Trees are useful things to man in three important (way) they give us wood and other useful things, they give us cool places, they help to stop drought and flood.
Unluckily, in many parts of the world, man has not found that third of these points is the most important. Man wants to make money from trees, so he has cut them down large numbers, only to find that without them he has lost the best friends he had. And also, he is usually too careless (plant) and look after new trees. So the forests (slow) disappear.
This does not only mean that man will have fewer trees. The results are even (bad) : for where there are trees, their roots break up soil allowing the rain to sink in and also bind (固定) the soil, thus preventing it from (wash) away easily; but where there are no trees, the rain (fall) on hard ground and flows away, causing floods and carrying away the rich top-soil. When all the top-soil is gone, nothing (leave) but useless desert.
