Autumn is here. That means shorter days and cooler nights. Across Colorado, autumn means trees shed their leaves after a final colorful salute to summer. Unlike bright colors of flowers, which attract pollinators, or “warning colors” of many kinds of animals, the vibrant colors of fall foliage are a by-product of chemical changes as the trees go dormant. Fall leaf color has no apparent biological function or significance.

North America is one of the few places on the planet where deciduous trees produce vibrant fall colors. In Germany, for example, sugar maples turn a dull yellow, whereas in Vermont, an identical sugar maple will turn nearly fluorescent yellow. Though several factors influence this phenomenon, the key is climate. Throughout much of central, northern and eastern North America, the climate is ideal to stimulate leaf color changes.

Great fall color in trees needs a combination of crisp, cold nights and clear, sunny days. Sunlight is especially important, and nearly all trees planted in full sun yield bolder colors than those planted in shade. As daylight gets shorter and shorter, this change in photoperiod indicates to trees to begin getting ready for winter.

As green chlorophyll disappears from the leaves, the leaves turn yellow and orange. Small amounts of these colors have been in the leaves all summer, but are covered by green chlorophyll. The trees then produce anthocyanin (an-thuh-‘si-hu-nuhn) pigments. The bright red and purple colors come from anthocyanin pigments. In leaves, like maples, these pigments are produced from trapped glucose. Some scientists believe the pigment protects the leaves from the sun and lowers their freezing point, giving some frost protection and helps trees keep their leaves longer. If leaves remain on the tree longer, more sugars, nitrogen and other valuable substances can be used before the leaves fall.

Brown colors come from tannin, a bitter waste left in the leaves. Orange colors come from carotene (‘kar-uh-teen) and the yellows from xanthophyll (‘zan-thuh-fil). These common pigments are also found in flowers and food like carrots, bananas and egg yolks. Some scientists believe their exact role in leaves is involved in photosynthesis.

As fall colors appear, other changes take place at the point where the leaf stem is attached to the tree. A special layer of cells develops and gradually severs the tissues that support the leaf. At the same time, the tree seals the cut, so when the leaf is blown off by the wind or falls from its own weight, it leaves behind a leaf scar and a bud for next year’s growth.

Every autumn, as nature provides the beauty of the fall colors, it is easy to take trees for granted. Their shade and the water they transpire cool the asphalt; streets and buildings absorb and release heat. Noise levels ease as trees absorb and deflect sound waves. Trees clean the air by filtering and trapping fine particles and prevent erosion with topsoil. Trees make our lives livable.

For every ton of new wood a tree grows, it produces 1.07 tons of oxygen and removes 1.47 tons of carbon dioxide. Trees provide sustenance and medicines for the world’s population, along with a host of products that keep us warm and dry. Trees also create sanctuaries for shade, shelter, nesting, food and storage for wildlife. As the weather becomes cooler, the colors of fall may warm the heart.

PlantTalk #1728, Why Leaves Change Color in Fall

The authors have received training through Colorado State University Cooperative Extension's Master Gardener program and is a Master Gardener volunteer for Larimer County.

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This page updated:  November 1, 2006