Apart from providing people with timber，forests help regulate climate，conserve water and soil，wall off wind，fix sands，improve water quality，keep away smoke and dust，sterilize（purify）the air，reduce noise，beautify surroundings，maintain and improve ecological balance. Man would not be able to exist without green plants producing oxygen and absorbing harmful gases all the time. Man's health is closely related to the quality of the air，which in turn is decided by the size（extent）of forests.