As leaf cells break down after the growing season is over, the leaves tend to turn some shade of brown or tan due to a reaction between leaf proteins and tannins stored in the cell vacuoles. Prior to going completely tan or brown, the leaves usually demonstrate a wide variety of colors as they go through various stages of degeneration. In the chloroplasts of mature leaves are several groups of pigments such as green chlorophylls and carotenoids including yellow carotenes and pale yellow xanthophylls.
These pigments play various roles in photosynthesis. The green chlorophylls are usually found in higher concentrations and during the season of active growth, they are able to mask the other pigments. As the chlorophylls break down during the fall, the other colors become apparent. The breakdown of chlorophyll is not completely understood, however, it appears to be tied to the gradual reduction in day length. Anthocyanin, a common red pigment, and betacyanin a second red color may also accumulate in the cell vacuoles as fall progresses. Anthocyanins are red if the cell sap is slightly acidic and blue if the sap is more alkali (basic). Betacyanins are restricted to several plant families, including cacti and beets. While some trees demonstrate brilliant fall displays of chlorophyll breakdown, others such as birch trees have a single shade of color in their fall leaves.