Serviceable tree volume: An alternative tool to assess ecosystem services provided by ornamental trees in urban forests

Abstract

Ornamental trees bearing outstanding flowers bring notable ecosystem services to cities. Existing research often treated ornamental trees as a sub-category of urban trees. To investigate the contributions of ornamental trees, this study aimed to: (1) assess their provision of ecosystem services; and (2) examine their ecological composition and diversity. Comprehensive tree survey in 93 public housing estates in Hong Kong identified 12,516 ornamental trees. To characterise the ability to provide ecosystem services, an index termed serviceable tree volume was devised. This innovative quantity aided the evaluation of ornamental trees by visual amenities, olfactory trait and ecological benefits while considering the compromise due to structural defects. The comprehensive tree survey showed a significant tendency of planting exotics for ornamental purposes. Most ornamentals could potentially deliver high-quality ecosystem services which were unfortunately restrained by prevailing very poor health conditions particularly attributed to structural defects on major branches. By relative importance value, several species dominated the ornamental tree inventory, namely Bauhinia ‘Blakeana’, Delonix regia and Bombax ceiba. By incorporating serviceable tree volume in calculating relative importance value, the importance ranking of ornamental species was re-ordered. Using tree stem as the unit of analysis, Shannon-Wiener Diversity, Reciprocal Simpson and Pielou’s Evenness of the ornamental tree stock stood at 3.08, 14.73 and 0.74 respectively, and they were reduced when the three indices incorporated serviceable tree volume. Diverse native ornamentals could be planted extensively in the urban forest in tandem with the preservation of important exotic species.