Pod Insights Short Wave Fall foliage is still a mystery: Why do some leaves turn red? About this episode Scientists know why leaves turn yellow in the fall: Chlorophyll breaks down, revealing the yellow pigment that was there all along. But red? Red is a different story altogether. Leaves have to make a new pigment to turn red. Why would a dying leaf 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.
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0:00 Speaker 00 This message comes from Planet Visionaries. Host Alex Honnold sits down this season with extraordinary doers, creating real climate solutions. Listen or watch wherever you get your podcasts. You're listening to Short Wave from NPR.
0:17 Emily Kwong? Hey, Shortwavers. 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.
0:39 Speaker 02 Along with some of those mutant squash you get at the farmer's market, the really wardy ones.
0:44 Emily Kwong? Hey, Nell.
0:44 Speaker 02 Hey.
0:45 Emily Kwong? That's NPR's Nell Greenfield Boyce. And Nell, from what I understand, you have been getting really into autumn leaves lately.
0:52 Speaker 02 Well, some of them, some of them, particularly the red ones.
0:55 Emily Kwong? I love the red ones. They're so beautiful.
0:58 Speaker 02 People like red leaves. And, you know, when I met up with this biologist named Nikki Hughes in North Carolina, she told me-
1:04 Speaker 03 My dad always was talking about his quest for the perfect red leaf.
1:09 Emily Kwong? Respect.
1:10 Speaker 02 So it's been this lifetime thing. But she's on her own quest now, and that is just to understand red leaves.
1:17 Emily Kwong? Why red leaves? Why not yellow?
1:20 Speaker 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?
1:54 Emily Kwong? Okay.
1:55 Speaker 02 And scientists agree on that. The yellow is there, it gets exposed. The red is this completely different thing.
2:01 Emily Kwong? Oh.
2:01 Speaker 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.
2:12 Speaker 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.
2:18 Speaker 02 So the question is-
2:22 Speaker 02 Why does the tree bother to make this red?
2:24 Emily Kwong? Especially since the leaf is about to fall and die anyway. Why go through this?
2:29 Speaker 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, OK? 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. Under a microscope, you see lots of red freckles.
2:55 Emily Kwong? Red freckles, that's so cute. OK, today on the show, the mystery of red leaves. What are scientists' best ideas about what a tree might get from going ginger?
3:04 Speaker 02 Join us as we rustle up some autumn leaf biology here on Short Wave, the science podcast from NPR.
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4:38 Emily Kwong? 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.
4:47 Speaker 02 Right, not all go red.
4:49 Emily Kwong? What do scientists think is going on?
4:50 Speaker 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.
5:00 Emily Kwong? Oh, but don't plants like light? Don't they eat it for breakfast?
5:05 Speaker 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.
5:37 Speaker 06 There are a lot of high-tech biochemical, physiological, experimental papers showing that one function is photoprotection.
5:47 Speaker 02 So photoprotection, protection from light. She told me all the plant physiologists are convinced this is a key thing.
5:53 Emily Kwong? What exactly is being protected, though? Because beautiful as these leaves are, aren't they just gonna fall off and die anyway?
5:59 Speaker 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.
6:17 Emily Kwong? It's like a last ditch, we're gonna help the tree out as it transitions. Fascinating, okay. Are there studies and experiments that support this idea?
6:26 Speaker 02 There are some, like there's one study that looked at yellow mutants of three deciduous species that normally turn red.
6:33 Emily Kwong? Okay, so three tree species that are normally red, but these ones were yellow.
6:38 Speaker 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.
6:52 Emily Kwong? Like the nitrogen didn't get into the tree itself, it was shed, without the red pigment. Fascinating.
6:58 Speaker 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.
7:09 Emily Kwong? Yeah.
7:09 Speaker 02 That help them get nitrogen.
7:10 Emily Kwong? Yeah.
7:11 Speaker 02 So they have plenty of nitrogen and they don't seem to bother to turn red.
7:14 Emily Kwong? Oh, well, that's a really revealing detail.
7:17 Speaker 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.
7:48 Emily Kwong? Oh.
7:48 Speaker 02 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.
7:56 Emily Kwong? 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 an intriguing explanation. What other ideas are kicking around when it comes to red leaves? Is it just a light thing?
8:14 Speaker 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.
8:36 Emily Kwong? Okay.
8:36 Speaker 02 About why some trees evolved to turn red. And it basically boils down to protection from insects.
8:42 Emily Kwong? Oh.
8:43 Speaker 02 So the kind that lay eggs in autumn.
8:46 Emily Kwong? Why, why?
8:47 Speaker 02 Well, basically, he thinks the red, you know, like in nature, red is often like a warning signal, right?
8:52 Emily Kwong? Yeah.
8:53 Speaker 02 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.
9:12 Speaker 07 But no, they thought I was an idiot because, you know, most insects can't really see red.
9:18 Emily Kwong? Oh, most insects can't see red?
9:20 Speaker 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.
9:40 Emily Kwong? Ah, I am so appreciative for my ability to see red now.
9:45 Speaker 02 It's kind of special. Like, when you see a bright red tree in autumn, that's like a very human experience.
9:50 Emily Kwong? Yeah, I was married in a red dress, too. So like, my whole color scheme would have been off without this.
9:55 Speaker 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.
10:24 Emily Kwong? That's wild. Well, why? Like, why would aphids want to avoid those leaves? Why is it a stop signal for them?
10:32 Speaker 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...
10:48 Speaker 07 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.
10:55 Emily Kwong? Yeah, it is co-evolution with the insects. Also a compelling idea. Uh, the light idea is compelling. It sounds like a lot more research needs to be done to figure any of this out. Um, 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.
11:16 Speaker 02 Well, it depends on where you are, right?
11:17 Emily Kwong? Yeah, true.
11:18 Speaker 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.
11:31 Speaker 03 I mean, you might think we know everything, but really, there's just not enough people studying these fun mysteries of life.
11:38 Emily Kwong? I love this tape.
11:39 Speaker 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.
11:58 Speaker 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.
12:10 Speaker 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.
12:21 Emily Kwong? And what did she find?
12:23 Speaker 02 Well, she told me the red trees had fewer scale insects. Um, 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.
12:35 Emily Kwong? 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?
12:46 Speaker 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.
12:55 Speaker 03 You'll have a single tree, and some branches will be purple, and some of them 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?
13:07 Emily Kwong? It seems incredibly complicated to figure this out.
13:10 Speaker 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,
13:20 Emily Kwong? Yeah.
13:21 Speaker 02 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.
13:34 Emily Kwong? Now, if this story inspires any more science in this area, please come back on Short Wave next fall and let us know.
13:41 Speaker 02 Yeah, maybe we can check in on Nikki's oaks. I mean,
13:44 Emily Kwong? Yeah.
13:44 Speaker 02 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.
13:57 Emily Kwong? Do you think you will find a perfect red leaf this fall?
13:59 Speaker 02 I mean, I'll look. I don't know. If I find it, I'll send you a picture.
14:05 Emily Kwong? 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.
14:15 Speaker 02 Thank you.
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15:03 Emily Kwong? 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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