Every autumn, cottonwood, quaking aspen and willow are transformed into colorful hues of gold, orange and russet. Before long, their leaves will fall and again become part of another cycle that feeds the soil. What causes this yearly cycle, and what determines which color the leaves turn?
During spring and summer, leaves actively produce foods necessary for plant growth. This food-making process takes place in the many cells within the leaf. Within these cells are chloroplasts, which contain the chlorophyll pigments that are responsible for the green color of plants. The leaves also contain lesser amounts of other pigments, primarily xanthophylls (yellows) and carotenoids (yellows, oranges, and reds).
Most of the year, these other pigments are masked by the greater amounts of chlorophyll in the leaves. But in fall, when changes in temperature and the period of daylight occur, the leaves stop their food-producing activity. Soon the chlorophyll begins to break down, the green color disappears, and the yellows, oranges and reds slowly begin to emerge, giving the leaves their fall splendor.
The intensity of color is determined by the plant's response to complex gradients of temperature and moisture. Fall weather conditions favoring formation of brilliant autumn color are warm, sunny days followed by cool nights with temperatures below 45F (7C). Sugar production increases during the daytime, but cool nights prevent movement of sugar from the leaves.
From the sugars trapped in leaves, the pigment called anthocyanin is formed. When fall weather is consistently cloudy or rainy, and the nights warm, the leaves usually have less intense coloration. The smaller amount of sugar made during periods of less sunlight moves out of the leaves during the warm nights, reducing the conversion of excess sugars into pigments.
Before the leaves can gracefully spin from their leafstalks, a special layer of cells develops and gradually severs the tissues that support the leaf. A small leaf scar is the only evidence that leaves once adorned these deciduous plants. --Anne Halford, Botanist