Fall Colors Part 2: Why Do Leaves Change Color in the Fall?

Joshua VanBrakle Tuesday, 27 October 2015

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In my last post, I talked about why leaves change color in the fall. But as I wandered my local woods this October, another question came to me. Why do different trees turn different colors? Quaking aspen reliably turns a golden yellow. Sugar maple gets more of an orange hue. And red maple—as its name suggests—turns a bold scarlet.

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What’s up with that?

It comes down to three different chemicals found inside leaves: chlorophyll, carotene, and anthocyanins.

I talked about chlorophyll in my last post. It’s a green pigment, and it’s what plants use to absorb sunlight and convert carbon dioxide into sugar the plant can use to grow. In the fall, plants stop producing chlorophyll, and as it breaks down, other colors show through.

Those other colors come from different pigments, and the big two are carotene and anthocyanin. Carotene gives us the bright yellows and oranges (think “carrots”). Most plant leaves have carotene in them, and it’s present in pretty constant levels throughout the year. As a result, there’s little difference from one fall to another in how intense the yellows and oranges are.

Anthocyanin is different. It’s the pigment that give trees like red maples their astounding red color. But while carotene is always in the leaf, anthocyanin is only produced in the fall. When the tree cuts off its flow of nutrients to the leaf, the leaf is still making some food for the plant in the form of sugar. Anthocyanins form when that sugar reacts with other chemicals inside the leaf.

Here’s the catch though. Those reactions can only occur when the leaf’s sugar concentration is high, and it gets high when the leaf loses moisture. That’s why fall reds vary from year to year. In falls with dry, sunny days and cool (but not freezing) nights, more water evaporates from the leaf. That leads to more concentrated sugars, more reactions making anthocyanin, and ultimately bolder, deeper reds. Get a wet, soggy fall, and the reds will be muted.

But hold on. That still doesn’t explain why some trees turn red while others turn yellow. Why the difference?

It turns out that not all tree species produce anthocyanin, and those that do produce it in different amounts. The species known for their deep red and purple fall colors like red maple, red oak, and sumac produce a lot of anthocyanin, overwhelming the carotene. But our reliably yellow quaking aspen doesn’t make anthocyanin, so the yellow carotene shows through.

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Sugar maple (left) and red maple (right) are closely related, but they turn different colors in the fall because red maple leaves produce a lot more anthocyanin than sugar maple leaves do.

That’s a lot of color science, but at the end of the day, what matters is that fall is an excellent time to get outside, walk in the woods, and enjoy the colors.

If you want to preserve those colors, an easy way to do it is to build a plant press. Pressing leaves removes the moisture so they’ll stay intact and colorful long after the season ends.

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