Leaves near the Knox College softball field are red while others remain green Friday afternoon.
Warm weather foils foliage shift
Fall colors not yet showing; Scenic Drive less than scenic?
Saturday, September 22, 2007Summer is not kindly yielding the stage to fall.
And the hot season's reluctance to leave will have an impact on the area's foliage.
While vendors and gawkers prepare for traditional fall events like the Knox County Scenic Drive, unseasonably warm temperatures could translate into a delay before October offers its full palette of fall colors.
"I know there isn't a lot of change going on right now," Miava Reem said. "I would be surprised to see a lot leaves change color until you start to see cooler temperatures - at least some cool temperatures at night."
Reem serves as academic support staff for Knox College's biology department and has overseen the college's greenhouse for 14 years.
Galesburg's average temperature this September is 69.2 degrees - above the normal average of 64.7 degrees. Those higher temperatures is one of the factors that retards many leaves' change from green to yellow, orange and red.
But temperature isn't the only factor that determines when leaves change color.
"The thing is, you just never know," Reem said. "The two main factors that determine when leaves change are the length of the day and temperature.
"In general - first and foremost - the changing color is based on the length of the day. When it starts to get dark earlier, photosynthesis stops."
Monmouth College's Tim Tibbets, a professor of biology, said a leaf's fall colors are always present. But yellow, orange and red hues are covered by green during most of the leaf's life cycle.
"I tell my students that a leaf's color is a lot like being at a party," Tibbets said. "You have really loud music playing and you also have conversation. If the music suddenly stops, you find you are talking loudly to a person right next to you.
"The loud music is the green of the leaf. When the green stops, the other colors come through."
The green color of a leaf or other plants is caused by chlorophyll. When chlorophyll isn't exposed to light, it breaks down and gives way carotenoids and anthocyanins.
Carotenoids produce yellow, orange and brown colors, while anthocyanins produce shades of red.
Tibbets said red colors are generally enhanced by temperature fluctuations like warm days and cool nights.
"If you don't see a lot of leaves turning red, it could be because of the red and orange stage was brief," Tibbets added. "But it could also depend on the species of tree. Many species don't ever turn red or orange."
Knox College professor Stuart Allison said he thought the change "might be a little late."
Allison, who specializes in plant ecology and is the director of the Green Oaks Field Research Center, outlined the conditions many feel are necessary for a colorful fall.
"The ideal is to have moderate rainfall and cool - but not freezing - nights," Allison said. "Those conditions are thought to lead to the most spectacular leaf shows."
Allison, Tibbets and Reem didn't want to predict whether the Knox County Scenic Drive, held the first two weekends of October, would have brilliant fall colors as a backdrop.
"The last few months have been all over the place," Allison said. "It rained a lot in August and not much in September. It's been warm, but we did have a cool period a little more than a week ago.
"The one thing I have noticed is that the cottonwoods shed their leaves without much color change. We'll see how the rest go."
Reem said conditions could change quickly.
"Right now, I would be surprised to see leaves at their best coloration by the first or maybe even second week of the Scenic Drive," Reem said. "But you never know. It is getting dark earlier. If we get a lot of cool nights in the next couple of weeks, it could speed things along."
- Chlorophyll gives leaves their basic green color and is necessary for photosynthesis, the chemical reaction that enables plants to use sunlight to manufacture sugars for their food. Trees in the temperate zones store these sugars for their winter dormant period.
- Carotenoids, which produce yellow, orange, and brown colors in such things as corn, carrots and daffodils, as well as rutabagas, buttercups and bananas.
- Anthocyanins, which give color to such familiar things as cranberries, red apples, concord grapes, blueberries, cherries, strawberries and plums. They are water soluble and appear in the watery liquid of leaf cells.
Both chlorophyll and carotenoids are present in the chloroplasts of leaf cells throughout the growing season. Most anthocyanins are produced in the autumn, in response to bright light and excess plant sugars within leaf cells.
During the growing season, chlorophyll is continually produced and broken down and leaves appear green. As night length increases in the autumn, chlorophyll production slows down and then stops and eventually all the chlorophyll is destroyed. The carotenoids and anthocyanins present in the leaf are unmasked and show their colors.
Source: USDA Forest Service
September 2007 temperatures
- Average - 69.2
- Average high - 81.4
Normal September temps (1971-2000)
- Average - 64.7
- Average high - 74.7
August 2007 temps
- Average - 77.3
- Average high - 86.3
Normal August temps (1971-2000)
- Average - 72.6
- Average high - 81.9
Source: National Weather Service, Central Illinois Weather Forecast office
- September 2007 - 0.68 inches
- Normal - 2.55 inches
- August 2007 - 6.2 inches
- Normal - 4.07 inches