1 Introduction
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.