Trees perform various ecosystem functions within urban green space, yet little is known about the magnitude of change in urban tree DBH, and its potential response to urbanization. Field investigation was used to determine current tree DBH within Urban Function Units (UFUs) in the coastal city Zhanjiang in China. The cover of each UFU was determined via visual interpretation of satellite images. We recorded 12,434 individuals within Zhanjiang green space belonging to 185 species, 137 genera, and 51 families. The dominant DBH range was 5-15 cm, which accounted for 43.72% of the total stems. The DBHs of 33 individuals were larger than 90 cm - 20 of these individuals were Ficus species. The average tree DBH within commercial areas was (32.29 cm ±1.74 cm), which was the highest among all UFU types, and lowest within woodland areas (7.11 cm ± 0.56 cm). Tree DBH was significantly positively correlated with imperious surface rate, and significantly negatively correlated with green space surface rate. Variation partitioning analysis showed that impervious surface rate had the highest explanatory power, followed by construction age, then patch density. These three prediction variables, however, only explained 20% of the total observed variation - this suggests that DBH was strongly influenced by several additional factors. Understanding urban tree DBH structure and its influencing factors can promote the stable development of the urban forest.
