Why some leaves change color in the Fall

ELISSA NADWORNY, HOST:

If you're in a part of the country with some great fall foliage and wondering, why do some leaves change color in the fall? - well, we've got you covered. Emily Kwong and Brit Hanson from NPR's Short Wave podcast explored that very question with the botanist and plant ecologist Tanisha Williams. She told them the changing leaves start with environmental cues, like the days getting shorter.

TANISHA WILLIAMS: So once they have these cues saying, like, oh, wintertime is coming, the falling of the leaves and the changing of the colors are all in preparation for the tree to hunker down and basically hibernate in the wintertime. Then all of this magical stuff starts to happen in the leaf.

EMILY KWONG, BYLINE: OK, like what?

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BRIT HANSON, BYLINE: Well, it actually has to do with the magic of - drumroll, please - photosynthesis.

KWONG: Yes. OK.

HANSON: Right? Yeah, photosynthesis. It's this cool process of how plants harness energy from sunlight and water and carbon dioxide to make food for themselves.

KWONG: Yeah, casual plant alchemy.

HANSON: Exactly. It's pretty amazing. And as you may remember from biology class, in order for photosynthesis to happen, as a first step, a tree needs to find a way to capture the sunlight. And leaves are how that happens. But to be even more specific, it's the pigments inside the leaves that do this, that capture the sunlight.

KWONG: And by pigments, do you mean colors?

HANSON: Sort of. So stick with me here, Emily. There are a handful of different pigments in each leaf, and each of these pigments absorbs sunlight and reflects the rest of it.

KWONG: OK.

HANSON: So the color that we see on the leaves depends on the light that each of the pigments is reflecting.

KWONG: Oh.

HANSON: So let's start with the color green.

WILLIAMS: So the reason why we see all of this green color in the leaves is because of the pigment known as chlorophyll. And that is what we see during the summer and spring months because there's a lot of sunlight.

KWONG: So basically, the more sunlight there is, the more green we see.

HANSON: Yes, because of chlorophyll, the green pigment in the leaf. And this is the pigment that's the most responsible for kick-starting that process - photosynthesis. But that starts to change once those environmental cues set in.

KWONG: Yeah, I love the word cues because it reminds me of stage actors being cued...

HANSON: Yes.

KWONG: ...For a costume change.

HANSON: Yep.

KWONG: Right? Like the leaves are, oh, it's my turn to transform. But in this case, it's less daylight and colder temperatures.

HANSON: Exactly. So as the tree starts to get ready for winter and the chlorophyll has stopped showing off this, you know, sort of green show...

KWONG: Yeah.

HANSON: ...We can start to see another pigment - carotenoid.

WILLIAMS: Carotenoids are where we're going to get that yellow and orange color.

KWONG: Oh, OK. So chlorophyll equals green leaves, and carotenoids equals yellow and orange leaves.

HANSON: Yes. And for some trees, right here, this is where the colors stop. But for others, Tanisha says there's one more pigment to go.

WILLIAMS: Then we can get into the red color. And the red color is produced by pigments called anthocyanins.

HANSON: So one thing that Tanisha pointed out is that sort of in the background of this whole process, why the leaves are changing color, the tree is also entering what she calls hibernation mode, so sort of slowing down the flow of nutrients out to the limbs; basically hunkering down to conserve energy so it can make it through the winter.

KWONG: This sounds like what I do.

HANSON: (Laughter) Exactly.

KWONG: I will not move from this couch.

HANSON: No one can make you.

KWONG: No.

HANSON: So after a leaf has cycled through its pigments, it starts to run out of energy.

WILLIAMS: And it starts to die off. And also, it starts to weaken at the stem right there. So wind and as we're walking through leaves, we're kicking them and things like that to drop the leaves off the tree.

KWONG: I had no idea so much was going on inside each leaf.

HANSON: And, you know, when I was talking with Tanisha, Emily, I kind of realized, like, when I take my afternoon walk through my neighborhood, I haven't really been appreciating how hard the leaves are working, you know, for this amazing production.

WILLIAMS: Once you start learning about them...

HANSON: Yeah.

WILLIAMS: ...Then your lenses change. You just see the world differently.

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NADWORNY: That's botanist Tanisha Williams speaking to NPR's Short Wave. Transcript provided by NPR, Copyright NPR.