From the red fruits of a tomato to the blue flowers of an iris, plants exhibit a remarkable array of colors. Perhaps no color display is more spectacular than the fall colors of deciduous trees. Every autumn, in the eastern United States, southern Canada, eastern Asia, southwestern Europe and other temperate regions of the world, deciduous trees explode with color.

    There are three major groups of pigments responsible for autumn leaf color: 

1. Chlorophylls
       These pigments are green, fat-soluble, and are stored in the chloroplasts, the sub-cellular factories for photosynthesis. Chlorophyll is necessary for this sugar-making process, so plants have lots of this pigment, and not surprisingly, leaves and other photosynthetic structures are green; 
   
2. Carotenes and xanthophylls
       These yellow and orange pigments, like chlorophyll, are fat-soluble and are stored in chloroplasts. One of their major functions is to help chlorophyll to do its job of absorbing light for photosynthesis. However, because carotenes and xanthophylls occur in much lower concentration than chlorophyll in the chloroplast, we can't usually observe their presence in leaves - they are hidden behind the green curtain of chlorophyll. In some cases carotenes and xanthophylls are responsible for the color of flowers and or fruits. For example, ripe tomatoes are colored by lycopene, a red xanthophyll; and 
   
3. Anthocyanins
       These pigments are water-soluble and found in the cell sap. They range from red to blue to purple and their color is affected by pH. As a generalization, flowers and other brightly-colored structures owe their beauty to these pigments.

    During the growing season, most leaves are green because of the high concentration of chlorophyll relative to the other pigments. Autumn leaf colors are the result of changes in the pigment composition in the leaf as the plant prepares for winter. As winter approaches the length of the day decreases. The shorter days initiate hormonal changes within the leaf, which in turn, stimulate the formation of a layer of cells at the base of the leaf stalk. This "abscission" layer ultimately severs the leaf connection with the stem and seals the wound resulting in a scar on the stem.

    At the same time, in response to the hormonal changes and the reduction of nutrient and water supplies to the leaf, chlorophyll production shuts down and pre-existing supplies are degraded and recycled into other materials including sugars. As the chlorophyll disappears, the more chemically-stable carotenes and xanthophylls appear from behind the green curtain of chlorophyll, giving the impression that Jack Frost has worked his autumnal magic. Red oak is an example of a plant whose autumn color result primarily from the emergence of carotenes and xanthophylls.

    In other plants, the excess sugars in the leaf are converted into anthocyanins. These sugars are produced from the decomposition of chlorophyll and other materials that are being salvaged and from the ever-declining levels of photosynthesis. The leaves of these species, such as the maples, sumacs and white oak, are brilliantly-colored and are the ones that inspire the greatest awe among "leaf-peepers".

    The conditions that favor the best fall colors are bright sunny days, cool but not freezing nights, with adequate moisture. Sunny days stimulate the leaves to produce more sugar via photosynthesis and is also important for enhancing anthocyanin production. Apple pickers know that the fruit is always reddest on the sunny side of the tree and similarly, fall colors are often brightest on the sunny south side. Cool temperature is important for the formation of the abscission layer which traps the pigments in the leaf prevents the loss of sugar for anthocyanin production. And, the conversion of starch into the sugars that are used for additional anthocyanin production is promoted by cool temperature. Adequate moisture is also required for sugar production.

    The following is a listing of plants and their fall color: reds/purples - dogwoods, sumac, burning bush; bright-yellow/fiery red - sugar maple; yellow-green/yellow - aspens and poplars; yellows - ash, birch, basswood; bright scarlet - red maple; red - white oak; brown - red oaks. Enjoy the fall colors behind the pine curtain.

For more information, check out one of the references.  To keep tabs on the progress of color in the state, visit the Minnesota Department of Natural Resources Fall Color web site.

References:

• Thimann, Kenneth (1950) "Autumn Colors" Scientific American. October.
• "Why Leaves Change Color", USDA Forest Service, St. Paul, MN web site
• Palevitz, Barry. 2001.  Why leaves turn colors in the fall. The Scientist 15 (24): 1.  available on-line.