The central staging site for Green Heart Louisville, where trees were stored before transport to a cluster of neighborhoods. (Aruni Bhatnagar)
BY BISHOP SAND
Aruni Bhatnagar looked up.
“This tree right here, it’s got a lot of good leaves so you can stick a lot of air pollutants in it,” Bhatnagar, a cardiology researcher, said as he gestured toward a magnolia tree on the U.S. Capitol grounds.
Bhatnagar, silver haired and wearing a black turtleneck, was in D.C. for the World Forum on Urban Forests to speak about his $15 million Green Heart Louisville project — an initiative aimed at showing a causal connection between greenness and human health, and a potential model for U.S. cities looking to measure the effects of their tree planting.
In 2018, Bhatnagar, a University of Louisville medical school professor, decided that he wanted to “do something” about air pollution in Louisville, which has repeatedly earned failing grades for air quality from the American Lung Association. His contribution, he decided, would be to find the connection between trees and better heart health using the gold standard for evidence: clinical trials.
“The idea is to learn to examine everything, no matter how obvious they may seem,” he says.
Bhatnagar is well aware of the massive forest of urban tree research available, but much of it involves observational health studies, in which scientists measure potential correlations between urban trees and residents’ health.
“What I thought was we really don’t know if trees are beneficial for health,” Bhatnagar said.
To get beyond that, he proposed the Green Heart Louisville initiative, which launched in 2018. Over time, contractors and volunteers have planted nearly 8,000 trees and shrubs in a cluster of lower-to-middle-income neighborhoods in southern Louisville and measured health data from nearly 500 residents.
Today, the project involves more than 50 researchers, four universities, four nonprofit groups, five state and local government agencies, and the U.S. Forest Service. It began as a collaboration between Bhatnagar; Louisville philanthropist Christina Lee Brown; former Louisville mayor Greg Fischer; and Ted Smith, Louisville’s then-chief innovation officer. Roughly $9 million from the Nature Conservancy got things moving. The National Institute of Environmental Health Sciences provided another $3 million, and local donors contributed $3 million as well.
The work is focused in neighborhoods that — like many poor urban areas — have fewer trees compared with more affluent parts of the city. The neighborhoods are mixed racially and ethnically: 54 percent White, 29 percent Black and 11 percent Hispanic. A highway runs right through the areas — providing an unhealthy baseline of air pollution.
Bhatnagar collects an almost-obscene amount of data that includes blood panels, urine, hair samples, wastewater runoff, air pollution samples, soil and leaf samples, bat sounds, LiDar scans, temperature and humidity measurements, crime data, psychological surveys and sleep surveys. It is all being parsed, and relationships are starting to emerge, he said.
Among the tantalizing hypotheses Green Heart is testing: whether trees filter air pollution that can stiffen human arteries. Another is whether trees reduce stress and improve sleep by buffering noise. Some trees seem to be better at filtration than others — evergreens, for instance, filter air throughout the year and those with needles absorb harmful pollutants more efficiently than broad-leafed trees.
Another hypothesis is that trees release a suite of chemicals into the air that reduce blood pressure and stress. Bhatnagar has seen these chemicals’ metabolites show up in urine samples at higher concentrations where people have more exposure to trees and other greenery.
Cities around the country are set to receive funding from the Inflation Reduction Act this year to plant trees, and already many local governments spend millions every year on planting and maintaining trees. Cities often maintain detailed records of size and health of every tree for every block, and LiDar scans from aircraft paint a more complete model of the urban tree canopy. Medical professors also study green spaces and trees’ effects on aging. And psychologists have observed that stress levels and depressive states are less in greener areas of the city.