[简答题]

Forests remove carbon dioxide from the air, conserve soil and water, and are home to a variety of species. (46) They are also repositories of potentially valuable new products, such as pharmaceuticals, and as a source of building material and firewood they provide employment for millions worldwide.
In 1990 forests took up about a quarter of the planet’ s land surface ( not including an additional 13 percent of other woody vegetation, such as sparsely covered woodland and brush land). Russia accounts for perhaps a fifth of the globe’s forest. Brazil for about a seventh, and Canada and the U.S. each for 6 to 7 percent. (47) Historically, virtually all countries have experienced deforestation, mostly because of the need for new farmland, pasture, fuelwood and timber. In the U. S. , forests now cover 22 percent of the land area, a decline of perhaps 40 percent since European colonization began. (48) Forest acreage, however, has remained about the

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