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.