The Deciduous trees of the tropical dry forest serve a great purpose. When there is precipitation, the trees (which have formed a dense canopy) collect the water. Because they are deciduous, the leaves fall off hence the water goes into the soil and allows the trees and other plants to grow. The water that is still on the leaves (a lot of water) transpires and is taken up by the atmosphere. The cycle continues. A small amount of water “runs” away. This is called runoff.