Autumn Leaves Changing in Denver-How Do They Do That?

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Why Do Autumn Leaves Change Color
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Why Do Autumn Leaves Change Color?

Skies luminous like a paint palette of blended colors, as greens transform to various hues of red, gold, yellow, brown, and purple in autumn. Each year, leaves change colors and create beautiful scenery, but why and what exactly gives them their bright colors?

As we enjoy autumn, we can also take pleasure in understanding the color changing process in leaves. "Numerous factors including geography, growth habits, exposure to light, plant pigments, and weather conditions all play important roles in the fascinating process," said Jim Skiera, Executive Director of the International Society of Arboriculture.
Where leaf colors are the most vivid

Only a few places in the world have the combination of tree species and climatic conditions necessary for vivid autumn foliage. Deciduous forests and trees, with their many broad leaves that change color almost in unison, display the most noticeable autumn colors. Evergreen species also display changes in color, just slowly and gradually.

The best locations with deciduous trees and favorable conditions for brilliant autumn colors are:
    •     the eastern U.S.;
    •     southeastern Canada;
    •     scattered mountainous locations in western North America; and
    •     Seattle, Portland, and San Francisco surrounding areas.

Why do leaves change color?

A chain reaction occurs when sunlight strikes the pigment in leaves, allowing us to see various leaf colors throughout the year. Four broad categories of pigment play a crucial role in autumn beauty: chlorophylls, carotenoids, anthocyanins, and tannins.

During the spring and summer, leaves are the principal site for the photosynthetic process that transforms carbon dioxide and water into the carbohydrates that fuel tree growth. This food-making process takes place in numerous cells containing chlorophyll, the pigment that gives leaves their green color.

As days shorten and temperatures cool in autumn, the synthesis of new chlorophyll drops off, green color disappears, and the rate of photosynthesis declines. Trees become frugal and more efficient, pulling nutrients such as nitrogen and phosphorus into twigs and branches for winter storage, further enhancing the loss of chlorophyll.

Along with chlorophyll, leaves contain yellow or orange carotenoid, the same pigment that gives carrots their familiar color. Masked for most of the year by chlorophyll, the carotenoids reveal themselves in autumn, most noticeable as yellows and golds in tree species such as Norway maples, Ohio buckeyes, sycamores, birches, and hickories. For example, the golden yellows seen in beech leaves result from the presence of tannins and carotenoid pigments.

The vivid pink, red, and purple leaves seen on maples, sassafras, sumacs, white and scarlet oaks, and many other woody plants are formed by reactions between various sugars and complex compounds called anthocyanins. A mixture of red anthocyanin pigment and yellow carotene often results in the bright orange color seen in some species of maples.

"Autumn is a unique, distinctive season. It displays an array of vivid bold colors that beautifies the surroundings," says Skiera.
Fast Facts: Autumn Colors
    •     Fertile soil enhances the intensity of reds in leaves.
    •     The more light a leaf is exposed to, the more likely it is to turn red.
    •     Weather conditions most favorable to brilliant color are warm sunny days followed by cool nights with temperatures below 45 degrees (not to freezing levels).
    •     Rainy or cloudy days near "peak coloration time" will dampen the intensity of autumn color by limiting photosynthesis.
    •     Freezing temperatures and heavy frost can kill the brilliance of autumn color by severely injuring leaves before pigments are fully developed.

The International Society of Arboriculture (ISA) is a nonprofit organization supporting tree care research around the world. Headquartered in Champaign, Ill., ISA is dedicated to the care and preservation of shade and ornamental trees. For more information, contact a local ISA Certified Arborist or visit www.treesaregood.com.

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