Fall colors replace green of summer

Posted: Sunday, September 22, 2002

There's a tang in the air, and for many of us it's time to watch "termination dust" making its way down the mountainsides. But before winter comes we can look forward to one of nature's most colorful art shows.

On the mountainsides, along the river, and often in our own back yards the leaves of certain trees and smaller plants are turning brilliant yellow, red, orange, and even purple. How leaves do this is a fascinating process, one that is keyed to both falling temperatures and declining hours of daylight.

Most of us know that the green we see in leaves during spring and summer comes from chlorophyll, the group of pigments that plants use to produce food through photosynthesis. It's chlorophyll that captures energy from the sun (photo-) so leaves can manufacture simple sugars from water and carbon dioxide (-synthesis). The sugars then provide food for the plants' growth and development.

During the food-making process, chlorophyll continuously is breaking down and being used up; but during the growing season healthy plants continuously replace it, so their leaves stay green. Come fall, cooler temperatures and shorter daylight cause photosynthesis and chlorophyll production to slow down. The amount of chlorophyll dwindles, and the green in leaves starts to fade. Suddenly we begin seeing yellow and orange - pigments called carotenoids that were in the leaves all along but were masked by the green.

Some of the most brilliant shades of yellow in Southeast's fall foliage are seen on cottonwoods, willows, deer cabbage, and devil's club.

But what about the leaves of blueberry, nagoonberry, highbush cranberry, and bunchberry, or dwarf dogwood? What makes their leaves turn shades of red or purple?

These plants produce pigments called anthocyanins, which also are found in cranberries, red apples, cherries, and strawberries. Unlike yellow and orange carotenoids, anthocyanins are not present in leaves year round. They're produced only in late summer, and only by plants that are genetically designed to do so. They develop in the sap of leaf cells when phosphates move out of the leaves, and sugars trapped in the sap break down in the presence of bright sunlight.

The exact colors anthocyanins produce depend on the pH (or acid/base level) of sap and the amount of sugar trapped in the leaves. But their color, too, shows only as the masking green from chlorophyll is depleted; and they, in turn, mask the yellow carotenoids that are also present in the leaves.

The yellow, gold, and orange colors in leaves remain fairly consistent from year to year because the pigments that cause them are always present. But red pigments are brighter when the weather is cool and sunny. That's because the leaves produce more sugar in sunlight, but cool temperatures keep the sugar sap from flowing out of the leaves into the stem or trunk of the plant.

The most spectacular fall colors usually are seen in the alpine, near glaciers, and in bogs or muskegs. Two places particularly easy to get to in Juneau are along the shore of Mendenhall Lake and in the subalpine just above the Mount Roberts Tramway. Last year the peak of fall color around Juneau was in mid to late September.

Southeast Wild is provided by the Juneau Audubon Society. Contact Audubon members at ckent@alaska.net.

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