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PORTLAND, Ore. (Portland Tribune) — Trees have been used to symbolize life for thousands of years.

Now there’s growing scientific evidence that living near trees helps keep people alive.

Thirty years of urban tree planting by the Portland-based nonprofit Friends of Trees was linked to significant reductions in cardiovascular-related deaths and non-accidental mortality overall, a recent study led by the U.S. Forest Service showed.

As a result, the economic benefits of planting trees where people live, the researchers concluded, dramatically outweighed the costs of planting them.

It’s not a new concept that urban trees offer benefits beyond their aesthetic beauty, said Geoffrey Donovan, lead author of the study and researcher with the Forest Service. They pull pollution out of the air, provide crucial cooling during heat waves and boost property values. Their ability to sequester carbon also is seen as key to fighting human-caused climate change.

In 2013, Donovan conducted a study that showed the loss of trees due to the emergence of the invasive emerald ash borer was associated with increased human mortality. The beetle is considered the most destructive forest pest in North America and was first discovered in the Portland area this June.

“That was an interesting result, but since then I had this sort of nagging thought: ‘Is the opposite true as well?’” Donovan said.

Previous research has shown that exposure to the natural environment is associated with lower non-accidental mortality — deaths that aren’t the result of accidents such as car crashes.

But such research has been limited for two reasons, Donovan said. First, the natural environment generally changes slowly, making it hard for studies to capture enough variation. Also, previous studies have mostly relied on satellite imagery, which may not be sensitive enough to detect changes in the environment’s vegetation and can’t distinguish between types of vegetation.

Luckily for Donovan and his colleagues, Friends of Trees has maintained detailed records of its tree-planting efforts, with data showing where and when more than 49,000 trees were planted in Portland between 1990 and 2019.

The researchers analyzed each of Portland’s 140 U.S. Census Bureau tracts — roughly neighborhoods — pairing tree planting data with Oregon Health Authority data showing each census tract’s mortality rate due to cardiovascular, respiratory and non-accidental causes.

Their calculations accounted for demographic characteristics that influence mortality rates such as age, race, income and education level to help isolate the impact of tree planting. Additionally, they factored in existing tree canopy coverage, among other controls.

The results of the study, which was published online this October in the journal Environment International, were remarkable, Donovan said.

The more trees planted, the lower the mortality rate of the census tract, the study shows. Specifically, planting 11.7 trees in each neighborhood — the average annual number of trees planted in a tract — was associated with 15.6 fewer non-accidental deaths and five fewer cardiovascular deaths per year on average.

Furthermore, the longer the trees had to grow, the connection between tree planting and reduced mortality grew as well. Trees planted in the preceding one to five years were associated with a 15% drop in mortality and those planted 11 to 15 years ago were associated with a 30% drop, the study shows. The finding “suggests that preserving existing mature trees may be particularly important for public health,” the researchers wrote.

The study found also that the link between tree planting and reduced mortality was strongest among males and people over the age of 65.

A forest economist by training, Donovan said the cost-benefit implications of the study are important for policymakers to consider.

“You need to understand what the benefits are, not just have a general warm, fuzzy feeling that trees are nice, ” Donovan said. “Tree being nice isn’t very useful from a policy point of view. You want to know how nice.”

Assigning the statistical value of an adult human life at $10.7 million — the value used by the U.S. Environmental Protection Agency — the researchers calculated that planting one tree in each of Portland’s 140 census tracts amounts to an annual life-saving cost benefit of $14.2 million.

The cost of maintaining those 140 trees, the researchers estimate, is between about $3,000 and $13,000 annually. That amounts to a cost-benefit ratio of about 1,700-to-1, Donovan said.

“Trees are cheap, human life is valuable and health care is expensive,” he said.

It’s also important for people to understand the limitations of the study, Donovan said.

The study was observational, meaning it reveals a correlation between tree planting and reduced mortality, rather than showing that tree planting directly causes fewer people to die.

Additionally, it doesn’t account for all the trees planted in Portland or how many died during the 30-year timeframe. The researchers calculated, however, that the trees planted by Friends of Trees between 2015 and 2019 made up 78% of all street trees planted in Portland, with individuals and governments planting the other trees.

It’s also possible that healthier people happen to prefer to live in neighborhoods with recent tree plantings, the study notes.

The future of Portland’s efforts to expand its tree canopy became uncertain this year.

Although the city’s tree canopy increased between 2000 and 2015, it declined between 2015 and 2020, according to two studies — one by Portland Parks & Recreation and another by researchers at Portland State University — released this year. The net loss was the size of Mount Tabor Park every year, the city said at the time.

Shortly after those studies, Portland ended its contract with Friends of Trees. Officials said the move was intended to make the city’s tree-planting efforts more efficient and realign tree-planting responsibilities between the parks department and the Bureau of Environmental Services. Meanwhile, Friends of Trees’ leaders say they aren’t going anywhere and will continue their planting work.

More recently, the city has proposed a $40 million budget for tree planting and maintenance over the next five years to be provided through grants from the Portland Clean Energy Fund.