FALL FOLIAGE

Learning to Observe Changing Colors

While animals prepare for winter by hibernating, migrating, or storing food, trees prepare by shedding their leaves. But first the leaves turn colors. Sentimental pictures of Jack Frost with a paintbrush imply that fall colors are caused by the frost, but a frost that came before the colors would in fact prevent them. The whole process begins way back in June, but nothing shows until fall.

When a tree feels the days shortening after the summer solstice on June 21, it begins to change its relationship to its leaves. During spring and early summer the tree had been providing the leaves with plenty of minerals and fluids to support their food-manufacturing processes. But around midsummer it begins to pull its resources back in. It gradually deprives the leaves of their life-support system and finally abandons them altogether.

If you look closely at a leaf that is just beginning to show color, you will see advanced signs of this changing relationship between the tree and its leaves. A dying leaf is at first part green and part yellow. The green is chlorophyll, which was necessary to the leaf's food production, but which begins to break down at the end of the growing season. The yellow is caused by pigments called carotenoids and xanthophylls, the same pigments that color carrots, egg yolks, cream, and butter. These substances were present in the leaf all summer, but they were masked by the dominant green of the abundant chlorophyll. When the chlorophyll disappears, however, the underlying yellow becomes the dominant color.

The color changes in many species of trees stop here. Birches, aspens, poplars, and hickories contribute shades of yellow to fall's array of colors. But other trees produce new pigments in response to continuing changes within their leaves. The reds of maples, sumacs, and some oaks are caused by a pigment called anthocyanin, which also makes geraniums and apples their different shades of red. Anthocyanin forms in the cell sap of the dying leaves. The more sugar in the cell sap, the more brilliant the red. Likewise, the more acid, the more red.

Two other colors related to anthocyanin contribute to the richness and variety of fall foliage. Purples indicate anthocyanin in an alkaline cell sap, and flaming oranges form as the emerging reds mix with the underlying yellows. Because anthocyanin responds to light by becoming yet more brilliant, clear fall days invite trees to achieve their peaks of color before they finally shed their leaves for winter.

Although the browns of late fall are not as spectacular as the yellows, reds, purples, and oranges of Indian summer, they too tell us something about what the leaves are going through. As chemical changes continue in the dying leaf, the pigments oxidize, turning leaves brown just as a cut apple turns brown when exposed to air.

By the time leaves fall to the ground, they have undergone a number of chemical changes, but they're not finished yet. They are dead, but they still contain minerals and organic compounds. Worms, insects, and other organisms will feed on some of these substances, and water will leach others deep into the soil. Finally, one last color change will occur, unobserved and unobservable. The decomposing leaves will disappear as leaves, contributing the last of their shapes and colors to the rich dark humus that supports all forest life.

Copyright Gale Lawrence

A FIELD GUIDE TO THE FAMILIAR: Learning to Observe the Natural World by Gale Lawrence is available through the Naturalist's Almanac Bookstore. So is The Vermont LIfe Guide to Fall Foliage by Charles W. Johnson and Gale Lawrence: CLICK HERE