The Facts Behind Fall Foliage

Learn how the life cycles of deciduous trees across the nation contain the key to the brilliant autumn colors that grace us annually.

| September/October 2019

Photo by Adobe Stock/Tobias

Each year, the lush green of summer turns into the drabness of winter as trees lose their leaves. But the transition in fall typically offers a spectacular panorama of color. For some New England states, it’s a major economic factor. “Leaf peeping,” where tourists gather to see the fall colors, is a multibillion dollar industry. The shift from summer green to warm autumn hues is purely biological, so the beauty is serendipitous.

Although there’s variation from one autumn to the next, an overall pattern repeats itself every year because the display is part of a tree’s annual life cycle. Deciduous trees, primarily the hardwoods, are only active during the warm, freeze-free growing season. Leaf buds will expand in spring, and new leaves will emerge as the air warms. The “job” of the leaves is to gather the abundant sunlight of summer and use this energy to produce food: sugar. The leaves have a large surface area, better for collecting more light, and are typically thin to reduce weight. But these leaves can’t withstand the freezing temperatures of winter. Internal water would freeze, rupturing the vascular transport system. With the shorter days and cooler temperatures of fall, these trees “know” it’s time to shed their leaves and shut down for winter.

The pigment chlorophyll is an organic compound that has the ability to absorb energy from sunlight, which a plant can then use to produce sugar from carbon dioxide and water through photosynthesis. Chlorophyll primarily absorbs from the blue and red wavelengths of the spectrum while reflecting green — hence the greenish color of deciduous leaves. Once the leaves emerge in spring, they begin to produce more and more chlorophyll, and they turn from light green to a darker, richer hue. As fall sets in, less chlorophyll is produced, and the leaves begin to lose their green color.

There are other plant chemicals in leaves that have distinctive colors. Carotenoids, which produce yellow, orange, and brown hues, are found in leaves throughout the growing season, but are masked by the chlorophyll green. Anthocyanins, which are associated with reds and purples, are produced mainly in fall, and aren’t present in all trees. When chlorophyll production ceases in a leaf, the green color fades, and the other two pigments become dominant. The actual fall leaf color depends on the species of tree.

Fortunately, nature provides us with a full color palette in fall. Aspen and birch are known for their brilliant yellow leaves, which can stand in stark contrast to the surrounding evergreens of more northern latitudes. Oak leaves are typically brown, red, or russet. Sourwood and black tupelo, found mainly in the Southeast, are known for their brilliant crimson leaves. The flowering dogwoods of the Eastern states have purplish-red leaves. Also in the Eastern states, yellow leaves befit the yellow poplar. Hickories go golden bronze; beech become light tan. Some trees, such as elms, don’t take on bright colors; their leaves just turn brown and fall off.

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