Trees Are a Matter of Life and Death


“Everyday exposure to trees enhances your health now and promotes health across your entire lifespan,” said Dr. William Sullivan, Ph.D., a professor of landscape architecture at the University of Illinois, Urbana-Champaign, at a conference on the Washington, D.C. region’s urban tree canopy organized by Casey Trees. Some 150 urban forestry policymakers, experts, and designers heard Sullivan make the striking argument that the social and psychological benefits of trees and other greenery may even eclipse their ecological benefits. Research, based in real data, is now clearly demonstrating that exposure to trees brings people together, reduces crime, and lowers stress. Furthermore, trees are even a matter of life and death — their presence is a predictor of death rates for many.

Given that social ties are a predictor of our health and well-being, we need healthy, strong ties across our lifetimes. “Social ties are what glues us together. And people with stronger social ties have better health outcomes.” Sullivan outlined how social support buffers stress hormones, reduces blood pressure, increases chances of adopting healthy patterns, all of which lead to reduced mortality and morbidity rates and healthier lives.

In case the audience didn’t understand what he meant by social ties, Sullivan laid it all out. Social ties involves people getting together to see each other. There’s a progression in human relations. At first, there’s nodding, then smiling, then chatting. “Some people you chat with become friends.” Strong social ties involve those people we rely on. Weak social ties though are also hugely important. But social tie formation isn’t just dependent on how social you are, your environment also plays a large role. Sullivan pointed to a standard example of a sprawled out bedroom community and explained how these places reduce social tie formation, while green landscapes, streets improve these crucial ties.

To back up his points on the role of trees in boosting socialization, Sullivan pointed to a number of research studies, including a few of his own undertaken in public housing complexes in Chicago. He said these facilities are the “perfect place to research the effects of landscape on health” because there are people living in similar conditions but with varying levels of tree canopy. There are areas with no trees, some areas with up to 7 courtyard trees, and others that look out on dense forest. In his examination, Sullivan found that at Robert Taylor homes, “any trees mattered a lot” in terms of how many people were outside socializing. “Smaller spaces with trees were a big predictor of people hanging out.” Doing “real science,” Sullivan and his team interviewed  more than 140 people and found that the presence of trees had an impact on socialization with nearby neighbors and creating a local sense of community.

As a side-note, Sullivan also looked at trees and crime in the public housing complexes. His argument was that given trees encourage people to get outside and socialize, doesn’t that also mean that there are more eyes on the street and neighbors can then shush away bad guys? Using police and F.B.I. archival crime data over the past 2 years, Sullivan found that there was 52 percent less crime in high-tree density areas. Crime, however, could have moved on to more barren, treeless areas.

There are other research studies that demonstrate the powerful impact of trees on health, particularly for those with lower income. Pointing to a study by Mitchell and Popham in The Lancet in 2008, which offers up “empirical evidence” of some 40 million people in the United Kingdom, Sullivan said there is a relationship between lifespan and trees, particularly for medium and low-income residents. For medium income group, the presence of trees means they “are less likely to die.” For low-income residents, green spaces means they are “much less likely to die.” Interestingly, for high-income residents, there was no direct relationship. “The density of vegetation had no impact.” So Sullivan said what blew away even the assembled tree experts: “trees are a predictor of death rates. Trees are about life and death.” This study brings up issues related to equity and justice. The disparities between high and low-income people are dramatically reduced by the “power of living in a green neighborhood filled with trees.”

But stress, which is a contributor to early death, really takes its toll on almost everyone these days. Sullivan said stress impacts the central nervous system by flooding it with hormones. While it “sharpens your focus, it also shuts down digestion and prepares you for emergencies.” In today’s world, filled with commutes, kids, jobs, people suffer from “chronic stress, which is a disease.” Chronic stress leads to immune system repression, hypertension, damage to nerve cells, and insulin resistance. MacArthur studies have found that stress also leads to “impulsive actions, reduced cognitive function, increased cardiac diseases and mortality.” Unfortunately, “we have designed a society that gives us long term stress.”

To test whether a “dose of nature” has any impact on stress, Sullivan said researchers found 300 people who live in standard sprawled-out communities in the Midwest and brought them into labs. They were given a cortisol test when they got into the lab, then asked to give a 5-minute speech about their dream job (“highly stressful”) or asked to subtract 16 from a range of huge numbers (“people hate this”), and then their cortisol was tested again. But some of these poor guinea pigs were shown a 5-minute video of green street scenes and tested again. Sullivan said the study found a “curved linear relationship,” with the presence of trees in the video reducing cortisol.

To sum up, Sullivan argued that “living without trees has a significant cost.” He said the location of trees also really matters. While “big central parks matter, they are not enough. Too many people live in barren landscapes. A 30-40 percent tree cover adds a lot to health. We need trees at every doorstep.”

Explore Dr. Sullivan’s exciting research.

Image credit: Robert Taylor Homes, Chicago / University of Illinois at Chicago

4 thoughts on “Trees Are a Matter of Life and Death

  1. smithadrian 11/01/2012 / 10:17 am

    I enjoyed this article and I’m happy to hear that what we have felt intuitively as landscape designers is being proven true by scientific studies, since that tends to be what most people believe.

    I wanted to add one point about the subject of life and death when it comes to trees: in light of the recent spat of hurricanes in the Northeast (Irene in 2011, Sandy just now in 2012) where there is a climate that supports a canopy of large, leafy trees, do we as designers need to consider how near to homes we plant trees?

    Several of the fatalities in the region were due to trees crashing thru the roofs of homes. Do we build stronger buildings or move trees away from them? What will this do to our ability to live close to trees and the benefits associated with that proximity?

  2. David Licata 11/01/2012 / 10:30 am

    Wonderful post.
    If anyone is looking for a lengthier treatise on the economic, health, and spiritual value of trees to people, you might want to read Jim Robbins excellent book, The Man Who Planted Trees. Robbins is a science writer who contributes regularly to the New York Times. Highly recommended.

  3. Alisa Rose 11/01/2012 / 12:17 pm

    Such an important topic…A literal tree-hugger since early childhood, I too have always simply known that the presence of trees in our living environment is absolutely essential to physical, mental and emotional well being, and our ability to thrive.

    Our built environments directly and continuously impact our social interactions. Where treeless expanses of hardscape dominate, hearts become hardened just trying to survive.

    About this week’s devastating tree fallings: The confluence of high winds and high water presents a severe challenge (a ‘coincidence’, or destructive climate change? real?!), and what Hur. Sandy is yet again showing is that the overall design and placement of our built environment is crucial.

    All over the planet, the earth has been massively paved…We need to ‘delete concrete’ – to find a way to get permeability back across the land, even in the biggest cities, by de-paving and by farming and gardening organically.

    And of course, while vulnerable in some instances, trees also act as wind blocks…

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