Why do leaves change color in the fall?

During the growing season, trees and shrubs produce chlorophyll, which uses energy from sunlight to transform CO2 and H2O into food. The presence of chlorophyll gives leaves their green pigment. As the amount of sunlight decreases in early fall there is no longer enough energy to support food production (photosynthesis). The chlorophyll (and associated green pigment) begins to dissipate allowing other pigments in the leaf tissue to become visible.

Both yellow and orange colors are caused by carotenoids which are always present in the leaf tissue but are usually masked by the presence of chlorophyll. Since these are constantly present in stable amounts, the intensity of yellow/orange colors changes little year to year.

Anthocyanins produce the red and purple fall colors. Unlike carotenoids, these pigments are produced in autumn as sugar concentrations increase within the leaf tissue. Weather often effects the concentration of sugar in the leaf tissue thus effecting the intensity of color. In general, a series of warm, sunny days with cool (but not freezing) nights produce the best colors.

Once chlorophyll, carotenoids, and anthocyanins disappear, only tannins (a bitter waste product) remains leaving leaves a dull brown. As the decreasing sunlight curtails energy production, trees enter into dormancy, much like some animals hibernate. Metabolism, energy consumption, growth, etc. all begin to slow down. This allows trees to use stored energy (starch) in an optimal manner since no additional energy will be produced for months. Basically, trees remain alive but relatively inactive. In a few months, several environmental cues will begin to alter the balance of hormones and enzymes involved in promoting and inhibiting growth and the cycle will begin anew.

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