Earth Notes: Autumn Colors

Cooler breezes crisp the air, as it shifts from the warmth of summer to the sharp invigoration of fall. Autumn is a season for the senses. The hues of stark crimsons and fiery oranges light up the hillsides, a cascading mix of nature’s brilliance. Even the cottonwoods transition into golden tones, the willows into bright yellows.

Though most think of autumn colors as the tree’s aging, the glory of fall color begins long before autumn and is a result of chemistry, not poetry. From the green of chlorophyll to the yellow of carotenoids and red of anthocyanins, these “pigments” mark a chemical change in the life of a leaf.

Chlorophyll, the chemical responsible for photosynthesis, enables plants to make sugars from sunlight. As the days grow shorter and nights colder, chlorophyll production slows and comes to a gradual halt. The carotenoids and anthocyanins present in the leaf are then unmasked, showing their true colors.

The production and function of these chemicals indicate the health of each tree. A Canyon maple, one of the first native trees to make the change in Arizona, may display brilliant red hues later than usual after a period of drought.

From the golden glow of quaking aspen to the deep reds of Arizona ash, the biotic shifts of these broadleaf trees are truly brilliant. The season’s colors are even more dazzling when you consider the chemical and evolutionary processes that bring those colors to life.

This Earth Note was written by Danika Thiele and produced by KNAU and the Sustainable Communities Program at Northern Arizona University with funding from the Charles Redd Center for Western Studies.