I think trees are one of the most precious things we have on earth, because they help humans as well as animals in numerous ways. First of all they help preserve the balance of gases in the atmosphere. A tree's leaves absorb carbon dioxide from the air. They also produce oxygen and release it into the atmosphere. These two processes are necessary for people to live, because we could not be able to survive if the air had too much carbon dioxide and too little oxygen.
Trees also help conserve soil and water.
In open country, they act as windbreaks and keep the wind from blowing away topsoil. In addition their roots prevent soil from being washed away by heavy rains. These roots also help store water in the ground. In mountain regions, trees prevent sliding snow from causing avalanches. They also provide shelter for animals and many of them live and depend on trees.