$\bullet$ The forests act as rainwater harvesters, they replenish the ground water and help in maintaining the water cycle.
$\bullet$ Desertification refers to the conversion of fertile land into deserts due to deforestation. The roots hold the soil and hence prevent soil erosion which stops desertification.
$\bullet$ The oxygen that we breathe in, is also given out by the trees. They take in carbon dioxide and balance these two gases in the atmosphere.
$\bullet$ Thus, if forests disappear soil erosion and desertification will occur.