4.2 street tree air purification benefit and urban geography
In most cases, along with the urban-suburban gradient, trees/green space is increasing (Berland 2012, Ren et al. 2012, Malkinson et al. 2018), but in our case, the PKB which indicates ability for providing air purification benefit by street trees of a specified area showed a decreasing trend. Berland and Hopton (2014) also found that street tree assemblage doesn’t show an increasing trend. Actually, they even didn’t find a decreasing trend along the urban-suburban gradient. They found the peak of stormwater interception benefit provided by street tree assemblage is in the inner-ring suburban area and the lowest one shows in the outer-ring suburban area. They thought the temporally lagged growth of the newly planted trees in the outer-ring suburban is the main reason for that. However, it isn’t true in our case. The ATB which indicate the average ability for providing air purification of a tree showed an increasing trend along the urban-suburban gradient. That implies there are more big/old trees in the outer-ring suburban area than in the old town. The main reason for our case might be that there are more trees in the old towns than in the outer-ring suburban area (the amounts of street trees for per kilometer in B1, B2 and B3 are 228 trees/km, 194 trees/km and 160 trees/km respectively). That also probably reflects the fact that the investment of street trees planting in the old town is more than the outer-ring suburban area.