Here’s What Determines Fall Leaf Color

A colorful capture of Beech trees, courtesy Valiphotos on Pixabay.

Fall approaches, so I thought I’d explore the reasons behind the annual burst of color. We in the south won’t see real changes to our trees for a few months yet, but folks in Northern climes should start seeing variety in their verdure by the end of this month. Dwindling daylight triggers the transition, with moderate fall temperatures producing the best color.

A beautiful but dying Red Maple leaf. Cells develop at the base of the leaf’s stem that lock in pigment-producing sugar. This generates brilliant color, but ultimately severs the leaf from its branch. Photo courtesy Dave Hoefler at Unsplash.

Of the three pigments that determine a leaf’s color, green-hued chlorophyll dominates during the long summer days. It soaks in sunlight and converts the solar energy to sugars that feed the tree–the process known as photosynthesis. (This process also produces oxygen. In fact, photosynthesis creates nearly all the oxygen on the planet, with about one-third coming from land plants and two-thirds from ocean plants like plankton and kelp.) Photosynthesis depletes chlorophyll, but sunshine creates new chlorophyll to perpetuate the process.

As days shorten, chlorophyll production in the leaves of deciduous trees and bushes sputters and finally collapses. The other two pigments in the leaves seize the opportunity and assert themselves. These are carotenoid pigments, which produce reds, yellows, and oranges; and anthocyanin pigments that contribute reds, purples and blues.

As the long-suppressed pigments emerge, the splashiness of their display depends on the weather. Warm, sunny days allow the leaves to produce as much sugar as possible before they fall off. Too many autumn clouds, or an early freeze, will short-circuit sugar production and with it the season’s color.

Finally, we can’t forget evergreens. It’s not true that they never shed their foliage. Evergreen conifers, which include pine trees, replace older needles, but not necessarily every year. Those that drop change colors first, turning deep copper or light gold before dropping The difference between deciduous trees and evergreens is that evergreens always retain a healthy supply of green to carry on photosynthesis, while deciduous varieties give us brilliant colors to enjoy every fall.

Evergreen conifers, courtesy galaphix at Pixabay. Evergreens retain leaves that produce chlorophyll and photosynthesize all year long. The oldest living tree in the world is a conifer–a Bristlecone Pine in California with an estimated age of 4,852 years.

A Houston favorite, the Bald Cypress is a deciduous conifer that produces orange and brown leaves in autumn. Bald Cypresses are the oldest living wetland species in the world. Image by supa-dimz from Pixabay.
Deciduous Aspens are abundant in the American West and are famous for the “music” the wind makes as it passes through their quaking leaves. The aspens in this photo are clones of each other and part of a single root system believed to be from 14,000 to 80,000 years old. Some scientists label this stand of aspens the oldest organism in the world. Photo courtesy J. Zapell at Wiki Commons.
Beech leaves, courtesy claude05alleva from Pixabay. Beech trees are deciduous, but their leaves are stubborn; they hang around until they’re forced off by strong winter winds.

Works Cited

“Autumn Leaf Color,” Wikipedia.

“Autumn Leaves: How Plants Prepare for Winter,” Science Made Simple.

Garcia, Julie, “Houston is Having a Vibrant Fall Foliage Moment. Here’s the Science Behind Why,” Houston Chronicle. 26 Nov 2019.

“Deciduous,” Wikipedia.

“Photosynthesis,” Wikipedia.

“Save the Plankton, Breathe Freely,” National Geographic.

“Science of Fall Colors,” U.S. Forest Service.

Taylor, Lindsey, “Photosynthesis in Pine Trees,” Sciencing. 22 Nov 2019.

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