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Ever wondered why trees ditch their leaves each fall?

What looks like pretty scenery to us is a matter of deadly serious survival for trees.

(Getty Images)

Autumn arrives this week, and that means pumpkins, football and piles of fresh, crackly leaves. Did you ever wonder why trees throw away an important part of their anatomy each year? After all, wouldn’t it be similar to people losing all their hair — or even weirder, their skin — just as our part of the world gets colder?

While it might seem strange from the point of view of a human, to a plant, losing leaves makes perfect sense.

Trees are solar-powered. Each leaf is loaded with a pigment called chlorophyll (pronounced CLORE-o-fill), which absorbs light and helps convert water and carbon dioxide into energy. The process is called photosynthesis (fo-toe-SYN-thuh-sis).

But there’s a problem. In parts of the world that experience seasons, winter means less and less sunlight each day. It also comes with biting cold that can freeze the liquids inside leaves. These two factors hamper the tree’s ability to make energy.

A full-grown oak tree might have more than 60,000 leaves, and each one requires valuable nutrients. So when fall turns into winter, trees discharge their leaves as a cost-cutting measure. If it had to spend resources on all those leaves through the winter, not only would the leaves freeze, but the tree would die.

However, evergreen trees have a different strategy, says Mason Heberling, assistant curator of botany at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania. Instead of dropping and regrowing their leaves each year, pine trees and other evergreens evolved short, thick “leaves” that can withstand winter’s wrath. Of course, we call them “needles.”

In addition to having a different shape, pine needles are covered in an insulating wax, and inside they have compounds that act like antifreeze in a car to keep the tree from freezing solid.

Needles and leaves are two ways to solve the same problem. And considering that each has been around for hundreds of millions of years, it’s clear that they’re successful survival strategies. But leafy trees do have one bonus that their needly cousins do not: fall colors.

Remember how leaves are full of chlorophyll? Well, they also have pigments called carotenoids, which create orange and yellow hues. Most of the year, the green from the chlorophyll overpowers the carotenoids, says Heberling, but in the fall, as the chlorophyll starts to break down, all those bright, tawny colors start to show.

Some trees, such as black cherry, flowering dogwood and white oak, also begin to produce a pigment called anthocyanin each fall. This gives their leaves a brilliant red color. Scientists aren’t sure why some trees make this extra pigment, but the leading theory is that it helps protect the leaves from the sun for their last few weeks of life.

“Anthocyanin is kind of a sunscreen for plants,” says Heberling.

So the next time you see a pile of crimson, amber and tangerine-colored leaves on the ground, think about all the biological processes that had to happen for them to exist. Then by all means, jump in.

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