I was asked recently if I knew why leaves changed color in the fall. Even though I could make a few educated guesses, I really didn’t know the science behind it. If yo live in Maine, and enjoy the change of the seasons, it’s a good trivia question to be ready to answer!
First you need to understand what leaves are and what they do. Essentially they are nature's food factories. Plants take water from the ground through their roots. They collect carbon dioxide from the air. Plants use sunlight to turn water and carbon dioxide into glucose. Glucose, a sugar, is used as food and the way plants turn water and carbon dioxide into sugar is called photosynthesis. Chlorophyll helps make photosynthesis happen and it is Chlorophyll that gives plants their green color.
 As summer ends and autumn comes, the days get shorter and shorter. During winter, there is not enough light or water for photosynthesis. The trees will rest, and live off the food they stored during the summer. They begin to shut down their food-making processes. The green chlorophyll disappears from the leaves. As the bright green fades away, we begin to see yellow and orange colors. Small amounts of these colors have been in the leaves all along. We just don't seem them in the summer, because they are covered up by the green chlorophyll.
The bright reds and purples that 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 turn this glucose into a red color. The brown color of trees like oaks is made from wastes left in the leaves.
Best Conditions A succession of warm, sunny days and cool, crisp but not freezing nights seems to bring about the most spectacular color displays. During these days, lots of sugars are produced in the leaf but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. These conditions - lots of sugar and lots of light - spur production of the brilliant anthocyanin pigments, which tint reds, purples, and crimson. Because carotenoids are always present in leaves, the yellow and gold colors remain fairly constant from year to year.
The amount of moisture in the soil also affects autumn colors. Like the weather, soil moisture varies greatly from year to year. The countless combinations of these two highly variable factors assure that no two autumns can be exactly alike. A late spring, or a severe summer drought, can delay the onset of fall color by a few weeks. A warm period during fall will also lower the intensity of autumn colors. A warm wet spring, favorable summer weather, and warm sunny fall days with cool nights should produce the most brilliant autumn colors. It is the combination of all these things that make the beautiful colors we enjoy in the fall. |
