Fall Foliage: Why Leaves Change Color

For years, scientists have studied how leaves prepare for the annual show of fall color. The molecules behind bright yellows and oranges are well understood, but brilliant reds remain a bit of a mystery.

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. 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."

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

Ruddy mystery

The red color comes from anthocyanins, 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 signal 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.

Stress management

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 LiveScience. "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 pretty sight.

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, which spoke for the trees, leaf color could one day tell us what trees are feeling.

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