4.3 street tree air purification benefit and public policy
Public policy probably is a main driver of vegetation management.
Irga et al. (2017) in their research pointed out that policy could encourage green infrastructure installation in Australia. Harrington and Hsu (2018) also showed that policy would take the lead role in driving green infrastructure installation in US. Our results also support this statement. Shenyang start to redevelop old industrial area (district T) and develop new industrial in the selected areas (district D and Y) from 2002. The city planning of the old industrial area and new industrial areas at that time would aim to improve the environment and redevelop the old industrial areas as livable areas to attract more citizen to move in. Green infrastructure construction is an important mean to achieve the redevelopment target, therefore, the government would take political and economic action to promote the green infrastructure construction in these areas. As Hopkins et al. (2018) said, integrating green infrastructure into control plans by policy means is an important method to improve green infrastructure installation. It reflects that government plays a key role in promoting green infrastructure installation and improve ecosystem services in city. In another word, if the government has done nothing in green infrastructure construction, such as taking action to improve it, or preventing it from being occupied by building or something else, the green space in the city would substantially degrade and shrink (McWilliam et al. 2015).
Also, we should notice there is a time leg effect of public policy on development of green infrastructure because of trees needing time to grow up. The highest benefit values were found in district T and D,which were starting to redevelop in 2002 and 2003 respectively. The benefit value of district Y (starts to develop in 2010) were found around the average value (USD 1048.75 for PKB) of the 7 districts. The newest district HN which was set up in 2014 had the lowest value under the average value. But we noticed that the ATB of HN is not as low as its PKB among the 7 districts (Fig. 3C). That give us a caveat that when planting new trees in a developing area, the most important thing is preserving the high-quality heritage tree (in generally, the tree with DBH over 20cm should be preserved. That would be a good foundation to construct street tree landscape with rich layers, walkable pedestrians and livable environment for residents.