Beneath the floor of a forest lies a hidden network so fine you would need a microscope to see its threads and a map larger than the forest to trace them all. Fungi weave through the soil in filaments called hyphae, and these connect to the roots of trees, and — in many cases — link one tree to another underground. In the 1990s a memorable nickname stuck: the wood wide web. It has since become one of the most beloved ideas in popular science, and also one of the most quietly contested.
A deal struck in the dark
The foundation of the story is solid and genuinely wonderful. The vast majority of land plants form partnerships with fungi at their roots, a relationship called mycorrhiza — literally "fungus-root." It is one of the oldest and most important symbioses on Earth, dating back hundreds of millions of years, and it may have helped plants colonise land in the first place.
The deal is straightforward. The fungus threads its hyphae through the soil far more finely and extensively than roots can, and is superb at scavenging water and nutrients, especially phosphorus and nitrogen. It delivers these to the plant. In return, the plant — which can photosynthesise, as the fungus cannot — pays the fungus in sugars made from sunlight and carbon dioxide. Both partners profit. This much is textbook, and not in doubt.
Because a single fungal individual can connect to many plants at once, its network can physically link neighbouring trees below ground, forming what scientists call a common mycorrhizal network. And here is where the story took flight.
The claims — and the pushback
In a series of influential experiments beginning in the 1990s, the Canadian ecologist Suzanne Simard used radioactive and stable carbon isotopes to show that carbon could move between trees connected through these fungal networks — even between trees of different species. She and others went on to make bolder claims: that big, old "mother trees" nurse their seedlings by funnelling carbon to them, that trees warn each other of insect attack through the network, that a forest behaves almost like a single cooperative superorganism. Simard's work reached millions through a popular book and TED talk, and the idea inspired novels and films, the fungal internet becoming a symbol of hidden natural cooperation.
Then came the reckoning. A number of mycologists and forest ecologists reviewed the actual evidence and argued, pointedly, that the popular story had raced far ahead of the data. Their criticisms are worth knowing, because this is a rare, clear case of science publicly correcting a beloved narrative. Yes, carbon can move between plants underground — but it is often unclear how much travels specifically through fungal networks rather than through the soil by other routes, whether the amounts are large enough to matter to a mature tree, and in which direction it flows on balance. The evidence that mother trees deliberately favour their own kin, or that trees send warning signals to one another through fungi in the wild, is far thinner than the confident retellings imply. Some celebrated claims, the critics noted, rest on a small number of studies, sometimes done with seedlings in controlled conditions rather than mature forests.
Nor is cooperation the only possibility. Fungi are not selfless couriers working for the trees' benefit; they are organisms pursuing their own interest, and a shared network can just as easily be a route for competition, parasitism or the spread of harmful organisms as for altruistic sharing. To picture the wood wide web as a gentle socialist utopia of the trees may say more about human hopes than about what fungi actually do.
None of this unmakes the real marvel. Mycorrhizal symbiosis is ancient, near-universal and essential to life on land; forests genuinely are underpinned by an immense, mostly invisible fungal infrastructure. The honest position is the interesting one: the underground network is real and vital, but the warm story of trees consciously talking and sharing through it is, for now, more hypothesis than established fact. The forest floor keeps its secrets, and good science means admitting how many of them we have not yet dug up. That the debate has been so public, and so pointed, is itself a healthy sign: a reminder that even the most enchanting scientific stories must earn their keep against the evidence, and that a fact repeated in a hundred documentaries is not thereby made true.
Quiz nuggets
- Mycorrhiza is the ancient symbiosis between fungi and plant roots; the term means "fungus-root."
- The fungus supplies water and nutrients such as phosphorus and nitrogen; the plant repays it with sugars from photosynthesis.
- The nickname "wood wide web" describes common mycorrhizal networks linking multiple trees underground.
- Ecologist Suzanne Simard's isotope experiments from the 1990s showed carbon can move between trees, popularising "mother trees."
- Many scientists have pushed back, arguing that claims of trees "talking" or deliberately sharing through fungi outrun the current evidence.