Trees are useful to man in three very important ways: they supply him with wood and other products; they give him shade; and they help to prevent droughts and floods. 　　Unfortunately, in many parts of the world, man has not realized that the third of these service is the most important. In his eagerness to draw quick profit from the trees, he has cut them down is large numbers, only to find that without them he has lost the best friends he had. 2 000 years ago a rich and powerful country cut down its trees to build warships, with which to gain itself an empire. It gained the empire but, without its trees, its soil became hard and poor. When the empire fell to pieces, the home country found itself faced floods and starvation. 　　Even if a government realizes the importance of adequate supply of trees, it is difficult for it to persuade the villager to see this. The villager wants wood to cook his food with; and he can earn money by producing charcoal or selling wood to the townsman. He is usually too lazy or too careless to plant and take care of new trees. So, unless the government has a good control system, or can educate the people, the forests will slowly disappear. 　　This does not only mean that the villager’s sons and grandsons have fewer trees. The results are even more serious: for where there are trees their roots break the soil up ― allowing the rain to sink in ― and also bind the soil, thus preventing its being washed away easily; but where there are no trees, the rain falls on hard ground and flows away from the surface, causing floods and carrying away with it the rich topsoil ,in which crops grow so well. When all the top soil is gone, nothing remains but a useless desert. What does the passage mainly talk about