Tree canopy loss in cities may be linked to higher death rates, according to a new study. Researchers have found that Chicago neighbourhoods that lost more tree cover over 11 years also recorded higher mortality, especially in areas with hotter summer temperatures. Researchers say the findings show that cities should not only plant new trees but also protect mature ones.The study was published in the journal GeoHealth, Science X reported. Researchers examined annual changes in tree canopy between 2011 and 2021 and compared them with more than 220,000 death records from the same period. I“Previous studies have mostly looked at how health outcomes are associated with static tree canopy coverage,” says the study’s senior author Daniel Horton of Northwestern University.“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 adds.Tree canopy refers to the parts of trees made up of leaves and branches that spread over the ground when viewed from above. The larger the canopy, the more shade the trees provide. Mature trees usually have much wider canopies than young trees.
Tree canopies across Chicago
The project grew out of the Buffett Institute’s Defusing Disasters Working Group. The group brings together climate scientists, doctors, public health experts, city officials and community partners to study how extreme weather affects people in Chicago and how cities can reduce those risks.“The Defusing Disasters Working Group set out to determine what makes Chicago residents vulnerable to extreme heat,” Horton says. “We found that a number of different factors contribute to heat vulnerability, and trees are one part of the equation.”The idea for studying tree canopy also came from personal observations made by lead author Harrison Garcia during a walking tour of Chicago’s North Lawndale and Douglas Park neighbourhoods.“I noticed how much more exhausting it felt to walk through areas with little tree cover compared to nearby blocks lined with mature trees,” Garcia says.“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.”
Chicago neighbourhood data showing tree canopy, summer temperatures and mortality (2011–2021). Credit: GeoHealth (2026).
To test this idea, scientists used satellite images to measure tree canopy every year across every census tract in Chicago between 2011 and 2021.The satellite information was then combined with more than 220,000 death records collected by the Illinois and Chicago Departments of Public Health.Many things can affect health besides trees. To make sure tree canopy was being studied as fairly as possible, the researchers built a statistical model that considered several other factors.The team found that simply having more trees in a particular year was not strongly linked with lower mortality. Instead, what mattered most was whether tree canopy increased or decreased over time.Neighbourhoods where tree canopy increased were consistently associated with fewer deaths from cardiovascular disease, respiratory disease, several musculoskeletal conditions and mental health-related illnesses.
Why mature trees are important
Planting trees is important, but planting new saplings does not immediately replace the benefits of older trees. Young trees take many years to grow large enough to produce broad canopies that provide shade and cooling.Mature trees already perform these functions. When mature trees are removed because of disease, storms or development, decades of growth can disappear almost immediately.Study co-author Raed Mansour says cities should focus on protecting existing trees as well as planting new ones. “Chicago must both maintain the canopy it has and expand the canopy where it’s needed most,” Mansour says.“That means protecting mature trees from unnecessary removal, providing ongoing care for existing trees, replacing removed trees and investing in their long-term survival,” Mansour adds.To understand what greater tree cover could mean, scientists also created a model based on a small annual increase in canopy. They estimated that increasing Chicago’s tree canopy by about 0.03% each year, roughly equal to planting around 14,000 trees annually, was associated with about 105 fewer deaths each year.Trees provide benefits that go beyond reducing heat. They help improve air quality, encourage people to spend time outdoors and contribute to healthier neighbourhoods. Previous research has also linked trees with lower stress levels, reduced anxiety, lower blood pressure and stronger community connections. Trees can also reduce soil erosion and support biodiversity by providing habitats for many plants and animals.Mansour says, “Many folks see trees primarily as aesthetic features or amenities, but they should be recognised as part of our public health infrastructure.”
