Emily Kwong (2:00)

Okay.

Nell Greenfield Boyce (2:00)

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 mottled red on yellow.

Guest Scientist or Interviewee (2:17)

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

Nell Greenfield Boyce (2:24)

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

Emily Kwong (2:30)

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

Nell Greenfield Boyce (2:35)

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 under a.

Guest Scientist or Interviewee (2:58)

Microscope, you see lots of red freckles.

Emily Kwong (3:01)

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?

Nell Greenfield Boyce (3:10)

Join us as we rustle up some autumn leaf biology here on shortwave, the Science podcast from NPR.

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Emily Kwong (4:21)

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.

Nell Greenfield Boyce (4:30)

Right? Not all go red.

Emily Kwong (4:32)

What do scientists think is going on?

Nell Greenfield Boyce (4:33)

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.

Emily Kwong (4:43)

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

Nell Greenfield Boyce (4:48)

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. 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.

Emily Kwong (5:22)

Biochemical, physiological, experimental papers showing that one function is photoprotection.

Nell Greenfield Boyce (5:30)

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

Emily Kwong (5:36)

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

Nell Greenfield Boyce (5:42)

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.

Emily Kwong (5:53)

Okay.

Nell Greenfield Boyce (5:53)

And you know, the red pigments, the chemical sunscreen, maybe protecting that chemical activity.

Emily Kwong (6:00)

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?

Nell Greenfield Boyce (6:09)

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

Emily Kwong (6:16)

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

Nell Greenfield Boyce (6:21)

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.

Emily Kwong (6:36)

Like, the nitrogen didn't get into the tree itself. It was shed without the red pigment. Fascinating.

Nell Greenfield Boyce (6:41)

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.

Emily Kwong (6:57)

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

Nell Greenfield Boyce (7:00)

Yeah, I mean, it's, 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. And 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.

Emily Kwong (7:39)

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?

Nell Greenfield Boyce (7:57)

No. Okay, so there's this Whole other line of thinking. And I was talking with one evolutionary biologist who's named Marco Archetti 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 insect. Oh, so the kind that lay eggs in autumn.

Emily Kwong (8:29)

Why? Why?

Nell Greenfield Boyce (8:30)

Well, basically, he thinks the red. You know, like, in nature, red is often like a warning signal.

Guest Scientist or Interviewee (8:35)

Right.

Nell Greenfield Boyce (8:35)

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.

Announcer (8:55)

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

Emily Kwong (9:01)

Oh, most insects can't see red?

Nell Greenfield Boyce (9:03)

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.

Emily Kwong (9:23)

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

Nell Greenfield Boyce (9:28)

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

Emily Kwong (9:33)

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

Nell Greenfield Boyce (9:38)

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.

Emily Kwong (9:52)

Okay.

Nell Greenfield Boyce (9:52)

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.

Emily Kwong (10:07)

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

Nell Greenfield Boyce (10:15)

He's got some evidence that aphids actually have a worse 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.

Announcer (10:31)

Answer would be, I don't know.

Nell Greenfield Boyce (10:33)

But if I had to bet, I.

Announcer (10:35)

Would bet that it's some kind of co evolution with the insects.

Emily Kwong (10:38)

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 out 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.

Nell Greenfield Boyce (10:59)

Well, it depends on where you are, Right?

Emily Kwong (11:00)

True.

Nell Greenfield Boyce (11:01)

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.

Guest Scientist or Interviewee (11:14)

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

Emily Kwong (11:21)

I love this tape.

Nell Greenfield Boyce (11:22)

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.

Guest Scientist or Interviewee (11:41)

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 in the exact same temperature.

Nell Greenfield Boyce (11:53)

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.

Emily Kwong (12:04)

And what did she find?

Nell Greenfield Boyce (12:06)

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.

Emily Kwong (12:18)

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?

Nell Greenfield Boyce (12:29)

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

Guest Scientist or Interviewee (12:38)

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?

Emily Kwong (12:50)

It seems incredibly complicated to figure this out.

Nell Greenfield Boyce (12:52)

Yeah, Yeah. I mean, you 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 effects 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.

Emily Kwong (13:17)

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

Nell Greenfield Boyce (13:24)

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 myst.

Emily Kwong (13:40)

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

Nell Greenfield Boyce (13:42)

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

Emily Kwong (13:48)

Shortwavers, if you find a perfect red leaf, please send it to us@shortwavepr.org so we can admire your discovery. Nell Greenfield Boyce, thank you for coming on the show.

Nell Greenfield Boyce (13:58)

Thank you.

Emily Kwong (14:01)

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 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 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 Burleigh McCoy. It was edited by our showrunner, Rebecca Ramirez, and fact checked by Tyler Jones. Kwesi Lee was the audio engineer. Beth Donovan is our vice president of podcast testing. I'm Emily Kwong. Thank you for listening to short wave from NPR.

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