题型：阅读理解 题类：常考题 难易度：普通
北京市丰台区2019届高三上学期英语期末考试试卷
As global temperatures rise, trees around the world are experiencing longer growing seasons, sometimes as much as three extra weeks a year. All that time helps trees grow faster. For the past 100 years, trees have been experiencing fast growth in temperate regions from Maryland to Finland, to Central Europe, where the growth rate of some trees has even sped up nearly 77% since 1870. Assuming wood is just as strong today, those gains would mean more timber(木材) for building, burning, and storing carbon captured from the atmosphere. But is wood really as dense as it used to be?
Hans Pretzsch, a forest scientist at the Technical University of Munich in Germany, and his colleagues wanted to find an answer. They carried out a study of the forests of Central Europe. They started with 41 experimental plots in southern Germany, some of which have been continuously monitored since 1870. Pretzsch and his team took core samples from the trees—which included Norway spruce, sessile oak, European beech, and Scots pine—and analyzed the tree rings using a high-frequency probe.
They found that in all four species, wood density has decreased by 8% to 12%, they report online in Forest Ecology and Management. “We expected a trend of the wood density like this, but not such a strong and significant decrease,” Pretzsch says. Increasing temperatures, and the faster growth they spur, probably account for some of the drop. Another factor, Pretzsch says, is more nitrogen in the soil from agricultural fertilizer(化肥) and vehicle exhaust. Previous studies have linked increased fertilizer use to decreased wood density. Above all, the study suggests that the higher temperatures—combined with pollution from auto exhaust and farms—are making wood weaker, resulting in trees that break more easily and wood that is less durable.
“I am getting worried,” says Richard Houghton, an ecologist at the Woods Hole Research Center in Falmouth, Massachusetts, who was not part of the new study. As the density of the samples dropped, so did their carbon content, by about 50%. That means forests may suffer more damage from storms and may be less efficient at soaking up the greenhouse gas carbon dioxide (CO2) than scientists had thought, Houghton says.
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