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

September 28, 2026

AI Summary

5 min read

Every autumn, as chlorophyll breaks down and leaves turn yellow, a smaller subset of trees does something stranger: they manufacture brand-new red pigment, even though the leaf is about to die and fall off within days. Biologist Nicky Hughes in North Carolina, who grew up watching her father hunt for the perfect red leaf, now studies this puzzle herself. As she told NPR’s Nell Greenfieldboyce, the red isn’t hiding underneath the green the way yellow is—it’s made from scratch in the fall. If it were there all along, the leaf would look purple. So why would a tree bother spending energy to create a new color for a doomed leaf? That question has divided plant scientists for decades.

The sunscreen hypothesis

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What you'll learn

1 (00:24) **A Fall Mystery Introduced** - Emily Kwong and Nell Greenfieldboyce set up the seasonal puzzle: why do only some leaves turn red in autumn, while yellow is just exposed?
2 (02:45) **Red Pigments: A Chemical Sunscreen** - The first major hypothesis: red pigments act as photoprotection, shielding the leaf from excess light.
3 (05:56) **Protecting the Nutrient Heist** - The sunscreen idea is linked to the tree’s final task: recovering precious nitrogen from the dying leaf.
4 (07:26) **A Geographic Clue: Europe vs. the US** - A correlation between sunlight intensity and red leaf prevalence offers more support for the sunscreen hypothesis.
5 (08:13) **An Alternative Idea: Insect Deception** - Evolutionary biologist Marco Archetti proposes a completely different function: red leaves are a warning signal to insects.
6 (09:58) **Evidence for the Insect Hypothesis** - Studies show that aphids actively avoid red leaves, and they have a worse survival rate on them.
7 (11:10) **The Astonishing Lack of Answers** - The episode acknowledges the surprising mystery at the heart of a common autumn phenomenon.

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Show Notes

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 [email protected].

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