SEPTEMBER/OCTOBER

WHAT MAKES FALL COLOR?

The intensity of fall color and time of peak color vary and are determined by complex environmental factors, as well as the genetic makeup of the plants themselves. These factors vary by plant and by geographic area.

What happens to leaves in the autumn? In summer, leaves are green because of a group of pigments known as chlorophylls. Chlorophylls are vital to the tree's food-making process, called photosynthesis. Leaves manufacture simple sugars from water and carbon dioxide, using energy captured from the sun by the chlorophylls. These sugars are the sole source of the carbohydrates needed for the tree's growth and development.

In the food-making process, chlorophylls break down and are continually "used-up". However, the tree replenishes them all through the growing season. As long as replacement remains high, the leaves stay green. As fall approaches, influences inside and outside of the plant cause the chlorophylls to be replaced at a slower rate. Shorter days cause a layer of cork cells to form at the base of each leaf, gradually closing off the flow of water and minerals into the leaf. This is the location where the leaf will eventually separate from the tree and fall to the ground. As the supply of chlorophylls dwindles, other pigments that may have been present in the leaf all along are slowly unmasked and begin to show through.

Unmasked pigments include the carotenoids , which result in brilliant yellows and oranges. Some trees where carotenoids are conspicuous include ash, maple, aspen, birch, black cherry, cottonwood, tulip tree, and sycamore.

Another group of pigments, called anthocyanins, are responsible for the reds, purples, and blended combinations of these colors. Unlike the carotenoids, these pigments have not been present in the leaf all season. Instead, they develop in late summer in the sap of the leaf cells. Their formation depends on the breakdown of sugars in the presence of bright light while the level of phosphate in the leaf is reduced.

Phosphate is at a high level during the growing season, but in autumn it moves out of the leaf and into the stem of the plant. When this happens, the sugar breakdown process changes, leading to the production of anthocyanin pigments. The brighter the light, the more anthocyanins are produced and the more brilliant the color. The brightest colors develop when autumn days are sunny and cool, along with nights that are chilly but not freezing. All of this information frees "Jack Frost" from having anything to do with fall coloration!