CORVALLIS, Ore. - Autumn leaves are the grand finale of the growing season. What creates all that color?
It’s all about photosynthesis, according to Pat Breen, Oregon State University professor emeritus of horticulture.
The word “photosynthesis” means “to transform with light.” That is just what happens inside leaf cells as chlorophyll uses the sun’s light to transform water and carbon dioxide into food for the growing plant.
Throughout the summer, green plants produce a continual supply of chlorophyll to keep the transformation going. But when days get shorter and nights get cooler, plants slow their production of chlorophyll. As the amount of chlorophyll declines, the green color of plant leaves starts to fade and other pigments begin to shine through.
Carotenoids, for example, are found in most green plants, and are necessary for capturing sunlight. When chlorophyll fades, the carotenoids that are left create yellow and sometimes brilliant gold color. We see carotenoids at work in big-leaf maples this time of year.
A second pigment, anthocyanin, is produced in the leaves of only a few kinds of trees. It creates shades ranging from pink to red to purple, as in our native vine maples.
When the colorful pigments finally fade, leaves turn brown from the remaining tannin. Tannins are found in almost all trees, and are especially abundant in Oregon white oak, which keeps its autumn color to a conservative brown.
Weather plays a part in the show of autumn leaves. The most vivid color tends to unfold when autumn days are sunny and nights are cool but above freezing. The red-color anthocyanins are produced in strong light when sugars are trapped in the leaf as stems begin to shut off in preparation to drop. If the weather holds, enough sugars are produced to create brilliant red and orange color in the leaf. Rainy weather blocks warmth and sunshine, and so inhibits sugar production. Leaves fade without much color change. Likewise, early frost can kill leaves, turning them brown.
You can orchestrate your own grand finale by planting trees that promise autumn color. Many kinds of native trees and shrubs are already brightening the woods this time of year. A trip to the local nursery will reveal an even larger selection. Consider Norway maple or tulip tree for yellow and gold color; sugar maple and sweet gum for bright orange and red; and scarlet oak for deep red. All these should do well throughout most of Oregon.