Why leaves change color

Fall is my favorite time of year. We have two blaze maple trees that turn a reddish-orange, and are simply stunning.

Karen Bennett is an extension forestry specialist at the University of New Hampshire. She says leaves are green due to chlorophyll production, which makes food for the tree. But as days become shorter and the nights longer, the tree is signaled it needs to prepare for winter. The leaves stop making chlorophyll and as it fades away, underlying yellow and orange pigments begin to take its place.

"These pigments are in a broad class called carotenoids. They're the same pigments that make carrots orange, bananas yellow," says Bennett. "There's one other really large group of pigments, and we call those anthocyanins. Anthocyanins are pigments that make blueberries blue, they make cranberries red. They're the reddish purplish kinds of pigments."

Bennett says the anthocyanins are different from the carotenoids in that they are usually not already in the leaf. They are only produced in autumn when sugars feed trees in the warmth of the day, and are trapped in the leaves during cool nights.

This means the key to a brilliant color show is autumn weather conditions.

"If we have warm, sunny days followed by cold, but not freezing nights, we tend to have our most colorful autumn. And that's because that's when the sugars get trapped in the leaves and they get turned into anthocyanins," she explains.

The quality of color can also be negatively affected by weather. Cloudy, windy, and rainy spells decrease the intensity of color by limiting the sugars available for anthocyanin production.