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. They built statistical models that controlled for factors like summer temperature, air pollution, income and residential segregation, which past studies show are tied to higher levels of mortality.

The team then grouped neighborhoods by their degree of canopy cover, and teased out how the heat and mortality data related to that metric. That helped them pinpoint where the effects of canopy loss or gain were strongest.

It's a rigorous design, Shandas said, but like any observational study it has limits: it can show canopy loss and mortality moving together, not prove that one causes the other. Plus, satellite measurements can miss newly planted trees and thus underestimate canopy cover, whereas finer-resolution tools could potentially pick up young, small-canopied trees that coarser satellite data misses.

It's still not entirely clear why tree cover seems to help, although the data suggest that the shade provided by trees isn't the whole story. That's because canopy loss was tied to higher death rates from a range of conditions, not only those strongly related to heat.

Geographically, the patterns highlighted in the study echoed Chicago's history of disinvestment. The South Side and West Side saw the sharpest canopy losses, hottest summers and highest mortality rates, despite sometimes starting with above-average tree cover. Lakefront neighborhoods stayed cooler by contrast, with lower mortality.

Study co-author Harrison Garcia, a Northwestern medical student, said the project began with a walking tour of sparsely treed Chicago blocks. "I noticed how much more exhausting it felt to walk through areas with little tree cover compared to nearby blocks lined with mature trees," he said in a statement.

The team modeled a hypothetical citywide canopy increase of 0.03% a year — roughly matching Chicago's current pace of planting 14,000 trees annually — and estimated it would prevent about 105 deaths per year, thanks to canopy growth.

Shandas said that figure is best read as a useful estimate of scale rather than a precise promise, since it comes from a statistical model rather than a count of trees planted and lives actually saved. He added that a newly planted tree doesn't deliver a mature tree's shade overnight, and prior research in Portland found planted trees were tied to greater reductions in human mortality as those trees aged.

This pattern, Shandas said, only reinforces the study's central point that protecting existing, mature trees may matter more than planting new ones.