People in Portland Planted Trees. Decades Later, a Stunning Pattern Emerged

Money can't grow from trees but it can grow from something else.

A study led by the USDA Forest Service found that each tree planted in a community was associated with reductions in cardiovascular and non-accidental mortality.

According to the study's authors, the yearly economic benefits of planting trees dramatically exceed the cost of maintaining them.

Exposure to nature has been shown to have a number of human health benefits. A major factor for mental health is access to nature, and that doesn't necessarily have to be wilderness. The benefits of urban forests and street trees are shown in research.

The authors of the new study note that several longitudinal studies show that exposure to more vegetation is associated with lower non-accidental mortality.

Payam Dadvand is a researcher with the Barcelona Institute for Global Health and the senior author of the new study.

Dadvand and his colleagues used a well- documented tree-planting campaign that took place in Portland, Oregon, between 1990 and 2019. The Friends of Trees planted nearly 50 thousand street trees in Portland.

They kept records of where and when the trees were planted. The researchers were able to look at the number of trees planted in a particular neighborhood over the course of a decade.

Each census tract's tree data was linked to the mortality rate due to cardiovascular, respiratory, or non-accidental causes.

The results show a lower mortality rate in neighborhoods with more trees planted, and the researchers say this negative association is significant for both cardiovascular and general non-accidental mortality, especially for males and anyone above the age of 65.

As trees grow taller, the association grows stronger. There was a correlation between trees planted in the prior one to five years and a drop in mortality.

Larger trees were associated with reduced mortality. This finding shows that preserving large trees that already exist is more important for public health than planting new trees.

While these links don't explain how trees benefit human health, the seemingly greater protection from larger trees would make sense.

The first author of the study is an economist from the USDA.

According to some US federal agencies, planting one tree in each of Portland's 140 census tracts would generate about $14.2 million annually in lives saved.

It would cost between $3,000 and $13,000 a year to maintain those 140 trees.

Dadvand says that the results give an important evidence base for interventions to increase longevity.

The study was published in an international journal.