Podcast Summary: Short Wave

Episode: Fall foliage is still a mystery: Why do some leaves turn red?
Hosts: Emily Kwong & Nell Greenfield Boyce (NPR)
Date: November 7, 2025
Duration: ~15 minutes

Overview

This episode dives into the enduring mystery of why some tree leaves turn brilliant red in the fall, while others stick to yellow or orange. Host Emily Kwong and science correspondent Nell Greenfield Boyce explore the leading scientific theories, from photoprotection to evolutionary insect deterrence. With input from researchers, the episode uncovers what is known, what’s still guessing, and why the humble red leaf captivates scientists—and all of us—each autumn.

Key Discussion Points & Insights

The Basics of Fall Color Change

• Chlorophyll Breakdown

  • Trees pull nutrients from their leaves as days get colder, causing chlorophyll (green pigment) to break down. This reveals pigments that were already in the leaves.
  • Nell Greenfield Boyce: "So yellow is in the leaves all along...and when [chlorophyll] breaks down...this exposes yellow." (01:26)

• Red is Different

  • Red pigments (anthocyanins) are produced newly in the fall, unlike yellow or orange pigments.
  • Nikki Hughes (biologist): "The red was not there beforehand, or you would have seen it. The leaf would have been purple because green plus red equals purple." (02:17)

Why Make Red Pigment at All?

• Scientific Mystery
  • The big question: Why would a tree invest energy in creating red pigments right before shedding the leaf?
  • Emily Kwong: "Especially since the leaf is about to fall and die anyway. Why go through this?" (02:30)

The Photoprotection (Sunscreen) Theory

• Red as Light Protection

  • Many plant scientists think red pigments protect leaves from excess sunlight, which is suddenly overwhelming as chlorophyll vanishes.
  • Suzanne Renner (Washington University in St. Louis): "There are a lot of high tech biochemical, physiological, experimental papers showing that one function is photoprotection." (05:22)
  • Red pigments absorb some light and mop up free radicals that could damage the leaf.

• Supporting Evidence

  • A study showed that yellow mutant leaves in typically red-leaved tree species retained more nitrogen (less was recovered by the tree), implying red pigments might help the nutrient recovery process. (06:09)
  • Trees in areas with more sunlight (like the Northeast US) show more red foliage, possibly supporting the sunscreen idea. In Europe, where sunlight is lower, trees are less red. (07:00)

The Insect Defense Theory: Red as a Warning

• Evolutionary Idea

  • Some scientists think red leaves evolved as a warning signal for insects (like aphids) not to lay eggs, because red may indicate leaves are less suitable hosts.
  • Marco Archetti (Penn State): "The red could basically be telling them, don't lay your eggs here." (08:30)
  • Notably, most insects can't literally see red as humans do, but they can distinguish leaf colors and show preferences.

• Experimental Support

  • Studies found aphids avoid red leaves and apple trees with redder leaves had fewer aphids than green or yellow. Aphids also had lower survival rates on red-leaved hosts. (09:52)
  • Even so, the research is early-stage and this theory is still controversial among scientists.

Human Connection: Our Unique Perception of Red

• Humans are unusual in their ability to see the color red. This may have evolved so primates could detect young, nutrient-rich red leaves.
• Emily reflects, "I am so appreciative for my ability to see red now." (09:23)

Open Questions and Ongoing Mysteries

• It’s not universally agreed why some species turn red and others do not. Environmental factors, nitrogen relationships, and genetics all play a role.
• Nikki Hughes: "You might think we know everything, but really there's just not enough people studying these fun mysteries of life." (11:14)
• Intriguingly, individual trees—even on the same site—sometimes display yellow, orange, or red leaves, adding to the mystery.
• Hughes’ "parking lot science" found that in one study, red-leaved oaks had fewer scale insects, but no nitrogen difference (12:06).
• Some species, like sweetgums, can produce a rainbow of fall colors simultaneously on different branches—another puzzle for scientists. (12:29)

Notable Quotes & Memorable Moments

• Emily Kwong (on yellow pigment): "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...this exposes yellow." (01:26)
• Nell Greenfield Boyce: "The question is, why does the tree bother to make this red? ...within days, the leaf is doomed." (02:30)
• Suzanne Renner: "There are a lot of high tech biochemical, physiological, experimental papers showing that one function is photoprotection." (05:22)
• Marco Archetti: "You’d think the entomologists, the people who are into bugs, would have liked it [the red-as-insect-deterrent theory]...But no, they thought I was an idiot because, you know, most insects can't really see red." (08:55)
• Nell Greenfield Boyce: "It's kind of special, like when you see a bright red tree in autumn, that's like a very human experience." (09:28)
• Nikki Hughes: "You might think we know everything, but really there's just not enough people studying these fun mysteries of life." (11:14)
• Nell Greenfield Boyce: "With her students, she's actually done like parking lot science...I mean, it's a very convenient field site." (11:53)
• Emily Kwong: "If you find a perfect red leaf, please send it to us@shortwavepr.org so we can admire your discovery." (13:48)

Important Timestamps

• Fall Foliage Basics: 00:17 – 02:40
• Why Red Pigments Exist: 02:41 – 04:33
• Photoprotection Theory & Evidence: 04:33 – 07:39
• Insect Deterrence Theory: 07:57 – 10:38
• Open Questions & Human Perception: 10:38 – 13:18
• Sweetgums and Rainbow Leaves: 12:29 – 12:50
• Invitation to Listeners: 13:48

Conclusion

Despite the dazzling display each autumn, scientists still don’t have a definitive answer to why trees produce red leaves. Leading theories point to protection from light and deterrence of insects, with each supported by intriguing but incomplete evidence. The episode celebrates curiosity and the beauty of not-knowing, encouraging listeners to look at the colorful world around them with scientific wonder—and maybe participate in some "parking lot science" themselves.

For further listening, review, or to send your own perfect red leaf, reach out at shortwavepr.org.