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

Episode Date: September 28, 2026

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 reports on the leading hypotheses out there.Read more of Nell's reporting on this topic. Interested in more seasonal science? Email us your ideas at [e-mail] 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. See 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

You're listening to Shortwave from NPR. Hey, shortwavers, Emily Kwong here. Now, depending on where you live in the U.S., 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. Along with some of those mutant squash, you get at the first. farmer's market, the really warty ones. Hey, Nell. That's NPR's Nell, Greenfield-Boice,

and Nell, from what I understand, you have been getting really into autumn leaves lately. Well, some of them, some of them, particularly the red ones. I love the red ones. They're so beautiful. People like red leaves. And, you know, when I met up with this biologist named Nikki Hughes in North Carolina, she told me, my dad always was talking about his quest for the perfect red leaf. Respect. So it's been this life. time thing, but she's on her own quest now, and that is just to understand red leaves. Why red leaves? Why not yellow? 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, 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. Yeah. And this exposes yellow, right?

Okay. And scientists agree on that. The yellow is there. It gets exposed. The red is this completely different thing. Oh. Because the red is actually made brand new in the fall. Like, Nikki picked up this one maple leaf, which was this kind of modeled red on,

yellow. The red was not there beforehand, or you would have seen it, so the leaf would have been purple, because green plus red equals purple. So the question is, why does the tree bother to make this red? Especially since the leaf is about to fall and die anyway. Why go through this? 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 responsive. for the leaves we see as orange. Okay? Like Nikki Hughes picked up this other leaf from like a tuplow tree that was red and dark orange.

And she said if you looked at what appeared to be orange. Under a microscope, you see lots of red freckles. 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? Join us as we rustle up some autumn leaf biology here on Shortwave, the science podcast from NPR.

Okay, now, 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. Right. Not all go red. What do scientists think is going on? 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. Oh, but don't plants like light? Don't they eat it for breakfast?

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. There are a lot of high-tech biochemical, physiological experimental papers showing that one function is photo protection. So photo protection, protection from light. She told me all the plant physiologists are convinced this is a key thing. What exactly is being protected, though, because beautiful as these leaves are, aren't they just going to fall off and die anyway? 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.

Okay. And, you know, the red pigments, the chemical sunscreen, may be protecting that chemical activity. 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? There are some. Like there's one study that looked at yellow mutants of three deciduous species that normally turn red. Okay.

So three tree species that are normally red, but these ones were yellow. 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. Like the nitrogen didn't get into the tree itself. It was shed without the red pigment. Fascinating. And people have also noted that trees that live with nitrogen fixing bacteria. So, you know, there's the, these species that live kind of like in relationships with bacteria that help them get nitrogen. Yeah. So they have plenty of nitrogen and they don't seem to bother to turn red.

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

No. Okay. So there's this whole other line of thinking. And I was talking with one evolutionary biologist who's named Marco Arquetti at Penn State. And he's been skeptical about this idea that red pigments are there to protect the tree from sunlight. as is 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. Oh. So the kind that lay eggs in autumn. Why?

Why? 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 physiologist 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.

But no, they thought I was an idiot because, you know, most insects can't really see red. Oh. Most insects can't see red? 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. I am so appreciative for my ability to see red now.

It's kind of special. Yeah. When you see a bright red tree in autumn, that's like a very human experience. Yeah. I was married in a red dress too. So like my whole color scheme would have been off without this. 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 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. That's wild. well, why? Like, why would aphids want to avoid those leaves? Why is it a stop signal for them? 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 if I had to bet, I would bet that it's some kind of co-evolution with the insects. Yeah, it is co-evolution 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. Well, it depends on where you are, right? True. 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. me, there's just this dearth of research in general. Like, we don't know why some species go red and others don't. I mean, you might think we know everything, but really it's just not enough people studying these fun mysteries of life. I love this tape. 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. 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. 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.

And what did she find? 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. 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? Yeah, it's really interesting, right? Yeah. She's been studying sweetgums. She says these are trees that turn all the autumn colors, like including red. 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?

It seems incredibly complicated to figure this out. 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. Now, if this story inspires any more science in this area, please come back on Shorewave next fall and let us know. Yeah, and 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. Do you think you will find a perfect red leaf this fall? I mean, I'll look. I don't know. If I find it, I'll send you a picture. Shortwavers, if you find a perfect red leaf, please send it to us at shortwave at npr.org. So we can admire your discovery.

Nell Greenfield Boys, thank you for coming on the show. Thank you. All right, shortwavers, 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 Morningstar 0222 who called us an excellent podcast and said of all the podcasts I listened 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. Morningstar 0222. 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. Quasi Lee was the audio engineer. Beth Donovan is our vice president of podcasting. I'm Emily Kwong, thank you for listening to Shortwave from NPR.