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,” 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 whether 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.

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 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 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.

Beyond their role as a sunscreen, anthocyanins may also protect plants from damage by acting as antioxidants, some researchers think. When exposed to intense sunlight, leaves produce highly reactive “free radicals” of oxygen, which can damage their biological machinery and destroy cell membranes. Anthocyanins might be scavenging the free oxygen radicals before they do harm.

Scientists also recognize anthocyanins as helpful antioxidants in the human diet, a major reason why it is healthful to eat plenty of fresh fruit or drink red wine in moderation.

“They’re more effective than vitamin C, than vitamin A,” notes David W. Lee, a plant physiologist at Florida International University in Miami. “They’re particularly high in leaves.”

There’s just one rub – the anthocyanins accumulate in the vacuole, or storage area of leaf cells, and not in the chloroplasts, where the oxidation damage occurs.

This fall, scientists at Harvard Forest, which is owned by Harvard University, are monitoring a batch of dogwoods – including some mutants that can’t produce anthocyanins – to see if they can detect additional nutrient production among the trees with the red pigment. In previous tests, no difference in nitrogen was found, but there may be some phosphorus variations, says N. Michele Holbrook, a biology professor at Harvard.

If anthocyanins help protect leaves from sunburn and enable them to store more food over winter, why don’t all trees produce the pigment? Leaves that turn yellow or orange must have other biological mechanisms for dealing with the sunlight, researchers say, though no one knows what those are.

Other scientists wonder if the pigments help protect leaves from freezing temperatures. Anthocyanins are water-soluble, says Linda Chalker-Scott, associate professor of horticulture at University of Washington, and may help leaves retain water in their cells. In the Pacific Northwest, she notes, anthocyanins seem more prevalent in trees on mountain slopes likely to be subject to early frosts.

“The colors get more intense as you get into higher altitudes,” she says.

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,” says the Forest Service’s Schaberg. “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.

Meanwhile, even hardcore scientists admit that it’s hard to be completely analytical while studying fall foliage. “It’s fun, because we’re a very visual species and color-oriented,” says Holbrook.

For more information on why leaves change color in the fall, and for good places to see foliage at its peak, go to www.foliagenetwork.com.