Why do leaves change color in the fall?
"Evergreen" trees or conifers do not lose their leaves in the fall because the leaves are protected from freezing by a thick waxy layer and by other adaptations that prevent freeze damage to their leaves and stems. Deciduous or broad-leafed trees like oaks, maples, and aspens lose their leaves in winter because their leaves are tender and unable to withstand freezing. Leaf loss (abscission) is an active process in deciduous plants that involves complex physiological changes in the plant. A thin layer of cells where the leaf joins the branch is sensitive to a plant hormone called auxin that is produced by the leaf. As long as the leaf produces auxin, the cells in this layer stay attached. In the fall, or when a plant is stressed, auxin production drops or stops, the cells separate, and a layer forms that seals the break so that the plant does not lose sap.
Leaves are the food-producing parts of a plant. The leaves take in carbon dioxide and water and use sunlight to produce sugars for energy and for growth. This is the process of photosynthesis. Pigments, large carbon and nitrogen-containing molecules, absorb the light that provides the energy for photosynthesis.
The most abundant pigment in the leaves is chlorophyll, which absorbs red and blue wavelengths of light and therefore makes the leaves look green. Leaves also contain small amounts of other pigments such as xanthophyll and carotene that absorb different wave-lenghts of light and appear orange or yellow, but these colors are masked by the large amounts of chlorophyll present. As daylight decreases and temperatures drop, the tree slows down the process of producing chlorophyll. Because chlorophyll is a relatively unstable molecule, as production slows and eventually stops, the chlorophyll present in the leaf is gradually broken down and reabsorbed by the plant, disappearing from the leaves and unmasking the other pigments. In addition, large amount of sugars in the leaves, lower temperatures, and bright sunlight stimulate the production of another pigment, anthocyanin, which appears red or crimson. The hues and brilliance of fall foliage are dependent upon the species of tree and the weather. Day/night temperatures, the amount of sunshine, and even summer rainfall will affect how the pigments are expressed in the leaves. Shades of red result from the relative amounts of anthocyanin, carotenoids, or even a little remaining chlorophyll. Anthocyanin and chlorophyll produce brownish colors. Anthocyanins and carotenoids produce orange hues. In some plants the color production is quite uniform. In other plants, leaves vary between individuals, within an individual, or even within a single leaf.
Fall colors of a Siberian Elm
