Year-to-year increases in urban tree canopy, rather than existing canopy levels, were associated with lower all-cause and cause-specific mortality across Chicago neighborhoods in an 11-year longitudinal study published in GeoHealth.
Researchers from Northwestern University and collaborating institutions analyzed annual data from 801 Chicago census tracts between 2011 and 2021, integrating US Department of Agriculture tree canopy estimates, summer maximum temperatures, mortality records, nitrogen dioxide concentrations, and neighborhood demographic characteristics.
The analysis included 220,711 deaths and evaluated associations between tree canopy and all-cause as well as disease-specific mortality while adjusting for population size, income, neighborhood characteristics, air pollution, and regional differences across the city. Researchers also examined interactions between canopy change and heat exposure and used K-means clustering to identify neighborhoods sharing similar environmental and mortality patterns.
Existing tree canopy was not significantly associated with all-cause mortality. In contrast, each 1% year-to-year increase in tree canopy was associated with approximately 10% lower all-cause mortality. Similar associations were observed for cardiovascular, respiratory, mental health, and selected physical activity–modifiable musculoskeletal causes of death. The association varied with temperature and was particularly pronounced in the hottest neighborhoods, where greater year-to-year canopy losses were associated with higher predicted mortality.
Spatial analyses showed that neighborhoods on Chicago's South and West sides had higher temperatures, greater canopy loss over time, and higher mortality rates despite some areas having relatively high baseline tree canopy.
Using the study's regression model, researchers estimated that a hypothetical annual 0.03% citywide increase in tree canopy, equivalent to planting approximately 14,000 trees, would correspond to about 105 fewer deaths per year. The estimate was modeled rather than observed, and the study could not establish causality.
Sensitivity analyses restricted to the pre-COVID-19 period continued to show an association between year-to-year increases in canopy and lower mortality using both tree canopy and vegetation measures.
“Protecting tree canopy cover from losses over time, particularly in vulnerable areas with higher temperatures, may present a key opportunity to reduce mortality and mitigate impacts of climate change,” wrote Harrison C. Garcia of the Feinberg School of Medicine, Northwestern University, Chicago, Illinois, and colleagues.
The analysis also lacked information on how long decedents had lived at the addresses recorded on their death certificates, limiting assessment of individual long-term tree canopy exposure. The 30-m resolution canopy data may have missed smaller or newly planted trees, and residual area-level confounding could not be excluded.
The authors reported no conflicts of interest relevant to the study. The research was supported by the Defusing Disasters Working Group at Northwestern University's Buffett Institute for Global Affairs.
Source: GeoHealth
