In 1997, a young forest ecologist named Suzanne Simard published a paper in Nature that would eventually inspire the glowing neural network in James Cameron's Avatar. She had traced radioactive carbon isotopes moving between a paper birch and a Douglas fir, proving that trees were trading resources underground through fungal threads. The magazine dubbed it the "Wood Wide Web," and a scientific finding became a cultural phenomenon.
The problem? Twenty-seven years later, scientists still can't agree on what's actually happening down there.
The Fungal Middleman
Mycorrhizal fungi wrap around or penetrate tree roots, extending threadlike filaments through soil far beyond where roots can reach. The arrangement is transactional: trees pump 30-70% of their photosynthesized carbon into these fungi, and in return, the fungi deliver phosphorus, nitrogen, and water that roots can't access alone. For some trees, fungi provide up to half their nutrient supply.
The exchange happens at a contact zone where plant and fungal cells meet. Each nutrient molecule must cross two plasma membranes—the fungus's and the plant's—governed by proton-coupled transporters and electrical gradients. It's the strongest force outside the subatomic world, and it's happening beneath every footstep in a forest.
Individual trees can connect with 100-200 different fungal species simultaneously. These fungi don't just serve one tree; their filaments branch out, touching the roots of neighboring trees, sometimes of different species. This creates a network, and networks can theoretically move things from node to node.
What Simard Found—and What She Claims
Simard's original experiment showed 6% of carbon moving between two trees. She's since expanded this into the "mother tree" hypothesis: old-growth trees act as network hubs, nurturing saplings and sharing resources with younger trees that struggle in the understory shade. When a Douglas fir gets attacked by insects, the theory goes, it sends warning signals through the fungal network, and neighboring trees ramp up their chemical defenses before the threat arrives.
A 2015 study by Song and colleagues seemed to support this. When researchers defoliated one Douglas fir, its ponderosa pine neighbors showed increased defense responses. The trees were connected by ectomycorrhizal networks, and something—carbon, chemical signals, or both—appeared to move between them.
The narrative is appealing. Forests aren't battlegrounds of competition but cooperative communities where elders support youth and neighbors warn each other of danger. Peter Wohlleben's 2015 book The Hidden Life of Trees sold millions of copies in over 40 languages. The European Parliament cited tree communication in 2022 forestry recommendations. Amy Adams and Jake Gyllenhaal bought the film rights to Simard's memoir.
The Skeptics Push Back
In 2024, ecologist Justine Karst led a team through 28 field experiments on nutrient transfer between trees. Only five suggested it might be happening. Brian Pickles, who studied under Simard, found that carbon transfer between trees amounted to less than 1%—nutritionally negligible.
The critique goes deeper than quibbling over percentages. Torgny Näsholm, a forest ecologist at Sweden's University of Agricultural Sciences, invokes Carl Sagan: "Extraordinary claims require extraordinary evidence." He points out that competition, not cooperation, has been documented in forests since 1926, when Finnish researchers watched Scandinavian pines crowd each other out. Modern studies confirm that mature trees often outcompete seedlings for resources rather than nurture them.
Pierre-Henri Gouyon, a biologist at Paris's National Museum of Natural History, calls the public embrace of the Wood Wide Web "collective hysteria." His concern isn't just scientific accuracy—it's that forest management policies based on incomplete research could backfire. If we assume forests self-regulate through fungal networks and adopt hands-off conservation strategies, we might be neglecting ecosystems that actually need active management.
What We Know for Certain
Strip away the controversy, and some facts remain solid. Mycorrhizal networks exist and function at enormous scale. They're ancient—perhaps 400 million years old, possibly enabling plants to colonize land in the first place. They move nutrients between fungi and trees efficiently and constantly.
What's contested is whether these networks facilitate meaningful resource sharing between trees or act primarily as individual supply lines that occasionally overlap. The difference matters. One scenario describes forests as integrated superorganisms. The other describes trees using the same fungal infrastructure while remaining fundamentally self-interested.
The warning signal question is similarly murky. Plants do respond to threats from neighbors—that's established. But whether those signals travel through fungal networks, through the air as volatile organic compounds, or through soil in ways we don't yet understand remains unclear. Correlation isn't mechanism.
When Snow Melts Too Early
Climate change is running an unplanned experiment on these systems. In the Rocky Mountains, a 30-year warming study showed that a 2°C temperature increase shifted grasslands to shrublands with fewer mycorrhizal fungi. Snow insulates soil, letting fungi decompose organic matter even when air temperatures drop below freezing. But early snowmelt advances fungal growth by a week while plant root growth stays on its old schedule. The mismatch means nutrients leach into streams before plants can absorb them.
Whether mycorrhizal networks share resources between trees or not, they're clearly essential to forest function. Damage them, and forests suffer in ways we're only beginning to measure.
The Uncomfortable Middle Ground
Science advances through controversy, and this one isn't resolved. Simard's Mother Tree Project continues, now incorporating Indigenous knowledge from British Columbia First Nations. Karst and other skeptics keep demanding better-controlled experiments. Both sides agree the networks exist and matter—they just can't agree on what the networks actually do.
The public wants a simple story: forests are communities, trees are generous, nature is wise. But forests might be more complicated than that—simultaneously competitive and cooperative, selfish and interconnected, depending on conditions we're still learning to measure. That's a harder story to tell, but it's probably closer to the truth growing beneath our feet.