Temperate climates provide a clear and beautiful sign of the approaching winter when autumn leaves begin to change color. This brilliant show inspires many travelers to head north for breathtaking fall color tours. But what actually causes leaves to change from summer's green to the signature colors that precede the fall leaf drop? To understand this transformation, it is helpful to first know what the job of a leaf actually is. Why do trees have leaves in the first place?
Leaves Are Plant Food Factories
Plants, and other photosynthesizing organisms, have the singular talent of being able to turn sunlight into food. Can you imagine if you could just stand in the sunlight and produce your own satisfying meal whenever you got hungry? It's a pretty neat trick that only photoautotrophs can do (photo=sun; auto=self; troph=feeder).
In order to make food energy, in the form of glucose (a type of sugar), plants need water, carbon dioxide (CO2) and energy from the sun. The plant then uses the glucose as food energy to live and grow. To capture sunlight energy, plant leaves have a green pigment called chlorophyll. This pigment makes plants' leaves appear green.
The Food Factory Shuts Down in Winter
As winter approaches, the days get shorter and cooler. These changes in day length and temperature trigger trees to essentially hibernate for the winter. It is very energetically expensive for a tree to run its leafy food factories in the winter, when, due to freezing temperatures, water transport (from the ground into the tree's trunk and leaves) becomes a problem. It is more energy efficient for the tree to shut down operations in the winter and go dormant.
The Pigments in Leaves
When a tree begins its preparations for dormancy, the chlorophyll pigment begins to break down. But chlorophyll is not the only pigment that a plant has at its disposal. There are also other colored pigments, such as carotenoid and anthocyanin, but their appearance is typically masked by the green chlorophyll. Carotinids are pigments that create the bright yellows and oranges that we see in some fruits and vegetables. Anthocyanins impart a red color to plants, such as that seen in cranberries, red peppers, cherries, and strawberries.
In autumn, in preparation for leaf drop, the tree stops making new chlorophyll, and the existing chlorophyll breaks down. With the masking green color gone, the bright carotenoid and anthocyanin pigments are able to show through. So the fantastic array of leaf colors that we see in fall are always there, but remain hidden until the changing season allows them to shine through.
Why Isn't Fall Color the Same Every Year?
Varying annual weather conditions during spring and summer affect the autumn colors in the fall, so fall leaf colors are a little bit different every year. Temperature and moisture both impact the intensity of the show. The most brilliant colors of fall are produced from a warm wet spring and summer followed by a series of alternating warm days and cool nights.
"Why Leaves Change Color", USDA Forest Service
"The Science of Fall Colors", USDA Forest Service
"Why Leaves Change Color", SUNY College of Environmental Science and Forestry