A study led by a U.S. Forest Service researcher suggests that a Philadelphia program to increase tree cover across the city would prevent hundreds of premature deaths citywide, particularly in its poorest neighborhoods.
The city’s goal under its Greenworks program has been to boost tree canopy cover to 30% in each neighborhood by 2025.
The new research suggests that increasing the canopy to that degree could result in around 400 fewer premature deaths annually, because of a variety of factors.
Even a more modest increase, however, would allow more Philadelphians to live longer. Further, growing the canopy could have the most dramatic impact in poorer areas, which tend to have the lowest tree canopy.
“To the best of our knowledge, our report is the first citywide health impact assessment of estimated effects of a tree canopy policy on premature mortality,” the authors wrote in an article published in April in Lancet Planetary Health.
The lead author, Michelle Kondo, is a research social scientist with the Forest Service. Her research team included scientists from Pompeu Fabra University in Spain, the National Socio-Environmental Synthesis Center, Colorado State University, and Drexel University. The study was funded by the Spanish Ministry of Science, Innovation and Universities and the Catalonian government. Kondo had connections in Spain through a sabbatical in Barcelona where she learned a research method she applied to Philadelphia.
Kondo, of Narberth, said tree canopy is associated with a number of factors that influence mortality risk.
“One of the primary ways trees can help improve our health and increase our lives is through social connections,” Kondo said. “Trees bring us together, for example in parks. And we know that social isolation in old age is a big killer. Stress and mental health is another big one. Trees can reduce our stress and improve our mood.”
The recent COVID-19 stay-at-home orders demonstrated how vital green space is to escaping stressful situations.
Kondo said other research has shown that replacing vacant lots and greening those areas is associated with reduced gun violence and drug-related crimes.
A solid tree canopy can also reduce summertime temperatures by 10 or more degrees, cutting heat-related illness and death. Large neighborhoods in Philadelphia experience the heat island effect, which means they are warmer than surrounding areas because they have so much concrete, asphalt, and tar roofs, and so few trees.
The authors predicted how many premature deaths would be prevented every year if the tree canopy was increased on an ambitious scale, a moderate plan, or a lower one. They considered physical activity, air pollution, stress, noise, heat, and exposure to green space using a tool developed by public health researchers in Spain and Switzerland called the Greenspace-Health Impact Assessment. The tool provides a formula using data on millions of deaths globally.
Philadelphia was a good focus for the study, the authors said, because it is one of the largest urban areas in the United States and also one of the poorest, with a comparatively high mortality rate. Plus, the city has an established tree plan, and also has quality data showing existing tree cover.
The authors drilled down to the census tract level, looking at tree cover and socioeconomic data in each.
Their study comes on the heels of a plan by Philadelphia officials to reverse a loss of leafy shade that has amounted to the equivalent of 1,000 football fields in the last 10 years. In December, the city announced a 10-year Urban Forest plan, working with arborists, educators, and community leaders organized by Philadelphia Parks and Recreation and the Office of Sustainability.
Current tree canopy coverage in Philadelphia ranges from 2% to 88%, with an average of 20% coverage based on 2014 data.
Overall, canopy cover is better in the wealthier northern and western parts of the city, where there are more parks. The most densely urbanized and industrialized areas, including Center City and neighborhoods along the Delaware River, have sparse tree cover.