The science of leaves
Herald-Standard
Locally, changing leaves should hit their peak color in mid-October.
Photo courtesy of GO Laurel Highlands
Cucumber Falls in Ohiopyle
With summer officially behind us, it’s time to wait for the one true symbol of fall: changing leaves.
But did you know that the colors that will come out in a few short weeks are there are all year long?
Dr. Mark Tebbitt, a professor in the Department of Biology, Geology and Environmental Sciences at California University of Pennsylvania, said green leaves contain various pigments that capture energy from sunlight in the process of photosynthesis.
“One group of pigments are the chlorophylls, which absorb much of the sun’s energy, but not that in the green part of the light spectrum,” Tebbitt said. “Since this unused green portion is reflected from the leaf, leaves with lots of chlorophyll in them appear green to us.”
Tebbitt said other photosynthetic pigments include yellowish-orange carotenes and pale yellow xanthophylls. But, during the spring and summer months, the abundance of chlorophylls in the leaf obscures those other colors with a dominant green, leaving behind the remaining colors of fall leaves.
In fall, Tebbitt said, the decreasing length of the day and cooling temperatures act as cues for the trees that winter is coming. That, in turn, causes them to store the green pigment before the trees shed their leaves.
“From an energy standpoint, it’s better for the leaves to be lost and replaced with new ones each year,” he explained.
While some of the other pigments are also taken from the leaves to be recycled, Tebbitt said an interesting question to ask is why those pigments aren’t completely broken down and recycled like the chlorophylls.
“It has been suggested that leaves retain some of the colorful pigments right to the end of their lives because these colors signal to insects that the leaves are protected by toxins,” Tebbitt said. “So perhaps the bright colors act as a defense signal, protecting the leaf as it recycles much of its valuable compounds. We don’t know this for certain but it’s an intriguing idea and might explain the bright colors we see each fall.”
Lee Hendricks, a meteorologist with the National Weather Service in Pittsburgh, said it won’t be long until the leaves in the Laurel Highlands start changing.
“Some trees are already starting to get the colors we’re used to,” Hendricks said. “For the best time around here, we’re looking at pretty much mid-October when things will start to peak.”