(ORDO NEWS) — A 30-year tree planting campaign in Portland, Oregon allowed researchers to show that the amount of vegetation on the street is associated with reduced mortality, and that this association becomes stronger as the trees grow.
For the past 30 years, Friends of Trees, a non-profit organization, has been planting trees along the streets of Portland, Oregon, USA.
Now, the study found that every tree planted was associated with significant reductions in accident and cardiovascular disease deaths (by 20% and 6%). Scientists have calculated that the annual economic benefits of planting trees far outweigh the costs of maintaining them.
Health in the city
“Most studies use satellite images to estimate the vegetation index, which do not distinguish between different types of vegetation and cannot be directly translated into real measures,” says Pyam Dadwand, ISGlobal researcher and senior author of the study.
So, scientists took advantage of a natural experiment that took place in the city of Portland. The research team looked at the number of trees over the previous 5, 10 or 15 years.
They linked this information to deaths from cardiovascular, respiratory, or non-accidental causes in the same area using data from the Oregon Health Authority.
As a result, in areas where more trees were planted, the mortality rate (deaths per 100,000 people) was lower.
This association was significant for cardiovascular and non-accidental (i.e., all-cause, excluding accidental) mortality, especially for men and people over 65 years of age.
In addition, the relationship increased as the trees aged and grew: the reduction in mortality associated with trees planted 11-15 years ago (30%) was twice that of trees planted in the previous 1-5 years (15%).
For example, old and large trees are associated with a significant reduction in mortality and their conservation may be of particular public health importance.
The benefits of trees
Of course, the study does not provide a direct understanding of how trees improve health. However, the fact that large and old trees have a positive effect on health speaks for itself, since.
For example, trees are better at cleaning air from pollution, reducing temperature and noise (three factors associated with increased mortality).
“We saw an effect in both green and less green areas, which suggests that street tree planting benefits both,” says Jeffrey H. Donovan of the USDA Forest Service.
The analysis took into account other factors that can affect mortality: income, education and the racial composition of the inhabitants of the area.
The authors estimate that the benefits of planting trees far outweigh the costs: the annual cost of planting and caring for a single city tree in each of Portland’s 140 census tracts is somewhere between $3,000 and $13,000, while it would bring in about $14.2 million dollars.
“Our results provide an important evidence base for tangible interventions to increase the life expectancy of city dwellers,” concludes Dadwand.
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