Can Tree Mortality Predict Human Mortality?

A tree saved your life today. You might not know it, but living near trees is drastically improving your health.

We already know that regular contact with the outdoors (you know, plants and natural light and so on) seems to raise students’ test scores and help people recover from surgery more quickly. Now new research indicates that the benefits of trees--and the costs of their absence--might be even more important.

Geoffrey Donovan is a researcher at the U.S. Forest Service’s Pacific Northwest Research Station. He has long been interested in how trees affect life for city dwellers. And a devastating pest, the emerald ash borer, presented him with an opportunity to find out what happens when a huge number of trees disappear from residential areas on a large scale.

The emerald ash borer, an invasive beetle from East Asia, was first discovered in North America in 2002. It infects all species of the North American ash, and infected trees almost never survive. By 2012, it had killed an estimated 100 million trees in 15 states.

Donovan and his colleagues gathered 18 years of data from all the counties where the ash borer had wreaked its havoc--mostly around the Great Lakes--and looked for correlations between the loss of trees in those areas and human mortality.

“I wasn’t surprised that we found an effect,” he says, “but I was a little surprised by the size of the effect.”

According to their mathematical model, the presence of the borer, and the subsequent loss of trees, was associated with 6.8 additional deaths per year from respiratory causes and 16.7 additional deaths per year from cardiovascular causes per 100,000 adults. That’s more than 21,000 deaths in total.

To try to make sure this was a case of causation, and not just correlation, they modeled the relationship between the borer and mortality across space and time simultaneously, and controlled for demographic factors like race and income.

So how exactly do trees help our heath? Donovan doesn’t know precisely. It could be by improving air quality, reducing stress, increasing physical activity, moderating temperature, or a combination of several factors. Regardless, it’s becoming increasingly clear that protecting our arboreal friends is more than hippy tree-hugging--it can keep us humans vibrant and salubrious.

As Donovan says, “Perhaps we should start thinking of trees as part of our public-health infrastructure.”

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