When Trees Die, People Die
The curious connection between an invasive beetle that has
destroyed over 100 million trees, and subsequent heart disease and
pneumonia in human populations nearby
~maja*majika~/Flickr
The blight was first detected in June 2002, when the trees in Canton,
Michigan, got sick. The culprit, the emerald ash borer, had arrived
from overseas, and it rapidly spread -- a literal bug -- across state
and national lines to Ohio, Minnesota, Ontario. It popped up in more
distant, seemingly random locations as infested trees were unwittingly
shipped beyond the Midwest.
Within four years of first becoming infested, the ash trees died --
over 100 million since the plague began. In some cases, their death has
an immediate impact, as they fall on cars, houses, and people. In the
long term, their disappearance means parks and neighborhoods, once
tree-lined, are now bare.
©American Journal of Preventive Medicine
Something else, less readily apparent, may have happened as well.
When the U.S. Forest Service looked at mortality rates in counties
affected by the emerald ash borer, they found increased mortality rates.
Specifically, more people were dying of cardiovascular and lower
respiratory tract illness -- the first and third most common causes of
death in the U.S. As the infestation took over
in each of these places, the connection to poor health strengthened.
The "relationship between trees and human health," as
they put it, is convincingly strong. They controlled for as many other demographic factors as possible. And yet, they are unable to satisfactorily explain why this might be so.
In a literal sense, of course, the absence of trees would mean the
near absence of oxygen -- on the most basic level, we cannot survive
without them. We
know, too, that trees act as a natural filter, cleaning the air from
pollutants, with measurable effects in urban areas. The Forest Service
put a 3.8 billion dollar value on the air pollution
annually removed by urban trees. In Washington D.C.,
trees remove nitrogen dioxide to an extent equivalent to
taking 274,000 cars off the traffic-packed beltway, saving an
estimated $51 million in annual pollution-related health care costs.
©Science
But a line of modern thought suggests that trees and other elements
of natural environments might affect our health in more nuanced ways as
well. Roger Ulrich demonstrated the power of having a connection with
nature, however tenous, in his classic 1984 study with patients
recovering from gall bladder removal surgery in a suburban Pennsylvania
hospital. He manipulated the view from the convalescents' windows so
that half were able to gaze at nature while the others saw only a brick
wall. Those with
trees outside their window recovered faster, and requested fewer
pain medications, than those with a "built" view. They even had slightly
fewer surgical complications.
Environmental psychologists Rachel and Stephen Kaplan attributed
nature's apparent restorative ability to something they termed "soft
fascination": Natural
scenes, they theorized, are almost effortlessly able to capture
people's attention and lull them into a sort of hypnotic state where
negative thoughts and
emotions are overtaken by a positive sense of well-being. Indeed, an
analysis of numerous studies in BMC Public Health found evidence for natural
environments having "direct and positive impacts on well-being," in the form of reduced anger and sadness.
The effect, it has been suggested, can have subtler effects than a mere elevation of mood. A 2010 study
looked at the presence of parks and forests in the
vicinity of people's homes and their ability to act as a "buffer"
against stress. They ending up finding that the presence of "green
space" was more closely related to physical -- in
terms of minor complaints and perceived general health -- than
mental well-being. While nature wasn't enough to make the participants
forget about stressful life events, it appeared to quell their
psychosomatic complaints.
The increases in mortality identified by the Forest Service study,
meanwhile, were more pronounced in counties where the median household
income was above average. The disparity
highlights what we intuitively know about the presence of trees: In
wealthier communities, they increase the market value of homes, while
parks in poor
neighborhoods attract crime and are thus undesirable. The
researchers hypothesize that the rich communities that thrived before
the blight arrived thus experienced its destructive effects
more potently.
Which is all to say that there is something fascinatingly mysterious
about the entanglement of our health with that of nature. The suspicion
that this may be so, of course, is seen well outside of the scientific
literature on the topic. Maurice Sendak knew it, as he spoke of his appreciation for the trees seen from
his window in the final months of his life. And Henry David Thoreau, writing in The Atlantic in
June 1862, said, "I think
that I cannot preserve my health and spirits, unless I spend four
hours a day at least -- and it is commonly more than that -- sauntering
through the woods and
over the hills and fields, absolutely free from all worldly
engagements."