When I was a little girl I was told that when the leaves turned colors in the fall, it was because a little elf named Jack Frost flew from tree to tree on the first really cold nights of fall and painted the trees red, yellow and orange.
It is always a little exciting to see the first color changes of the season, the vivid red maples. Although I believe in elves, I have learned that trees do not need Jack Frost to burst into color each year. Did you ever wonder how this magic happens every year? What makes perfectly beautiful green-leaved trees switch wardrobes and eventually go nude, just as the weather gets colder? Somehow that just seems wrong, doesn’t it?
Trees, like all growing things are really food factories. Trees and plants create energy and growth with sunlight, water, carbon dioxide and chlorophyll. This is the magic we call photosynthesis – turning sunshine into living plants. Trees contain colorful substances in their leaves: green chlorophyll, orange and yellow carotenoids and red anthocyanins. Chlorophyll is the most prevalent, so the leaves are green.
Solar fuel is what the tree counts on to keep its food factory running smoothly. As long as the sun shines for long daylight hours, photosynthesis churns out oxygen for the earth and glucose that keeps the tree healthy and growing. When the earth tips and the hours of sunshine shorten, (September to November, depending on where we live) the tree begins to notice that its factory is running low on solar energy – one of its essential production raw products.
Rather than stop all production, which would shut down the factory and kill the tree, it pulls water and chlorophyll back out of the leaves into the tree branches and trunk. This process helps deciduous trees recover as much as 50 percent of the nitrogen and phosphorous from its leaves to save for making new leaves in the spring. The branches and trunk store the precious nutrients needed to stay alive and begin production again when more solar power is available in a few months.
With the green disappearing from the leaf cells as the chlorophyll is drawn from the leaves, the other substances in the leaves, the yellow carotenoids and red anthocyanins turn the leaves into an artist’s palette. Depending on the weather, the colors may be more vivid from year to year. If the fall is sunny and warm, the tree makes more anthocyanins and leaves will be a more vivid red.
As the tree draws more into itself the tip of the stem on each leaf where it joins the tree loses its flexibility and dries out until the leaf drops. Any nitrogen and phosphorus left in the leaf that degrades on the ground becomes nutrient rich mulch that can feed the plant life under the tree. This makes deciduous trees Earth’s prettiest recycling factories.
So why don’t all trees do this? What’s with evergreens? Evergreens and other trees that stay green all winter are conifers. Conifers have groups of needles instead of leaves. This “not a leaf” design helps increase surface that absorbs sunlight and preserves nutrients. Conifers also produce a chemical that is like an antifreeze to limit damage caused by cold weather. In this way, the conifers do not have to recycle leaves like deciduous trees do to stay alive.
Somehow, knowing the science of how a deciduous tree changes its colors each fall doesn’t ruin the wonder, even if it’s not Jack Frost. Somehow, it adds to the magic of the season. Nature’s ways are truly awesome.
Andrea Lee Witt is a retired registered nurse, kayaker, rock hound, Bickelhaupt Arboretum volunteer and Master Gardener.