In some places in North Africa, great cities once stood. There were gardens and trees and grass. But now these places are like deserts(沙漠). Scientists have begun to study this land to find out what happened. Many of them think the land died when the trees were cut down. Trees help other plants grow. Tree roots hold the earth in place. They stop the soil from being blown away. In forests, leaves on the ground keep rainwater from running off. Trees near farms protect growing plants from strong winds.

Once there were hundreds of farms in the Southwestern United States. Then the farms turned into fields of dry land, because few trees grow in the dry Southwest.

Now we try to save trees. In many forests, when a tree is cut down, a new tree is planted in its place. We do not want the land to die.