In response to the shorter days and diminished sunlight of early autumn, a layer of cells forms at the base of each leaf where its stem connects to the tree.  This layer gradually closes the veins that carry fluids in and out of the leaf.  These clogged veins trap sugars in the leaf and promote production of pigments called anthocyanins that give the leaf color.  Once this separation layer is complete and the connecting tissues are sealed off, the leaf is ready to fall due to wind, rain or gravity.

Adapted from U.S. Dept. of Agriculture

Autumn leaf color is a phenomenon that affects the normally green leaves of many deciduous trees and shrubs.  For a few weeks in the autumn season, trees change from green to different colors that range from red to yellow. 

This phenomenon is commonly called "fall colors" or "autumn colors," while the expression "fall foliage" usually indicates the viewing of a forest whose leaves have undergone the change.  In some areas of the United States and Canada, "leaf peeping" tourism in the autumn is a major source of economic activity.

Adapter from Wikipedia

This site contains original photography, much of which was taken in the Little Wolf Nature Preserve.