The healing power of trees

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Study shows tree-lined streets offer benefits similar to higher incomes

Tom Spears, Ottawa Citizen

A study of Toronto shows that adding 10 trees per city block makes residents healthier — by the same amount as raising their incomes $10,000 per household.

Pedestrians make their way across Bloor Street at Avenue Road in Toronto, where trees have been planted as part of a street revitalization project.

It’s well known that rich people enjoy better health than those in poorer neighbourhoods, the University of Chicago scientists who did the research knew.

But their close look at Toronto showed that greening up a neighbourhood is also effective in keeping people healthy.

Psychologist Marc Berman is an American who did post-doctoral work in Toronto, and he returned to the city because it has several data sets for the same areas, including:

  • Detailed maps and satellite photos of the “urban forest;”
  • Maps showing household income by area; and
  • The continuing Ontario Health Study, a survey from which they took responses from 31,000 people in more than 3,000 neighbourhoods, which they could overlay on the tree maps.

The survey included questions on heart disease, stroke, diabetes, obesity, cancer, depression and addiction. It also asked people to assess their own overall state of health.

All these health measures improved in communities with mature trees.

Planting 10 more trees on a city block “has equivalent health benefits to increasing the income of every household in that city block by more than $10,200 and having all of those households be moved to a neighbourhood that was $10,000 wealthier,” says the study led by Berman’s student, Omid Kardan.

The effect was twice as strong — like adding $20,000 to household income — for cardio-metabolic disorders (including heart disease and diabetes.)

And trees can even upset the usual pattern of healthier rich and not-so-healthy poor people.

People in poorer neighbourhoods with plentiful trees reported health that more closely matched the affluent residents, they found.

Similarly, affluent people who lived in neighbourhoods without large trees, such as new housing developments, had health reports resembling those of people in poorer communities.

And there’s another quirk: Trees lining the street are more helpful than trees in backyards.

The study is published in a research journal called Scientific Reports. Why do trees help our health? The study team only has educated guesses. But these include the fact that trees reduce air pollution, they cool the summer air, and they have a hard-to-measure — but very real — stress-busting ability.

There’s a hint in the study that evergreens may help us more than deciduous trees, but the team isn’t sure.

The study concludes that adding 10 trees per block to tree-deprived neighbourhoods would involve boosting Toronto’s overall number of trees by some four per cent. That, says the team, is not too lofty a target.

Berman is looking at further things that trees and other natural surroundings can do. The research interests on his home page include “the ability of natural environments to improve affect, attention and memory.”