As urban tree shade shrinks, mortality rises

As Chicago's urban tree cover diminishes, the city's mortality rates are on the rise, according to a new study by Northwestern University researchers. The study, published in the journal GeoHealth, tracked death rates and tree canopy cover over an 11-year period in Chicago. Areas experiencing significant losses in tree canopy, which refers to the green area under trees and their leafy crowns, also saw higher mortality rates, especially in the hottest neighborhoods of the city.

The study's senior author, Daniel Horton, an associate professor at Northwestern's Weinberg College of Arts and Sciences, emphasized the importance of prioritizing the preservation of mature trees alongside efforts to plant new ones. The researchers used satellite data to analyze changes in tree canopy across Chicago's census tracts from 2011 to 2021, and combined this with over 220,000 death records from local health departments.

By examining year-to-year changes in tree canopy, the team found that a neighborhood's overall tree canopy was not significantly linked to mortality. Instead, it was the changes in tree canopy over time that mattered most. Neighborhoods with increasing tree canopy experienced fewer deaths from cardiovascular disease, respiratory disease, musculoskeletal conditions, and mental health-related illnesses.

The study highlighted geographic disparities in Chicago, with neighborhoods on the city's South and West sides facing hotter temperatures, larger canopy losses, and higher mortality rates compared to other areas. To combat these disparities, the researchers recommended protecting mature trees from removal, providing ongoing care for existing trees, replacing those that are lost, and investing in their long-term survival.

This approach would not only increase tree canopy but also strengthen the city's resilience against extreme heat and other climate-related challenges.