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.