The Healing Power of Trees

Until now, that is. Green Heart is the first project to back up the commonsense idea that trees are good for people with rigorous medical research—and the first clinical results, released this past August, are already attracting national attention.

In a way, Louisville is used to the spotlight. The river city of about 600,000 people sits at the center of Kentucky’s vaunted bourbon industry, drawing tourists to its craft distilleries. Downtown, not far from the Muhammad Ali Center (the self-proclaimed “Greatest” was born here), a landmark factory still makes Louisville Sluggers–Major League Baseball’s official bat. And, of course, for 150 years, the city has hosted the Kentucky Derby, one of the most-watched horse races in the country.

Outside of the first Saturday in May, however, the people who live in the shadow of Churchill Downs have sometimes felt overlooked.

“Louisville has a lot of distinctions that are not great,” says Chris Chandler, a Louisville native who directs TNC’s Cities and Strategic Partnerships Program. “We have super-poor air quality. The air pollution is, in some cases, generated from industries and activities in Louisville, but it also moves up and down [the Ohio] River.” Because of the topography, he adds, poor-quality air sticks around longer during the summer.

There’s a direct connection, scientists have long known, between air quality and human health. Polluted air can lead to cancer, heart disease and even obesity. These conditions have contributed to a recent two-year drop in life expectancy between 2011 and 2021. And the harms are not evenly distributed: People who live in more prosperous, predominantly white parts of Louisville live more than 15 years longer than the residents of more diverse, poorer neighborhoods—which, not coincidentally, also tend to have fewer trees.

Such disparities have long troubled Dr. Aruni Bhatnagar, chief of the Division of Environmental Medicine and director of the Christina Lee Brown Envirome Institute at the University of Louisville. He was aware of the longstanding associations between greenery and human health, and of the ability of plants to mitigate air pollution, including by serving as a physical barrier against dirty air, absorbing chemicals into their cells and even collecting particles with their leaves, in the same way that insects are trapped by flypaper.

Still, the connection between greenery and health had only ever been demonstrated by observational studies. Randomized controlled trials are generally accepted to be the gold standard in medical research. Most pharmaceuticals on the market have multiple such studies behind them. So Bhatnagar intended to treat nature like medicine.

“The best way to do it would be to plant trees and then see what happens,” Bhatnagar says. “That’s the clinical scientific methodology. The trees are the treatment.”

It was a simple idea, but the logistics would prove more complicated. Bhatnagar realized that, to make a significant difference over the limited time that could be allotted to a clinical trial, the research team would have to plant very large trees, not saplings. And they would need a lot of them.

Planting trees may be an unorthodox clinical intervention, but it was in the wheelhouse—and the budget—of The Nature Conservancy, which over the past decade has been conducting more work in cities, places where the well-being of nature and humanity intersect in clear ways. That’s sometimes been about ensuring clean water or air, or safeguarding habitat. Here, the goal would be improving people’s health.

“We are an imperiled species,” says Chandler, who helped lead TNC’s involvement in the collaboration that became Green Heart. “We are just as important as any other system or any other species on the landscape, and humans need to see ourselves in the work of healing and restoring our planet.”

The project began with community engagement, as the local nonprofit organization Louisville Grows worked to ensure that people wanted trees in their yards, and that they would do their part to help care for them. Many residents were excited; community groups had after all been working for years to get more infrastructure investment, including in parks and street trees. Others expressed less enthusiasm about watering and raking leaves, or disbelief that the trees were actually free.

“We spent a lot of time knocking on doors and sitting on front porches,” Chandler says. “It’s conversations like that where you build trust, you build understanding.”

Meanwhile, Bhatnagar’s research team recruited about 750 people, both from the treated neighborhoods and from a control area south of downtown Louisville, who would have their health monitored before and after the trees arrived.

Then, in 2019, volunteers and contractors began planting.

“We planted trees in every typology that you could find in the city,” Chandler says. “We planted them in parks, on private property—backyards, front yards. We planted them all the way from the neighborhood street to arterial roads to the federal highway. We planted in church parking lots, at community
centers, even a couple commercial properties like gas stations.”

Jerry Englehart, a social worker who lives two blocks from the expressway, accepted more than two dozen trees on his .11-acre lot, including a handsome, flowering yellowwood in his front yard and a row of arborvitae out back. The difference has been remarkable, he says, noting that traffic noise is down, while more birds have come to roost as the trees grow. 

“It’s really nice to be able to go into our backyard and be in a green space, without having to drive somewhere,” he says. “We can just go out there and get green therapy.”

Englehart and his wife, who recently welcomed a baby daughter, also signed up for the health study. “I think research is important,” Englehart says, “and to be able to participate in this, it makes me feel like I’m actually contributing in a way that’s helping.”

This past August, the research team at the University of Louisville reported their first results. Within the treatment area, where homes are now surrounded by more than twice as many trees, study participants had significantly lower levels of a blood marker known as C-reactive protein, which is a strong indicator of inflammation.

Part of the body’s natural defense system, inflammation is triggered by injury, infection or irritants, summoning immune cells to combat pathogens and heal damage. But so-called chronic inflammation, which lasts for months or years and is associated with ongoing exposure to irritants like cigarette smoke and air pollution, actually degrades the body’s functions and contributes to problems like cardiovascular disease, diabetes and some cancers. The 13-20% reduction in C-reactive protein the Louisville researchers found translates to a proportionally lower risk of these inflammation-associated conditions.

It’s great news for residents of the newly green neighborhoods, and an astounding finding for the research team.

“It was hard to convince ourselves!” Bhatnagar says. “Certainly we didn’t expect this, within such a short time.”

There’s plenty more to learn, Bhatnagar says, cautioning that the dramatic drop in inflammation levels, though heartening, is still just the first result. The researchers are continuing to track participants’ health, including effects on blood pressure, immunity and mental well-being. Other studies are looking at environmental effects, such as how the trees have affected air pollution, temperature and biodiversity.

Meanwhile, Bhatnagar is fielding calls from researchers at other institutions hoping to replicate the Green Heart findings. And already other cities are beginning to look to Louisville as a leader in connecting urban greenery with human health. That’s been a source of pride for this city by the Ohio River, as well as for everyone involved with Green Heart.

“Until we got these first results, I’m sure there was continued skepticism out there,” says Phemister. “But seeing those initial results, and how quickly this showed up in the data, was a real eye-opener. Think about the billions of dollars that are invested in our road systems, in our utility systems—all essential, all important for human communities. Nature needs to be a part of those investments, too.”

“Nature is critical infrastructure in cities,” he adds. “And its conservation could be a key public health strategy.”