Overview of Forests on Forests
This Radiolab episode expands the story of forests beyond the underground “wood wide web” and into the canopy. Instead of focusing on what’s beneath the forest floor, the episode explores the hidden ecosystem living high above it: soil, roots, fungi, insects, birds, and even small aquatic creatures thriving in the treetops. The central idea is that forests are not just layered downward into the earth — they are also layered upward into the sky.
Key Discoveries
Forest canopies are living ecosystems
Ecologist Nalini Nadkarni, known as the “queen of the canopy,” helped pioneer the study of treetop life. What scientists once assumed was mostly empty space turned out to be a dense, active habitat full of life and complexity.
There is actual soil in the trees
In temperate rainforests and redwoods, thick mats of moss, lichens, and decomposing plant matter form canopy soil on branches and limbs. In some cases, this soil can be up to a foot or more deep.
Life in the canopy is surprisingly rich
Scientists found:
- Earthworms and invertebrates living in canopy soil
- Salamanders hundreds of feet above the ground
- Copepods, tiny aquatic-like creatures, in redwood canopies
- Even new trees and seedlings growing in the treetops
The episode emphasizes that canopy life may account for about 50% of all terrestrial life on Earth.
How Trees Use the Canopy
Trees grow roots upward, not just downward
Ecologist Karina Mifune discovered that some trees send roots into canopy soil. These roots can branch into the soil high above the ground and absorb nutrients there.
The canopy becomes a nutrient reserve
During the spring growing season, the forest floor can be short on key nutrients like nitrogen and phosphorus. But canopy soils can be comparatively rich in those same nutrients, giving trees access to a kind of hidden “snack cabinet” above the competition below.
A “forest in a forest in a forest”
The episode’s most memorable idea is that the forest canopy is not just a copy of the ground-level forest. It’s a distinct and productive world of its own — with layers of moss, soil, roots, fungi, and even miniature nurseries for young trees.
Bigger Themes
Forests are more three-dimensional than we imagine
The episode challenges the idea that forests are mainly a flat landscape rooted in the ground. Instead, they are vertical ecosystems with complex interactions at every height.
Competition and adaptation
The canopy offers another strategy for survival. When resources are scarce on the ground, mature trees can tap into treetop soil, showing how forests adapt by exploiting multiple layers of habitat.
Scientific discovery depends on changing where we look
A major takeaway is that many ecological mysteries remained hidden simply because scientists weren’t looking in the right place. Once researchers climbed into the canopy, they found a whole new world.
Notable People
- Nalini Nadkarni — ecologist and canopy pioneer
- Karina Mifune — researcher on canopy soils and treetop nutrient cycling
- Annie McKeown — producer of the episode
- Robert Krulwich — Radiolab co-host, returning to frame the story
Final Takeaway
The episode’s closing image is that forests are not just “turtles all the way down” — they are trees all the way up. Beneath the forest floor is a hidden network, but above it is another one: a living, layered, nutrient-rich world that turns the treetops into a second forest.