A delicate ecosystem
Rainfall is absorbed by the trees and pulled up to the canopy where it evaporates into the air.
Trees and plants receive most of their nutrients from decaying matter (fallen leaves and organic debris).
Plants and trees hold down the topsoil and keep it from washing away in heavy rains.
When the land is cleared and crops are planted, the soil doesn't stay fertile for long without the nutrients from the decaying matter.
Once the foliage is removed, heavy rains wash away the topsoil.