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发布时间：2006-06-21 20:59     点击：

Companies engaged in intensive timber cropping naturally take a dim view of alders suppressing more valuable evergreen trees. But times are changing; a new generation of foresters seems better prepared to Include in their management plans consideration of the vital ecological role alders, play.

Among the alder's valuable ecological contributions is its capacity to fix nitrogen in nitrogen-deficient soils. Alder roots contain clusters of nitrogen-fixing nodules like those found on legumes such as beans. in addition, newly developing soils exposed by recent glacier retreat and planted with alders show that these trees are applying the equivalent of ten bags of high-nitrogen fertilizer to each hectare per year. Other chemical changes to soil in which they are growing Include a lowering of the base content and rise In soil acidity, as well as a substantial addition of carbon and calcium. to the soil,

Another important role many alders play in the wild, particularly in mountainous areas, is to check the rush of water during spring melt. In Japan and elsewhere, the trees are planted to stabilize soil on steep mountain slopes. Similarly, alders have been planted to stabilize and rehabilitate waste material left over from old mines, flood deposits, and landslide areas in both Europe and Asia.

31. What does the passage mainly discuss?

(A)Differences between alder trees and Douglas fir trees

(B)Alder trees as a source of timber

(C)Management plans for using alder trees to improve soil

(D)The relation of alder trees to their forest environments

32. The word "dense" in line I is closest in meaning to

(A) dark

(B) tall

(C) thick

(D) broad

33. Alder trees can suppress the growth of nearby trees by depriving them of

(A) nitrogen

(B) sunlight

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