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Good Question: Why Do Trees Change Color?

(WCCO) It's the time of year when the nights get longer and cooler, and many of our trees begin to show their bright orange and red colors. Why do trees change their colors?

"It's a strategy that trees have developed in this part of the world," said Dr. Adrian Hegeman, a plant biologist at the University of Minnesota.

According to Hegeman, the burst of autumn orange is about survival.

"They're using as much energy as they can from what is available," said Hegeman. The green isn't disappearing, rather, "to some extent, its being reabsorbed by the plant."

The long nights act as a trigger, telling the tree that it's time to eat and store energy for the winter.

Trees make their food through photosynthesis. Chlorophyll is the chemical that turns the leaf green.

"They're green because they absorb all the red and blue light out of the spectrum, the green is what they're not using," said Hegeman.

According to Hegeman, it's a super-efficient process harnessing 60 percent of the sun's energy as nutrition.

"A gas-powered engine gets 18-20 percent recovery," he said.

Calling it color "change" isn't exactly accurate, either. It's more like a color reveal.

"It turns out leaves have a lot of different color pigments that are there all the time," said Hegeman.

The pigments live inside the cells of plants, "this is a process called senescence, a delicate orchestration of the withdrawal of the resources from the tissue about to be shed," he said.

Hegeman provided a comparison: "If you had a mixture of paints, and you have 90 percent green paint in a container and you have a little bit of red and yellow and orange, and then you were somehow able to say, 'OK, all of the green paint is going to go away.'"

Maple trees are among the brightest when it comes to fall colors, so why have some trees changed and others are still green?

"That's a good question," said Dr. David Zlesak, a horticulture educator with the University of Minnesota's Extension Service.

"Depending on what latitude the maple was collected at will influence when it colors up. If it was from a more northern latitude they typically color up sooner," Zlesak said.

Also, different varieties of maples color up at different rates. Sometimes individual trees show variation in color change.

According to Hegeman, that's not because of a variation in exposure to the sun.

"When you start to see a tree that has one half that lost its leaves… it may be a sign of disease, maybe a fungal infection," he said.

The University of Minnesota has actually developed a new type of maple tree to satisfy the demand for trees with vibrant fall colors. It's the Firefall Maple Tree, a hybrid between two different types of maples.

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