Fall color in the forest

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By Daniel Carpenter

Kentucky’s forests are home to one of the most diverse hardwood species mixes in the United States, second only to Florida. With more than 12 million acres of forests, a rainbow of color is on display in Kentucky almost every fall.

The intensity of fall color and time of peak color vary and depend upon complex environmental factors, as well as the genetic makeup of the plants themselves. What happens to leaves in the fall that produces the colors we love so much? 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 chlorophylls. These sugars are the sole source of carbohydrates needed for the tree’s growth and development.

In the food-making process, chlorophylls break down and are continually “used-up.” The tree, however, replenishes them all through the growing season. As long as replacement remains high, the leaves stay green. As fall approaches, influences inside and outside the plant cause chlorophyll 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 chlorophyll 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 and that leads 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 cool, frost-free nights. Colors may be less vibrant after an early frost.

Leaf color can also be a helpful way to identify trees. Here’s a legend for a few of your favorites:

Blackgum – bright red, orange or purple

Birch – bright yellow

Black Walnut – yellow, yellow-brown

Scarlet Oak – scarlet red

White Oak – yellow, yellow-brown, red, red-brown

Hickory – golden bronze

Poplar – gold, yellow

Red Maple – scarlet red

Sugar Maple – orange-red, yellow

Sourwood – deep crimson red

Dogwood – red, burgundy, purple

Sassafras – yellow, orange, pink, red

Winged Sumac – Red, maroon, purple

A fun site on which to follow fall foliage reports is http://www.kentuckytourism.com/seasons/reports/.

For more information on Kentucky forests, contact the LaRue County Cooperative Extension Service.

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