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I recently received a copy of a newsletter from the USDA Forest Service Pacific Northwest Research Station that included a summary of an article recently co-authored by one of the Station’s scientists on the effect of urban tree cover on pregnancy outcomes of new mothers in Portland, OR http://www.fs.fed.us/pnw/sciencef/scifi137.pdf
According to the summary, researcher Geoffrey Donovan and his colleagues found that babies born to pregnant mothers who lived in neighborhoods with high amounts of tree cover were more likely to have higher birth weights than babies born to mothers living in areas with less tree cover. Of course, being a skeptical (cynical?) Garden Professor my first reaction was, “Well, duh…expectant mothers that live in areas with more tree cover are probably living in better neighborhoods, are wealthier, better educated, and have lots of other things going for them that we typically associate with better pregnancy outcomes.”
The summary put out in the PNW Station newsletter was scant on these details so I went on-line and dug out the original paper (citation below). As I suspected the authors noted that, “Women with greater access to urban trees were more likely to be non-Hispanic white, younger, have fewer previous births, and live in newer, more expensive houses closer to private open space compared to women with less access to urban trees.” However, they were able to account for these effects in their statistical analysis and still found that birth weight increased with the amount of tree cover near the mothers’ homes.
As we’ve all heard many times, correlation is not causation and it is unclear exactly how tree cover improves pregnancy outcomes. One of the most likely explanations for the tree cover effect is that having more trees around reduces stress levels, resulting in better birth outcomes. Trees can also reduce noise and other forms of pollution as well. Regardless of the mechanism, this study may provide one more bit of ammunition for urban forestry advocates that go to battle for trees in our cities.
Donovan, G.H.; Michael, Y.L.; Butry, D.T.;Sullivan, A.D.; Chase, J.M. 2011. Urban trees and the risk of poor birth outcomes. Health & Place. 17(1): 390–393.
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do the study in NY and you’ll surprisingly find that proximity to Victorian brick correlates with higher birth rates….
This makes a lot sense to me : Mom gave birth to nine children (I’m #6). For the first four pregnancies, she & Dad lived on a plot of land with one tree, surrounded by treeless pastures & cropland. The next five pregnancies were spent on seven wooded acres (cropland just outside of the woods). Not much else about my parents’ circumstances changed in that time period (unless you count more mouths to feed making life harder). Guess which babies were bigger, healthier overall ? The last five !