Trees are useful to man in three very important ways. They provide him with wood and other products, they give him shade, and they help him prevent droughts floods.
Unluckily, in many parts of the world, man has not realized that the (three) of these services is the most important. In order to make money (quick) from the trees, he has cut them down in large numbers. Two thousand years ago, a rich and powerful country cut down thousands of trees (build) warships. It built up a strong empire (帝国). But without the trees, (it) soil became poorer and poorer. When the empire fell to pieces, the country (find) itself facing with floods and hunger.
Even though a government realizes the (important) of trees, it is difficult to make the people realize this. They cut down trees but are too lazy to plant and look after new trees. So, unless the government has good system of control, or the people are educated, the forests may slowly disappear.
This does not only mean that there will be fewer trees. The results are even (serious). If there are tree roots which break the soil up and allow the rain to sink in, the soil can be bound (受约束的).
This prevents the soil being washed away. But if there are no trees, the soil will become hard and poor. This causes floods. And the rain carries away the rich topsoil (表层土) in which crops grow. When all the topsoil is gone, nothing remains but useless sand.
