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  3. If trees fall in the middle of a city, does anyone make a sound? So far, not much of one—and that’s a problem. To most of us, ci

If trees fall in the middle of a city, does anyone make a sound? So far, not much of one—and that’s a problem. To most of us, ci

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问题    If trees fall in the middle of a city, does anyone make a sound? So far, not much of one—and that’s a problem. To most of us, city trees are to genuine forests what gardens are to jungles or fish tanks are to the ocean—pocket-size imitations of the wild world, decorative perhaps but playing no real role in the global ecosystem. But urban trees are a lot more important than that, and at the moment, they’re in decline.
   Tree cover from city to city has been measured by any number of studies, so direct comparisons of figures are hard, but across the country, things are trending downward. In the past few decades, Washington has lost half its tree cover; San Diego’s is off about a quarter; the cover in cities in Michigan, North Carolina and Florida has fallen to about 27% of what it once was; Chicago and Philadelphia are just 16%. "Urban deforestation," says Ed Macie, an urban specialist with the U.S. Forest Service in Atlanta, "compares with what’s going on in the world’s rain forests."
   As with so many other environmental problems, there are a lot of things causing the tree trouble. In the mid to late 20th century, Dutch elm disease wiped out millions of city trees, and now bark beetles, gypsy moths and emerald ash borers are chomping through millions more, thanks in part to climate change that makes cities more hospitable to the pests. The fact that so many trees exist in cities to be eaten in the first place is a tribute to our greener impulses. Many municipalities planted their now towering canopies right after World War II, but age is taking a toll as a lot of those trees simply reach the end of their life span.
   Development too much is a problem. In and around cities, single-family homes that once covered only a portion of their lot are being replaced by McMansions that don’t leave enough yard space to support large-canopied trees. Sprawl is causing the footprints of cities themselves to increase, with what was once relatively lush land being bulldozed and paved over.
   Urban fiscal woes have had a hand as well. Cities unable to keep up with the maintenance of large roadside trees swap them for pygmy-statured hawthorns and crape myrtles. Newly designed parks are replacing trees with no-fuss tennis courts and playgrounds. "Budgets are tight, and no one is giving extra money to plant larger trees," says Greg McPherson, director of the U.S. Forest Service Center for Urban Forest Research..
   All this hits the environment hard, starting with air quality. But it’s the thermometer that most noticeably reflects the loss of trees. A high canopy prevents sunlight and heat from ever reaching the ground; by contrast, unshaded asphalt soaks up thermal energy and radiates it back, creating what is known as heat islands.
   Local governments are finally responding to the problem. More than 2,000 big and small cities have launched long-term planting and preservation programs. For now, the most immediate answer is less the planting strategy than the preservation one, something that can best be achieved by curbing sprawl and downsizing our taste for too-big homes. For people who plan cities as well as those who live there, it’s important to remember that most of the time, sidewalks and sycamores are equally important.
According to the passage, what is the most direct impact of urban deforestation?
选项 A、Worsening of air quality.
B、Rise in temperature.
C、Unshaded asphalt.
D、Huge demand for thermometer.
答案B
解析
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