Trees are useful to man in three important ways. They provide him with wood; they give him shade; they help prevent drought (干旱) and floods. Unfortunately, in many parts of the world, man has not realized that the third one is the most important.
Two thousand years ago, a rich and powerful country cut down its trees to build warships, with which to gain itself an empire (帝国). It gained the empire; however, its soil became poor. When the empire fell to pieces, the country found itself faced by floods and starvation.
Even where a government realizes the importance of a plentiful supply of trees, it is difficult for it to persuade villagers to see this. The villagers want wood to cook their food with; and they want to make money from trees, so they have cut them down in large numbers, only to find that without them they have lost the best friends they had. And also, they are usually too careless to plant and look after new trees. So the forests slowly disappear (消失).
This does not only mean that the villagers’ sons and grandsons have fewer trees. The results are even more serious: for where there are trees, their roots break up soil—make the rain in—and also bind (结合) the soil, thus preventing its being washed away easily; but where there were no trees, the rain falls on hard ground and carries away with it the rich topsoil. When all the topsoil is gone, nothing is left but useless desert. What does the word "this" (line 2, para.3) mean