Forests on Forests

What It Covers

Radiolab revisits forest science by exploring tree canopies in old-growth forests, where ecologists Nalini Nadkarni and Karina Mifune discovered soil ecosystems, aerial root systems, and nutrient reserves hidden up to 100 feet above ground.

Key Questions Answered

• •Canopy soil depth: Old-growth redwood and maple trees accumulate up to three feet of soil on their branches, formed from decades of decomposing moss and leaves. This aerial soil hosts earthworms, invertebrates, fungi, and even aquatic copepod crustaceans with no known origin.
• •Aerial biodiversity estimate: Tree canopies, largely unstudied before the mid-1980s, now account for approximately 50% of all terrestrial life on Earth. Accessing them required innovation — including mountain climbing gear, cranes, elevated platforms, and French researcher Francis Saleh's dirigible balloon system.
• •Nutrient scarcity strategy: During spring growing seasons, forest floor nitrogen and phosphorus become scarce due to root grafting and mycorrhizal competition. Canopy soils hold significantly higher concentrations of both nutrients, functioning as a reserve that large trees can access when ground-level resources are depleted.
• •Aerial root adaptation: Large trees, including big-leaf maples, grow roots directly from their branches into canopy soil deposits. Ecologist Nalini Nadkarni traced wrist-thick roots back to their origin on the tree itself, confirming trees actively exploit elevated nutrient caches during resource-scarce periods.