The leaves aren't dying because they've been killed by frost. The changing of colors is an active process of the trees preparing for winter.
Robert Joly, professor of horticulture at Purdue University, says the first step to understanding the process is to know that tree leaves have three primary pigments:
By the time trees sport the bright colors of October, they are completing their preparation for winter, a process that began during the hot, hazy dog days of mid-August.
"Trees are tuned in to a number of things, such as changes in the day length, light quality and temperature," Joly says. "The trees respond to their changing environment and transform these environmental changes into biological changes."
In late summer, trees respond to the changing environment and stop the production of chlorophyll. The chlorophyll already present in the leaves begins breaking down into simpler sugars and starches, which are stored for the winter in the tree's twigs.
The tree will use the stored nutrients to make more chlorophyll -- and green leaves -- come spring.
As the chlorophyll is broken down, the other colors -- the familiar yellows, oranges, reds, and purples of autumn -- are unmasked.
For the best color show, clear days and cool nights are needed. The clear days allow photosynthesis to continue and allow the maximum production of anthocyanin. The cooler temperatures of autumn decrease the loss of nutrients through respiration and allow the pigments to accumulate. A mild or moderate drought also will increase the brilliance of the reds of autumn by stimulating anthocyanin production.
Writer: Steve Tally, (765) 494-9809; home, (765) 463-4355; Internet, email@example.com
NOTE TO JOURNALISTS: A color photograph and a computer graphic showing the scientific processes that cause fall colors are available. Contact Steve Tally at (765) 494-2096. To receive this story by e-mail, send this message: "send punews 9508f14" to firstname.lastname@example.org. Purdue News Service also maintains a searchable data base of experts and posts news releases on a web server at https://www.purdue.edu/uns and a gopher server at newsgopher.uns.purdue.edu. The web site also offers selected downloadable photographs.