Study Gives "Tree of Life" New Meaning

Jul 3, 2013

Emerald Ash Borer larvae tunnel along tree trunks searching for food.
Credit John Hritz / Flickr (user jhritz)

A sudden loss in the number of trees around you may slightly increase your chances for death. That's what a study from the US Forest Service published earlier this year suggests. Scientists found that areas with mass-tree deaths from beetle infestations had  increased numbers of cardiovascular and lower-respiratory related deaths.

The research is led by Geoffrey Donovan, an Economist with the US Forest Service in Portland Oregon.  He said he became interested in the link between human health and trees while studying something completely different: how trees affected real estate prices in the city of Portland. This more recent work uncovers a more profound, and startling, result. Aspen Public Radio's Ellis Robinson spoke with Dr. Donovan about trees and human health.

Geoffrey Donovan: We found that houses with more street trees sold for more. But as I was doing this study I really noticed that some blocks without trees just seemed so starkly different from similar blocks with a lot of trees that I thought there has to be a bit more going on.

Ellis Robinson: There is a very interesting natural experiment that took place over the past decade or so, can you tell us what that was?

GD: Tree diseases may give this unique opportunity to see what happens when a lot of trees die in a short period of time. Because if trees are good for our health, then removing a lot of them in a short period of time should be bad for  your health. And probably the nastiest bug in the last ten years is the Emerald Ash Borer. It was first discovered in Detroit, Michigan in 2002, and has since killed about 100 million trees in 18 states.

ER: In this study where you were able to link human health outcomes with the death of trees, what did you find?

GD: What we found was that counties infested with Emerald Ash Borer had higher rates of cardiovascular and lower-respiratory disease. And this is after we took into account demographic factors, so thing like race, or income, or education, for example.

ER: What could be going on between human health and the number of trees around?

GD: There's a number of things there. One is air quality, that's the obvious thing you can think of, and I looked at two causes of death that could be plausibly affected by air quality: cardiovascular and lower-respiratory health, for example.  The other thing is stress. We know that if you expose peopel to the natural environment, their stress goes down. And there's been lots and lots of work that shows that stress is related to cardiovascular disease, for example. So I think that's the mechanism.  But it's really important to say that this study didn't prove that it was one cause or another, but we were careful to look at two causes of disease that could plausibly be affected by the loss of trees: cardiovascular and lower-respiratory disease.

ER: Here in Colorado and other parts of Western North America, we've been dealing with the effects of an infestation of bark beetles, and subsequent tree death.  The US Forest Service is saying that it's at epidemic levels. Does the bark beetle present another natural experiment---might we learn more about this link by studying bark beetle infestation? 

GD: That's a really good question, and the answer is, I think, yes. It's a slightly different thing because Ash is a really popular urban tree, so you see a lot of Ashes in the city.  Unfortunately, it was often planted to replace Elm, which was killed from Dutch Elm disease. So when Ash die, people often see them die, they experience that death.  Whereas in Colorado, it's less likely that someone will have a lodgepole pine in their yard or something. So the loss of pine to pine beetle is a different type of loss, one that would be an interesting to study because it's often out in the woods as opposed to in our cities.

Geoffrey Donovan is an economist with the US Forest Service in Portland, Oregon. He spoke Aspen Public Radio's science reporter, Ellis Robinson.