Trees store carbon, reduce toxins in the air, and even fight crime. Now add this to the list of incredible things they do for the planet: According to new research, trees may improve human health where they are plentiful.
But invasive insects, drought, wildfires, and climate change increasingly threaten the nation’s trees, which leads to an obvious conundrum: If trees boost health, what happens when they die en masse?
Dr. Geoffrey Donovan of the U.S. Forest Service set out to answer this question by making the most of a horrible blight on tree populations across the Midwestern United States. The emerald ash borer, an invasive insect that made its way to the U.S. from Asia, has killed 100 million ash trees since it was accidentally introduced in the late 1990s, mostly in Michigan, Ohio, Indiana, and Illinois. Entire streets lined with ash trees can become treeless when emerald ash borers descend on a neighborhood—borers kill nearly all the trees they infest.
Donovan’s hypothesis was that if the presence of trees affects public health, than killing a lot of trees quickly would negatively impact health outcomes in neighborhoods where ash trees had largely disappeared.
He and his team pored over 18 years of data from 15 states, and found that their theory was correct. Americans living in neighborhoods most affected by the emerald ash borer suffered an additional 15,000 deaths from cardiovascular disease and 6,000 more deaths from lower respiratory disease compared to areas that were not afflicted.
“That affect got worse the longer the bug had been in that county,” Donovan said in an interview with TakePart.
The researchers controlled for demographic differences, like income, race, and education—all factors that are known to influence health outcomes. Interestingly, they found that the absence of trees affected wealthy neighborhoods disproportionately.
“Wealthier counties tend to have better more trees and better maintained trees. If you are getting a bigger public health benefit when the trees were there, then you would expect a larger negative affect when they’re not,” Donovan said.
Donovan is quick to point out at the study does not imply a causal link between the loss of trees and increased human deaths due to cardiovascular and lower respiratory disease. But the research does provide valuable evidence of a pattern—one that deserves further exploration.
“There seems to be this fundamental relationship between exposure to natural environments and human well-being,” Donovan said. “We need to not just think about trees as being nice in the neighborhood or keeping you cool on a hot day. Maybe we should start thinking about trees as part of our public health infrastructure.”
William Anderegg, a biologist at Stanford University who was not involved in the study, says Donovan’s research is “suggestive and intriguing,” in that it finds interesting patterns, but would be hard to use as definitive proof that dying trees lead to more human deaths. Proving a causal relationship is the hard part. “The study is relatively unique at looking at wide-scale ecosystem change and trying to correlate it to wide-scale public health outcomes,” Anderegg said.
Previous studies have examined the link between trees and smaller-scale health outcomes. Pioneering research found that the recovery of hospital patients after surgery was affected by how much of the natural environment patients saw outside their windows. Patients who looked out at trees during their time in the hospital experienced reduced stress, faster recovery times, and decreased use of strong painkillers than patients whose view was of a brick wall.
Anderegg believes that some of today's most encouraging forestry research connects trees to larger socio-economic and public health trends, providing a much needed antithesis to gloom and doom stories of forest loss. “There is really a lot of potential to use trees as part of the solution to some of the problems we face: smart management of forests could help minimize climate change; positive health affects can come from urban green spaces in cities and the replanting of trees,” he said. “We’re only starting to really appreciate what those positive benefits are. My hope is that we’ll make greater use of them in the future.”