The science of fall colors           

By John Fulton

[SEPT. 25, 2006]  It's fall leaf time again, and those interested in the phenomenon of fall leaf color (and science students who get an extra credit question) should be enthralled with this column. While peak color is still a few weeks away, many trees have begun the annual change from green leaves to various showy colors. Frost is often credited with causing the great fall colors, but it actually kills leaves, producing dull earth-tone colors. Bright fall colors are caused by chemical reactions in leaves, and these reactions are triggered by shortening day length and cool temperatures.

To understand the process that creates color, we need to know a little about basic tree growth. A tree has two parts in its vascular system, the xylem and the phloem. A tree's xylem cells can be thought of as thousands of minute soda straws packed end to end, going from the roots to the leaves. Water and nutrients are taken up by the roots and transported to the leaves through the xylem cells in the tree's sapwood. In the leaves, water and nutrients are converted into sugar, the energy that feeds the tree's growth. This conversion process, known as photosynthesis, happens in the presence of chlorophyll and sunlight.

The phloem is a thin layer of cells found in the inner bark of the tree. This is where the sugars move from the leaves to the roots and other storage sites within the tree. The location of the phloem shows how a tree can be severely injured or killed if its bark is damaged. If the phloem is disrupted, food can't flow through the phloem, and the roots starve to death.

Fall coloration starts with the onset of senescence, a natural process that disrupts the tree's vascular system. This is the orderly process in which the light-gathering and carbon-capturing substances in the leaves, including the pigments that capture sunlight and the proteins that use the captured energy, are disrupted and broken down. The change is started by the tree's genetic ability to "sense" day length and temperature variations. The long and warm days of summer produce high levels of the auxins and gibberellins that stimulate tree growth and low levels of growth inhibitors. Fall's shorter days, with less light and different light intensity, along with the cooler and longer nights, affect the production of growth regulators that trigger senescence. These stimulate a variety of changes, including the formation of corklike cells at the base of the leaf petiole, which produces a brittle zone around the vascular tissue so that it is easy for the leaf to break off from the branch. Eventually only the dead xylem cells are left holding the leaf on the tree. Heavy winds or rains can easily break this fragile connection, causing leaves to fall to the ground.

The shorter days and cooler temperatures get the tree ready for dormancy. Chlorophyll production drops dramatically from the high levels of the growing season to virtually nothing. The tree's priorities then switch to the production of sugars that will be stored for next season's growth. This reduction in chlorophyll production starts the visible fall colors. Chlorophyll is the predominant pigment, and it makes the leaves green during the growing season. Chlorophyll is also very fragile and must be replaced by plants on a continual basis until the days grow short and temperatures fall. The fading of the green color, due to much lower chlorophyll production, causes the other pigments once masked by the green chlorophyll to come through. These other pigments include yellow, orange, and buff colors of the carotenoid, xanthophyll and tannin pigments.

Carotenoids are always present in the leaves, so fall's yellow to orange colors are usually fairly consistent from year to year. Xanthophyll is a yellow to tan-colored pigment, and tannins are responsible for the brown earth tones found in oak leaves. A fourth pigment, called anthocyanin, does not naturally occur in the leaves but is a product of senescence and concentrated sugar sap in the leaf cells. Anthocyanins appear red and generate the varying shades of blue, purple and red that provide some of the most vibrant color displays. The actual color depends on the pH of the cell sap, with acidic saps causing red to orange, and neutral to alkaline saps will appear purple to blue. Not all trees produce anthocyanins. Sugar and red maples, dogwoods, sumac, black gum, sweet gum, scarlet oak, sassafras, persimmon, hawthorn, and white oak produce the most brilliant shades of red, maroon, purple and blue.

Hopefully this somewhat scientific explanation of fall colors will cause you to understand a little better what goes on within trees to bring about an abundance of fall color.

[John Fulton, unit leader, University of Illinois Extension, Logan County Unit]