Autumn Color in Trees

Why do leaves change color?

When autumn comes to the temperate forest, nature puts on a spectacular show of colors, painting the leaves of the trees and forests in bright oranges, reds, and yellows. The trees take on the a fabulous appearance, delighting the eye. Some people plan excursions of many miles just to see this splendor. The varied hues against a deep blue autumn sky can thrill the mind, soothe the soul, and set the imagination free. But just what makes the leaves turn the colors they do?

The answer is primarily the onset of autumn, with shorter days and cooler temperatures. As the length of the days shortens, the leaves stop their production of chlorophyll, which provides the green color. Other pigments, mostly yellow, that are present in the leaves then show through. The yellow color is the result of carotene and xanthophyll, and is most prevalent in aspen, birch, hickory, beech, tuliptree (yellow poplar) and willow.

Sugars that are trapped in the leaves as the trees prepare for winter form red pigments, the anthocyanins. Some trees with red or scarlet leaves in autumn are red and silver maple, dogwoods, sweetgum, red oak, black oak, scarlet oak, sumac and sassafras. Some trees will produce copious amounts of the red pigments, others produce little or none. Combinations of the red and the yellow pigmants create the pallette of hues that seem to range across the spectrum.

Some trees change leaf color hardly at all, their leaves dropping while still somewhat green. Others don't display much color, gradually turning dull yellow, then quickly becoming brown, usually due to high levels of tannins in the leaves. The most spectacular are those trees that produce the red anthocyanins, the red and sugar maples, sassafras, sumac, shadbush, and winged euonymus (burning bush). The intensity and combination of color can even vary on a single tree, with leaves on a sunny side often showing more red than leaves that are shaded. Even individual leaves can have a variety of colors.

The variation in color depends on many factors. Weather conditions during the weeks before autumn will have significant impact. Frost is not the key, and in fact can cause the leaves to die and wither before the colors develop. Low levels of sugars in the leaves will result in muted colors in the fall. Temperatures and cloud cover also have an impact: warm nights result in a more rapid break-down of the pigments, and cloudy days can reduce the amount of some pigments that are produced. Drought conditions can actually enhance the development bright red leaves. Of course, the genetic variation between species and between individuals in a species also has an effect on the colors produced. The best autumn colors are the result of clear, dry, and cool (but not freezing) weather.

© Copyright 1996, Russell E. Carlson

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This page was last updated on June 11, 2003.