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.

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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.