Saving old trees may protect people's health more than planting new ones, study of Chicago neighborhoods finds

Losing mature trees from a neighborhood may be a matter of life and death, a new study suggests.

The research, published Aug.

5 in the journal GeoHealth , tracked tree canopy cover and mortality across Chicago neighborhoods over 11 years.

It found that it wasn't the number of trees in a neighborhood that was most strongly linked to residents' survival — it was whether the overall tree canopy was shrinking or growing from one year to the next.

Neighborhoods that kept losing tree cover tended to see death rates climb, especially in the hottest parts of the city, while neighborhoods that kept gaining canopy saw death rates fall.

The study can't say for certain that losing trees is what spiked death rates; it only shows the two trends moving together.

But the finding adds to growing evidence that urban tree cover plays an outsized role in protecting people from extreme heat as the planet warms .

Trees cool city blocks by shading pavement and buildings and releasing water vapor that lowers the surrounding air temperatures.

The new study's finding is less about discovering trees are good for people's health — scientists have suspected that for years — and more about how the researchers tracked it, said Vivek Shandas , a professor of urban studies and planning at Portland State University who wasn't involved in the work.

Much of the past work linking tree canopy to mortality relied on a single snapshot of a neighborhood's greenery, including a 2022 analysis of heat-related mortality tied to tree cover across dozens of U.S. cities.

In this new study, researchers followed how canopy actually changed year to year, and it was that change rather than the raw number of trees that tracked with mortality.

The finding "reinforces what we've found in other cities," Shandas, who studies heat exposure and tree canopy loss in the U.S., told Live Science in an email.

"Across 33 U.S. cities , nearly three-quarters were losing greenery, and the hottest neighborhoods were generally losing more — or gaining less — than cooler neighborhoods." The effect size in the new study was notable: each year-over-year percentage-point increase in canopy was tied to roughly a 10% drop in mortality, with the greatest reductions in cardiovascular, respiratory and mental health-related deaths.

To reach their conclusion, the researchers combined annual satellite measurements of tree canopy for every Chicago census tract from 2011 through 2021 with more than 220,000 death records from state and city health departments.