3.2 Analysis along population gradient and urban-suburban gradient
The per kilometer and average tree air purification benefit of P1, P2 and P3 were USD 1049.75 and 5.51, 1336.56 and 7.19, and 770.08 and 4.98 respectively (Fig. 3A). P2 had the highest air purification benefit both for per kilometer and average tree, but not the lowest population density. While the lowest population density group P3 had the lowest PKB and ATB. The highest population density group P1 had a medium value of PKB and ATB. The result is contrary to the population density theory. Also, there is no a decreasing or increasing trend along with population density gradient.
The per kilometer of air purification benefit and average tree of air purification benefit showed opposite trends along the urban-suburban gradient (Fig. 3B). The PKB of B1, B2 and B3 are USD 1310.56, 1183.94 and 985.00 respectively, showing a decreasing trend along the urban-suburban gradient. While the ATB of B1, B2 and B3 are USD 5.75, 6.07 and 6.13 respectively, increasing along the urban-suburban gradient.