Why do leaves fall off trees in fall?

Why do trees begin to lose their leaves in the Fall?

• Why do trees lose their leaves in the Fall?

• Answer:

  As autumn approaches, certain influences both inside and outside the plant cause the chlorophylls to be replaced at a slower rate than they are being used up. In late summer, the veins that carry fluids into and out of the leaf are gradually closed off as a layer of special cork cells forms at the base of each leaf. As this cork layer develops, water and mineral intake into the leaf is reduced, slowly at first, and then more rapidly. It is during this time that the chlorophyll begins to decrease. During this period, with the total supply of chlorophylls gradually dwindling, the "masking" effect slowly fades away. Then other pigments that have been present (along with the chlorophylls) in the cells all during the leaf's life begin to show through. These are carotenoids and they provide colorations of yellow, brown, orange, and the many hues in between. The conventional reasoning behind leaf fall is that leaves may be unable to survive winter, or that they may run short of water and minerals, although plants leaves can and do survive to winter, and even aquatic plant shed leaves. Evergreens are misnamed, as they also shed their leaves albeit not in autumn. Brian J. Ford has proposed that the shedding of the leaves is a primary means of plant excretion. There have been suggestions that leaf shedding may be a response that provides protection against diseases and certain kinds of pests such as leaf miners and gall forming insects.