in order to make more farmland and build buildings and roads,many trees have been cut down in the last thirty years,more and more land is becoming desert.the weather in my hometown is much worse now.
      trees are useful to us in four ways :
      first,they help to prevent drought and floods.
      secondly,they help to prevent air pollution.
      thirdly,they help to prevent rich soil form being washed away easily. 
      finally,trees can take in carbon dioxide and turn it into oxygen.
      in a word,people and trees need each other ,we plant and care for trees in return for what they do for us.unluckily,we do not think of how much we need trees until we have lost them.