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The mysteries of fall color revealed

Sterling Pond at The Morton Arboretum
Sterling Pond in autumn.

All across The Morton Arboretum, trees are changing color. Some trees light up the hillsides like red flames. In other places, a walk in the woods surround you with gold.

Why do leaves change color before they fall in autumn? And why do they turn different colors?

Trees’ fall color is a side effect of going dormant—the process of shutting down temporarily to conserve resources during the cold winter. As they prepare to shed their leaves, trees stop producing chlorophyll, the food-making pigment that makes the leaves green, says Ed Hedborn, manager of plant records at the Arboretum. When the green drains away, it reveals other colors.

In many leaves, such as cottonwoods, buckeyes, and many sugar maples, it’s yellow that appears, from the same pigments that make daffodils yellow and pumpkins orange. Other leaves simply turn brown, the color of dead cells, giving the woods a touch of antique gold.

Red is more of a mystery. To turn their leaves red, trees actually have to start producing red and purple pigments called anthocyanins.

“They’re the same ones as in grapes and apples and pink flowers like peonies,” Hedborn says.

Scientists still aren’t sure why trees would go to that trouble for leaves they’re about to abandon.

But in the woods, red plus yellow is magic, creating landscapes with all the oranges and reds of a sunset over Lake Michigan.

Day length is probably the biggest factor in determining when a tree will change color, Hedborn says; shorter days are a signal that winter is coming. But sunlight, temperature, moisture, and stress also play roles. After a drought summer, trees are likely to change color and drop their leaves sooner.

And of course, different species have evolved to turn different colors at different times. Red sumacs, such as the Staghorn sumac, are among the first plants to show color, and oaks may hold their antique gold leaves all winter.

In general, the tree color show is most spectacular when there are sunny days and cool nights, Hedborn says. If it’s hot in September, the color is likely to be early and muted. If it’s cool and rainy, the show may be delayed.

Conditions vary not just from year to year but even within the tree. Leaves on the outside of the leaf canopy tend to change color and fall first, Hedborn says, because they are most exposed to sunlight and therefore to the signal of shorter days. Leaves in the interior often stay green and hang on longer. Trees in the forest, where they are shaded by other trees, tend to change color and lose their leaves later than trees along streets or in yards that are more exposed.

The vibrant red leaves of a Japanese maple

Most of the trees that make the color show so rich at the Arboretum are native sugar maples. But trees from 40 countries—bright red Japanese maples, amber dawn redwoods from China, orange Persian ironwood—add to the tapestry and make fall at the Arboretum an experience exclusive to the champion of trees.