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As summer heat fades into brisk autumn weather, nature offers many parts of the world one more vibrant burst of color before the gray of winter takes over—fall leaf color in deciduous trees.
Regions with the climatic conditions particularly favorable for fall color—warm autumn days and cool autumn nights—such as the northeastern United States, southeastern Canada, southwestern Europe, and parts of eastern Asia, have trees that are known for their fantastic displays of bright fall colors. The red, yellow, orange, and purple hues that emerge in the leaves of these trees attract admirers to forests and parks in these areas of the world every autumn.
While many people look forward to this yearly phenomenon, fewer understand why it occurs. For those who do understand the process of fall coloration, other factors, like the appearance of early fall color, may remain a mystery.
What Causes Fall Color?
Fall color in a tree’s leaves is one of the results of the tree’s yearly cycle of food production and food storage. According to William Chaney, professor of tree physiology at Purdue University, deciduous trees make the food they need to grow in their leaves. This food production occurs in a process called photosynthesis, which takes place mainly during the spring and summer months. During photosynthesis, leaves use energy from the sun to transform carbon dioxide and water into carbohydrates, which are used as food for the tree.
Chlorophyll
Photosynthesis occurs within chloroplasts, Chaney explains. Chloroplasts are specialized organelles in a leaf’s cells where the pigment chlorophyll is found. Chlorophyll is constantly being produced and destroyed within leaves, and it gives them their green color. Leaves remain green when the production of new chlorophyll is equal to the destruction of old chlorophyll, thus creating a balance.
According to Chaney, as the days become shorter and cooler in the fall, the amount of sunlight available for leaves to absorb decreases. With the corresponding cooler temperatures, the production of new chlorophyll slows down, and the amount of the pigment found in leaves is reduced. As a result, their green color begins to disappear.
As chlorophyll fades, other pigments in a tree’s leaves gain prominence. These pigments—carotenoids, anthocyanins, and tannins—are the basis for the vivid color changes in leaves before a tree becomes dormant for the winter.
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International Society of Arboriculture
