Why do leaves change colors in the fall
Leaves are an important part of any tree. During the spring and summer, the chlorophyll inside them uses sunlight to turn water and carbon dioxide into oxygen and sugar in the form of glucose in a process called photosynthesis. Like an animal preparing for hibernation, plants create extra glucose to help them through the winter. As the shorter, colder days of fall begin, plants must consider what to do with their leaves. Some plants, called evergreens, have leaves built specifically to survive the winter, either with minimal surface area or waxy coatings. Other trees prepare to lose their leaves, but as they do they create a wonderful palette of color.
Greens, yellows, and orange
The green found naturally in leaves is created by chlorophyll. Another chemical in leaves, called auxin, maintains a part of the leaf called the abscission layer. As fall begins, the cooler days prevent the creation of more auxin. The abscission layer grows and prevents the tree and leaf from sharing any more water or nutrients. Without water, chlorophyll begins to fade, allowing other colors to show through. Oranges and yellows are often already present in the leaves, in the form of carotene and xanthophyll.
Reds and purples
Some trees, like maples, use glucose trapped in the leaves to create a chemical called anthocyanin. Some scientists have suggested anthocyanin may help a tree keep its leaves longer, as it lowers the freezing point of the leaves. This may allow the tree to collect as much nutrition from the leaf as possible.
Browns
Brown colors in leaves are caused by a waster product named tannin.
Where to find the best colors
Weather plays a large role in color production. In order to produce the best colors, the trees need to experience a warm and wet spring, followed by a temperate summer and an autumn with bright sunny days and cool nights. The national forest service provides a hotline which offers information about the best color displays. The scenic byways in thirty-one states have also been designed to best display the fall colors.
When looking for a particular color, keep in mind that colors are often characteristic of a species. Oaks, high in tannins and anthocyanin, are often reds and browns. Hickory leaves turn a golden bronze, while aspens are golden yellow. Dogwoods can nearly be magenta, with their combination of purple and red. Beech trees typically have tan leaves. Black tupelo leaves are crimson red in the fall. Maples vary by species. Red maples, with their extra glucose, turn scarlet. Sugar maples allow a little more of the carotene to shine through and are a fiery orange-red. Black maples are a brilliant yellow and striped maples nearly lose all their color.
Not a leaf wasted
The abscission layer of the leaf eventually loosens the leaf enough that it will fall easily. These leaves serve as food for all sorts of decomposers, which in turn help to restock the nutrients in the soil and help hold water. Scientists have also proposed that chemicals like anthocyanin in leaves may help avert the growth of other species.
For those that love the fall colors, and would like some science experiments to accompany a leaf exploration, Science Made Simple has some great experiments.
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