The Science Behind Aspen Leaf Color Change

Quaking aspen hold a special place in the hearts of Coloradans today, and have played a prominent role in cultures native to Colorado in centuries past.

According to Ute legend, aspen leaves tremble as punishment for not paying the proper respect to the Great Spirit. As the legend goes, the Great Spirit visited Earth during a special full moon, and all of nature trembled before it — all except the aspen trees, that is. The furious Great Spirit decreed that from that moment forward, aspen leaves would tremble when looked upon. This characteristic inspired the species’ scientific name: Populus tremuloides.

Another Ute story suggests the fall aspen colors resulted from an encounter between the Great Bear and a hunter. The hunter’s fire angered the Great Bear, and when they fought, yellow cooking grease and red blood splattered the leaves of the aspen forest.

Of course, science also explains the color change, though I admit that it is a bit drier than the awe-inspiring Ute legends. Interestingly, aspen leaves in a grove tend to change color at the same time. This is attributable to the fact that most groves consist of genetically related individuals, collections known as clones. Each of the individual trees (“ramets”) in a clone arise from the root system of a single parent. Because trees from the same parent share the same genetics with their neighbors, the leaves on each tree within the clone change color and fall at the same time. In fact, one way to determine the different clones in an aspen stand is to compare when the trees’ leaves change color; each clone will have slightly different timing. This clonal reproduction gives aspen an advantage following fires and other canopy-reducing disturbances and allows the species to quickly sprout new trees. It also results in expansive clones that sprawl across dozens of acres.

Aspen trees in the same clone change color at the same time, which may be different than other neighboring clones.

Aspen trees in the same clone change color at the same time, which may be different than other neighboring clones. The yellow trees on the right, while surrounded by unchanged neighbors, likely stem from different parent roots.

The leaf changing process begins when sunlight becomes less available as days grow shorter toward the end of the summer. When nights grow long enough (the threshold differs by species), the tree reacts and causes the cells near the juncture of the leaf and stem to rapidly divide without expanding. The dense, newly-formed layer is called an abscission layer, and it begins to limit the exchange between the leaf and the branch. Carbohydrates produced by photosynthesis flow more slowly to the branch, and the flow of minerals from the roots to the leaves slow and eventually cease.

When the tree stifles the transfer of material between leaf and branch, the result is a reduction in chlorophyll, a green pigment in the leaf that absorbs energy from sunlight and powers photosynthesis. The photosynthetic process naturally depletes chlorophyll and trees must replenish it throughout the growing season. The US National Arboretum compares this breakdown as similar to how colored paper fades in sunlight. But as the days grow shorter late in the summer and the tree sends fewer resources to the leaves, the chlorophyll eventually fades away. As it does, the other pigments in the leaf emerge — xanthophylls (yellow), carotenoids (orange), and anthocyanins (reds and purple) — and produce the brilliant fall colors we love. These other pigments are always present in the leaf — without chlorophyll, we would see them year-round.

The rate of leaf change varies by species, and is controlled by a variety of environmental factors, namely temperature, sunlight, and soil moisture. Chlorophyll breaks down more quickly with ample sun and low temperatures, especially after the formation of the abscission layer. The combination of cool temperatures, particularly at night, and sunlight also stimulate more anthocyanins, the red and purple pigment in the leaf. Of course, if the cool nights are too cold they will destroy the leaf and end the production of anthocyanins. Much as a late spring frost can be disastrous to crops, an early fall frost can be disastrous to fall colors. Similarly, drought can expedite the end of the season and cause leaves to prematurely fall. The common knowledge is that a wet growing season followed by an autumn filled sunny days and cool typically foster the best colors in the fall. Leaf fall may also be dictated by local weather such as a strong storm or heavy winds.

What a cool process. I love that aspens have inspired vengeful and gory legends and fascinating science. We haven’t even dug into the ethnobotanical uses of the tree, which can help treat urinary tract infections, diarrhea, and painful bowel movements. Perhaps we will save that for another day, though. For now, we encourage you to find a free weekend this fall and make an effort to check out the changing leaves in Rocky Mountain National Park or another aspen-heavy locale near you.

— Aaron Sidder, Editor in Chief