题型：阅读理解 题类：常考题 难易度：普通
广西钦州市钦州港经济技术开发区中学2017-2018学年高一上学期英语期中考试试卷
Windbreaks are barriers (屏障) formed by trees and other plants. Farmers plant these barriers around their fields, which help prevent the loss of soil, stop the wind from blowing soil away. They also keep the wind from damaging or destroying crops. Besides, extra trees and plants can be cut down and used or sold for wood.
Windbreaks can be highly valuable for protecting grain crops. For example, in parts of West Africa grain harvests were as much as twenty percent higher in fields protected by windbreaks compared to fields without them.
But here is something interesting about windbreaks. They seem to work best when they allow some wind to pass through the barrier of trees or plants around a field. If not, then the movement of air close to the ground will lift the soil. Then the soil will be blown away. For this reason, a windbreak works best if it contains only 60 to 80 percent of the trees and plants that would be needed to make a solid line. An easy rule to remember is that windbreaks can protect areas up to 10 times the height of the tallest trees in the windbreak.
There should be at least two lines in each windbreak. One line should be large trees. The second line, right next to it, can be shorter trees or other plants with leaves. Locally grown trees and plants are considered the best choices for windbreaks. Trees reduce the damaging effects of wind and rain. Their roots help protect soil from being washed away. And trees can provide animals outdoors with shade from the sun.
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