Tree and Human Health May Be Linked

Our deep connection with nature has roots seemingly as old as humanity itself. Ancient Celts viewed trees as sacred symbols of fertility and rebirth. Conservationist John Muir believed that everyone should have a sanctuary where “nature may heal and give strength to body and soul.” A recent study from the U.S. Forest Service now suggests that this primordial bond goes beyond metaphor, providing scientific evidence that tree and human health may be intertwined.

“I basically tagged onto one of the oldest ideas in the word,” said lead author Geoffrey Donovan, a researcher at the Forest Service's Pacific Northwest Research Station. But “the quantification is a new thing.”

Donovan and colleagues combed through demographic, human mortality, and forest health data from about 1,300 counties in 15 states from 1990 through 2007. Their findings? In counties where trees were plagued by the invasive emerald ash borer, about 15,000 more deaths from cardiovascular disease and approximately 6,000 from lower respiratory disease were reported compared to uninfested areas. The researchers focused on these ailments in particular since trees can filter particulates and other pollutants, improving air quality. Even after controlling for demographic variables, such as race, income, and education, they observed the same trend.

Since earlier studies of nature’s effects on human health often looked only at cross sections of the human population, they didn’t control for community influences on health. Other studies spanned only a year or a few years, but environmental changes happen slowly, and capturing their effects often requires much longer studies.

But Donovan didn’t have those problems, since the ash borer affected a broad swath of the country -- wiping out 100 million trees in the eastern and midwestern U.S. -- and trees can die quickly once attacked. A swarm of emerald ash borers can kill several trees at the same time, leaving once tree-lined streets barren within a few years of infestation.

Donovan’s study shows an association between loss of trees and human death due to cardiovascular and lower respiratory disease -- suggesting, but not proving, a causal link. “This isn’t so much hypothesis proving as hypothesis generating,” Donovan said. “I hope this is ongoing research.”

He added that the results, published last month in the American Journal for Preventive Medicine, offer an understanding of health that could help inform policymaking in a tenuous economy. Although studies have shown that people living near parks tend to be healthier, building a park might not be feasible in a densely populated urban area. Tree planting could be an easier, more practical alternative.

"You need to know what you get for your investment. It turns out now if we plant trees or protect trees, there might be a public health benefit," he said.  "Maybe we should think of trees as our public health infrastructure and look at it as a basic component of our well-being.”

Read More:

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Image by iStockphoto/shanovrustam

Melissa Pandika is an editorial intern at Sierra and a graduate journalism student at Stanford University. Her interests include environmental health and justice, urban environmental issues, and conservation biology. She has a soft spot for cetaceans.