Fantasy, Facts & Fall Color

While these fictional tales are entertaining, we can also take pleasure in understanding the truths of fall's color change. Geography, growth habits, the physics of light and color, plant pigments, leaf physiology and anatomy, and weather conditions all play important roles in the story of autumn.

WHERE COLORS ARE BRIGHT AND BEAUTIFUL

Only a few places in the world have the combination of tree species and climatic conditions necessary for vivid fall foliage. Deciduous forests and trees, with their many broad leaves that change color almost in unison, display the most noticeable fall color. Evergreen species also display fall colors, only slowly and gradually. Among the best locations with deciduous trees and favorable conditions for brilliant fall color are: the eastern U.S.; southeastern Canada; a few mountainous locations scattered in western North America; and areas around Seattle, Portland, and San Francisco.

WHY LEAVES CHANGE COLOR

Thanks to the physics of sunlight striking pigments in leaves, we see various leaf colors throughout the year. Four broad categories of pigments play a crucial role in fall 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 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 the days shorten and temperatures cool in the fall, 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 the fall, most noticeably as yellows and golds in the quaking aspen, Norway maple, Ohio buckeye, sycamore, birches, hickories, ashes, and many other tree species. The golden yellow seen in beech leaves, for example, 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, shadbush, winged euonymus, and many other woody plants are formed by reactions between various sugars and complex compounds called anthocyanidins. A mixture of red anthocyanin pigment and yellow carotene often results in the bright orange color seen in some species of maples.

FAST FACTS ABOUT FALL LEAF COLOR

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 (but not freezing).
Rainy or cloudy days near "peak coloration time" will dampen the intensity of fall color by limiting photosynthesis.
Freezing temperatures and heavy frost can kill the brilliance of fall 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

Taken from www.treesaregood.com