Spatial differentiation and landscape-ecological assessment of heritage trees in urban Guangzhou (China)
Introduction
For millennia, humans have transferred selected elements of nature into settlements to mimic nature’s pleasantries. Different cultures have cultivated urban trees for their multiple benefits and functions, and city greeneries often reflect the history of urban development and associated environmental changes. Trees have become the surrogate and icon of nature in cities. Consciously or unconsciously, urban trees contribute many goods and services to urban communities, including recreational (Dwyer et al., 1992) and medical-health (Ulrich, 1984) functions, social and psychological (Kuo and Sullivan, 2001) and amenity uses (Hayward and Weitzer, 1984), increased property values (Anderson and Cordell, 1988), improved urban environmental quality (Bussotti et al., 1993), microclimatic amelioration (Feng, 1992), and socio-economic benefits (Gold, 1972). Despite such important uses, the need for trees to co-exist with human settlements is often taken for granted.
Trees are planted in heavily built-up to sparsely developed urban enclaves. Cities are hostile environments for trees (Bernatzky, 1978, Bradshaw et al., 1995), with competition from artificial structures and surfaces. Consequently, trees are commonly relegated to remnant spaces with growth limited by surrounding buildings, carriageways, cables and pipes both above and below ground (Jim, 2003), and factors such as compacted soil (Jim, 1993), and polluted air, water, and soil (Bassuk and Whitlow, 1988). City development, landscape planning and design, citizen preferences, horticultural traditions and fads, and urban forestry management systems, are key determinants of urban forests (Sanders, 1984, Jim, 2002a).
Due to anthropogenic and natural stresses, urban trees seldom realize their biological potentials (Jim, 1997). However, genotype–site interactions and chance may permit some trees to survive for decades to centuries, expressing fully their innate life expectancy, dimension, structure, health and robustness. They form a heterogeneous group, including spontaneous or cultivated trees, and native or exotic species. These outstanding remnant specimens are widely respected as landmarks or heritage trees by urban dwellers. They bring ecological, amenity and cultural values, make significant contributions to cityscape, and furnish living landmarks that have witnessed the history of human tenure in cities.
Heritage trees are significant because so few trees can excel under urban stresses. To echo the intimate association between the prized trees and the community, residents have commonly bestowed on them religious, spiritual or other symbolic values (Boyer, 1996, Read, 2000). Some citizens have developed emotional attachment to them. Cities have established official registers to ensure statutory protection of the oldest living components as natural-cum-cultural heritage, and labelled them variously as ancient, beautiful, big, champion, elite, famous, heritage, historic, old, outstanding, remarkable, specimen, veteran trees (Randall and Clepper, 1977, May, 1990, Mitchell et al., 1990, Alderman and Stevenson, 1993, Jim, 1994a, Pelt, 1996, Parkenham, 1997, Lewington and Parker, 1999, Read, 2000, Browne, 2001, Meyer, 2001). Most studies on heritage trees hitherto were rather descriptive and did not adopt an in-depth, scientific and analytical approach.
Guangzhou city in south China has a high canopy cover with a significant cohort of heritage trees. By tree number, density, coverage and species diversity, Guangzhou’s urban forest is one of China’s best (Guangzhou Municipal Government, 1928, Yang, 1991). With a 2500-year planting history, both the pre-urbanization and planted forests have influenced the present tree composition (Jim and Liu, 2001a) with 1400 plant species and many native species (Hou, 1956). As one of the earliest cities open to other countries in the 19th century, many exotic species, notably from Australia and Southeast Asia, have been introduced (Wang et al., 1994). The principal city in south China has grown rapidly since the ‘Open and Reform’ policy in 1978. The population increased from 2.29 million in 1980 to 5.67 million in 2000 (Guangzhou Statistical Bureau, 2001). Due to the intensity and pervasiveness of high-density development, Guangzhou’s urban forest is being degraded (Jim and Liu, 1997, Jim and Liu, 2001a). Old neighbourhoods have been converted into high-density residential and commercial uses, and new developments often leave little plantable space with adverse effect on remaining trees.
The objectives of this study were to (1) assess the species composition of the heritage trees in Guangzhou, (2) analyse the spatial pattern and differentiation of the heritage trees by districts and landuses, (3) evaluate the impacts of human activities and other urban factors on their growth and landscape-ecological contributions and (4) suggest measures to strengthen their conservation and management.
Section snippets
Study area
Guangzhou (Fig. 1) is situated in the southern part of the subtropical-humid climatic zone with mean annual precipitation of 1690 mm and mean air temperature of 21.8 °C. Typhoons and thunderstorms occasionally damage trees (Jim and Liu, 1997). Pre-urbanization natural vegetation was evergreen broad-leaved rain forests, dominated by tree species in the families Lauraceae, Moraceae and Caesalpiniaceae (Jim, 2002b). Although the primary vegetation has been destroyed by millennia of agricultural
Species composition and landscape-ecological value
Comparison between the urban-tree population (UTP) and the heritage-tree population (HTP) provided insights on species composition. The city had 384 heritage trees, of which 338 were over 100 years old. UTP had 254 species, dominated by 29 common ones (frequency>1%) that constituted 75% of the population (Table 1). HTP had 25 species, with the top five contributing 87.4% of the trees (Table 2). Dominance by common species was more pronounced in HTP than UTP. Frequency distribution of heritage
Management implications and conclusion
In China, urban trees are one of the major landscaping components that have been planted in cities since several thousand years ago (Du et al., 1986), leaving its legacy on cities in other parts of the world (Erdberg, 1936). However, recent urbanization has degraded the quality of urban environment in some fast-growing cities (Jim and Liu, 2001b). Under a degenerating growth regime, the management and conservation of heritage trees in large Chinese cities demand enlightened and urgent attention
Acknowledgements
Special thanks are hereby conveyed to Z. Zhong and staff of the Landscape Bureau of Guangzhou, and H.H.T. Liu. The research grant provided by the Outstanding Researcher Award and Hui Oi Chow Trust Fund of the University of Hong Kong is gratefully acknowledged.
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