5 Conclusions
The distribution of street tree assemblage and its associated air purification benefit can’t be explained by population density theory and don’t vary along the urban-suburban gradient. That is probably due to the social characteristic of street trees. Street trees’ installation is closely related to roads/streets’ construction, and roads/streets’ development is closely related to economic development. Therefore, where the government invests more for economic development and streets/roads, and promote more people to settle down, there would be more street trees and the associated air purification benefit. From the ecosystem services perspective, it is a good phenomenon that where there are more people, there is more ecosystem services. However, street tree assemblage is just one of the natural components in cities, we need further researches to illuminate the distribution patterns and the affected factors of other natural components and their associated ecosystem services in cities respectively, then we can modify and promote green infrastructure installation to meet the need of city dwellers.
Public policy is a main driver of street trees planting and government take lead role in promoting street trees installation. Therefore, it is important for city sustainable development that public policy maker conceive eco-consciousness when they are making decision and the environmental decision-making ability of a government should be strengthened.
Public policy would have a time leg effect on ecosystem services provided by street trees for trees needing time to grow up. High-quality heritage trees should be preserved during area redevelopment and city sprawl.