Catch Them Before They Fall

Leaves crunch underfoot. The first whiffs of smoke drift from a far-off fireplace. Autumn has arrived in Colorado—and there's no better time to take a drive into the mountains. Here, four can't-miss routes covering nearly 250 miles of fall splendor.

Fields of Gold

Why Colorado's aspens turn that famous color.

Let's face it, Colorado's fall foliage isn't going to make anyone's official Top Ten list for brilliant color. We don't have the reds, oranges, and purples it takes to compete with the trees of Appalachia and New England. Coloradans still adore the color that spreads across our state—especially as it's coupled with mountain scenery that's second to none—but there is one question that always floats from the back seat: Why do we only have yellow leaves?

During spring and summer, leaves manufacture food necessary for a tree's growth. This food-making program takes place in cells containing chlorophyll—the chemical that gives leaves their green pigment. This chemical absorbs from sunlight the energy that is used in transforming carbon dioxide and water to carbohydrates (photosynthesis). Along with chlorophyll, leaves contain other pigment chemicals called carotenoids (yellows and oranges) and anthocyanins (reds and purples). For most of the year these other chemicals are masked by huge amounts of chlorophyll. But when the cooler days of fall arrive, trees go into a growth-cessation phase—meaning they stop producing green chlorophyll. The chlorophyll breaks down and the leftover carotenoids and anthocyanins become visible in all their nongreen glory.

Of course, each tree is different and carries different pigment chemicals in its leaves. In the East, crimson-colored sugar maples and purple dogwoods display the results of high levels of anthocyanins. In Colorado, however, the change in seasons means yellow, yellow everywhere. "Aspens have predominantly yellow pigments living in their leaves," says Cecil Stushnoff, Ph.D., a professor in the Department of Horticulture at Colorado State University. "You can sometimes find aspens with an orange or red tint; that's an environmentally controlled circumstance, and I don't think anyone knows why it happens for sure." Stushnoff does say that variants like early cold temps, insect damage, or mechanical injury might lead to a reddish or orange hue—but that those highly sought-after colors don't manifest themselves in the same trees every year. "I have found a few decent-size patches of orange in Rocky Mountain National Park," says Stushnoff, "but they don't stick around year to year."