Why do leaves change color in the fall?

Mapple trees always one of the most beautiful scene for me (even tough i never see the real before, just from the movie or pic or else), especially when this yellow leaves fall down in the fall season...Amazing!!! I wish i have one here---But then, i wonder about why leaves change it's color while fall down, why they don't fall down with green color or normal color? Why must change? Or what make them transform-----


Many of the colors we see in fall are always present, but normally they’re hidden from view, says UW-Madison Arboretum native plant gardener Susan Carpenter.

The leaves of trees and other plants contain three main pigments: carotene, anthocyanin, and the photosynthetic pigment, chlorophyll, which captures the sun’s energy to make food for plants. As the most abundant pigment, chlorophyll is what gives leaves their green hue in spring and summer.

Another chemical in leaves, auxin, controls a special band of cells at the base of each leaf stem, called the abscission layer. During the growing season, auxin prevents this layer from fully developing and blocking the tiny, internal tubes that connect each leaf to the rest of the tree’s circulatory system.

In fall, however, cooler and shorter days trigger an end to auxin production, allowing the abscission layer to grow and cut off the circulation of water, nutrients and sugar to the leaves.

When this happens, chlorophyll disintegrates rapidly, letting carotene shine through as the yellow in maple, aspen and birch leaves. Anthocyanin, meanwhile, provides the oranges and reds of maples, sumacs and oaks. When there’s less sun, anthocyanin isn’t as chemically active and leaves are more orange or yellow than red.

source:

http://whyfiles.org/2007/why-do-leaves-change-color-in-the-fall/