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The science of fall color change

Forever, I've chalked those colors up to: The trees are dying, winter's coming. If you think that's all of it, you better buckle up.

GOLDEN VALLEY, Minn. – It’s amazing how stupid I feel when I learn, again, how smart nature is.

The mind-exploder-de-jour explores the science behind the stunning fall foliage we get here in the Upper Midwest.

Forever, I’ve chalked those colors up to: The trees are dying, winter's coming. If you think that's all of it, you better buckle up.

Truth is they're not dying, they’re shedding, akin to snake skin.

And those stunning reds and yellows and browns all have different purposes in the land of leaves.

You see, while some of us get the fall blues, trees get fall clues that winter is coming.

This alert tells the tree that the amounts of sunlight, water retention and nutrient availability will be changing.

So, like a tree’s best buddy, the squirrel, you got to start stocking up for winter.

“What they are going through is a process called senescense,” said Eric Singsaas, director of Wood Products and Bioeconomy at the University of Minnesota Natural Resources Research Institute in Duluth. “The leaves drop and die because in the winter it's too cold anyway for photosynthesis. So, they know ahead of time they are supposed to shed their leaves before winter.”

But here's the thing, many deciduous trees store about 80 percent of its nitrogen is in the leaves. Trees roots work darn hard for that nitrogen; they don’t want to waste all that work by just dropping that precious nitrogen in dead leaves. So, trees begin the process of sucking that nitrogen from the leaves back into the trunk for the winter.

And it's this process—slowing down photosynthesis and reabsorbing nitrogen—that triggers the color changes.

“When there is too much sunlight, more sunlight than the leaf can use, that’s what can be damaging to the leaf,” said Singsaas.

There's two basic kind of pigments that go into making the characteristic fall colors: The reds and purples are called anthocyanins, and the oranges and yellows are called carotenoids. They are both responding to the same thing, but they are responding in different ways, according to Singsaas.

The red pigments—found in various maples and oaks—act as a sunscreen for the leaves. They just can't take that much sun when they aren't doing a whole lot of the photosynthesis thing. So the red helps to limit sun damage while the tree preps for winter.

The yellow pigments, rather, are a reaction to a leaf getting too much sun. It acts like a circuit breaker that cuts off the leaf’s ability to absorb sun, also preventing more sun damage to leaf tissue.

Singsaas says the best conditions for prime fall colors are cool and sunny.

“You get those chilling temps in the low 50s, definitely in the 40s, plus bright sunlight,” said Singsaas.

This provides the optimal stressors that trigger the most pigment.

According to the Smoky Mountains Fall Foliage Map, which predicts the peak times for the best foliage, anywhere south of Duluth will see peak colors between October 8th and October 15th.