Response of urban tree DBH to fast urbanization: Case of coastal Zhanjiang in south China

Urban trees perform various ecosystem services, and tree DBH (trunk diameter at breast height, 1.3 m above the ground) can act as a surrogate measure of these important functions. However, DBH has seldom been studied in relation to spatial variation, possible underlying drivers, and response to urbanization. Our field investigation collected data from tree species within Urban Function Units (UFUs) in south China's coastal city Zhanjiang, which has recently experienced rapid urbanization. The land cover of UFUs was determined by visual interpretation of satellite images. Trees with DBH > 2 cm were assessed, and vines were excluded. We recorded 12,434 trees in the green space belonging to 185 species, 137 genera, and 51 families. The dominant DBH range, 5-15 cm, accounted for 43.72% of the total stems. The DBH of 33 individuals exceeded 90 cm, 20 of which were Ficus species. At 32.39 cm, the average DBH in commercial areas was the highest among the UFU types, while woodlands had the lowest at 7.11 cm. GLMM analysis showed that UFU types, landscape configuration, land cover, and construction age were all significantly associated with tree DBH. Tree DBH had a significant positive correlation with impervious surface area and a negative correlation with green space surface area. The largest trees were mainly distributed in urban villages, schools, parks, and transportation areas. Understanding urban tree DBH structure and its related factors can promote sustainable urban-forest development and management.