Urbanization effects on spatial-temporal differentiation of tree communities in high-density residential areas

Abstract

The changing mode of urban development through time can bring a varied landscape mosaic accompanied by spatial-temporal differentiation of urban vegetation. Hong Kong as an ultra-compact city generates intense interactions between trees and urban fabric to highlight urbanization effects on tree communities. The study areas cover public housing estates which accommodate about half of the 7.26 million population. Thirteen site factors related to estate, landform and habitat traits were measured or computed as surrogate urbanization effects. Species composition and diversity of tree communities in 102 estates were assessed by field surveys, including four estate groups: older or newer ones situated respectively in urban core or new towns. They contain 48,823 trees belonging to 232 species with heavy exotic representation. Total tree density and native tree density in newer estates were significantly higher than older ones. Differences in species richness and diversity and native species richness between older and newer estates were not significant, expressing to a certain extent the floristic-homogenization phenomenon. Multi-response Permutation Procedures (MRPP) results showed significant difference in species composition between older and newer estates, which could be explained by variations in development age, density, town plan and pre-urbanization land cover. Nonmetric Multidimensional Scaling (NMDS) results found tree distribution patterns in four estate groups strongly associated with estate area, open space area, estate population, estate age, Shannon index (H′) at planting strip and tree pit, and tree site quality index. Urban-forest management could be improved by adopting more native species and providing high-quality and spacious planting sites to accommodate more and larger trees. The research methods and findings could be used by policy makers and planners in similar large and developing cities to evaluate, design, maintain and enhance urban biodiversity.