Why Leaves Fall
In temperate regions most trees and shrubs shed their leaves in autumn in response to shorter days, decreased sunlight, and cooler temperatures. Two chemicalsabscisic acid, a hormone, and ethyleneappear in increased quantities in the leaf. They cause structural changes that, in effect, kill the leaf. As a result, transpiration stops and water that would have been lost as vapor through pores in the leaves flows back into the branches, trunk, and roots. Thus, the shedding of leaves conserves water during winter when it is difficult for plants to obtain moisture from the frozen soil.
In early autumn a layer of cells at the base of the petiole softens, dries, and becomes corky, separating from the stem. The leaf is then held only by the tiny tubes that pass through the petiole. When these are broken by wind, rain, or frost, the leaf falls.