More trees near homes connected to healthier newborns

The link between proximity to greenspace - including trees and parks- and healthy birth outcomes is well established. Now new data from researchers at Drexel University's Dornsife School of Public Health adds to our understanding of these health benefits, accounting for other factors that may influence this link, such as education, income and body mass index, but also taking the body of knowledge a step further by exploring the effect of residing near newly planted trees. The findings were recently published in the journal Science of The Total Environment.

Using a unique dataset - the planting of more than 36,000 trees between 1990 and 2020 in Portland, Oregon by the nonprofit group Friends of Trees - the researchers measured the number of new trees planted within 100 meters of a mother's address for the first 10 years after their child's birth, (using Jan. 1, 2015 - Dec. 31, 2020 birth data from the Oregon Health Authority), existing tree cover and road coverage in that area.

Controlling for factors that can influence birth weight, such as mother's race, whether they're pregnant for the first time, mother's BMI and education level, the researchers found a link between number of trees planted and higher birth weight. Additionally, nearby tree planting, including new and existing trees, were associated with three key measures of newborn health: higher birth weight, lower risk of small-for-gestational-age birth and decreased risk of pre-term birth.

For example, each tree planted within 100 meters of a mother's home within 10 years before a child's birth was associated with a statistically significant 2.3-gram increase in birthweight. In contrast with previous studies that also looked at greenspace and pregnancy, the current study controlled for existing tree coverage to focus on the role newly planted trees may play in the health of newborns.

Although there is benefit from well-established trees, we're finding that newly planted trees are also associated with healthy birth weight. This is another data point showing that planting trees is a relatively easy and low-cost way to improve public health from the earliest stages of a life."

Yvonne Michael, ScD, senior author, professor and interim chair in the Dornsife School of Public Health

Living within 100 meters of at least 10 trees was associated with about a 50-gram increase in birthweight, according to the researchers. The birthweight benefit from trees generally tops out at about 10 trees, the researchers found.

"Fifty grams may not seem like a lot, but if every baby in our sample gained 50 grams at birth, that means 642 fewer babies considered small for gestational age, and at higher risk for worse development later in life," said Michael, who noted that there were 2,879 babies with that diagnosis in the study.

Stress is associated with increased likelihood of delivering a baby preterm and poor health outcomes later in life. The authors speculate that established older trees near one's address may provide more benefit than newly planted trees by also providing psychological restoration, as other studies have suggested that developed natural environments foster a "soft fascination" - in contrast with psychologically draining characteristics of buildings and other aspects of built environments.

"It's rare to have detailed information on large-scale tree plantings for research," said Michael. "Existing tree cover is often closely tied to factors like income, education, and race, making it difficult to fully account for other possible explanations when studying birth outcomes. By focusing on newly planted trees, we were able to reduce the bias -- essentially treating it as a natural experiment. Plus, we observed the improvements in birth outcomes after trees were planted, establishing temporal order."

In the current study, existing trees, but not newly planted ones, helped to mitigate some of the negative impact of road density - ratio of the length of roads in an area relative to the overall land area - on birthweight. The authors speculated that that this may be attributed to more developed leaves of established trees, which are better equipped to combat road noise and reduce air pollution.

Although the authors observed decreasing birthweight among babies during the study, they said this is likely attributed to other factors linked to lower birthweight, such as rising air temperatures and hazardous smoke from wildfires (in 2020) in the area. The authors' dataset found that race, education and even existing tree canopy had no correlation with new trees planted in an area.

The authors note that a randomized controlled study is needed to definitively prove that the trees cause these positive health outcomes. However, the current study offers some of the best data available to support the link between trees and positive birth outcomes.

The latest study follows multiple papers delving into the effects of greenspace on health, including a 2013 study published by Michael and colleagues which found that the loss of 100 million trees to an invasive pest, called the emerald ash borer, coincided with an increase in heart disease deaths and lower-respiratory-tract illness in counties with more tree loss due to bug infestation.

In addition to Michael, other authors on this paper include Geoffrey H. Donovan, Jeffrey P. Prestemon and Abigail R. Kaminski from the USDA Forest Service.