Colorful MYSTERY OF FALL

Theories: For decades, scientists thought leaves changed color as a byproduct of trees preparing for winter. Now there is evidence that it indicates stress.

Medicine & Science

October 20, 2003|By Timothy B. Wheeler | Timothy B. Wheeler,SUN STAFF

When the days grow shorter and the mercury starts to dip, millions of Americans head for the mountains to gawk at forests ablaze with crimson, orange and golden leaves. It's nature's last fling before drab winter sets in.

Yet for all its beauty, the spectacle of fall foliage may represent something more serious. The brilliance of the color -- especially the reds -- could be a sign that trees are fighting off injury from insects, pollution or drought.

"It may be an indication of stress," says Paul Schaberg, a plant physiologist with the U.S. Forest Service in Burlington, Vt.

Schaberg and other biologists are vying to solve what most people don't realize is still a mystery -- why some broadleaf trees turn red in the fall, as opposed to yellow or orange. They're also trying to determine if environmental factors influence the timing and intensity of the color change.

"It seems like such a basic thing," Schaberg says, "yet the more we look into it, the more questions we find."

The answers are of more than passing interest in the East and Midwest, where fall-foliage tourism is big business. Heavily forested Vermont, for example, hosts an estimated 1.5 million "leaf peepers" every year. They pump $1 billion into the New England state's economy.

But there's not much science can do -- for the time being -- when leaves don't turn on schedule. Weather can affect the timing and intensity of fall foliage. This year's warmer-than-normal temperatures have delayed peak color by a week or more in much of the Eastern United States, Schaberg says.

Leaves usually are at their brightest in northern New England by early October, and in most of the mid-Atlantic by the end of the month.

But this year's delay hasn't deterred many foliage fans. In mountainous Western Maryland, about 65,000 turned out Oct. 11 for the parade marking Garrett County's 36th annual Autumn Glory Festival.

"Up until two days before the festival, we were gre-e-en," says Peggy Santamaria, spokeswoman for Garrett's chamber of commerce. But by the weekend, bright blue skies helped bring out the "rich golds and wonderful flame-orange of the sugar maples," she said.

For decades, biologists thought that the color change in leaves was an incidental byproduct of trees and other leafy vegetation going dormant in preparation for winter.

In spring and summer, plants and tree leaves are green because they produce chlorophyll, a pigment that uses sunlight to help make food from carbon dioxide, water and other nutrients. But shorter days and cooler nights in autumn prompt plants to shut down their photosynthetic activity. The veins that carry water and food to and from the leaf plug up. The cells attaching the leaf to the tree deteriorate, until the leaf breaks away and drops to the ground.

Before the leaves fall, however, they change color, revealing pigments that had been masked by the overpowering green of the chlorophyll they were producing during the growing season.

Some trees' leaves turn yellow from the xanthophyll they harbor; others turn orange from carotene -- the same pigment that gives carrots their color.

But many trees generate yet another pigment in the fall, anthocyanin, which makes their leaves turn red or purple. Scientists once thought anthocyanin had no purpose and was merely a product of sugars trapped in the leaf as its veins clogged. Now, says William A. Hoch, a plant physiologist at the University of Wisconsin-Madison, "We know nature's more efficient than that."

The past five years or so have seen a flurry of research and scientific conjecture into what roles anthocyanin plays in trees such as maples, oaks, dogwoods and viburnums. Biologists trade pet hypotheses -- that the pigment acts as sunscreen, antifreeze, antioxidant or pest repellent.

Many scientists now believe that anthocyanin helps shield leaves from excess sunlight, enabling trees to continue photosynthesis a bit longer in the fall and store more food. "They're taking apart their photosynthetic machinery ... and trying to recover as many nutrients as possible before discarding their leaves," says Hoch.

Without chlorophyll, leaves become more vulnerable to the bright sunlight typical of clear fall days. That "sunburn" could damage the leaves, cutting short photosynthesis.

The notion that anthocyanins act as a light-screen first surfaced a century ago, but until the past decade or so, scientists couldn't measure light's impact on leaves. Now, with fiber-optic probes, they can monitor photosynthetic activity by flashing light on a leaf's surface and calibrating the response.

Using that technology, scientists ran tests on red-osier dogwoods in Harvard Forest, a 3,000-acre preserve in central Massachusetts, and found that leaves containing anthocyanins suffered less damage and recovered more quickly after exposure to intense light. Leaves from the same trees without the pigment did not fare so well.

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