Put down the apple. It’s the tree that may help keep the doctor away.

In urban areas, trees shade sidewalks, suck up air pollution, soften traffic noise – and are just plain nice to look at. And by taking climate-warming carbon out of the atmosphere, trees are good for the planet, too.

It turns out the health gains of all that greenery add up.

A recent study conducted in Portland, Oregon, found that in neighborhoods where a nonprofit planted more trees, fewer people died.

The paper, by researchers at the U.S. Forest Service, adds to a budding body of research into the health benefits of living around greenery. Its findings amount to a prescription for policymakers to plant more trees.

“Urban trees are an essential part of a public health infrastructure, and they should be treated as such,” said Geoffrey Donovan, the Forest Service researcher who led the study published in the December issue of the journal Environment International.

GREEN HEALTH CARE

For three decades, the Portland nonprofit Friends of Trees planted nearly 50,000 oaks, dogwoods and other arboreal species around the city, giving Donovan and his colleagues granular data on how its canopy has changed over time.

Using a mathematical model to control for factors such as race, income, age and education, the team found that for each 100 trees planted, there was roughly one fewer nonaccidental death a year.

“Across the board, the benefits of trees are astounding,” said Yashar Vasef, executive director of Friends of Trees, which plants across six counties in Oregon and Washington. “And they come at a lower cost than many other solutions.”

The health benefits of living among trees bloomed as the trees themselves grew. As the trees got older, grew taller and cast their leafy limbs wider, the mortality rates among people nearby went down, the study found.

Or as Donovan summed it up: “Bigger trees, bigger impact on mortality, which is what you would expect.”

MORE TREES, FEWER DEATHS

The findings are in line with results from other researchers suggesting nature is good medicine for many ailments, including depression and high blood pressure. Another recent study in the British medical journal The Lancet suggested a third of premature deaths from a 2015 heat wave in Europe could have been avoided with 30 percent more tree cover.

“Many other global studies have looked at similar research questions but use different study designs,” said David Rojas-Rueda, an assistant professor of epidemiology at Colorado State University who has studied the health benefits of vegetation but was not involved in this latest paper. “Most evidence confirms that tree planting is beneficial in reducing premature mortality.”

There are several reasons trees may boost health, including better air quality, reduced stress and increased physical activity among those in tree-lined neighborhoods. The link between planting trees and lowering death rates held in both already leafy neighborhoods, which tend to be wealthier, and tree-deprived neighborhoods, which tend to be poorer.

“Studies have found links between exposure to the natural environment and improved health in a wide range of different cities and countries,” Donovan said. “We certainly know that air pollution, stress, and sedentary behavior are bad for people no matter their race or socioeconomic status.”

The reverse, unfortunately, seems to be true, too. Mortality rates appear to go up in areas that lose tree cover.

In a previous study, Donovan and colleagues saw an uptick in deaths related to cardiovascular and lower-respiratory-tract illness in counties from Minnesota to New York that lost trees to a wood-burrowing pest called the emerald ash borer.

The study out of Portland has its limitations. Donovan’s team did not have access to individuals’ health records. Instead, the team looked at neighborhoods’ overall deaths. And the researchers did not have other detailed data that could shed light on other factors affecting residents’ health, such as a neighborhood’s smoking rate.

The paper stopped short of asserting a direct cause-and-effect relationship between trees and mortality.

Still, Donovan has taken this research to heart. A resident of Portland since 2002, he has planted fig, plum and pear trees in his yard in Oregon.

“I’m quite fond of fruit trees,” he said. “When you get a sun-warmed fig in that bowl of yogurt and honey, it’s not so terrible.”

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