• Photo of park visitors enjoying sunset from the Alpine Ridge Trail in Rocky Mountain National Park.

    Rocky Mountain

    National Park Colorado

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Intensity of Fall Colors

Most folks who enjoy the outdoors especially appreciate the coming of fall with the attendant color changes in many plants. Aspen (Populus tremuloides) are among the most colorful and wide-spread color-changing trees in Rocky Mountain National Park. Experience suggests to us that aspen produce more or less colorful leaves from year to year, and that these differences seem to relate to weather patterns, soil fertility, and the amount of moisture they received during the growing season.

Recently scientists put forward an intriguing alternative explanation for intense fall colors in some trees (Ecol. Lett. 6, 807 (2003)). Mountain birches in Norway may use intense fall colors to signal leaf-chewing insects not to infest them. The intensity of color seems to be an indicator of how much chemical defense compound the tree can produce. In the case of the mountain birches, an inchworm (geometrid) moth lays eggs on the trees in the fall. The following spring the eggs hatch, and the moth caterpillars eat the trees' leaves. Trees that can produce larger amounts of chemical defenses to make their leaves unpalatable receive less damage. The trees with the most intense leaf colors in the fall also have the least damage the following spring, suggesting a direct relationship between chemical defenses and intense colors. Over time, perhaps the moths have learned to avoid laying eggs on trees with the most highly colored leaves!

We don't know whether the same thing happens in Rocky Mountain National Park's aspen or other trees. We do know there are many different representatives of the inchworm or geometrid moth family in the park. However, whether you enjoy fall colors because they are beautiful or because they may reveal scientific secrets, Rocky Mountain National Park offers an excellent opportunity to experience a glorious autumn.

Did You Know?

a photo of a Rocky Mountain Parnassian butterfly

This Rocky Mountain Parnassian butterfly is a strong flier, even on the windy alpine tundra at 12,000 feet. More...