Remember back to school biology and the basic lesson that chlorophyll gives plants their pigment. Chlorophyll is also important in photosynthesis as plants convert light into nutrients in order to grow. So what happens inside leaves in the fall when days get shorter, the air gets colder, and the leaves change color and explode into the incredible display we so love at this time of year? It’s all about the pigments, says UVM Extension Professor Leonard Perry.
“Chlorophyll is the green one, which makes the oxygen and sugars that the plant needs," says Perry. "It uses light to do that, mainly blue and red light." As days grow shorter and nights get longer, the leaf forms an abscission layer at the point where the leaf and stem meet the branch. As the abscission layer forms, it cuts off the flow of sugars and nutrients to the leaf, and the plant stops producing chlorophyll. What we see when leaves turn in autumn are the other pigments that are hiding in the background: the yellow pigments (xanthophyll) and orange pigments (carotenoid) are noticeable when the green pigment (chlorophyll) stops getting produced. Carotenoid and xanthophyll take different light wavelengths than chlorophyll does, says Perry. “There’s just a lot more [chlorophyll], because it’s the one doing the main business of the plant,” he said. “But when it breaks down, these others are what we see.”
"There are all these processes going on while we're looking at these pretty leaves. We take such joy at looking at leaves dying, but it's a natural process." - Leonard Perry
The red pigments (anthocyanins) are produced a little differently. “[The leaves] basically use up residual sugars,” says Perry. “There are sugars in the leaf that the chlorophyll made, it’s not going to waste those.” Brown leaves, meanwhile, are colored by a pigment called tannins. “If the layer forms fairly quickly, then those leaves drop, that’s why you have those colored leaves on the ground,” he said. “But if it kind of hangs on the leaves, then eventually those pigments will go away too, and the leaves turn brown.”
So why do leaves change earlier than usual in the season? One reason is that the tree may be under stress. “If a plant is stressed, say a branch has a weak point where it meets the main trunk of the tree, then it’s really not getting all its nutrients. That’s a signal to make those red pigments sooner,” Perry explains. “So that’s why you sometimes get a tree or branch that’s starting to turn color, that branch or tree is under stress. It may not be getting enough nutrition, or there may be some damage to the roots.”
Another factor is temperature. Longer nights mean cooler temperatures that can trigger leaf change. “That’s why we get leaves turning in fall, because the days get shorter, the nights are getting longer -- as that happens, the leaves start turning,” says Perry. “We’ve had a lot of fairly nice, sunny days and cool nights, and that’s what you want for the best color,” he says. “I think it’s really shaping up to be a pretty good fall.”
