More trees in a city bring surprising benefit, Portland study finds

You've heard all the obvious benefits of urban trees -- shading buildings, sheltering wildlife, filtering air pollution, stopping erosion. A new Portland study suggests a more surprising benefit: healthier newborns.

Researchers used satellite images to compare tree cover around the houses of 5,696 women who gave birth in Portland in 2006 and 2007. Pregnant women living in houses graced by more trees were significantly less likely to deliver undersized babies.

Tree cover made no difference in the rate of pre-term births, but researchers found a consistent link to the prevalence of infants who were small for their gestational age. For each 10 percent increase in tree coverage within about 50 yards of a home, the rate of undersized newborns decreased by 1.42 per 1000 births. As it stands, about 70 of every 1,000 newborns in Portland are small for gestational age.

"Maybe it sounds a bit daft at first," says lead author Geoffrey Donovan, a scientist with the U.S. Forest Service's Pacific Northwest Research Station in Portland. But he says it's plausible that having lots of trees nearby counteracts the stress experienced by pregnant women.

Studies in animals and people make clear that maternal stress is harmful to a developing fetus and can increase the probability of underweight birth. In a variety of human clinical trials, exposure to nature and greenery significantly reduced people's stress levels and helped them withstand high-stress situations.

"That may be the mechanism," says Donovan, a specialist in forest economics whose work for the Forest Service includes studying urban trees and their effects on crime, energy use and health. The birth study, co-authored by researchers with Multnomah County Health Department, Drexel University and the National Institute of Standards and Technology, was published online by the journal Health & Place.

Dr. Stephen Fortmann, a senior investigator at the Kaiser Permanente Center for Health Research in Portland who was not involved in the study, finds the results intriguing. "It points out that some of the neighborhood level factors that effect health might work in ways we haven't thought about," Fortmann says.

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