A rainbow of autumn colors: Explaining the hues of fall
LISLE, Il – As the days begin to shorten and the temperature cools, trees begin their transition into the beautiful spectrum of red, yellow, and brown shades Illinois residents have come to know as the first signs of autumn. The question remains, however, why do leaves change color?
According to the Illinois Department of Natural Resources (IL DNR), the fall season beings to take place in northern Illinois counties around the third or fourth week of September with leaves beginning their color change in central Illinois by the second week of October, and by the last two weeks of that month, southern Illinois will have made the color change transition as well.
Julie Janoski, the plant clinic manager at Morton Arboretum, cites the slow down in the production of the plant chemical chlorophyll as one reason leaves being to lose their bright green hue.
“Fall colors on the leaves means that the trees are preparing for dormancy. Chlorophyll is the key component in a plant’s ability to turn sunlight into glucose, which feeds the trees. When it starts to get colder outside during the fall season, Chlorophyll production begins to stop and what you get left is that yellow to brown color in most trees,” Janoski said. “The yellow color is caused by a chemical called Flavanol which is always present in leaves, but because of Chlorophyll being abundant in the spring and summer seasons, that’s why you only see that yellow color in the fall.”
However, other types of trees contain additional compounds known as Carotenoids and Anthocyanins that are responsible for the orange and red shades of color. While Carotenoids, like Flavanols, are always present in leaves but not seen because of Chlorophyll, Anthocyanins (the chemical responsible for red and purple leaves), is produced by some species of trees to better survive the fall climate.
IL DNR states, “These pigments develop from the sugars that are trapped in leaves. On warm, sunny fall days, the leaves produce sugars. At night, as the air cools, plant transport slows, and the sugars are unable to be transported from the leaves. The brighter the fall sunlight, the more sugars are produced, and the more brilliant the colors.”
Tom Simpson, a research field station ecologist from the McHenry County Conservation District, says in addition to temperature changes, weather conditions can also affect the colors on leaves.
In an email correspondence, Simpson explained, “I don’t have a crystal ball concerning what this fall will be like, but in general, an early cold snap will bring on fall color earlier than normal, whereas a cool, wet fall can delay color change. I have been told that a more abrupt transition from summer to fall weather increases the intensity of the leaf color change.”
Janoski warns that if temperatures get to be too cold however, trees can lose their leaves without them ever changing color.
“The main factors in getting those rich autumn colors all comes down to sunlight, temperature, and weather. Residents can help maintain their trees healthy by making sure they give them plenty of water especially during those hot summer days. Examples of trees that change into those deep red, bronze and orange leaves are red oaks, sugar maples, and sumacs. Trees that will produce yellow leaves include cherry trees, birch, hickory, cottonwood, and again, the sugar maple tree,” Janoski said.
With Illinois hosting around 250 species of trees, fall is sure to bring about a diversity of colors making autumn in Illinois a beautiful time of year.
For more information about trees or questions regarding plant life, email the Morton Arboretum’s Plant Clinic at [email protected] or call (630) 719-2424 open 11 am to 3 pm Monday through Friday.
