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How the Death of Trees Means the Death of People

June 21, 2013
Jason Kane / PBS NewsHour

The hypothesis: Trees improve people's health. The experiment: Remove 100 million trees in the eastern and midwestern US over the course of 10 years and see what happens. What happened? People died. In the 15 states infected with the emerald ash borer -- which killed all 100 million of those trees -- an additional 15,000 people died from cardiovascular disease and 6,000 more from lower respiratory disease compared with uninfected areas.


How Removing Trees Can Kill You
Jason Kane / PBS NewsHour

(JUNE 10, 2013) -- The trees died first. One hundred million of them in the eastern and midwestern United States. The culprit: the emerald ash borer, a beetle that entered the US through Detroit in 2002 and quickly spread to Iowa, New York, Virginia and nearly every state between. The bug attacks all 22 species of North American ash and kills nearly every tree it infests.

Then came the humans. In the 15 states infected with the bug starting, an additional 15,000 people died from cardiovascular disease and 6,000 more from lower respiratory disease compared with uninfected areas of the country.

A team of researchers with the US Forest Service looked at data from 1,296 counties, accounted for the influence of other variables -- things like income, race, and education -- and came to a simple conclusion: Having fewer trees around may be bad for your health. Their findings, published recently in the American Journal of Preventive Medicine, suggest an associative rather than a direct, causal link between the death of trees and the death of humans.

Geoffrey Donovan, a research forester at the Forest Service's Pacific Northwest Research Station, joined the NewsHour recently to discuss why.

NEWSHOUR: Geoffrey Donovan, thank you so much for joining us. It's an interesting premise. What made you want to study this?

DONAVON: Well my basic hypothesis was that trees improve people's health. And if that's true, then killing 100 million of them in 10 years should have an effect. So if we take away these 100 million trees, does the health of humans suffer? We found that it does.

Researchers have shown this in other ways in the past. There's been some famous research showing that people recover faster from surgery and take fewer drugs if their hospital room has a view of trees. Other research -- including some of my own -- has shown that mothers with more trees around their homes are less likely to have underweight babies.

It's been shown that if you put people in a natural environment, it can reduce their blood pressure, heart rate and other measures of stress. Obviously we also know that trees can improve air quality. And that's why I looked at these two causes of death. I didn't look at pancreatic cancer or something like that. I looked at cardiovascular disease and respiratory disease because both can be affected by air quality and stress.

NEWSHOUR: So the emergence of the emerald ash borer presented a new opportunity to study the effect?

DONAVON: Exactly. This is what we call a natural experiment. If the emerald ash borer were to come around your house, you would probably never see it because the beetle itself has no direct effect on people's health. All it really does is serve as a tree removal agent. It just gets rid of the trees -- kills them with no other effects, almost like the trees were beamed up into space or something.

That's a really unique opportunity. Imagine if you were trying to look at the effect of trees growing on someone's health and I got 100 people, I put them in 100 identical houses, and I planted trees in front of 50 of those houses and then waited. It would take 40 or 50 years before you found anything because trees grow really slowly.

It's hard to see significant changes quickly. On the other hand, trees die really quickly. That's why you have this unique opportunity to see a big change in the natural environment in a short amount of time.

NEWSHOUR: And what did you find?

DONOVAN: Increased rates of death from cardiovascular and lower respiratory mortality in the counties with emerald ash borer. And interestingly, what we found was the effect got bigger the longer you had an infestation, which makes sense because it takes two to five years for a tree to die typically.

We looked across space and time and saw this repeated over and over again in places with very different demographic make-ups. So you're seeing it in Michigan but then you're seeing it in Ohio, you're seeing it in Indiana, in New York, Maryland and Tennessee. So it's happening again and again in very different places. Places with high education, with low education, with great income, with low income, with different racial makeups.

NEWSHOUR: So what's the takeaway message here?

DONOVAN: I put it in terms of a question. Maybe we want to start thinking of trees as part of our public health infrastructure. Not only do they do the things we would expect like shade our houses and make our neighborhoods more beautiful, but maybe they do something more fundamental. Maybe trees are not only essential for the natural environment but just as essential for our well-being. That's the message for public health officials.

For ordinary people: Get involved in planting trees. In most cities, either the city itself or nonprofits will help with tree planting efforts. Also, spend time in the natural environment. I think people intuitively know that. There's a reason that we like to go walk in the woods or that we like to spend time in the park.

The only thing that's new here is we're trying to quantify it. If you talk to a painter or a poet or a writer, do you think they understand that trees are part of our well-being? Look at things like the tree of life metaphor in the Bible.

Look at how often trees get painted as symbols of well-being or used in literature. The idea that trees and humans are linked is as old as humanity. So I think you need to look at my research in that context.

NEWSHOUR: Geoffrey Donovan, thank you so much for joining us.

DONOVAN: Thank you.

Do Dying Trees Lead to More Human Deaths? The Debate Continues
Jason Kane / PBS NewsHour

(June 14, 2013) -- The hypothesis: Trees improve people's health.

The experiment: Remove 100 million trees in the eastern and midwestern United States over the course of 10 years and see what happens.

What happened: People died.

In the 15 states infected with the emerald ash borer -- which killed all 100 million of those trees -- an additional 15,000 people died from cardiovascular disease and 6,000 more from lower respiratory disease compared with uninfected areas of the country.

After studying data from 1,296 counties and accounting for other variables, research forester Geoffrey Donovan and his team at the US Forest Service concluded that having fewer trees around may be bad for your health. Their study -- published recently in the American Journal of Preventive Medicine -- concluded that there's an associative link between trees health and human health. Proving a direct, causal link will take much more research.

When the PBS NewsHour published a discussion with Donovan earlier this week, many viewers remained quite skeptical. One wrote that "this hypothesis has no more scientific validity than a theory that the darkness of night causes mice to spontaneously generate." The questions multiplied when the story was posted on reddit and Slashdot.

Joining us once again to answer a representative set of those questions is Geoffrey Donovan.

Comment 1, from reddit user eloiserat: What if there was some other thing making the trees susceptible to the tree borer that's killing them? How did you account for other causes of death?

Donovan: An enormous range of things can affect our health, so it is right to be skeptical of our claim that we were able to isolate the effect of trees. We took into account all the other causes of death in three ways.

First, we accounted for trends over time. For example, health care improved during the 18-year study period, and we wanted to make sure that we weren't accidentally picking up this effect.

Second, we accounted for demographic differences between counties. For example, we know that poorer people tend to have more health problems, so we were careful to account for income.

Finally, and this is difficult to explain, we accounted for the things that we couldn't measure. How on earth do you take account of something you can't measure? You compare mortality in the same county across multiple years. Some counties will have higher or lower mortality for reasons you can't explain, but you don't need to know why to take account of it.

For example, if a county had a high cardiovascular mortality rate after being infested with emerald ash borer, we looked at the mortality rate before the bug arrived. If it was equally high, then maybe the bug wasn't to blame. If it was significantly lower, then maybe the bug was to blame. All these steps happened simultaneously in a fairly complex statistical model.

Comment 2, from NewsHour viewer mollycruz: It strikes me that when trees go, roads and cars come, and I suspect air pollution could be responsible for the curve upward in diseases long associated with exhaust fumes. Did you look into the link between pollution and the death of both trees and humans?

Donovan: I agree that the loss of trees may result in worse air quality. That's why we looked at two causes of death (cardiovascular and lower respiratory disease) that are influenced by air quality. However, I don't think this is because of increased development. That said, our results don't tell us for sure what mechanisms link trees and health. I can speculate, but this is a question that needs more study.

Comment 3, from Slashdot user dkleinsc: Your basic hypothesis was that "trees improve people's health." There's no particular reason why that hypothesis would be true. And I say that as someone who enjoys walking around in the woods. In fact, for those with nasty allergies, trees can be positively bad for your health.

Donovan: You're right to point out that trees can increase allergies. Indeed, a recent study in New York showed this. However, trees can also uptake pollutants. In addition, they lower temperatures, which helps reduce the production of ground-level ozone.

Most research shows that, on balance, trees improve air quality. We also know from several experiments that putting people in a natural environment reduces markers of stress such as heart rate, blood pressure, and salivary cortisol. And stress has a well-documented effect on cardiovascular disease.

Comment 4, from reddit user sylphs: If the study's conclusions are true, are there higher rates of illness in desert areas? Cities?

Donovan: Great question. The narrow answer is that this study was limited to the Midwest and East Coast, so it doesn't apply directly to people living in the desert. However, I suspect that exposure to a sweeping desert vista would also have some health benefit. Trees are just one component of the natural environment.

In addition, people in the desert plant and maintain trees in environments that they normally wouldn't grow in. Indeed, several desert cities, including Phoenix, have explicit goals to expand tree planting. This suggests that even desert residents appreciate trees.

Comment 5, from reddit user suninabox: Correlations and the associated inductive reasoning (of this kind) are some of the weakest forms of evidence science has to offer. In fact, almost no major scientific discoveries have been made using such evidence alone.

For specialists in the field, such studies can be useful to hone in future studies, but the number of possible variables that aren't accounted for in these studies make them absolutely useless to laypeople. It convinces a bunch of people something is true (trees dying is bad for your health) when in fact all it really is is one very minor data set in a far grander tapestry, that could mean anything until you piece together all the other millions of pieces to see what it makes up.

Do you agree with this assessment? What's the value of a study like yours if it only shows a potential relationship? Should the media refrain from reporting on a study like this?

Donovan: I appreciate the questioner's skepticism. Human health is indeed an enormously complex thing, it would seem impossible to pin down the effect of something like trees. This is why the spread of the emerald ash borer was such a unique opportunity. It killed 100 million trees in 10 years across counties with very different demographic makeups.

Even so, it took a lot of additional data collection and some tough statistical analysis that brings back bad memories to isolate the effect of the bug. Did this analysis prove a causal relationship? Absolutely not, but this type of observational research has made some major breakthroughs when it was impractical, unethical, or prohibitively expensive to conduct an experiment.

For example, in 1854, John Snow discovered how cholera is transmitted using the same sort of basic study design as we used. Observational research also discovered a link between smoking and lung cancer and between lead exposure and mental development in children. These studies provided invaluable insight without forcing people to drink lead paint or raw sewage.

The charge that we used inductive reasoning is odd, because, outside mathematics, all science is inductive. People want absolute answers, but science can only offer degrees of certainty. This is not to say that reasonable people can't disagree about the strength of evidence, but if you're seeking complete certainty, all science will be a disappointment.

Should the press report these studies? Yes, I strongly believe they should. Will a reporter always get the story right, and will readers always understand all the nuances of a study? No. However, I'm a publicly-funded scientist, and I have a responsibility to produce research that helps people make better informed decisions, and they can't make better informed decisions if they don't know about the research. Also, people have a right to know how their taxes are being spent.

It can be tough trying to explain my research to a general audience, but I welcome the opportunity to try, and I'm very grateful that people are interested enough in my work to ask me questions.

Comment 6, from reddit user Fencepost7: Where was this study conducted? Ohio, which has one of the largest infestations, has a small percentage of ash trees. Seems unlikely that a small decline in the total numbers of trees would cause all these problems.

Donovan: If ash trees were evenly distributed across an area, then this might be an issue. However, some neighborhoods have a lot of ash, as these two pictures by Dan Herms of Ohio State University illustrate.

Here's a photo of a street in Toledo in 2006:
[Graphic of street covered with a canopy of leafy green trees.]

Here's the same street in 2009, just three years later:
[Graphic of same site. Trees are barren, dying and dead.]

Is it so difficult to imagine that the loss of these trees might harm the health of someone who already had serious health problems? This is an important distinction. I'm not suggesting that the death of a tree would kill an otherwise healthy person. However, losing trees might contribute to the death of a person with major health problems.

Comment 7, from reddit user johnnyfiv3: The Great Recession happened to overlap with this era of beetle infestation. Could the human death rate have been tied to the recession during this period rather than a decrease in the number of trees present?

Donovan: You're absolutely right to point out that there was a major recession during the study period. There was also a decrease in smoking and an improvement in health care. In fact, cardiovascular and lower-respiratory mortality rates declined during the study period. We accounted for these, and other, broad trends in our statistical models.

A final word: Without six smart and committed coauthors, this study would never have been done.

Posted in accordance with Title 17, Section 107, US Code, for noncommercial, educational purposes.




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