ash tree in fall (31438 bytes)

Why leaves change colors in the fall

By John Pohly, Colorado State University Cooperative Extension

Mother Nature has been working overtime to color Colorado's mountainsides a glorious gold. Here's a primer about how those leaves change colors.

During spring and summer, leaves manufacture most of the foods needed for a tree's growth. The food manufacturing occurs in cells that contain pigment chlorophyll, which gives leaves their green color. Leaves also contain yellow or orange carotenoids, the same material that gives carrots their orange color. Most of the year, these yellowish colors are masked by the greater amounts of green. In the fall, however, changes in daylight and temperature stop the food-making process in the leaves. Chlorophyll breaks down, the green color disappears and the yellowish colors become prominent - thus the fall splendor. Other chemical changes give rise to reddish and purplish colors of some trees. Brilliant red leaves are more apt to form when warm, sunny days are followed by cool nights. Sugar is made by day, but nighttime cool prevents its movement from the leaves. From this sugar, the red pigment, anthocyanin, is formed.

Photo: Judy Sedbrook

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Date last revised: 01/05/2010