Chasing Color: Reasons for the seasonal colors
September 22, 2013
Behind every brilliant show of bright fall colors is an abundance of darkness.
When the evenings grow long enough late in the summer, the trees have their cue.
A layer of corky cells forms in the stem of the leaves, local naturalist Karen Vail explains, after she plucks a green aspen leaf outside her home.
This layer quickly blocks the flow of minerals and chlorophyll to the leaves.
Without this chlorophyll, the yellow pigments called xanthophylls and orange pigments called carotenoids are revealed.
They've been hidden in the leaves for months by the green color of the chlorophyll.
Vail said the fall color show almost always starts at the same time of year because it is dependent on the length of the nights.
Here in Colorado, that means the nights are long enough to spur the change around late September. But the show's intensity depends on temperature and moisture.
"If we get an early freeze, we usually don't get very good colors," Vail says, explaining the jolt of extreme cold can cause the corky layer to form much more rapidly and ruin what usually is a gradual change of colors.
The perfect ingredients for a fall show? An abundance of summer moisture followed by cool but not freezing nights and warm days.
"That is what is going to give you those absolutely gorgeous colors," she said.
With so much late rainfall, Vail said it will be hard to predict the intensity of the fall color show this year.
Although the leaf change depends so much on science, forecasting how spectacular the colors will be can be a crapshoot.
"The exact timing and intensity of the annual turning of the leaves is indeed trickier to predict than the weather itself," the National Weather Service in Pueblo wrote in an early forecast of the fall colors.
In Northwest Colorado, colorful aspen trees will dominate the landscape.
But as brilliant as they are, Vail said, we shouldn't get too caught up in just the aspens.
"A red geranium leaf at your feet could make a cool photo surrounded by the golden aspen leaves," she said. "Don't forget to look down."
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