Spatial differentiation and landscape-ecological assessment of heritage trees in urban Guangzhou (China)

Abstract

Recent rapid urbanization of Guangzhou in south China has degraded its rich urban-tree endowment, scattered in roadside, park, and public and semi-public grounds. Based mainly on age, 384 outstanding trees of 25 species were officially designated as heritage specimens. They were evaluated in the field for tree dimensions and habitat types to establish landscape-ecological impacts and spatial differentiation by districts and landuses. To aid the analysis, new quantitative indices were developed, namely landscape-ecological value (LEV), biomass–landscape congruence (BLC), and species spatial differentiation (SSD), respectively, for species presence, abundance, fidelity and clustering. Cultivated natives and five common species dominated heritage trees, accompanied by 11 rare and 9 single ones. Comparisons between the urban- and the heritage-tree populations, and between the LEV of individual species, indicated the uniqueness of heritage species composition and relative landscape-ecological contributions of species groups. Tree count, LEV and species diversity increased markedly with district age, culminating in the second oldest district and then declined somewhat in the oldest; species richness peaked in the mature district and dropped in the old and oldest. Older districts had a narrow range of similarly stressful habitats accommodating mainly inherited old trees; mature districts had more diversified habitats and a dual provenance of inheritance plus recent recruits; young districts had few of both types. Stressful roadside landuse was dominated by tree count, but it was poor in species diversity and richness, and had a rather monotonous native-foliage character. Park provided ample opportunities for a complex species cohort of disparate geographical origin, dimension, age, and amenity function, but its species diversity fell short of expectations. The public and semi-public grounds harboured mainly common species except religious grounds which had higher species diversity and some notably large and unique trees. BLC and SSD results echoed the differential operation of environmental and human influences on tree biomass, configuration and landscape impacts, resulting in notable variations between districts and landuses. The management implications of the results and prognosis for the future of urban heritage trees were explored with reference to Guangzhou and other cities.

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.