Fall foliage: Why leaves change color

Every autumn, nature transforms into a riot of red, orange, yellow and gold. But colorful fall foliage is more than just a pretty sight — it's also a signal of change brought on by weather, light and a healthy dose of seasonal stress.

In response to chilly temperatures and fewer daylight hours, leaves stop producing their green-tinted chlorophyll, which allows them to capture sunlight and make energy through photosynthesis. Because chlorophyll is sensitive to the cold, certain weather conditions like early frosts will turn off production more quickly.

Meanwhile, orange and yellow pigments called carotenoids — also found in orange carrots — shine through the leaves' washed out green.

"The yellow color has been there all summer, but you don't see it until the green fades away," said Paul Schaberg, U.S. Forest Service plant physiologist. "In trees likes aspens and beech, that's the dominant color change."

Related: Are carrots orange because of a Dutch revolutionary?

Ruddy mystery

Scientists know less about the radiant red hues that pepper northern maple and ash forests in the fall.

The red color comes from anthocyanins — pigments which, unlike carotenoids, are only produced in the fall. They also give color to strawberries, red apples and plums.

On a tree, these red pigments beneficially act as sunscreen, by blocking out harmful radiation and shading the leaf from excess light. They also serve as antifreeze, protecting cells from easily freezing. And they are beneficial as antioxidants. Trees produce them in response to stresses in the environment like freezing cold, UV radiation, drought and fungus.

But red leaves are also signals of distress. If you see leaves of a tree turning red early, in late August, the tree is most likely suffering from a fungus or perhaps a ding from a reckless driver, according to Schaberg.

Trees blaze orange around Lake Superior in September 2020, in this NASA satellite image. (Image credit: NASA/ Joshua Stevens)

Why would a tree put its energy toward making new ruddy anthocyanins, just when the leaf is about to fall off?

"People have speculated that maybe it's something that helps leaves deal with stress," Schaberg told Live Science. "If making the anthocyanins helps the leaf stay on the tree a little longer, it may help the tree absorb some of the good things before it falls off. The tree can use those resources for the next growing season."

By nailing down the correlation between red leaves and environmental stresses, fall foliage may become more than just a late-year delight. Scientists hope studying anthocyanins will clue them into the degree to which some trees are stressed, which could provide a better picture of environmental problems early on. Like Dr. Seuss's character the Lorax, who spoke for the trees, leaf color could one day tell us what trees are feeling.

Originally published on Live Science.

Corey Binns lives in Northern California and writes about science, health, parenting, and social change. In addition to writing for Live Science, she's contributed to publications including Popular Science, TODAY.com, Scholastic, and the Stanford Social Innovation Review as well as others. She's also produced stories for NPR’s Science Friday and Sundance Channel. She studied biology at Brown University and earned a Master's degree in science journalism from NYU. The Association of Health Care Journalists named her a Centers for Disease Control and Prevention Health Journalism Fellow in 2009. She has chased tornadoes and lived to tell the tale.