Street trees and health in Toronto, and other tales

On National Tree Day, September 21, 2016, the City of Ottawa invited the community to the Horticultural Building at Lansdowne for a celebration.  About 100 people showed up. Featured speaker was University of Chicago’s Marc Berman who described himself as an “environmental neuroscientist.”  He presented highlights of the paper he had co-authored using data for Toronto:

Omid Kardan et al., “Neighbourhood greenspace and health in a large urban center,” Nature/Scientific Reports 5, 11610 (1.1 MB), doi: 10.1038/srep11610 (2015)

Here is the Abstract of the paper:

Studies have shown that natural environments can enhance health and here we build upon that work by examining the associations between comprehensive greenspace metrics and health. We focused on a large urban population center (Toronto, Canada) and related the two domains by combining high-resolution satellite imagery and individual tree data from Toronto with questionnaire-based self-reports of general health perception, cardio-metabolic conditions and mental illnesses from the Ontario Health Study. Results from multiple regressions and multivariate canonical correlation analyses suggest that people who live in neighborhoods with a higher density of trees on their streets report significantly higher health perception and significantly less cardio-metabolic conditions (controlling for socio-economic and demographic factors). We find that having 10 more trees in a city block, on average, improves health perception in ways comparable to an increase in annual personal income of $10,000 and moving to a neighborhood with $10,000 higher median income or being 7 years younger. We also find that having 11 more trees in a city block, on average, decreases cardiometabolic conditions in ways comparable to an increase in annual personal income of $20,000 and moving to a neighborhood with $20,000 higher median income or being 1.4 years younger.

Or to put it another way:

“…if we consider two families, one earning $10,200 more annually than the other, and living
in a neighborhood with the same higher median income, it is predicted that the more affluent family who is living in the richer neighborhood perceives themselves as healthier people. Interestingly, however, that prediction could turn out to be wrong if the less affluent family lives in a neighborhood that has on average 10 more trees beside the streets in every block. Regarding cardio-metabolic conditions, the same scenario is expected to hold true for an income difference of $20,200.” (p8)


“In summary, our results show that street trees are associated with a significant, independent and reliable increase in health benefits in urban populations and that small increases in the number of trees along the street could improve health markedly and in cost-effective ways.”

In its opening section the paper provides a summary of recent and not so recent studies of the relationship between health and a green environment.  In his presentation Dr. Berman touched on several of these, including the link between crime and nature, and cognition and nature.

He went into some detail about a test of the Attention Restoration Theory, which makes the distinction between “directed attention fatigue” (which can be depleted) and involuntary attention (which is less depletable).  Thirty-four University of Chicago students were tested on a standard memory retention test (repeat a series of numbers backward), then sent out to a 50-minute walk — one half to a nature park, the other to city streets — and then tested them again.  Those who had walked in nature showed a 20% increase in ability compared to the street walkers.  Repeating the test a few weeks later with the assignments reversed yielded the same results.  The results held even if the walkers didn’t like the walk (at -20C).  A similar test was conducted by subjecting people to just pictures of nature, with comparable results.  The restorative effects of exposure to nature were even stronger when tested on people who were diagnosed with depression.

Finally, Dr. Berman, in a part of his presentation dubbed “Deconstructing Nature,” showed that just three parameters of a picture — colour, structure and hue — predict 80% of how “natural” a picture appears to be.  These three parameters explain 50% of the variance in preference for natural pictures in test subjects.

Dr. Berman stayed shy of stating that exposure to nature (real or pictured) causes the effects observed.

Comment: One City staff member quipped afterwards: “We don’t need parks, just street trees will do!”  (He could have gone further: We don’t need street trees, pictures of them will do!)  As it happens, in the Toronto study, a variable that captures tree canopy in the neighbourhood (such as in neighbourhood parks) performed quite poorly: The wrong sign when regressed against health perception, and in both the health perception and cardio-metabolic regressions, deeply non-significant.  Yes to planting more street trees, but let’s value our parks too.