Experts explain why autumn leaves change color

Autumn brings a bold color palette change to the landscape, with pigments ranging from bright yellows and oranges to rich purples and reds. 

In a Sept. 16 article published by Utah State TODAY,  Jaydee Gunnell, Utah State University Extension agriculture and natural resources specialist and Cache County director, explains why green leaves undergo a color change every year during autumn months. 

Day length 

Gunnell notes length of day is the primary internal trigger causing leaves to change color. 

“When days become shorter and nights become longer, a process within the plant causes cells around the base of the leaf – the petiole – to divide rapidly but not elongate,” he explains. “This process forms an abscission layer where the leaf will eventually separate. This abscission layer blocks or prevents sugars from escaping the leaves.” 

Gunnell further explains, during the growing season of warm spring and summer months leaves constantly produce sugars through the process of photosynthesis. These sugars are moved through the plant for growth and storage. 

“It is during this active production time the vibrant green colors are dominant,” he says. “Chlorophyll – the green pigment – is found in nearly all plants and is a key component in photosynthesis. It breaks down readily in sunlight and is replaced constantly throughout the growing season.”

Leaf color

While day length is the leading cause of color change in autumn leaves, Gunnell notes the brilliance of the pigment depends on weather conditions before and during the time chlorophyll depletes, with temperature and moisture as the two main factors. 

Additionally, pigment differs among plant species and can vary year to year.

“Colors are most vibrant when plants have had adequate moisture throughout the season, followed by sunny autumn days and crisp, cool nighttime temperatures,” Gunnell states. 

Likewise, an article published by the U.S. Forest Service (USFS) titled “Science of Fall Colors,” states, “Like the weather, soil moisture varies greatly from year to year. The countless combinations of these two highly-variable factors assure no two autumns can be exactly alike.”

“A late spring or a severe summer drought can delay the onset of fall color by a few weeks. A warm period during fall will also lower the intensity of autumn colors. A warm wet spring, favorable summer weather and warm sunny fall days with cool nights should produce the most brilliant autumn colors,” the USFS article continues.

While carotenoids and xanthophylls – orange and yellow pigments – are produced throughout the growing season to aid in photosynthesis, they are often masked by chlorophyll. However, as photosynthesis slows and chlorophyll breaks down, these pigments become more apparent and fade at a slower rate.

Gunnell notes a few examples of trees with high carotenoid and xanthophylls production include quaking aspen, ginkgo, Norway maple, ash, birch and honey locust. 

Pink, purple and red pigments are attributed to the production of anthocyanin and can vary from year to year. The amount of sunny days and cool – but not freezing – nights are two determining factors influencing anthocyanin production in a plant. 

According to USFS, anthocyanin is produced when sugar is made during hot, bright fall days, but as nights cool, plant veins gradually close and prevent these sugars from moving out of the leaf. 

Gunnell shares examples of species with these pigments include certain maples, burning bush, flowering pear, sumac and dogwood. 

He also points out tannin – the brown pigment – is the last to break down before a leaf falls off of a tree. 

“Oaks or other non-showy species notorious for having leaves containing tannin are the final reminder winter doldrums are soon to follow,” he concludes. 

Hannah Bugas is the managing editor of the Wyoming Livestock Roundup. Send comments on this article to roundup@wylr.net.