The changing color of leaves in the fall never fails to delight me. But did you ever wonder why maples turn red, birches yellow, oaks stay brown, and there is such a wonderful display of many colors in between?
To answer that question, we first need to understand the purpose of leaves, and how they do their job. So, here's a little science lesson.
Leaves are food factories for trees and other plants. Using water taken up by their roots and carbon dioxide from the air, they produce glucose (sugar) for growth, and oxygen. Then, combined with the chemical chlorophyll plus sunlight, a process called photosynthesis occurs. The chlorophyll is what gives leaves their green color.
In addition to chlorophyll, though, two other color pigments are produced by the leaves. Carotenoids are color pigments that are responsible for yellow, orange and brown colors, while anthocyanin produces red. The carotenoids are in the leaves all year, but there is so much more chlorophyll that the leaves just look green to us. Not all tree leaves make anthocyanin, however.
I know, I know, this all sounds complicated — and it is — but here comes the "pretty" part. As autumn approaches, the days get shorter, and temperatures drop. The trees sense the approach of winter and begin to shut down their leafy food factories. They respond to lessening sunlight by producing less of the green chlorophyll. As the green color disappears, the carotenoid (yellow, orange and brown) and anthocyanin (red) pigments, which were there all along, become visible, giving us the beautiful fall colors.
As that process is going on, and before the leaves begin to fall off the tree, the glucose in the leaves is sent down to the roots to sustain the tree through the winter and to give the tree enough energy to begin growth again when spring comes.
I'm sure you've noticed that some years produce brighter leaf colors than others. The best autumn colors come when there has been a warm, wet, spring, a summer that's not too hot and dry, and a fall with warm, sunny days and cool nights. We got the right combination this year, didn't we?
Now, the leaves need to get off the tree. Here's how that is accomplished. Because leaves contain a lot of water, and freezing them would burst the cell walls, the veins that carry sap in and out of the leaves gradually close as sunlight decreases. Meanwhile, a layer of cells is formed at the base of the leaf where it connects to the branch. When this "separation layer" is complete, the leaf falls.
Evergreens do not lose their leaves, or needles, in winter because they are covered with a heavy, waxy coating. This, along with the substance inside their cells that resists freezing, means evergreen leaves can live for several years before they fall and are replaced with new growth.
When on the ground, all fallen leaves are broken down by fungi, bacteria, earthworms and other organisms. The resulting humus helps replenish the soil with nutrients and absorbs rain. Thus, nothing goes to waste. Isn't Mother Nature wonderful?
Coming up: This is the last week to register for Winter Dreams, Summer Gardens, the all-day gardening symposium sponsored by the Oregon State University Extension and Jackson County Master Gardeners. The symposium offers 40 classes from which participants choose four. Held on Saturday, Nov. 2, at the RCC/SOU Higher Education Center, at 101 S. Bartlett St., in downtown Medford, the event costs $40, which includes lunch. Call 541-776-7371 for a registration packet.
Carol Oneal is a past president of the OSU Jackson County Master Gardeners Association. Email her at email@example.com.