1. Passage ThirteenTrees are useful to man in three important ways: they provide 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.In his eagerness to make money from trees， he has cut them down in large numbers， only to find that without them he has lost the best friends he had.And besides， he is usually too careless to plant and look after new trees.So the forests slowly disappear.This does not only mean that man will have fewer trees.The results are even more serious: for where there are trees， their roots break up soil——allowing the rain to sink in——and also bind the soil， thus preventing it from being washed away easily; but where there are no trees， the rain falls on hard ground and flows away， causing floods and carrying away the rich topsoil.When all the topsoil is gone， nothing remains but worthless desert.Two thousand years ago a rich and powerful country cut down its trees to build warships， with which to gain itself an empire.It set up the empire but， without its trees， its soil became poor and it grew weak.When the empire fell into pieces， there is nothing left except floods and starvation.What is the most important service of trees to man according to the passage?