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Why Leaves Change As the days grow shorter, and nights grow longer and cooler, biochemical processes begin in leaves that create Nature's autumn palette. These leaf color comes from pigments that are natural substances produced by the leaf’s cells. The factors influencing autumn leaf color are the leaf pigments present in each type of tree and the weather. The three main pigments that color leaves are chlorophyll, carotenoid and anthocyanins. Chlorophyll is the most important of the three and gives leaves their basic green color. It is necessary for photosynthesis, the chemical reaction that enables plants to use sunlight to manufacture sugars for their food. Trees in the temperate zones store these sugars for their winter dormant period. Carotenoid produce yellow, orange, and brown colors. Both chlorophyll and carotenoid are present in the leaf cells throughout the growing season. Anthocyanins are produced in the fall, responding to bright light and excess plant sugars within leaf cells and produce the reds, purples, and crimson hues. Anthocyanin production is increased by warm, sunny days and cool, crisp but not freezing nights, and these conditions bring about the most spectacular color displays. During spring and summer, chlorophyll is continually being produced and broken down and leaves appear green. As autumn progresses, chlorophyll production slows down and then stops and eventually the remaining chlorophyll is destroyed. The carotenoid and anthocyanins that are present in the leaf are then unmasked and show their colors. Weather conditions, both before and during the autumn season, will affect the amount and brilliance of the colors that develop, with temperature and moisture conditions being the main influences. Warm, sunny days encourage the manufacture of the anthocyanins that produce the red and purple hues. The yellow, gold and orange colors created by carotenoid are always present in leaves, and the amount does not change in response to weather. A severe frost will kill the leaves, turning them brown and causing them to drop early. The amount of rain in a year also affects the magnitude of autumn leaf color. Drought conditions can delay the arrival of autumn colors and a warm, wet period during the color season will diminish the intensity of the colors. The best color show occurs when there's been a warm, wet spring, a summer that's not too hot or dry, and a fall with plenty of warm sunny days and cool nights.
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