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.
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.
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