain vegetation of different growth forms, ranging from mosses and lichens, to ferns and herbs, and to shrubs and trees. The wall plants are due to spontaneous colonization unassisted by human actions. As mimicry of nature's rocky precipices, the wall habitat is expected to harbor a similar plant assemblage. The humid-tropical environment of Hong Kong, with an inherently rich native flora enriched by exotic components, has many candidate species with different degrees of

Ecological association between walls and trees

Wall dimensions apparently do not affect tree frequency. Overall, large walls do not necessarily harbor more trees or more species (Table 1; Fig. 4Fig. 5). The smaller walls are often the best endowed with tree flora. Most trees are attached to walls with stones slightly to moderately weathered, implying an increased affinity for old walls. Alternatively, it implies that it takes time for walls to be gradually colonized by trees, thus an old wall can accumulate more trees. Highly weathered

Vicissitudes of wall vegetation

In nature, the uncommon vertical surfaces are mainly rocky cliffs with trying conditions for survival, mainly the absence of an effective soil layer and the impermeability of stones. This is compounded by shortage of available moisture and nutrients, and exposure to wind and insolation. In cities, artificial vertical faces abound on buildings and other structures, yet most are too inhospitable for even the most tolerant species. Stone retaining walls, however, are human emulation of nature's

Long-term management strategy and conclusion

In Hong Kong, intensive and relentless urban development in a difficult terrain over 150 years has bequeathed many fine stone retaining walls. They testify the elaborate masonry work of a bygone era, entailing much hard manual labor and proud workmanship. Stone walls were built to stabilize a vertical engineered surface. Any subsequent plant colonization is a coincidental byproduct, a result of the interactions between nature and culture. A harsh and cold vertical wall face is softened by

Acknowledgements

I would like to convey my gratitude to the Caltex Green Fund for financial support of this project. The help in field work provided by my student research assistants Wing-yee Ho, Oi-man Lee, Shun-wa Lee, Esther Li, Jeannette Liu, Wai-ling Tam and May Wong is gratefully acknowledged.

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