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There’s nothing like the fireworks of Mother Nature to herald in the fall season. Starting in early October trees all across the United States begin to lose their summer green and don their brilliant oranges, purples and golds. But Mother Nature didn’t produce such a display for our sole viewing pleasure. The turning of the leaves in fall is an essential process in preparing the trees for the long winter ahead.
As we all learned in elementary science class, the green in tree leaves is due to chlorophyll. This pigment is responsible for absorbing the sun’s rays and, through an efficient chemical process, converting that sunlight into sugars and starches – necessary food for growing trees. During the fall, however, the days become shorter and the amount of sunlight absorbed through the leaves is reduced. The drop in temperature signals the tree to stop its food production cycle for the dormant winter period. Both the reductions in sunlight and temperature cause a chemical reaction in the tree that limits the amount of chlorophyll produced in the leaf. As the green pigment fades away, the brilliant colors of fall appear.
The oranges and golds so common during the fall are actually present year round. Tree leaves have the pigments of fall colors in their leaves regardless of the season. But during the spring and summer months, when the food-production cycle is at its peak, the green pigment of chlorophyll dominants the entire leaf. Not until the food cycle declines for the winter does the green color fade unmasking the other brilliant colors of the tree. Although not necessarily involved in photosynthesis like chlorophyll, the secondary pigments such as carotene (orange) and xanthophyll (yellow) are believed to protect the tree from too much sunlight.
Anyone who has seen the fall colors in full display would have noticed that certain trees tend to turn particular colors. The leaves of dogwoods and sumacs, for example, will invariably turn deep purple or red. Aspens, however, are known for their bright yellow leaves while oak leaves are brown. The actually color of the leaves depends on the mixture of pigments and the residue of sugar remaining in the leaf. According to Patricia Hauslein of the Department of Biological Sciences at St. Cloud State University in St. Cloud, Minnesota, brighter days during fall mean more sugar will be trapped in the leaf of a sugar maple resulting in its brilliant orange color.
Nature is quite efficient and for a tree to survive the winter, it must conserve its energy. Patricia Hauslein notes that photosynthesis – a process in full swing during the summer – is responsible for pulling water up through the tree. That water is then evaporated by sunlight from the surface of the tree leaves. During the winter, the tree’s water sources are usually frozen. If the tree were to maintain its green leaves throughout the entire winter, precious water that it needs to survive would be whisked away by the sun, leaving it vulnerable to dehydration until spring arrived. To ensure their survival, deciduous trees must shed their leaves.
The best display of fall color depends on the right mixture of the natural elements of sunlight and temperature. These two factors vary between different regions of the United States, and therefore the trees begin to turn at different stages during the fall season. For those in the northwest of the country, the areas furthest from the coast will see their peak colors in early to mid October. The northwest coastal regions will see the best displays towards late October. The mid-Atlantic region has fall colors from mid-October to early November with the best shows in the northern states during October and working down to the southern states by November. The Great Lakes region of the mid-west will see their leaves change as early as late September, but the southern portions of the mid-west must wait until late October. Only the northern reaches of the southeastern United States will see any significant fall color and then not until early November. Certain pockets of the southwest will see the leaves change throughout the month of October, as will certain areas of the north central U.S.
So as the temperature begins to drop and the sunlight fades with early sunsets, deciduous trees across America are hard at work conserving their energy for the coming winter. The gradually turning of the leaves is their final curtain bow to us and a brilliant reminder that Mother Nature is only entering a temporary period of dormancy, so that come spring she’ll bloom once again.
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