Declining Urban Tree Canopy Linked to Higher Mortality in Chicago Neighborhoods

Losing mature trees in an urban neighborhood may be more than an environmental concern—it could also affect residents’ health and survival, according to a new study of Chicago communities.

Published Aug. 5 in the journal GeoHealth, the study examined tree canopy coverage and mortality rates across Chicago neighborhoods over an 11-year period. Researchers found that the year-to-year change in urban tree cover was more closely associated with mortality than the total number of trees in a neighborhood at any single point in time.

Neighborhoods with steadily declining tree canopy tended to have higher mortality rates, particularly in the hottest parts of the city. By contrast, areas where tree canopy increased over time generally recorded lower mortality rates.

Urban tree canopy and mortality rates

The study does not prove that tree loss directly causes higher mortality. Instead, it identifies an association between shrinking tree canopy and rising death rates. Even so, the findings add to growing evidence that urban trees play an important role in protecting public health, particularly as climate change increases the risk of extreme heat.

Trees can cool city streets by shading sidewalks, roads and buildings. They also release water vapor through a process known as evapotranspiration, which can lower surrounding air temperatures and make neighborhoods more comfortable during hot weather. Protecting urban tree cover may therefore help reduce exposure to extreme heat.

The study’s significance lies not only in showing that trees are associated with better health outcomes, but also in the way researchers measured the relationship. Earlier studies often compared mortality with a single snapshot of neighborhood greenery. This research instead tracked how tree canopy changed over time.

“Many past studies linking tree canopy to mortality relied on a single snapshot of a neighborhood’s greenery,” Vivek Shandas, a professor of urban studies and planning at Portland State University who was not involved in the research, told Live Science.

Shandas noted that the new findings are consistent with research from other U.S. cities. Across 33 U.S. cities, nearly three-quarters have experienced a loss of greenery, with the hottest areas often showing the greatest vulnerability.

Tree canopy growth associated with lower mortality

The estimated effect was substantial. Each year-over-year percentage-point increase in tree canopy was associated with approximately a 10% reduction in mortality rates. The largest decreases were linked to deaths from cardiovascular disease, respiratory disease and mental health-related conditions.

To conduct the analysis, researchers combined annual satellite measurements of Chicago’s tree canopy by census tract from 2011 through 2021 with more than 220,000 death records from city and state health departments. Their statistical models accounted for several factors that can influence mortality, including summer temperatures, air pollution, income and residential segregation.

The research team grouped neighborhoods according to their level of tree canopy and examined how changes in greenery related to heat exposure and mortality. This approach helped identify where tree canopy loss or growth appeared to have the strongest relationship with health outcomes.

Chicago neighborhoods show unequal tree loss

Like all observational studies, the research has limitations. Although tree canopy loss and higher mortality occurred at the same time, the analysis cannot prove that one caused the other. Satellite imagery may also miss newly planted trees or underestimate smaller areas of canopy that higher-resolution tools could detect.

It is also unclear exactly why declining tree cover was associated with mortality from several different conditions, rather than only illnesses strongly connected to heat exposure. Shade is one possible explanation, but trees may also influence air quality, physical activity, stress levels and neighborhood conditions in other ways.

Geography played an important role in the results. Chicago’s South and West sides experienced some of the greatest tree canopy losses, the hottest summers and the highest mortality rates. Some of these neighborhoods began the study with above-average tree cover but lost greenery over time. Lakeside neighborhoods, meanwhile, generally remained cooler and had lower mortality rates.

Study co-author Harrison Garcia, a medical student at Northwestern University, said the project began after a walking tour of Chicago’s sparsely wooded neighborhoods.

“We noticed how tiring it was to walk in areas with less tree cover compared with nearby blocks lined with mature trees,” Garcia said in a statement.

Protecting mature trees may be especially important

Using their statistical models, the researchers estimated that a 0.03% annual increase in Chicago’s citywide tree canopy—roughly consistent with the planting of about 14,000 trees per year—could prevent approximately 105 deaths annually.

However, the researchers emphasized that this figure is an estimate based on statistical modeling, not a direct count of lives saved. Newly planted trees also cannot provide the same level of shade and cooling as mature trees, which may take decades to reach their full size.

Previous research in Portland found that the health benefits of planted trees increased as the trees grew older.

For that reason, the study’s findings suggest that preserving existing mature trees may be just as important as planting new ones. Maintaining healthy urban forests could help reduce heat exposure and support better health outcomes in communities already facing environmental and economic disadvantages.