Why do Leaves Turn Colors in the Fall?

The turning leaves of deciduous trees are a welcome sign of autumn and its longer, cooler nights. Vibrant reds, yellows, and oranges make up the color scheme of fall, and they are all part of the cyclical nature of the seasons. Leaves on trees, shrubs, and plants turn colors when they begin the process of going dormant for the winter.

Leaves are the parts of plants and trees that make the food necessary for sustaining life. Trees use sunlight, water, and carbon dioxide taken from the air to produce energy. The carbon dioxide and water are converted to glucose, which is a type of sugar. The glucose is then used as energy for the tree to live and grow. This entire process is called photosynthesis. Leaves are green because they contain the chemical chlorophyll, which facilitates photosynthesis.

Once the days and nights begin to cool and the nights get longer, the trees sense that fall and winter are approaching. In the trees' preparation to go dormant, circulation to the leaves, which are no longer needed to produce food, is cut off. Due to the longer nights and reduced sunlight, the production of chlorophyll is reduced, meaning that the green will eventually disappear from the leaves. Some of the pigments responsible for causing the vibrant colors in the leaves, such as carotenoids, are already present in the leaves, but are obscured by the green chlorophyll. Carotenoids are responsible for yellows, oranges, and browns.

The pigments that cause bright reds, russets, and purples are called anthocyanins. These are produced in the leaves of some species in the fall due to the excessive amounts of glucose in the leaves, along with the bright light of fall. This is why certain species of trees produce bright red leaves as opposed to others that turn out yellows or golds.

Maples are known for their range of autumn palettes. Different varieties of maple turn bright red, purple, yellow, or nearly colorless in the fall. Oaks tend to produce browns and reds due to the waste in their leaves. Hickory trees have golden brown leaves, and aspens and poplars turn yellow. Dogwoods produce reddish-purple leaves, while sourwoods and black tupelos produce bright crimson red leaves. Elms can be a little disappointing, as their leaves usually turn brown and fall off without a big show.

Different species turn at different times. Trees in the eastern region of the United States tend to turn earlier, in September, as do those in the central Rocky Mountains. By the end of November, trees in the Smoky Mountains, Adirondacks, and Appalachians will all have the leaves of fall. Out west, trees in the mountainous regions tend to turn the latest.

Several factors have an impact on fall colors, making some years better or worse for viewing than others. The best fall colors are due to a warmer, rainy spring, good weather during the summer, and warmer, sunnier fall days and cool nights. A more disappointing fall show may be due to a later spring, a drought during the summer, and a warmer spell during the fall.