Fall coloration of trees in our area is always looked forward to with anticipation. The presence of a large number of plants having brilliant fall foliage is more unusual than you think, as the only other places in the world with a similar abundance of foliage colorations are northern China, Korea, and Japan. A common question this time of year is: will the colors be good or not? The answer is meteorological.
The amount, duration, and brilliance of autumn color depend on weather conditions that occur before and during the time chlorophyll in the leaves is declining. If you remember your science class, chlorophyll is a biomolecule that makes leaves green and where photosynthesis takes place in plants. Without chlorophyll we would all starve.
Temperature, light and water supply are the primary factors that influence the synthesis of carbohydrates (sugars) that favor anthocyanin formation and bright fall color. Anthocyanin is a red color pigment in the leaf that is normally hidden by the green of the chlorophyll. Cool but not freezing temperatures favor anthocyanin production. Early frost is more likely to kill leaves, making them turn brown and drop sooner. Sunny days favor red coloration. Water supply also affects anthocyanin production, with mild drought favoring bright reds. Rainy days occurring near peak coloration will decrease color intensity. Late summer droughts can delay the onset of fall color by a few weeks. Temperature, sunlight and moisture are highly variable each year, assuring that no two autumns are ever alike.
The best fall foliage occurs when it’s dry during late summer to start formation of a barrier in the leaf stem (called the abcission layer) to trap sugar in the leaf. Then, to prevent leaves from falling too soon, rain is needed in early fall. An alternation of heavy rain and bright sunshine along with the gradual dropping of temperatures gives the most brilliant colors.
The prediction for Fall 2016 for our area is to have above average color due to lots of sunny summer weather and below average rainfall. I am concerned that the extended drought we’ve had will cause leaves to go to brown quicker than usual and may subdue the colors. Some rain would be good along about now to help keep leaves on the trees longer.
Information for this article came from Dr. Wayne Clatterbuck, professor of Forestry at UT. For more information on the why and how of fall coloration, check out this website: http://forestry.tennessee.edu/fallguide.htm.