For 30 years, the non-profit organisation Friends of Trees has been planting street trees in Portland (USA). Now, a new study shows that each tree planted was associated with a significant reduction in non-accidental and cardiovascular mortality (by 20% and 6%, respectively, if the trees were planted 15-30 years earlier). The researchers also estimate that the annual economic benefits of planting trees far outweigh the cost of maintaining them. The study, co-led by the Barcelona Institute for Global Health (ISGlobal), a centre supported by the "la Caixa" Foundation, and the USDA Forest Service, has been published in Environment International.
Increasing evidence points to an association between exposure to nature and lower mortality. "However, most studies use satellite images to estimate the vegetation index, which does not distinguish between different types of vegetation and cannot be directly translated into tangible interventions", says Payam Dadvand, ISGlobal researcher and lead author of the study.
Thus, the authors took advantage of a natural experiment that took place in the city of Portland: between 1990 and 2019, Friends of Trees planted 49,246 street trees (and kept records of where the trees were planted, and when). The research team analysed the number of trees planted in a given area (specifically, a census tract, where approximately 4,000 people live) in the preceding 5, 10 or 15 years. They associated this information with mortality from cardiovascular, respiratory and non-accidental causes in the same area, using data from the Oregon Health Authority.
The results show that in neighbourhoods where more trees had been planted, mortality rates (deaths per 100,000 people) were lower. This negative association was significant for cardiovascular and non-accidental mortality (i.e. all causes excluding accidents), especially for men and people over 65 years of age.
Moreover, the association was stronger as the trees aged and grew older: the reduction in mortality rate associated with trees planted 11-15 years earlier (30%) was twice that observed with trees planted 1-5 years earlier (15%). This means that older trees are associated with a greater decrease in mortality, and that preserving mature trees may be particularly important for public health.
This study does not provide direct evidence on the mechanisms by which trees improve health. However, the fact that large trees have a greater impact on health than small trees is telling, because larger trees are better at absorbing air pollution, moderating temperatures and reducing noise (three factors associated with increased mortality).
"We observed the effect in both green and less green neighbourhoods, suggesting that planting street trees benefits both," says Geoffrey H. Donovan of the USDA Forest Service and first author of the study. The analysis took into account other factors that can influence mortality, such as income, education and the racial composition of neighbourhoods.
In the end, the authors estimate that the benefits of planting trees far outweigh the cost: the annual cost of planting and maintaining one urban tree in each of Portland's 140 census tracts would range from $3,000 to $13,000, while generating an estimated $14.2 million annually in lives saved.
"Our results provide a strong scientific basis for guiding tangible interventions (e.g. tree planting) aimed at increasing the longevity of urban residents," concludes Dadvand.