Radiolab – “Forests on Forests”

Air Date: April 24, 2026
Hosts: Lulu Miller and Latif Nasser
Guests: Robert Krulwich, Nalini Nadkarni, Karina Mifune
Producer: Annie McEwen

Episode Overview

This episode explores the mysterious and vibrant world high above the forest floor—the forest canopy—and its hidden “soil in the sky.” Building on their previous hit “From Tree to Shining Tree,” which revealed the complex life and underground networks linking trees, Radiolab turns its gaze upward to discover entire ecosystems and nutrient economies thriving in the tree canopies. The episode features ecologists Nalini Nadkarni (the “queen of the canopy”) and Karina Mifune, whose research uncovers the vital, underappreciated role of canopy soils and their influence on forest health, biodiversity, and resilience.

Key Discussion Points & Insights

1. Recap of “From Tree to Shining Tree” (01:21 – 02:31)

• Networked Roots Below:
  • Underground, trees are interlinked by vast, complex networks of roots and fungal threads (mycorrhizae), exchanging resources and information.
  • “One tree was connected to 47 other trees all around it. It was like a huge network.” – Robert Krulwich (02:10)
• Shifting Focus to the Canopy:
  • The earlier episode revealed the “underground economy” of the forest; now, the spotlight shifts from below to above.

2. Into the Forest Canopy: Nalini Nadkarni’s Pioneering Work (03:45 – 06:52)

• Childhood Wonder Turns to Science:
  • Nalini Nadkarni, known as the “queen of the canopy,” found sanctuary and curiosity in treetops as a child, inspiring her scientific career.
• Challenging Scientific Preconceptions:
  • The canopy was considered mostly empty and not worth studying. Nalini defied this, convinced there was more than met the eye:
  • “It’s not enough to just stand on the ground and look up.” – Nalini Nadkarni (05:38)
• Climbing the Giants:
  • Nalini began using modified climbing gear to access and study the canopies of old-growth temperate rainforests in Washington.

3. Discovery: Rich ‘Soil in the Sky’ (06:53 – 08:31)

• A Surprising Find:
  • Instead of barren branches, she found thick carpets of mosses and soil—sometimes up to a foot deep—perched high in the trees.
  • “You’re digging your fingers into the soil that could be the soil that’s, you know, in your backyard garden, for goodness sake.” – Nalini Nadkarni (07:24)
• Canopy Soil Life:
  • Invertebrates, earthworms, fungi, and even entire, miniature forest floors existed hundreds of feet in the air.
  • “It’s almost like she stumbled into a perfect miniature of the forest floor she had just climbed up away from.” – Lulu Miller (07:59)

4. Biodiversity Above: Ecosystems Within the Canopy (08:31 – 10:43)

• Cranes and Dirigibles:
  • Other scientists soon joined in, using cranes and even balloons to reach and document this new ecosystem.
• Unexpected Residents:
  • Deep pockets of soil in old-growth redwoods host flowers, berry bushes, mosses, lichens, and some animals found only in the canopy—like salamanders whose entire lives are spent aloft.
  • “Scientists have found these tiny aquatic creatures… a species of something called a copepod.” – Lulu Miller (09:37)
• “Forests on forests”:
  • Up to 50% of terrestrial life may live in, or depend on, canopies around the globe.
  • “That’s a weird number that you’re saying—50% of it is up in the air somewhere.” – Robert Krulwich (10:25)

5. The Nutrient Economy of Canopy Soil (10:43 – 13:40)

• Karina Mifune’s Revelation:
  • Canopy soils are “forests within forests”—not mere replicas, but critical, resource-rich hubs distinct from the ground below.
  • In the spring, when nutrients like nitrogen and phosphorus are scarce in the soil below, the canopy soils are full of them.
  • “Downstairs there’s shortage, upstairs there's abundance. When it’s crumbs down below, up in the sky… is like a Thanksgiving dinner.” – Lulu Miller (12:32)
• Competition vs. Sharing:
  • Unlike the cooperative fungal networks on the ground, the race for resources above is fierce; yet the canopies hold an overlooked abundance.

6. The Superpower: Trees Rooting in the Sky (15:38 – 18:18)

• Trees Feeding Themselves from Above:
  • Nadkarni discovered roots—sometimes as thick as her wrist—growing from branches and directly into the canopy soils, allowing trees to access these nutrient reserves.
  • “The big tree… is growing roots from its branch and snaking underneath these mats of… canopy soil.” – Lulu Miller (17:11)
• An Emergency Snack Cabinet:
  • During droughts or resource scarcity, trees “tap into their canopy soils… like a secret cabinet that has all the good snacks in it.” – Karina Mifune (18:12)
  • “The expression I’m gonna eat my hat has now got a whole new meaning.” – Robert Krulwich (18:44)

7. New Frontiers & Fractals of Life (19:04 – 20:04)

• Endless Discovery:
  • “There is just so many things to be found high above the forest floor.” – Karina Mifune (19:04)
• Trees Within Trees:
  • Sometimes entire young trees take root in canopy soil, creating “nurseries” in the sky.
  • “You mean there’s a tree growing on the tree?… In the soil on the branch.” – Robert Krulwich & Lulu Miller (19:23–19:27)
• Fractal Ecosystems:
  • Life stacks on life: “It’s not turtles all the way down, but trees all the way up.” – Lulu Miller (20:04)

Notable Quotes & Memorable Moments

• “You get this sense of being in a place that looks very simple from the forest floor, but is actually this kaleidoscope of life.”
  – Nalini Nadkarni (06:41)
• “These tree canopies that up until the mid-80s everyone thought were just pretty much empty… hold about 50% of all terrestrial life on the planet.”
  – Lulu Miller (10:21)
• “During a drought or spring growing season, when resources on the forest floor are scarce, that’s when these big trees, that’s when they can tap into their canopy soils.”
  – Karina Mifune (17:56)
• “The expression I’m gonna eat my hat has now got a whole new meaning.”
  – Robert Krulwich (18:44)

Important Timestamps

• Recap of root networks: 01:36 – 02:31
• Nalini Nadkarni’s introduction and early canopy studies: 03:45 – 06:52
• Discovery of canopy soil: 06:53 – 07:59
• Description of canopy biodiversity (including aquatic copepods): 08:31 – 09:41
• Statistic: 50% of terrestrial life in canopies: 10:21
• Karina Mifune on canopy soil’s nutrient richness: 12:18 – 12:32
• Nadkarni’s discovery of branch roots accessing canopy soil nutrients: 16:24 – 17:15
• Trees growing in trees (“nursery” canopies): 19:04 – 19:31
• “Trees all the way up” wrap-up image: 20:04

Final Imagery & Tone

Radiolab’s signature blend of playful curiosity and deep reporting brings a magical yet grounded sense of wonder to the episode. The forest canopy is revealed not just as “the up,” but as a thriving frontier—full of secrets still being uncovered, and essential not just for forest life, but for understanding the fractal exuberance of nature itself. The final image is an evergreen one: forests stacked on forests, “not turtles all the way down, but trees all the way up.”

Credits

• Reported/Produced: Annie McEwen
• Special Thanks: Kiyomi Taguchi, Michelle Ma, Nina Ernest, Michael Werner, Joe Hanson, PBS Overview
• Featured Researchers: Nalini Nadkarni, Karina Mifune

For lush visuals of canopy life and more on this episode’s science, visit the Radiolab website or the PBS Overview team video.