4.1 street tree air purification benefit and population
density
Our result doesn’t support the population density theory, and is also
different from results found by Bigsby et
al. (2013) and Pham et al. (2017), who
found increasing trend of street tree cover along increasing population
density gradient. In our result, there isn’t certain trend of street
tree air purification benefit along population density gradient. The
highest benefit is neither in the area of the highest population density
nor in the area of the lowest population density, but in the area of
medium population density. While the lowest benefit is company with
lowest population density. Luck et al.
(2009) also found that the highest trees cover is in the mid-population
density area. But they found the cover at the lowest population density
is higher than cover at the highest density. The population density
theory is not supported any longer in some cases, because the
relationship between population distribution and green space is more
complicated under new situation, such as the improvement of ecological
consciousness which induced more investment on the green space
construction in old town where population density is high
(Harrington and Hsu 2018), the different
characteristics of different city natural component
(Per Bolund and Hunhammar 1999), and so
on. The change of original green space in the city might be explained by
the population density theory (Ren et al.
2012), while the artificially introduced ones would be explained by
other factors (Pham et al. 2017). In
theory, the area of lowest population density would have more space
available for trees. However, in our case, low population density also
relates to low economic development. We infer that is the main reason
for the low street tree benefit in the low population density, because
street tree assemblage is different from other conserved green space
such as a park which is based on the original green space. Street tree
assemblage is a natural element introduced totally artificially, and
related to the development of street/road which is also closely related
to economic development. Therefore, street tree assemblage development
is dependent on the investment by the government for the construction of
streets/roads and their related infrastructure.