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Autumn paints Colorado’s mountains in light and color.

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Barbara Demoing-Adams

Bands of yellow and orange aspen trees ripple across the slopes, flickering against the dark green of conifer forests.��The display is dazzling, but leaves don’t change color just for decoration. The transformation is the result of a chemical process inside each leaf that helps trees prepare for the coming winter,��says plant biologist��Barbara Demmig-Adams.

“Trees have developed different strategies so they can survive with limited resources and avoid competing with each other,” says Demmig-Adams, professor of distinction in the Department of Ecology and Evolutionary Biology. “That’s why we see some leaves turn yellow, some turn red and some stay green.”

Changing leaf color is a unique trait of deciduous trees, a group that includes aspens, maples and oaks. These species have broad leaves that they shed in the fall. Evergreen trees, like pines and firs, stay green through the seasons.

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During spring and summer, when sunlight is abundant, deciduous trees produce chlorophyll, a green pigment that captures light and turns it into food through photosynthesis.

As temperatures drop and days shorten in the fall, deciduous trees stop producing chlorophyll and begin pulling nutrients, especially nitrogen, back into their trunks and roots before shedding the leaves.

Without chlorophyll, the��yellow and orange pigments that have been present in the leaves all along shine through.

“The yellow pigment is mainly composed of carbon, which is abundant in the environment and in the plants, so trees are fine with letting it fall away,” Demmig-Adams says. ��

Red leaves tell a different story. Unlike the yellow pigment, trees make the red pigments, called anthocyanins, freshly in the fall.

When trees experience a succession of warm, sunny Autumn days, their leaves produce lots of sugar. If the nights become significantly cooler, the veins that carry fluids into and out of the leaves close off, trapping sugar inside. That sugar then binds with other chemical molecules to form anthocyanins.

This year, fall foliage started to emerge in the Front Range in late August, weeks earlier than the typical mid-September timing. This is mainly because of the drought conditions Colorado is experiencing, which triggered the color-changing process early as trees try to conserve nutrients.

“Trees are very smart. They’re always keeping track of all of the changes in the environment and respond promptly,”��Demmig-Adams says.