Street trees really do make people healthier

It’s easy enough to claim that being in nature makes people feel better. It certainly feels like it’s true. A weekend in the mountains, or even a few hours in a park after a long day at work, truly feels like it is somehow restorative.

There are some good reasons to believe that green space could have a causal relationship with health and happiness. For one thing, trees scrub pollution from the skies, allowing those nearby to breathe cleaner air. Exposure to nature has also been linked with reduced blood pressure and stress, and it seems to motivate folks to become more active and less sedentary. Then there’s the Japanese practice of shinrinyoku, or “forest bathing.” The Japanese believe that what essentially amounts to a nature walk promotes human health and wellbeing. Plants are also part of a complex food web that, together, provides things critical to our survival like oxygen to breathe, fresh water to drink, and food to eat. Even if all these things are true – and they probably are – that still doesn’t mean that it’s nature, per se, that’s having the apparent health benefit.

To make that claim we need real, quantifiable data. That’s where University of Chicago psychology graduate student Omid Kardan and University of Chicago professor Marc G. Berman come in. They and their colleagues looked to Toronto, Canada, a city for which there is plenty of satellite imagery (which allows them to measure green spaces) and self-reported health information through the Ontario Health Study. By using a set of common statistical techniques, the researchers were able to really see whether there’s anything to the idea that greenery makes people healthier.

But it wasn’t green spaces in general they were interested in; it was trees in particular. By leaving lawns and bushes out, the researchers hoped to zero in on what they thought was “potentially the most important component for having beneficial effects.” First, they took data on trees from two databases maintained by the city of Toronto: “Street Tree General Data” and “Forest and Land Cover.” Together, those databases provided information on street trees as well as those in parks and backyards. They chose Toronto in part to rule out the effects of health insurance; unlike in the US, Canadians are guaranteed universal publically funded healthcare, regardless of employment status or income level. Still, despite equal access, not all Canadians choose to avail themselves of healthcare in equal ways. Indeed, those with lower incomes and fewer years of schooling tend to see doctors less often, which is why the researchers made note of that sort of demographic data.

They found that those who live in areas with more street trees reported better health perception than those in neighborhoods with fewer trees. Regardless of their actual health, they felt they were healthier. But it turns out they were actually healthier too: they suffered from fewer cardio-metabolic conditions.

But that’s not all. To really drive the point home, Kardan reduced the findings to cold, hard cash.

His team found that by planting 10 more trees per city block, Toronto could improve health perception as much as if every household on that same block earned $10,000 more every year, or magically became seven years younger.

The results were even more striking for actual health. Planting just 11 more trees per city block would reduce cardio-metabolic conditions the same extent as if everybody living on that block earned $20,000 more each year or somehow became 1.4 years younger.

So what’s the secret? Kardan doesn’t know, and his study isn’t explicitly designed to get at the underlying mechanism. But a close look at the data offers up a suggestion. It wasn’t proximity to trees in a neighborhood that was the most important variable, but the number of trees on the streets. That suggests that it’s not necessarily that the trees are themselves providing important services (they do that, though that might not be what accounts for these health effects). Instead, it could be something as simple as peoples’ ability to literally see trees, and the most common place for most people to see trees is on the street. It’s also possible that street trees are disproportionately responsible for capturing street pollution, and that could be driving the team’s findings.

Maintaining a street tree for a year costs between $30 and $500, depending on where it is. In other words, planting ten or eleven trees per city block would be far cheaper than paying everyone $10,000-20,000 more each year. That should be good news for city planners. – Jason G. Goldman | 15 July 2015

Source: Kardan, O. et al. (2015). Neighborhood greenspace and health in a large urban center. Scientific Reports 5:11610. | DOI: 10.1038/srep11610.

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