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Here's how evergreen trees stay green all year

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Aspen pine trees at Lake Vermilion State Park
Aspen pine trees at Lake Vermilion State Park.
Brian Bakst | AP 2013

Evergreens are a beacon of hope in Minnesota winters — something green amid the gray skies and white landscapes.

So why are they able to stay green while deciduous trees (oaks, maples, birch, etc.) lose their leaves in the fall?

It turns out evergreens do lose their needles — just not all at once.

Every year, new needles grow and old needles drop. This time of year, you'll see new growth — those are needles that are lighter green coming out at the end of the stem.

Evergreens put needles on in groups and they tend to lose them in these same groups. It's kind of like your hair — all of your individual hairs have a cycle of growing and falling out, but they don't all fall out at the same time.

Evergreens usually keep their needles for two to three years. If you see red needles that are closer to the stem, those are the ones that are getting ready to drop.

And even though evergreens stay green during the winter, they're basically hibernating, according to University of Minnesota forest researcher Kyle Gill.

"They use the needles a little bit during the winter, but for the most part they're in a reduced activity state," said Gill. "A lot of that has to do with the roots being locked up in the frozen ground."

In addition, the strong, waxy coatings and shape of conifer needles makes them resistant to cold and keeps them from drying out during the winter.

But why do needles and leaves change color before they fall?

Those brilliant red, orange and yellow colors are already there, it's just covered up by the copious amounts of chlorphyll — the green pigment that plants use to absorb light.

"Those other pigments can photosynthesize also, but not as well as chlorophyll does," said Mary Meyer, horticulturist at the University of Minnesota Landscape Arboretum.

As fall comes and the weather changes, the chlorophyll starts to degrade and the other colors become visible, making leaf peepers very happy.

For more from Brains On, the MPR News science podcast for kids and curious adults, you can subscribe in iTunes or wherever you listen to podcasts.