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.