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This is why leaves change color in the fall

A leaf's fall color is in it's DNA. It gets revealed when the chlorophyll mask of summer fades.

DENVER — The leaves don’t turn color in the fall -- their true colors just get revealed.

A green mask conceals their real skin over the summer months.

“All the pigments have always been there in the leaves, its just that when the chlorophyll production ceases, it unmasks the yellows the oranges and the reds,” said arborist Tony Hahn with Denver Commercial Property Services.

RELATED: When and where to see the leaves change in Colorado

He said the green fades in the fall because there’s not enough sunlight during the shorter days to produce chlorophyll.

And the autumn color mystery that lies beneath is determined when the trees are born – it’s in their DNA.

“If you have a tree that’s always been yellow, you can’t do anything to make it more red," said Hahn. "It’s a genetically inherited characteristic.”

The yellow and orange pigments are called xanthophylls. And the red and purple pigments are called anthocyanins.

He said many leaves in Colorado have very little anthocyanins because our soils are not acidic enough to support that type of genetic development. Cottonwood leaves for example are almost exclusively yellow or orange.

Hahn said some aspens have anthocyanins but the reds and purples don't always show up because the xanthophylls typically have to get metabolized before the anthocyanins can be revealed.

It's the weather conditions that help cycle out the xanthophylls. It usually takes very cold nights with temperatures near 32 degrees, followed by days that are sunny but not hot enough to cause too much transpiration in the leaves.

Hahn said that is also the formula that can enhance how vibrant the leaves natural colors are.

RELATED: 9 of the best drives to see fall colors in Colorado

Credit: Deb Haas

And he said that trees usually disrobe their green from the crown to the ground, because they need a special hormone signal to stop producing chlorophyll.

“That’s why you see it at the tops of trees first because that’s where the hormones are produced, at those growing points at the tops,” said Hahn.

The leaves change at the higher elevations before the lower elevations because of temperatures. He said it’s still been too warm at night in the Denver area for many of the leaves to change.

RELATED: 9 hikes where you can see fall colors in Colorado

Friday was the first time this fall that the overnight temperature dropped into the 30s. Hahn said it usually takes several days below 35 degrees to convince trees to harden-off for the winter. That's usually when the leaves stop producing chlorophyll. 

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