A fall mystery: why do only certain leaves turn red?

Leaves turn yellow in the fall because Chlorophyll breaks down, revealing the yellow pigment that was there all along. But why red? Red is a different story altogether. Leaves have to make a new pigment to turn red. Why would a dying leaf go an extra mile to do that? Scientists don’t really know. NPR science correspondent Nell Greenfieldboyce https://www.npr.org/people/4494969/nell-greenfieldboyce reports on the leading hypotheses https://www.npr.org/people/4494969/nell-greenfieldboyce out there. Read more of Nell's reporting on this topic https://www.npr.org/2025/11/05/nx-s1-5599152/why-do-leaves-turn-red-fall-foliage-autumn. Interested in more seasonal science? Email us your ideas at [email protected] mailto:[email protected]. Support public media with NPR+ and enjoy perks for over 25 podcasts like this one. This show’s perks include sponsor-free listening. Learn more at plus.npr.org http://plus.npr.org. See pcm.adswizz.com https://pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences. NPR Privacy Policy https://www.npr.org/about-npr/179878450/privacy-policy

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  You're listening to Short Wave from NPR.

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  Hey, Short Wavers, Emily Kwong here. Now, depending on where you live in the US, you may be enjoying some fall foliage, you know, the result of colder days, trees preparing for winter, and in some, their chlorophyll, the green pigment, is breaking down. And instead of green, we're seeing these amazing colors, yellow, orange, and red, colors you might tuck into a home decoration.

• 0:45Speaker 02

  Along with some of those mutant squash you get at the farmer's market, the really warty ones.

• 0:50Speaker 01

  Hey, Nell.

• 0:50Speaker 02

  Hey.

• 0:51Speaker 01

  That's NPR's Nell Greenfield Boyce. And Nell, from what I understand, you have been getting really into autumn leaves lately.

• 0:58Speaker 02

  Well, some of them, some of them, particularly the red ones.

• 1:01Speaker 01

  I love the red ones, they're so beautiful.

• 1:04Speaker 02

  People like red leaves. And, you know, when I met up with this biologist named Nikki Hughes in North Carolina, she told me.

• 1:10Speaker 03

  My dad always was talking about his quest for the perfect red leaf.

• 1:15Speaker 01

  Respect.

• 1:16Speaker 02

  So it's been this lifetime thing, but she's on her own quest now, and that is just to understand red leaves.

• 1:23Speaker 01

  Why red leaves? Why, why not yellow?

• 1:26Speaker 02

  So, yellow is in the leaves all along. It's kind of hiding in there, right? And so you mentioned chlorophyll breaking down in the fall. And when that happens, it's because the tree wants to recover some key nutrients, like nitrogen. Nitrogen's really precious to a plant. It's needed for everything from photosynthesis to making proteins and DNA. And so, you know, the tree is going dormant kind of in winter, but it doesn't want to lose that nitrogen, it wants to hang on to it. So the chlorophyll gets busted up, the green is going away. And this exposes yellow, right?

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  Okay.

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  And scientists agree on that. The yellow is there, it gets exposed. The red is this completely different thing.

• 2:07Speaker 01

  Oh.

• 2:07Speaker 02

  Because the red is actually made brand new in the fall. Like Nikki picked up this one maple leaf, which was this kind of mottled red on yellow.

• 2:18Speaker 03

  The red was not there beforehand, or you would have seen it, because the leaf would have been purple, because green plus red equals purple.

• 2:24Speaker 02

  So the question is, why does the tree bother to make this red?

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  Especially since the leaf is about to fall and die anyway. Why go through this?

• 2:35Speaker 02

  Exactly. Like, within days, the leaf is doomed. So why go to the effort? Why go to the trouble of making new red colors? And I should note here that these red pigments are also responsible for the leaves we see as orange, okay? Like Nikki Hughes picked up this other leaf from like a tupelo tree that was red and dark orange, and she said if you looked at what appeared to be orange.

• 2:58Speaker 03

  Under a microscope, you see lots of red freckles.

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  Red freckles, that's so cute. Okay, today on the show, the mystery of red leaves. What are scientists' best ideas about what a tree might get from going ginger?

• 3:10Speaker 02

  Join us as we rustle up some autumn leaf biology here on Short Wave, the science podcast from NPR.

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• 4:49Speaker 01

  Okay, Nell, so you and I have talked about how in autumn, the level of green chlorophyll goes down, and the amount of red pigment goes up, at least in some trees.

• 4:59Speaker 02

  Right, not all go red.

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  What do scientists think is going on?

• 5:02Speaker 02

  It really depends on who you ask, okay? I think everybody agrees that the red pigments offer some kind of protection from light. They can act kind of like sunscreen.

• 5:12Speaker 01

  Oh, but don't plants like light? Don't they eat it for breakfast?

• 5:17Speaker 02

  Yes, but just like with us, too much light can be harmful. And so in the fall, it's cold, you've got chlorophyll going away, and the leaves end up with kind of an excess of light. And that means you've got free radicals forming, these are these destructive, unstable molecules. And it turns out, red pigments can mop those up, and they absorb some light. I was talking to Suzanne Renner at Washington University in St. Louis, and she told me that for decades, we have known that red pigments can protect against this extra light.

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  There are a lot of high-tech biochemical, physiological, experimental papers showing that one function is photoprotection.

• 5:58Speaker 02

  So photoprotection, protection from light. She told me, all the plant physiologists are convinced this is a key thing.

• 6:05Speaker 01

  What exactly is being protected, though? Because beautiful as these leaves are, aren't they just going to fall off and die anyway?

• 6:11Speaker 02

  So, okay, that is the question, right? I mean, the idea is that the tree is busy scavenging all this nitrogen and nutrients to store them away for the winter. And, you know, the red pigments, the chemical sunscreen, may be protecting that chemical activity.

• 6:28Speaker 01

  It's like a last ditch, we're going to help the tree out as it transitions. Fascinating. Okay, are there studies and experiments that support this idea?

• 6:37Speaker 02

  There are some, like there's one study that looked at yellow mutants of three deciduous species that normally turn red.

• 6:45Speaker 01

  Okay, so three tree species that are normally red, but these ones were yellow.

• 6:50Speaker 02

  Yeah, and when they shed their leaves, it turns out that those leaves had more nitrogen in them. So the idea is maybe without the red pigments, the plants weren't able to recover that nitrogen and get it out, as well as they would have normally.

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  Like, the nitrogen didn't get into the tree itself, it was shed.

• 7:07Speaker 02

  Yeah, exactly.

• 7:08Speaker 01

  Without the red pigment. Fascinating.

• 7:10Speaker 02

  And people have also noted that trees that live with nitrogen fixing bacteria, so, you know, there's these, you know, species that live kind of like in relationships with bacteria that help them get nitrogen. So they have plenty of nitrogen, and they don't seem to bother to turn red.

• 7:26Speaker 01

  Oh, well, that's a really revealing detail.

• 7:29Speaker 02

  Yeah, I mean, it's suggestive, right? I mean, there's a lot of kind of suggestive evidence out there. So like Suzanne Renner told me that trees in Europe tend not to be as red as in the US. And she did the study looking at the sunlight that comes in. Actually, you can look at it by seeing what hits solar panels. There's all this data out there on solar panels. And she found that trees in Europe get exposed to less solar radiation than trees in the Northeast US. So she's saying, like, maybe they just have less need for this kind of red sunscreen. Again, it's, you know, a correlation, it's suggestive.

• 8:08Speaker 01

  Yeah, I mean, it is an intriguing idea that because US trees are exposed to more sunlight, they may need a little bit more sunscreen via a red color. It's, it's an intriguing explanation. What other ideas are kicking around when it comes to red leaves? Is it just a light thing?

• 8:25Speaker 02

  No. Okay, so there's this whole other line of thinking. And I was talking with one evolutionary biologist, who's named Marco Arcetti at Penn State, and he's been skeptical about this idea that red pigments are there to protect, you know, the tree from sunlight as it's trying to recover its nutrients, you know, and store them away. He has this whole other idea. About why some trees evolved to turn red, and it basically boils down to protection from insects.

• 8:54Speaker 01

  Oh.

• 8:54Speaker 02

  So the kind that lay eggs in autumn.

• 8:57Speaker 01

  Why, why?

• 8:58Speaker 02

  Well, basically, he thinks the red, you know, like in nature, red is often like a warning signal, right? And so he thinks the red could basically be telling them, don't lay your eggs here. Like, this is not a good place for you. And when he published this idea years ago, it was really controversial, okay? The plant physiologists didn't like it. I mean, it's still kind of controversial. I mean, he says, you'd think the entomologists, the people who are into bugs, you'd think that they would have liked it.

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  But no, they thought I was an idiot, because, you know, most insects can't really see red.

• 9:29Speaker 01

  Oh, most insects can't see red?

• 9:32Speaker 02

  No, and, you know, even among mammals, incidentally, we humans are really unusual in our ability to see red. There's even this theory out there that we evolved, along with certain primates, that we evolved to see red, to see young, new leaves, which are often red, and they're kind of more digestible and nutrient-rich.

• 9:52Speaker 01

  Ah, I am so appreciative for my ability to see red now.

• 9:57Speaker 02

  It's kind of special. Like, when you see a bright red tree in autumn, that's like a very human experience.

• 10:02Speaker 01

  Yeah, I was married in a red dress, too, so like, my whole color scheme would have been off without this.

• 10:07Speaker 02

  It's a very potent color for us, right? Anyway, experiments have shown that even if insects can't see red, just like we do, they can still distinguish between different leaf colors, like, you know, green, red, and yellow. And it turns out, like aphids, they avoid red leaves. So there was this one study where Marco looked at apple trees and found that those with red leaves had fewer aphids on them than the trees with green or yellow leaves.

• 10:36Speaker 01

  That's wild. Well, why? Like, why would aphids want to avoid those leaves? Why is it a stop signal for them?

• 10:43Speaker 02

  He's got some evidence that aphids actually have a worst survival rate on red trees compared to aphids on yellow or green ones. I mean, there has not been a huge amount of experimental work on this, but he told me, if you ask him why some leaves go red, my answer would be, I don't know, but.

• 11:02Speaker 06

  If I had to bet, I would bet that it's some kind of coevolution with the insects.

• 11:07Speaker 01

  Yeah, it is coevolution with the insects. Also a compelling idea. The light idea is compelling. It sounds like a lot more research needs to be done to figure any of this out. But what I'm struck by, Nell, is just how scientists don't really know the purpose of red foliage. And it's astonishing, given that, you know, red foliage is everywhere right now.

• 11:27Speaker 02

  Well, it depends on where you are, right?

• 11:29Speaker 01

  Yeah, true.

• 11:29Speaker 02

  I mean, out west. I mean, worldwide, it's a minority of trees that go red. I was just talking to Nikki Hughes, and she told me there's just this dearth of research in general. Like, we don't know why some species go red and others don't.

• 11:43Speaker 03

  I mean, you might think we know everything, but really, there's just not enough people studying these fun mysteries of life.

• 11:50Speaker 01

  I love this tape.

• 11:51Speaker 02

  Like, other than tourism, like, what's the application here? It's not like it's important for agriculture or something, you know? But, I mean, she does notice lots of different, interesting things. Like, there are some species that can go one way or another, right? It's not always yellow, it's not always red. And, you know, she notices this a lot in parking lots.

• 12:09Speaker 03

  In a lot of parking lots, you'll notice that they have trees of a single species, but some are yellow and some are red and some are orange, even though they're all in the exact same sunlight and the exact same temperature.

• 12:22Speaker 02

  With her students, she's actually done, like, parking lot science. Like, she actually was looking at different colored willow oaks outside a home goods store, for example. I mean, it's a very convenient field site.

• 12:33Speaker 01

  And what did she find?

• 12:34Speaker 02

  Well, she told me the red trees had fewer scale insects. She and her students looked at nitrogen levels, too, and they didn't see any difference there. But, you know, that's just one little study, right? I mean, there's tons more work to do.

• 12:46Speaker 01

  Yeah. I want to go back to the thing you said a moment ago, that there are some trees that can turn a bunch of colors at once. They'll have, like, sections that will be, or, you know, an ombre effect. Like, what is going on there?

• 12:58Speaker 02

  Yeah, it's really interesting, right? I mean, she's been studying sweet gums. She says these are trees that turn all the autumn colors, like, including red.

• 13:06Speaker 03

  You'll have a single tree, and some branches will be purple, and some will be yellow, and some will be green. And I feel like that species is a fun clue, like, to work on. Like, what's the difference between those branches?

• 13:18Speaker 01

  It seems incredibly complicated to figure this out.

• 13:21Speaker 02

  Yeah, yeah, I mean, you've got different species, you've got different environments. I mean, is it one explanation for all of them? I mean, to me personally, it seems like it could be a bunch of things. I mean, she's been experimenting with different species of oaks to see what effect there could be to grow them with different levels of nitrogen in the soil. Like, how could that affect leaf color? I mean, there's just, like, a lot of cool, basic science people could do.

• 13:46Speaker 01

  Nell, if this story inspires any more science in this area, please come back on Short Wave next fall and let us know.

• 13:53Speaker 02

  Yeah, maybe we can check in on Nikki's oaks. I mean, I will totally be looking around at the autumn leaves with new eyes, because, you know, people notice things, and scientists notice things. And that's how science happens. It's all around you, the mystery, Emily, the mystery.

• 14:08Speaker 01

  Do you think you will find a perfect red leaf this fall?

• 14:11Speaker 02

  I mean, I'll look. I don't know. If I find it, I'll send you a picture.

• 14:16Speaker 01

  Short waivers, if you find a perfect red leaf, please send it to us at [email protected] so we can admire your discovery. The Nell Greenfield Boys, thank you for coming on the show.

• 14:27Speaker 02

  Thank you.

• 14:30Speaker 01

  All right, short waivers, we have a quick favor to ask. Wherever you're listening right now, leave us a rating or a review, because it really helps listeners find our show. I can tell you, we read everything that you all send in, like, MorningStar0222, who called us an excellent podcast and said, "Of all the podcasts I listen to, this is the show I try not to miss a single episode of. Informative, timely, and always contextual, and presented by people that love learning and sharing that knowledge. The only thing I like is there are times the short episode was not enough, and I want links for more information." That is a true short waiver right there. Thank you so much. We will work on adding more links, MorningStar0222.

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  So, yeah, be like them, send us a review, and we'll probably do exactly what you tell us to. This episode was produced by Burley McCoy. It was edited by our showrunner, Rebecca Ramirez, and fact-checked by Tyler Jones. Kwasi Lee was the audio engineer. Beth Donovan is our vice president of podcasting. I'm Emily Kwong. Thank you for listening to Short Wave from NPR.

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