Autumnal leaves in vibrant hues are a beautiful part of the season, but those leaves are also a vital part of keeping trees alive.
Trees that have leaves that change color in fall are deciduous. (Evergreen trees with needles, which stay green to continue the photosynthesis process through the winter, are coniferous.) Deciduous trees usually have large, broad leaves.
Most of the year, these leaves are green because of the chlorophyll they use to absorb energy from sunlight during photosynthesis. The leaves convert the energy into sugars to feed the tree.
As the season changes, temperatures drop and days get shorter. Trees get less direct sunlight, and the chlorophyll in the leaves breaks down.
The lack of chlorophyll reveals yellow and orange pigments that were already in the leaves but masked during the warmer months. Darker red leaves are the result of a chemical change: Sugars that can get trapped in the leaves produce new pigments (called anthocyanins) that weren’t part of the leaf in the growing season. Some trees, like oaks and dogwoods, are likely to produce red leaves.
How much and how fast leaves transform varies by location on the globe. The best colors are produced when the weather is dry, sunny and cool. Places that are cloudy, damp or warm won’t see the same degree of changing color.
Then, of course, the leaves fall. Trees start building a protective seal between leaves and their branches as the weather turns. They take in as many nutrients as possible from the leaves, but leaves wouldn’t survive the winter and would make trees vulnerable to damage if they remained. When the leaves are cut off from the fluid in the branches, they separate and drop to the ground.
In the fall, trees put on a pretty impressive fashion show. Leaves that were green all summer long start to turn bright red, orange, and yellow. But where do these colors come from?
Leaves are green in the spring and summer because that’s when they are making lots of chlorophyll. Chlorophyll is important because it helps plants make energy from sunlight—a process called photosynthesis.
The summer sunlight triggers the leaves to keep making more chlorophyll. But trees are very sensitive to changes in their environment.
As summer fades into fall, the days start getting shorter and there is less sunlight. This is a signal for the leaf to prepare for winter and to stop making chlorophyll. Once this happens, the green color starts to fade and the reds, oranges, and yellows become visible.