Winter appears to be settling in on us as average daily temperatures are decreasing and the first snows have fallen. But before we forget the beautiful fall color display of autumn leaves we so enjoyed leading up to this winter, wouldn't you like to know why and how leaves change color?
Why do some maple leaves turn bright red? Where do the yellows and oranges come from? And why do some oak tree leaves turn brown?
Leaves are nature's food factories. All plants take water from the ground through their roots. They also take a gas called carbon dioxide from the air. Plants use sunlight to turn water and carbon dioxide into oxygen and glucose. Oxygen is a gas in the air that we need to breathe. Glucose is a sugar. Plants use glucose as food for energy and as a building block to grow. This process is known as photosynthesis. Chlorophyll is what gives plants their green color and it is responsible for the photosynthesis process.
As summer ends and fall approaches, the days get shorter and shorter. This is how the trees "know" to begin getting ready for winter when water and sunlight will be in short supply. During winter, there is not enough light or water for reliable photosynthesis. Deciduous trees "rest," losing stored up food made during the summer months to survive the winter.
Leaves are attached to tree branches by stems called petioles. With the approach of fall, an abscission (separation) layer begins to form where cells swell and begin forming cork-like material, gradually cutting off the flow of water and other substances into and out of the leaf. Glucose and waste products get trapped in the leaf and chlorophyll begins to disappear as water flow into the leaf is cut off.
As the bright green fades away, we begin to see yellow (xanthophyll) and orange (carotene) colors. Small amounts of these colors have been in the leaves all along. We just can't see them in the summer, because they are covered up by the green chlorophyll.
The bright reds and purples (anthocyanin) we see in leaves are made mostly in the fall. In some trees, like maples, glucose is trapped in the leaves after photosynthesis stops. Sunlight and the cool nights of autumn cause the leaves to turn this glucose into a red color.
Some scientists believe that the anthocyanins help keep leaves on trees longer, allowing more time for sugars, nitrogen and other nutritious substances to move out of the leaf and into the tree before the leaves fall. The brown color (tannins) of trees like some of the oaks is made from wastes left in the leaves.
It is the combination of all these things that make s
the beautiful fall foliage colors we enjoy each year.
I just finished a brief lesson in the biochemistry of a tree's food factories, probably more than you cared to learn. But just think, the next time someone talks about the beautiful fall leaf colors, you can now explain what is happening and why. Without this process, deciduous trees would not be able to survive our winters.
Jim Leser retired to Cedaredge in 2007 after a career with Texas A&M University Extension in entomology. He is a member of the Cedaredge Tree Board and a Colorado Master Gardener.