听力原文： Windbreaks are barriers formed by trees and other plants. Farmers plant these barriers around their fields to protect their land. Windbreaks stop the wind from blowing soil away (33) . They also keep the wind from destroying or damaging crops. They are very important for protecting grain crops. There have been studies done on windbreaks in parts of West Africa, for example. These found that grain harvests can be twenty percent higher in fields protected by windbreaks compared to fields without such protection (33) . But windbreaks seem to work best when they allow some winds to pass through them (341. If the wall of trees and plants blocks the wind completely, then violent air motions will take place close to the ground. These motions will lift soil into the air where it will be blown away. There should be at least two lines in each windbreak. one line should be large treas. The second line, right next to it, can be shorter trees and other plants with leaves. Locally grown trees and plants are best for windbreaks. If the quality of the land is not very good, studies have shown that one kind of tree that can grow well is the white pine (35) . Windbreaks not only protect land and crops from the wind. Surplus trees can be cut down and used to provide wood (33) . (34)