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Chemistry of autumn colors

09/18/02 12:00AM By Ted Levin
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(Host) Commentator Ted Levin explains the many variables that give New England its distinctive blaze of fall colors.

(Levin) The chemistry of fall foliage is a lesson in simple mathematics. Autumn yellows are produced by subtraction, the subtraction of the green pigment, chlorophyll; autumn reds by addition, the addition of sugars and leaf waste, which are trapped inside the leaf.

Chlorophyll captures energy from sunlight. Using it to fuel the combination of carbon dioxide and water, chlorophyll produces a simple carbohydrate, glucose, basic food for the tree and the forest. But chlorophyll is volatile. Sunlight destroys it. A constant supply of water up through the trunk and into each leaf is required for the tree to continue manufacturing chlorophyll. Most of the fluids that travel into each leaf are transpired from tiny openings, called stomata, on the leaf's undersurface. Each leaf, in fact, is a conduit between the Earth and the sky. A small apple orchard of about forty trees returns sixteen tons of water to the atmosphere each day.

Cool September nights stimulate cells at the base of each leaf stem to dry out. Just inside these cells a layer of corky cells develops, stopping the flow of water into the leaf, stopping the export of carbohydrates and metabolic waste back to the tree. With the flow of water cut off, the production of chlorophyll stops. As the chlorophyll wanes, lesser pigments appear. Yellow xanthophylls, the pigment in egg yolk, and carotene, the pigment in carrots, both of which were hidden by chlorophyll, transform to golden yellow the leaves of American elm and quaking aspen, paper birch and black willow.

After the corky cells isolate the leaf from the tree, bright sunny days use up the remaining chlorophyll and stimulate the production of anthocyanin, which yields the red of sumac and blueberry and Virginia creeper, the blue of white ash, the purples of viburnum. If the leaf sap containing anthocyanin is acidic, leaves turn red. If the sap is alkaline they turn blue or purple.

If the skies of late September and October are overcast, little anthocyanin is produced. Leaves go from green to yellow. sunny days and cool nights generate reds and oranges and purples. An abundance of sugar maples, which contain all three pigments--xanthophyll, carotene, anthocyanin--gives the northeastern United States the most outstanding foliage on Earth. In Vermont, as go the sugar maples, so goes fall color. Each leaf has its own pattern. Yellow spreads from the leaf margins inward. Green retreats to the veins. Soon, the veins turn yellow. Finally, the whole the tree is orange, dusted with red, inlaid with yellow.

I've hiked in New Mexico on mountainsides of yellow aspen flanked by aquamarine Englemann spruce against a blue western sky. Although beautiful, the Aspen was yellow only, whereas sugar maple is moody, unpredictable, its colors an ephemeral reflection of the weather: varied, rich, and unstable. The genius of one sugar maple is worth an entire hillside of aspen. It's like comparing Mozart to Manilow.

This is Ted Levin of Gillette Swamp.

Ted Levin is a writer and photographer specializing in natural history.
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