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 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,” said 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 said, “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, has an estimated 1.5 million “leaf peepers” every year. They pump $1 billion into the New England state’s economy.

Why colors change

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.

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.

Maple leaves turn shades of red, purple and gold at Maple Grove Cemetery in Wichita. Biologists are studying the color changes to detect whether dark reds indicate a leaf's stress or disease.

Some 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 Hoch, a plant physiologist at the University of Wisconsin-Madison, “We know nature’s more efficient than that.”

Study of colors

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.

Anthocyanins could, in fact, play more than one role in protecting leaves from damage, many researchers believe. “These anthocyanins seem to have so many biological functions — antioxidant, antifreeze, sunscreen,” Schaberg said. “They’re kind of a general stress response, just to limit damage.”

To test that theory, Schaberg and others in Vermont have conducted experiments to find out if drought, pollution or physical damage prompts trees to produce more leaf-coloring pigment. They found that leaves on branches wounded by “girdling” produce twice as much anthocyanin — and turned a deeper red than those on unharmed limbs. This fall, the scientists are looking at the effect of reduced water supply on coloration. If these experiments bear fruit, Schaberg hopes biologists one day will be able to use the brilliance of fall foliage as a yardstick to measure the health of individual trees or entire forests.