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Why do Leaves Turn Colors in the Fall?

Posted by Jessi Loerch at Oct 21, 2021 04:42 PM |
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We know those colors are fun to look at, and make for excellent photos, but it also raises the question of “why?” Why do leaves turn different colors in the fall?

In the fall — as the days get shorter and all of us humans start putting on sweaters and debating the merits of pumpkin spice flavoring — trees are also preparing for winter. As far as we know, they’re not forming opinions about latte flavors. But we do know that they’re into the fall color palette.

Deep red fall leaves
Deep reds like these are lovely fall sight. Photo by Sue Kraemer.

At WTA, we have a lot of staff who vigorously defend fall as the best season for hiking. And part of the love for that season is because of the brilliant colors. We know those colors are fun to look at, and make for excellent photos, but it also raises the question of “why?” Why do leaves turn different colors in the fall?

To answer that question, we first have to explain why most leaves are green for most of the year. If you remember back to grade school science class, you probably learned about chlorophyll. It’s the green stuff in plants that, by a process that seems like magic, takes sunlight, water and carbon dioxide and turns them into food for the plant. (And, ultimately, for the whole world. Plants are so cool.)

So, for most of the year, you see green leaves as trees work hard to make lots of food. Come fall, as the tree senses the changing season — particularly as the hours of daylight dwindle — the tree begins to prepare for a nice winter snooze. One of the ways it does that is by producing less chlorophyll. But the tree doesn’t want to waste all of the chlorophyll. It can save the building blocks of it to use the following year. So, though it stops producing chlorophyll, the tree doesn’t immediately drop its leaves. It holds onto them as the chlorophyll levels decrease. And, as this is happening, the other colors in the leaves, some of which were there all the time but just being overpowered, begin to show through.

A fallen yellow leaf
A fallen yellow leaf in the river. Photo by Jackson Lee.

Depending upon the tree, the colors of the leaves will be different. Some trees end up a vivid yellow, while others are orange, red, brown or purple. The color depends on what pigments the tree has in its leaves, and how those colors show up through the remnants of the chlorophyll. Carotenoids (yellows and oranges) and anthocyanins (reds) are key pigments that determine what colors leaves turn in the fall. Carotenoids tend to be already in the leaves, but are revealed as the chlorophyll decreases. Anthocyanins are produced in the leaves as part of the chemical changes before the leaves fall from the tree. (It’s believed the red colors protect the leaves until they fall.) In some leaves, which show red, orange and yellow, the leaves progress from one pigment to the next, and put on a spectacular, ever-changing show.

And, in case you’re wondering: Why do trees even lose their leaves? We can answer that, too. In the winter, there’s less sun and the weather is cooler, and it would be more work for deciduous trees to take care of their relatively delicate leaves. Losing their leaves lets deciduous trees save energy and go dormant over the winter. Evergreen trees, on the other hand, use a different tactic, and have hardier leaves (needles are just a kind of leaf) that they hold onto over the winter. (Although some evergreen trees also do let go of some leaves in the fall. Cedars, for instant, let some foliage die back each fall to prioritize the foliage that’s getting the most sun.)

Now you know a bit about the science behind fall colors; it’s a great time to head out to the trail — and perhaps look a little more closely at the leaves you see. And if you do? File a trip report and help other hikers find the beauty of the changing season.