The science of autumn: Leaf-peeping season explained
Summer has given way to fall, and with that the green of the leaves is fading away for a burst of color before the trees lie dormant for the winter.
It is a given that each year the leaves will change color and fall to the ground, but what is happening inside a leaf that causes this phenomenon?
Well, as it turns out, quite a bit. Dr. Jason Kilgore, chair of the biology department at Washington & Jefferson College, explained the shifting daylight hours are the main cause for the change.
“The reason that plants start to change colors is due to the length of the nighttime,” Kilgore said. “There is a pigment that plants have called phytochrome. This blue-ish pigment will convert during the daytime to an activated form. At nighttime, it slowly goes back to the inactive form. If enough of it is inactive the next morning, that means it was a long night.”
This causes the phytochrome to slow the production of chlorophyll, which is what gives leaves their green color.
“A leaf is constantly producing chlorophyll. If you slow the production of chlorophyll, the plant actually starts to break down chlorophyll and move those nutrients back into the tree,” Kilgore explained.
When the chlorophyll breaks down, it will start to expose the pigments lying underneath. The yellow in the leaves of sugar maple and oak trees is caused by a pigment called carotenoids.
Sometimes leaves will appear red due to anthocyanins, a pigment they do not start to produce until they begin senescence, according to Dr. Janet Paladino, the director of programs in environmental science and studies at Waynesburg University. Simply put, senescence is the term for a leaf’s entire process of breaking down and dying.
“It’s really interesting because those pigments – they protect the photosynthetic apparatus,” Paladino said of anthocyanins. “Once the fall comes, we have quite a variation in temperature.”
The anthocyanins help to protect the leaf while the weather shifts between warm, sunny days and chilly nights.
Kilgore notes that some tree species do not always produce anthocyanin, and the stress of certain weather conditions can cause them to start.
“This year was bad,” Paladino said. “Temperature and a lack of precipitation at weird times in the summers. We had a really hot June, then it was kind of average and then we had a really hot August. And drought.”
According to Kilgore, we are already observing the effects of trees producing more anthocyanins as a result of drought.
“What we’re already seeing is a delayed fall, brighter reds and a shorter time between changing colors and leaf drop,” Kilgore said.
The leaves of some trees may not change at all. Paladino explained that some trees will react to the drought by producing an excess amount of chlorophyll.
“Chlorophyll produces sugar,” Paladino said. “They want to gather as much sugar as they can during their growing season so they can transport that sugar from the leaves to the roots, and keep it and store it until next spring when they’re going to produce new leaves.”