Autumn Colors Were Already There, Mostly

They’re changing now.

It’s that time here in Connecticut when deciduous trees change color and add drama to our forests and landscapes. Most of the colors we see in beautiful autumn foliage have been there all along. As day length and temperature decrease, the cells between the leaf and the petiole (stem) develop a corky abscission (separation) layer that begins to block materials coming into and going out of the leaf. As a result, leaves stop making new chlorophyll (the green pigment that makes energy for the plant using sunlight, carbon dioxide, and water) and their green color fades. The leaf’s yellow and orange pigments, called carotenoids, then show through. Yellow leaves from trees such as Witch Hazel (Hamamelis virginiana) are a good example.

Sweet Pepperbush’s (Clethra alnifolia) bright yellow leaves are caused by the carotenoid pigment called xanthophyll. This color was masked during the summer by the green chlorophyll. Photo by Andy Brand.

Some pigments are produced later by chemical changes and weren’t there all along. Anthocyanins give rise to the brilliant reds in the Sugar Maple (Acer saccharum), Sumac (Rhus spp.), and Northern Red Oak (Quercus rubra) and to the red-purplish color in Flowering Dogwood (Benthamidia florida).

The intense red, fall foliage of the Sugar Maple is caused by the pigment anthocyanin. Photo by Sarah Bailey

Weather certainly affects the intensity of colors. The drought we have been experiencing has stressed our trees. Some lost their leaves early or started turning color prematurely. So fall foliage will not be as vibrant. Adequate summer rains promote healthy trees that hold onto their leaves longer and thereby have richer autumn colors.

The right fall weather can cause anthocyanins to produce vibrant reds. This pigment needs sunlight for color production and the colors are enhanced by sunny days and cool nights. This is why fall color is so outstanding in the colder climes of Vermont and other areas of northern New England.

Owen Reiser, a biology and mathematics student from Southern Illinois University, filmed leaf color changes using time-lapse photography.

The purpose of leaf color is still shrouded in mystery, especially for the anthocyanins. Researchers have proposed various hypotheses. A physiological explanation is that anthocyanins act like a sunscreen, allowing the tree to better reabsorb nitrogen as chlorophyll is broken down. Nitrogen helps the plant in the next growing season. There is evidence is that redder leaves have less nitrogen when they fall from the trees, compared with leaves without anthocyanins.

The fall foliage of some kinds of plants is consistent every year. Leaves of the Oak-leaved Hydrangea (Hydrangea quercifolia) are usually maroon. Photo by Adam Wheeler, Broken Arrow Nursery.

One biological explanation is that color is camouflage for leaves to keep them from being recognized and eaten. Or the bright leaf colors may be an adaptation to dissuade insects from laying eggs if they think the leaves are toxic, thereby minimizing damage to the plant the following year. Another possible explanation is that bright colors are a flag to draw animals to eat the fruit and so disperse seeds away from the tree. Poison Ivy (Toxicodendron radicans) is thought to be an example of this—its bright red leaves in the fall may be a signal to birds that the plant’s white berries are ripe.

The red leaves of Poison Ivy'(Toxicodendron radicans) may be a signal to birds that its white berries are ripe and ready to eat. Photo by Famartin / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0).

Rather than raking, bagging, and putting your leaves to the curb, put them to good use. Remember that leaves make an outstanding mulch under your trees and shrubs and create important overwintering sites for native moths that provide food for birds for next year’s nesting season. You can easily make a leaf mulch container by a bending a four-foot (1.2 meter) high section of hardware cloth into a large circle supported with two stakes. Fill it with your leaves and by next summer or fall you’ll have an excellent mulch to add to your garden beds through the growing season.

Happy leaf peeping.

Published by Jim Sirch

Jim Sirch is the author of Beyond Your Back Door, a weekly blog about nature in your neighborhood. He is also Education Coordinator for the Yale Peabody Museum, a UConn Master Gardener and board member of his local land trust. As a trained naturalist, he brings a deep understanding of geology, plants and wildlife and how they interact within a particular ecosystem. He holds a B.S in Forestry from West Virginia University, a B.S. from Miami University in Science Education; and an M.S. in Environmental Studies Administration from Antioch University. He is also the 2014 Sigmund Abeles Award recipient from the Connecticut Science Teachers and Supervisors Association for outstanding science teaching and professional development.

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