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Photosynthesis
Chlorophyll is the green pigment in leaves that converts sunlight, carbon dioxide and water to sugar and oxygen through photosynthesis. Trees continually lose and replenish chlorophyll throughout the growing season. However, the rate of chlorophyll loss increases in fall. When the days get shorter, trees slow down the production of chlorophyll; eventually, photosynthesis stops altogether, because trees no longer need to produce sugars to fuel their growth. As chlorophyll declines, the leaves begin to lose their green color. In addition, leaves lose nitrogen and phosphorus in fall; these nutrients move from the leaves to the branches for storage over the winter, which contributes to the slowdown in photosynthesis.
Yellow Leaves
As chlorophyll declines, the other pigments in leaves become visible. Carotenoids give a yellow or orange color to daffodils, carrots, corn, and bananas, as well as the leaves of such trees as aspens, birches, hickories and sycamores. Although carotenoids are always present in leaves, they do not normally appear yellow during the growing season, because the presence of chlorophyll has a stronger effect on leaf color. The loss of chlorophyll in the fall gradually reveals the leaves' carotenoid content.
Brown and Gold Leaves
Tannins, which are byproducts of a tree's metabolism, gradually accumulate in autumn leaves. As yellow leaves age, tannins cause them to turn brown. When combined with carotenoids, tannins can produce a bright golden color.
Red, Orange and Purple Leaves
In fall, some trees produce anthocyanins, which cause leaves to turn red on sumac, dogwood and red maple trees. The colors that anthocyanins produce typically depend on the pH of the leaf cell. Acidic conditions produce a red hue; as the pH rises (i.e., becomes less acidic), purples and blues tend to appear. Leaves that contain both anthocyanins and carotenoids turn orange. Leaves that retain some chlorophyll while anthocyanins build up turn brown.
Weather Conditions
Weather conditions also affect the color of autumn leaves. Brilliant fall colors typically result when daytime weather is warm and dry, because this causes trees to produce a lot of sugars during their last days of photosynthesis. Cool nighttime temperatures that remain above freezing help the leaves to retain their sugars; this promotes the production of anthocyanins, resulting in vivid shades of red and purple. Warm, rainy weather during spring also yields intense fall foliage color.