U.S. Cities Are Losing Tree Cover Just When They Need It Most

 by Richard Conniff/Scientific American

Scientific evidence that trees and green spaces are crucial to the well-being of people in urban areas has multiplied in recent decades. Conveniently, these findings have emerged just as Americans, already among the most urbanized people in the world, are increasingly choosing to live in cities. The problem—partly as a result of that choice—is that urban tree cover is now steadily declining across the U.S.

A study in the May issue of Urban Forestry & Urban Greening reports metropolitan areas are experiencing a net loss of about 36 million trees nationwide every year. That amounts to about 175,000 acres of tree cover, most of it in central city and suburban areas but also on the exurban fringes. This reduction, says lead author David Nowak of the U.S. Forest Service (USFS), translates into an annual loss of about $96 million in benefits—based, he says, on “only a few of the benefits that we know about.” The economic calculation looked at just four such benefits that are relatively easy to express in dollar terms—the capacity of trees to remove air pollution, sequester carbon, conserve energy by shading buildings, and reduce power plant emissions.

Nowak and a USFS colleague, co-author Eric Greenfield, found tree cover had declined in metropolitan areas across 45 states. The biggest losers

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How Acting On a Hidden Environmental Threat Saves $11 billion

Prevention doesn’t come naturally to the human spirit. Even if we know better, paying the immediate cost often seems more daunting than the possibility of paying a much higher cost (death, say) at some unknown date in the future. This emotional disconnect is what makes it so hard to persuade people to eat less, exercise more, practice safe sex, or take action to prevent climate change.

Lately, the “why worry?” line of thinking has turned up in the debate over invasive species. If you think it’s hard making parents understand that their unvaccinated child could die from measles, just imagine how hard it is to convince them that they should also worry about, say, invasive beetles turning up in the backyard. To make prevention even less likely, some critics now argue that introduced species are often harmless, or in some cases beneficial, and that there are just too many of them to fight. At best, they say, we spend a great deal of money to slow down the invasion. But we can never stop it completely.

A new case study being published in the journal Frontiers in Ecology replies—if I may sum up the scholarship in a phrase—that this is a lot of crap. The coauthors look at the current poster child of invasive species, the emerald ash borer, a beetle with a glittering green carapace and a nasty habit of killing some of the most majestic trees lining the streets of North American cities, as well as in our yards and forests.

Emerald ash borers (also known as EAB) arrived from Asia by accident in the 1990s, and biologists first detected them in Detroit in 2002. In North America, they focus exclusively on ash trees (hence the name), and as their larvae bore into a tree, they block off its ability to transport nutrients and water.

Alarmed at the prospect of dying forests, the Nature Conservancy and the National Center for Ecological Analysis and Synthesis launched a wide-ranging series of studies on the problem. “By 2003, at least 5–7 million ash trees were dead or dying in a six-county area of southeastern Michigan,” the authors of one such study wrote early this year, “and it was becoming apparent that EAB had the potential to devastate ash on a continental scale.”

The new study looks at the preventive measure undertaken by 70 signatory countries, including the United States, to slow or stop the invasion. ISPM15, as it’s known, requires manufacturers in international trade to treat wood pallets and wood crating (like the box holding your supermarket clementines). Treatment typically involves heat or fumigation with an insecticide, at a cost of about $1.50 per pallet. That added up to a whopping $437 million in 2005, the first year of implementation, with the expectation of continuing costs at a much lower level as pallets run through their typical six-year life span.

So was it a waste of money?