Street Trees improve your health, Scientists have proved it.
There is a lot of research, going back several decades, indicating that exposure to natural environments improves our health. For example people who have views of nature from their hospital bed recover more quickly from operations. There is also statistical evidence, when disease reduced the number of trees in some areas of the USA, that the loss of trees from an environment leads directly to a deterioration in the health of residents of that area.
But until recently, nobody has investigated street trees and health. To address this Omid Kardan and Peter Gozdyra, at the University of Chicago, led a large international team to study this issue in the city of Toronto, which was chosen because it allowed a large scale study with a variety of street tree environments and good opportunities to ‘tune out’ other factors that might interfere with the statistical results. For example Canada has a universal healthcare system so local variations in healthcare are much less significant than in a US city.
The research used a geographical analysis of tree canopy, block by block, combined with data from a city-wide programme of health interviews (The Ontario Health Study) that covered a wide range of physical health indicators, mental health and self-perception of health. A sophisticated statistical analysis was undertaken to remove the effects of known health factors such as income, gender, age, diet, neighbourhood affluence and education.
Their study revealed that the presence of a substantial tree canopy on residential streets had a dramatic effect on the health of residents. As a standard measure they took a city block 140m x 140m with 10 trees around its perimeter, each with a crown area of 40 sq m. They found that this affected the health indicators of residents to a similar extent as having an increased annual income of CA$10,000 (£4,800), or being 7 years younger. They also found that increasing the amount of canopy beyond their standard measure brought about further improvements in health indicators.
I have questioned one of the lead authors about these measures as British cities do not have a regular pattern of city blocks. Kardan has explained (email to Chris Rust [CR], 15/01/2016), that the 10 trees per block was equivalent to one tree every 28m along the street. The size given (40 sq m) indicates a crown diameter of 7m, the mature Limes and similar trees on many Sheffield streets are of a similar size, CR measured two such trees on Ladysmith Avenue and found they were approx. 8m diameter
Replacement trees will take decades to reach that size so the health effects of the current programme will be significant. Now that this research has been published, it is possible that residents in streets where a substantial number of mature trees are removed may have a legal claim against Sheffield City Council or Amey for damages, due to the scientifically verified effects on their health.
The research makes a strong case for local authorities and health authorities to plant more canopy trees on streets, especially in areas where health is poorer than average. Some cities are doing that already.
Kardan, Gozdyra et al (2015) Neighborhood Greenspace and Health in a Large Urban Center, Nature Scientific Reports 5, Article number: 11610 (July 2015). [Available online at http://www.nature.com/articles/srep11610]