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.