Urban ecosystem is different from other natural ecosystems (e.g., river ecosystem, forest ecosystem, wetland ecosystem et.). It is an ecosystem consisting of natural and social-economic components, while predominated by one species, human being. Therefore, it is a severely impaired ecosystem deemed to be a trashed or damaged habitat by bio-ecologist. Even so, some bio-ecologist point out that it is more likely to be fixed if it can “localization” or “regional self-reliance” by properly introducing natural components which provide cities ecosystem services (Wu 2014). According to the study of Stockholm by (Per Bolund and Hunhammar 1999), there are seven types of locally natural components and street tree is listed as the first one (the other six are lawns and parks, urban forests, cultivated land, wetlands, lakes and sea, and streams).
Street trees assemblage is the foundational natural component in the city because street trees would be everywhere provided there is a road or street et. (Berland and Hopton 2014). Although it just accounts for a relatively small fraction of the entire urban natural components, it provides prominent ecosystem services for urban habitants (Mullaney et al. 2015, McPherson et al. 2016). Street trees can improve urban dwelling environment not only by environmental beautification and ornamentation (Silvera Seamans 2013), but also by air purification, stormwater reduction, energy conservation, noise reduction, carbon storage, shade providing and so on (Silvera Seamans 2013, Mullaney et al. 2015).
However, the distribution of street trees assemblage within a city is inequal and in most cases, is affected by social-economic development more than any other factors such as natural ones (Pham et al. 2017), except for some cases where physical geography is various (Lowry et al. 2011, Berland et al. 2015). In generally, the factors related to social-economic development can be divided into 3 categories: urban form (e.g., population density and urban morphology), urban geography (e.g., urban sprawl), and drivers of vegetation management (e.g., public policies, life style). According to the population density theory, the higher population density will lead to less physical space available for vegetation. Some researchers find negative relationship between population density and vegetation cover (Mennis 2006, Conway and Hackworth 2007, Troy et al. 2007, Luck et al. 2009). But recently, Bigsby et al. (2013) finds that there is a paradox in the result between Baltimore and Raleigh. (Pham et al. 2017) also found that there is a negative relationship between tree cover and population density, but a positive one between street tree cover and population density. That is probably some new situation coming out along with social-economic development. There is a need of more researches about the relationship between vegetation cover and population density to renew the population density theory. Nevertheless, we still concern about the relationship between vegetation cover as well as its associating benefit and population density when we explore the distribution characters of vegetation cover, especially when we focus on the ecosystem services provided by vegetation, because we want to know whether the demand of people is satisfied.
The distribution of urban natural component is also reported to be affected by spatiotemporal urban variation (Howe et al. 2017, Malkinson et al. 2018, Xing et al. 2018). In generally, because the urban core area is more affected by human disturbance than suburban area, the natural characteristics of the natural component in urban core area is more severely disturbed than in suburban area (Ren et al. 2012, Ossola and Hopton 2018). Concerning to the relationship between urban geography and urban green space, the degree of fragmentation of green space is increasing along with urban-suburban gradient (Shrestha et al. 2012, Inostroza et al. 2013, Malkinson et al. 2018), and the area of urban green space shows a decreasing trend with urbanization (Berland 2012). However, Berland and Hopton (2014) found that the street tree assemblage doesn’t significantly change along with urban-suburban gradient. He inferred that the management practices are relatively more important drivers of street tree characteristics than urban form.
As a natural component in cities where dominated by human, urban trees would be affected by policies made by human. However, how policies affect either urbanization or urban trees remains an issue to be explored (Wu 2014). Some researches found that policies would promote urban tree canopy cover increasing (Berland and Hopton 2014, Krafft and Fryd 2016). While Galenieks (2017) in their research about the comparison between city Loma Linda and Redlands, California suggested that policies are not always leading to positive effects on urban trees if the commitments to preservation and to practice management principles are not fulfilled. Hill et al. (2010) found some policies would effectively help to improve the status of urban trees and others not, even though all of them are initiated to improve trees conservation and planning. More researches on urban forest management and relation between urban forest benefits and existing policies (e.g. city planning policy, climate change adaptation, energy policy) are needed in the future (Krajter Ostoić and Konijnendijk van den Bosch 2015).
In this paper, we examined air purification ecosystem service which provided by street tree assemblage at the subdistrict scale, and explore its relationship with population density, urban sprawl and public policy respectively, to address the three following questions: (1) how does air purification benefit vary across the city at the subdistrict scale? (2) Whether the benefit of street tree assemblage varies along with the population density and urban-suburban gradients? (3) Does the public policy have a positive effect on the street tree benefit? From these questions, we intent to improve our understanding about the characteristics of street tree benefit distribution within a city and how urban form and urban geography influence the urban ecosystem services distribution, as well as how human modify urban natural component, thus urban ecosystem services, by public policy system.