When Missy Eppes was young, she loved to climb her family’s maple tree during the fall. “It turned the most amazing colors of orange and red and yellow,” she says. “I would sit up in that tree in the fall and feel like I was sitting inside the sun.”
Today, she is Dr. Eppes, a scientist at the University of North Carolina at Charlotte. She and her students have worked to help solve a puzzle about autumn leaf colors.
Scientists understand why summer leaves are green and why they change to yellow or orange in the autumn. But autumn reds are different. Some leaves turn red. Others don’t. It’s a mystery.
Inside all leaves, all summer long, a green pigment (colored chemical) called chlorophyll catches energy from sunlight for photosynthesis, a process that makes food for the plant. Yellow and orange chemicals called carotenoids are there through the summer, hidden by the chlorophyll. These lighter colors help catch sunlight and pass the energy to chlorophyll. In the autumn, photosynthesis stops, and the green chlorophyll breaks down. Then the yellow and orange carotenoids show through in the leaves.
But for most plants, the red color, anthocyanin, has not been hiding. The leaf doesn’t start making this red pigment until the fall. Dr. Eppes says, “Trees use up a lot of energy to make anthocyanin. So it has always been a mystery why they would go to all of that trouble at a time when they should be ‘going to sleep’ for the winter.”
Anthocyanins may help a leaf save nutrients that it stored up in the growing season. Dr. Eppes and her students ran a study to test that idea. In a protected area in North Carolina, they counted the kinds of trees that live in rich soil low on the hillsides, and the types that live in poor hilltop soils.
Low on the hillsides, they found more birches, beeches, and other trees that can’t make anthocyanins. These trees have yellow or brown fall leaves. On the hilltops, most of the trees were red maples, sweet gums, and others that can make anthocyanins, giving them red leaves in the fall.
“Trees that don’t make anthocyanins seem to naturally live in places with lots of ‘food,’ like floodplains where nutrients get added to the soil each time the river floods,” Dr. Eppes says. “Trees that make anthocyanins seem to live in places that don’t have this buffet.”
Dr. Eppes suspects the anthocyanins may lie in the soil after the leaves drop, fertilizing the tree in the spring. Other scientists think the chemical works like sunscreen, protecting the leaf while it moves nutrients into the tree and stores them for the winter.
The mystery isn’t solved yet, but scientists are a few steps closer!
Pronounce the Pigments
The pigment that makes plants green is chlorophyll (KLOR-oh-fill).
Yellow and orange colors are carotenoids (kah-RAWT-en-oyds).
Reds are anthocyanins (AN-thoh-SIGH-ah-nins).
Watch for Reds
Notice how sunlight helps give a touch of red, even on a single plant. For plants that make anthocyanins, the leaves that get more sun tend to turn redder than those in shade.
The red maple tree is named for the anthocyanins in its fall leaves and in its spring flowers.
Leaves of the yellow birch can’t make anthocyanin. When the green chlorophyll breaks down, the yellow carotenoids show through.
Sweet-gum leaves may turn yellow, red, or some hue in between, depending on how much sunlight hits them.