Ahh…autumn! How I relish the season! How I revel in its gentle breezes and azure skies plush with cottony clouds! And above all, how I eagerly await northern Arizona’s showcase of fall color!
For me, the harbinger of autumn is my own Virginia creeper’s leafy display of vibrant orange and burgundy in late August. Following that, the locust trees in Timberline switch from bright green to gold.
But it isn’t until September and October that autumn’s true palette shines forth in the High Country. This time of year, I oft give pause to wonder what fantastical chemistry must transpire within these leaves to give such enchanting displays of color.
If you share my curiosity, read on. Keep in mind I am discussing deciduous plants not evergreens. And my use of the word “tree” is usually interchangeable with “shrub” or “plant”.
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You are already aware the chemical chlorophyll gives leaves the color green. Through a process called photosynthesis, chlorophyll helps manufacture a sugar, glucose, that feeds the entire plant or tree. It does so by capturing energy from sunlight and taking in carbon dioxide and water. All plants are genuine photosynthesis workshops throughout late spring and summer, fabricating their own food for growth, reproduction, and winter food storage.
Deciduous trees would not be able to survive winter if they retained their leaves. Liquid water is in short supply during cold months, and broad, flat leaves allow too much water to escape through their numerous pores. With the onset of fall’s shorter days and cooler nights, deciduous trees become “aware” winter is in the near future (a small miracle in itself). A tree prepares to drop its leaves by slowing chlorophyll production. The pigment is broken down into its various nutrients and transported out of the leaves to be stored in the trunk, branches, and roots. These nutrients will feed the tree throughout winter and early spring.
Once leaves lose their chlorophyll, pigments that had been in the leaves all along are now able to manifest themselves. And so, begins autumn’s magnificent color showcase! Xanthophylls reflect yellow light, giving aspens, ashes, birches, locusts, and some oaks their brilliant golden hues. Carotenes (think carrots) lend sugar maples and other plants, like my sumacs, brilliant orange tones.
Anthocyanin pigments paint with the most vivid colors of all — reds, crimsons, and purples. Scarlet oaks, red sumacs, some aspen, and red maples can be truly breathtaking! Unlike other pigments, however, anthocyanins are not present throughout the life of the leaf. They are manufactured within the leaves in the fall as chlorophyll is broken down, and veins connecting leaves to branches start closing. In the process, some sugars become lodged within the leaves, triggering anthocyanin production.
Eventually, Abscisic Acid hormones stimulate trees to close all the veins connecting the leaves to branches, weakening the attachments. Hence, the leaves drop to the ground.
Now we might ask, “Does weather play a role in the tones and brightness of autumn color?” Yes, it does. Rain-free days in late summer followed by sunny days and cool nights in autumn make for ideal brilliant fall color. Here’s why:
Sunny late-summer days ramp up sugar production, and the more sugar produced, the more becomes trapped in the leaves. Cool fall nights take part by slowing the rate of sugars moving out of the leaves.
But of course, freezing temperatures cause any remaining leaves to freeze and fall off.
Plants have other amazing strategies to survive freezing temperatures. Notably, trees acclimate their cell membranes to allow water and nutrients to migrate out of cells and into intercellular spaces to be transported to branches, the trunk, or most often, the roots.
Additionally, plants may convert their starches to sugars, which we know as sap. Sap serves as an antifreeze in the plant kingdom. When this fantastical chemistry transpires in sugar maples, we humans may use a tad of chemistry of our own to convert the gooey stuff into syrup. Yum!
Cindy Murray is a biologist, co-editor of Gardening Etcetera. and a Coconino Master Gardener with Arizona Cooperative Extension.