What is the most important function of trees?

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 services is the most important. 2000 years ago a country cut down its trees to build warships to gain itself an empire. It gained the empire but, without its trees, its soil became hard and poor. 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. So, unless the government has a good control system, the forests will slowly disappear. 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.