Money may not grow from trees, but something even better does.
In a new study led by the United States Department of Agriculture’s (USDA) Forest Service, researchers found that every tree planted in a community was associated with a significant reduction in non-accidental and cardiovascular mortality among people living nearby.
In addition, the study’s authors conclude that the annual economic benefits of planting trees dramatically exceed the costs of maintaining them by a factor of more than 1,000.
Previous studies have linked exposure to nature to a number of human health benefits. Access to nature is an important factor in mental health, and that doesn’t necessarily require the green to be pristine wilderness. Research shows that urban forests and street trees can provide comparable benefits.
Several longitudinal studies have shown that exposure to more vegetation is associated with lower non-accidental mortality, the new study’s authors note, and some have also linked exposure to greenery to reduced cardiovascular and respiratory mortality.
“However, most studies use satellite imagery to estimate the vegetation index, which does not distinguish between different vegetation types and cannot be directly translated into concrete interventions,” says Payam Dadvand, a researcher at the Barcelona Institute for Global Health (ISGlobal) and senior author of the new Study.
For their study, Dadvand and his colleagues used a well-documented tree planting campaign that took place in Portland, Oregon, between 1990 and 2019. In those three decades, the nonprofit group Friends of Trees planted 49,246 street trees in Portland.
Crucially, they kept records of where and when each tree was planted. This allowed researchers to study the number of trees planted in a given neighborhood or U.S. census area — each home to about 4,000 people — over the past five, 10, or 15 years.
Using data from the Oregon Health Authority, tThey then linked each census tract’s tree data to its mortality rate from cardiovascular, respiratory, or non-random causes.
The results show lower mortality rates in neighborhoods with more tree planted, and the researchers report that this negative association is significant for both cardiovascular and all-cause non-accident mortality, particularly in males and those over 65 years of age.
The association also gets stronger as the trees get taller, according to the study. Trees planted in the previous 1 to 5 years were associated with a 15 percent decrease in mortality, while trees planted in the previous 11 to 15 years were associated with a 30 percent decrease.
Older, taller trees were therefore associated with greater reductions in mortality. While planting new trees is great, this finding suggests that preserving existing large trees is even more important to public health (as well as wildlife welfare).
While these links do not exactly To explain how trees benefit human health, the seemingly greater protection from larger trees would make sense, the researchers point out, since size increases a tree’s ability to mitigate known mortality factors such as air pollution, temperature and noise.
“We observed the effect in both green and less-green neighborhoods, suggesting that planting street trees benefits both,” said Geoffrey H. Donovan, USDA economist and lead author of the study.
If the value of a statistical adult human life is $10.7 million, as some U.S. federal agencies have found, the researchers would calculate that planting a tree in each of Portland’s 140 census tracts would cost about $14.2 million annually would contribute to saved lives.
The study authors estimate that maintaining these 140 trees would cost between $3,000 and $13,000 per year.
“Our results provide an important evidence base for concrete measures (e.g. planting trees) to increase the longevity of city dwellers,” says Dadvand.
The study was published in Environment international.