Chemistry Drives Fallfoliage

By Walter Witschey

On our small patch of land in Prince Edward County, a little ridge leads down to Buffalo Creek. Along the ridge, poplar trees are just beginning to show their yellow fall colors. They are the first to turn in what is otherwise a sea of green.

They remind us that Virginia, and the mountains of the eastern United States, regularly have the most spectacular display of autumn foliage to be found anywhere. Fall leaf colors are more than a pretty face – they are clues to tree behavior and tree chemistry.

Since spring, trees have been growing rapidly, taking in water and carbon dioxide and using chlorophyll in their leaves to manufacture sugars for food.

Since June 21, however, nights have been growing longer, and days shorter. By Christmas, nights will be 14 1/2 hours long.

Length of night, and cooler nighttime temperatures, are a primary signal for trees that the weather is changing and they must change with it. The water content of active growth areas, such as leaves, must be reduced to avoid freeze damage.

Broadleaf trees seal off their leaves from the transport of water and sugar, and shed them. New leaves grow in the spring when weather is warmer again.

As leaves are sealed off at the base of the leaf stem by a special growth layer, water escapes downward to the roots. Chlorophyll is no longer replenished. Sugars are trapped in the leaves.

As the bright green of chlorophyll fades away, we are able to see other pigments, some always present, and some newly formed in the fall.

Carotenoids, the orange pigment in carrots and the yellow in bananas, are present throughout the growing season. Disappearance of green chlorophyll reveals oranges and yellows.

Anthocyanins, the red of cranberries, cherries and grapes, form in autumn. They need good light (sunny days) and excess sugar trapped in the leaves.

Tannins, the bitter acids found in acorns and leaves, account for brown colors.

These four colors account for the variability we see among tree species, as well as the variability from one fall season’s colors to the next.

Poplars are destined to turn yellow. Dogwoods turn somewhat purple-red.

Summer and fall weather determines whether we experience a colorful fall. In particular, weather drives the production of anthocyanins, the red pigment.

Ideal conditions for anthocyanin production include sunny days and cool nights. This maximizes fall sugar production in the leaves, and its conversion to anthocyanins. Yellows, always present in leaves as carotenoids, do not vary so much from season to season.

Poor colors may be the result of warm weather in the fall, or a stressful summer. Either a late spring or dry conditions in summer can delay the appearance of fall colors up to several weeks.

And what about this year? It is a little early to tell.

The time of best viewing varies across Virginia. In the Tidewater area, peak foliage color is typically Oct. 20-31. For the Piedmont region, Oct. 15-25, and in the mountains, Oct. 10-20.

So now, let us abandon the science of carotenoids and anthocyanins and concentrate on Virginia’s great fall foliage color palette.

Walter R.T. Witschey is professor of anthropology and science education at Longwood University. Contact him at WWitschey@SMV.org and look for his column on the fourth Thursday of each month.

Originally published by Special Correspondent.

(c) 2007 Richmond Times – Dispatch. Provided by ProQuest Information and Learning. All rights Reserved.