As trees disappear from urban neighborhoods, lives may be lost with them.
In a new study, Northwestern University scientists tracked death rates and fluctuations in tree canopy cover across an 11-year period in Chicago. They found that areas experiencing greater losses in tree canopy - land covered by trees' leafy crowns - also experienced higher mortality rates, particularly in the city's hottest neighborhoods.
The findings suggest cities should prioritize protecting mature trees alongside planting new ones. Given the myriad health benefits associated with trees, tree maintenance has become even more crucial as human-caused climate change drives more extreme weather and intensifies heat risks in cities.
The study was published today (Aug. 5) in the journal GeoHealth.
"Previous studies have mostly looked at how health outcomes are associated with static tree canopy coverage," said Northwestern's Daniel Horton, the study's senior author. "These snapshot-style studies looked at mortality rates and the tree canopy at one moment in time. But cities and their tree canopies constantly change, so we looked at how the canopy and death rates change over the years. We found that as tree canopy decreases, mortality increases."
Horton is an associate professor of Earth, environmental and planetary sciences at Northwestern's Weinberg College of Arts and Sciences, where he leads the Climate Change Research Group (CCRG). He also is a faculty affiliate of the Paula M. Trienens Institute for Sustainability and Energy and the Roberta Buffett Institute for Global Affairs.
Exploring nature-based solutions
The study grew out of the Buffett Institute's Defusing Disasters Working Group, an interdisciplinary initiative that unites climate scientists, physicians, public health experts, city leaders and community partners to explore how extreme weather affects vulnerable populations in Chicago. Co-led by Horton, the group also develops practical strategies to help cities reduce health risks and strengthen climate resilience.
"The Defusing Disasters Working Group set out to determine what makes Chicago residents vulnerable to extreme heat," Horton said. "We found that a number of different factors contribute to heat vulnerability, and trees are one part of the equation."
A walking tour of Chicago's North Lawndale and Douglas Park areas inspired Harrison Garcia, the study's lead author and a medical student at Northwestern University Feinberg School of Medicine, to consider canopy coverage.
"I noticed how much more exhausting it felt to walk through areas with little tree cover compared to nearby blocks lined with mature trees," said Garcia, who also is a member of the Defusing Disasters Working Group. "I remember thinking that if the heat was affecting me so noticeably during a short walking tour, it must be shaping people's health over the long term."
To capture canopy changes, the team analyzed annual satellite measurements of tree canopy across every Chicago census tract from 2011 through 2021. Then, they combined satellite data with more than 220,000 death records from the Illinois and Chicago Departments of Public Health.
To isolate the relationship between tree canopy and mortality, the researchers developed a statistical model that accounted for differences in summer temperatures, air pollution, income, population size, racial/ethnic residential segregation and other neighborhood-level factors that can influence health. This approach enabled the research team to distinguish the effects of changing tree canopy from differences that already existed among neighborhoods.
Not all neighborhoods are affected equally
The team found that a neighborhood's overall tree canopy within each year was not significantly associated with mortality. Instead, what mattered were the year-to-year changes in tree canopy. Increases in canopy were consistently associated with fewer deaths from cardiovascular disease, respiratory disease, several musculoskeletal conditions and mental health-related illnesses.
"We used a technique called K-means clustering to group Chicago neighborhoods based on three factors: tree canopy, temperature and mortality," said study co-author Benjamin Barrett, a Ph.D. candidate in health and biomedical informatics at Feinberg and member of the Defusing Disasters Working Group. "That allowed us to identify patterns across Chicago and to pinpoint where protecting and expanding tree canopy is needed most."
The geographic disparities were striking. According to the model, neighborhoods on the South and West sides tended to experience hotter summer temperatures, greater absolute canopy losses and higher mortality than other parts of the city. Neighborhoods along Lake Michigan generally experienced cooler temperatures and lower mortality, while areas near the Forest Preserves had the city's largest tree canopy and experienced relatively lower mortality.
Mature trees matter
The study coauthors underscore an often-overlooked point: Planting trees and increasing tree canopy are not the same. While young saplings take years to develop broad, leafy crowns, mature trees already provide extensive shade and cooling. Losing those trees - to disease, storms or development - can erase decades of canopy growth almost overnight.
"Chicago must both maintain the canopy it has and expand the canopy where it's needed most," said study coauthor Raed Mansour, a member of the Defusing Disasters Working Group and of the Openlands Board of Directors. "That means protecting mature trees from unnecessary removal, providing ongoing care for existing trees, replacing removed trees and investing in their long-term survival."
To illustrate the potential benefits, the research team modeled what might happen if Chicago increased tree canopy by about 0.03% each year, which is roughly equivalent to planting 14,000 trees annually. The model estimated this increase was associated with roughly 105 fewer deaths annually.
Trees as public health infrastructure
While trees can cool neighborhoods by providing shade and reducing the effects of urban heat islands, they also have numerous other health benefits. Trees improve air quality, encourage outdoor activity and contribute to healthier neighborhoods.
"Trees provide dozens of benefits," Mansour said. "They decrease stress, anxiety and blood pressure and increase social cohesion. They also reduce soil erosion and support biodiversity. Maintaining the urban canopy and planting trees where they are needed are both important. Many folks see trees primarily as aesthetics features or amenities, but they should be recognized as part of our public health infrastructure."
The study, "Increasing urban tree canopy associated with reduced mortality: a longitudinal analysis of Chicago neighborhoods," was supported by the Buffett Institute for Global Affairs.
