Vivid Fall Foliage Sets Trees Ablaze With Color
The mythical Jack Frost brings reds and purples to the forest by pinching the leaves with his icy fingers. The hues of yellow, gold, and brown are mixed on his paint palette and applied with quick broad strokes of his brush as he silently moves among the trees to decorate them. Native Americans tell stories of celestial hunters killing a great bear in the autumn sky. The bear’s blood drips on forests, changing many leaves to red. Other trees are turned yellow by the fat that splatters out of the kettle as the hunters cook the meat.
While these fictional tales are en-tertaining, 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.
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.
Some of the best locations with deci-duous trees and fa-vorable conditions for brilliant fall color are:
• The eastern U.S.
• Southeastern Canada
• A few mountainous locations scattered in western North America; and areas around Sea-ttle, Portland, and San Francisco.
Thanks to the physics of sunlight striking pigments in leaves, we see va-rious leaf colors throughout the year. Four broad categories of pigments play a crucial role in fall beauty: Chloro-phylls, 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 en-hancing 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 carote-noids reveal themselves in the fall, most noticeably as yellows and golds in the quaking aspen, Norway maple, Ohio buckeye, sycamore, birches, hick-ories, 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.
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.
For more information, contact a local International Society of Arbori-culture (ISA) Certified Arborist or visit www.treesaregood.com.