The Science of Fall: Leaf Peeking Season Explained

Summer has given way to autumn, and with it the green of the leaves fades for a burst of color before the trees fall asleep for the winter.

It is a given that leaves change color and fall to the ground every year, but what happens inside a leaf that causes this phenomenon?

Well, as it turns out, quite a bit. Dr. Jason Kilgore, chairman of the biology department at Washington & Jefferson College, explained that changing daylight hours are the main driver of the change.

“The reason plants start to change color is due to the length of the night,” Kilgore says. “There is a pigment that plants call phytochrome. This bluish pigment will be converted into an activated form during the day. At night it slowly returns to the inactive form. If enough of it is inactive the next morning, that means it’s been a long night.”

This causes the phytochrome to slow down the production of chlorophyll, which gives the leaves their green color.

“A leaf continuously produces chlorophyll. If you slow down chlorophyll production, the plant starts to break down chlorophyll and bring these nutrients back into the tree,” Kilgore explains.

When the chlorophyll breaks down, the underlying pigments will be exposed. The yellow in the leaves of sugar maple and oak is caused by a pigment called carotenoids.

Sometimes leaves look red because of anthocyanins, a pigment they only begin to produce as they begin to age, according to Dr. Janet Paladino, director of programs in environmental science and studies at Waynesburg University. Simply put, aging is the term for the entire process of decomposition and death of a leaf.

“It’s really interesting because those pigments protect the photosynthetic apparatus,” Paladino said of anthocyanins. “Once fall arrives, we have quite a few temperature swings.”

The anthocyanins help protect the leaves as the weather changes between warm, sunny days and cold nights.

Kilgore notes that some tree species don’t always produce anthocyanin, and the stress of certain weather conditions can cause them to start.

“This year was bad,” Paladino said. “Temperature and lack of precipitation at strange times in the summer. We had a very warm June, then it was pretty average and then we had a very warm August. And drought.”

According to Kilgore, we are already seeing the effects of trees producing more anthocyanins as a result of the drought.

“What we’re already seeing is a delayed fall, brighter reds and a shorter time between changing colors and falling leaves,” Kilgore said.

The leaves of some trees may not change at all. Paladino explained that some trees will respond to the drought by producing an excessive amount of chlorophyll.

“Chlorophyll produces sugar,” Paladino said. “They want to collect as much sugar as possible during their growing season so they can transport that sugar from the leaves to the roots and store it until next spring when they start producing new leaves.”