The dry end of our summer may have delayed the turning of the trees but the shortening of the days will bring us a color show.

The chlorophyll in the leaves responsible for the green color has been destroyed, leaving the anthocyanin (red), xanthophyll (yellow) and carotenoids (orange) to be seen. The colors intensify with warm sunny days and cool, but not too cold, nights. Anthocyanin is only produced during the fall and triggered by cool temperatures; it is thought to act as sunscreen or antifreeze for the trees. When freezing temperatures rise, these colors also will cease production, leaving only the tannins (brown) behind.

These lengthened nights cause the cells to form an abscission layer at the base of the leaf to block the transport of carbohydrates to the branches and minerals from the roots to the leaves. They will become weakly attached and fall to the ground like confetti from a party.

Could our continued trend toward drought lessen the fireworks of the fall season? Scientists are discovering that severe drought during the growing season causes trees to begin to turn early and not last as long, or lead them to drop leaves before coloring. More moderate drought (like ours) can delay the onset of fall color.

Are there other reasons trees put on this color show? In the late 1990s, William D. Hamilton theorized that trees’ vibrant hues were indeed a signal, meant to ward off parasites that lay their eggs in autumn. He explained the behavior must be an outcome of co-evolution, in which two or more species reciprocally drive one another’s adaptations. However, other scientists and ecologists have not been able to prove Hamilton’s theory.

Are brilliant fall colors a sign of healthy trees in the expanse of the North American forests? The presence of brilliant red of the black tupelo, orange and purple of sassafras and the luminescent yellow of sugar maple may actually be affected by prolonged droughts or aid in deterring pests.

Trees with breathtaking fall color are black tupelo, scarlet oak, hophorn beam, sourwood, birch, ginkgo, katsura tree, poplar, service berry and sassafras. Shrubs to plant for fall color are oakleaf hydrangea, smokebush, witch hazel, fothergilla and highbush blueberry.

Kelly Allsup is the University of Illinois Extension horticulture educator in Livingston, McLean and Woodford counties.

0
0
0
0
0

Load comments