Why Do Fall Leaves Change Color?

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Warm, sunny days mixed with cool, but above-freezing nights appear to produce the brilliant red hues associated with peak fall foliage. Shane believes leaf-peepers judge whether it has been a "good" or "bad" fall based on the proportion of red leaves—the more, the better. "Other things being equal, that [ratio] changes more than anything else," he said. "The real question is: What's going on with these reds?"

Color Conundrum

Unlike carotenoids, which are always present in leaves, some species of trees produce red-hued anthocyanins, naturally occurring pigments that turn raspberries red and blueberries blue. What's more, trees produce the anthocyanin compounds found in leaves during the autumn season.

Exactly why this happens is uncertain. But scientists know that even nature's most arresting displays usually have a purpose.

Paul Schaberg is a research plant physiologist with the United States Forest Service in Burlington, Vermont. He notes that when trees produce anthocyanins "it ties up a lot of sugar and nutrients like nitrogen that you'd think [trees] would want to conserve."

"So it's a conundrum," he said. "Why do that when you're about to lose the leaf anyway? And why do some species do this and not others?"

Ideas abound. Researchers have variously suggested that the antioxidant acts as natural antifreeze, insect repellent, or sunscreen.

"The hypothesis is that if you can protect the leaf so that it stays on the plant a little longer, then you allow the tree a bit more time to recover important nutrients," Schaberg said.

Red Flags

Plant physiologist William Hoch has done extensive research into the sunscreen function of the red-hued pigment.

Using single-gene mutations, the University of Wisconsin-Madison researcher created test species from plants that would normally produce anthocyanins. The test species could not produce the compounds. The test plants were compared with natural relatives that could produce the red-hued pigment, as well as with plant species that do not develop red leaves.

"We found that pigments are being produced to protect the leaves from excess light during [the fall] period," Hoch said. "The mutants suffered more light damage to their photosynthetic systems, so that they weren't able to recover important nutrients in the leaves."

For these plants, sunscreen may be more necessary during the fall months than in the blistering days of summer.

Hoch notes that, while sunlight is brighter in June, plants that shed their leaves become more susceptible to sun damage in fall, when their leaves' natural systems are breaking down.

"[Leaves] can deal with high light levels when they are intact," Hoch said. "But as they are breaking down, they become unstable."

Yet many trees never develop red leaves. Aspens turn golden yellow. While some species, such as elms, change little before their leaves simply wither, die, and fall to the ground.

Hoch found that when he tested species like paper birch, which remains brilliant yellow, the tree recovered nutrients just as well as its red-leaved cousins.

"That tells us that nature's figured out multiple ways of doing things," Hoch said. "An analogy I like to use is the pollination of flowers: It happens by bees, birds, wind. Nature has evolved multiple ways of doing the same thing."

Schaberg's extensive research focuses on how red pigments signal a tree under stress. Any number of factors can create the strain. Schaberg has even produced red leaves on a branch-by-branch basis by selectively applying tourniquets to stress certain areas within a single tree.

"These antioxidants might be a response that's important for sun, cold, insects, fungal attacks, or anything," he said.

Schaberg believes that if science can understand trees' red-flag message, we might acquire a better understanding of overall forest health.

"We want to confirm a connection between a plant that's stressed and one that's producing red," he said. "If [red pigment] is produced for protection, does it make sense that a tree stressed by drought, pollution, or something else would have extra red? If so, maybe looking at the red of some trees could be a sort of litmus test of tree conditions."

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