Few scientific ideas have travelled as far or as fast as this one. Trees, it is said, are connected underground by fungal threads through which they share nutrients, support their own seedlings, and send chemical warnings to their neighbours. Forests are not collections of competitors but cooperative communities.
It has appeared in bestselling books, in novels, in documentaries and in blockbuster films. It has changed how a great many people picture a wood.
It is also, in its strong form, substantially less well supported than the popular version suggests — and the people saying so are mycorrhizal ecologists rather than sceptics of forests.
This is a story about a real phenomenon, a set of specific claims built on top of it, and what happened when somebody went back and checked the citations. Here is where the evidence actually stands.
What Is Definitely True
Start with the part nobody disputes, because it is remarkable on its own.
Most land plants form partnerships with mycorrhizal fungi. Fungal threads attach to and grow into plant roots, extending far into the soil, and the two organisms trade: the fungi supply water and mineral nutrients drawn from a volume of soil the roots could never reach, and the plant supplies sugars produced by photosynthesis.
This is standard biology, established for well over a century, and it is fundamental to how forests work. A tree without its fungal partners is a substantially poorer tree.
The next observation is also solid: fungal threads from a single fungus can be attached to more than one plant at the same time. That is what a common mycorrhizal network means — a fungal individual physically connecting two or more plants.
So the plumbing exists. The disagreement is about what flows through it, how far the networks extend, and whether any of it looks like the cooperation the popular account describes.
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The Claims That Were Built on Top
The famous version of the story goes substantially further, and it rests on three propositions.
The first is that these networks are widespread — that forests generally are underlain by extensive fungal webs linking most trees.
The second is that substantial resources move through them between trees, and that mature trees preferentially support seedlings, including their own offspring, in a way that improves seedling survival.
The third is that chemical signals pass between trees through the network, allowing one to warn another of insect attack or drought.
Together these produce the picture of the forest as a cooperative society, with older trees nurturing the young. That framing has enormous appeal, and it is the version that reached the public.
The Review That Checked
In 2023 a team led by Justine Karst at the University of Alberta, with Melanie Jones and Jason Hoeksema, published a review in Nature Ecology and Evolution examining those three pillars.
Their findings were pointed.
On prevalence: they concluded that the evidence for extensive networks across forests generally is thin. Networks have been rigorously mapped in only two of roughly 73,300 known tree species, and much of the influential work comes from a small number of forest types — notably Douglas fir forests in western Canada — which may not generalise.
On benefit to seedlings: they examined field studies and found no strong evidence of improved seedling performance. In only around 18 percent of the studies analysed did the benefits of network connection outweigh the negative effects of being connected to a larger competitor, which was insufficient to support a general conclusion.
On the citation record, the finding was the most striking of all. Among peer-reviewed papers published in 2022, fewer than half of the statements made about the original field studies could be considered accurate. A 2009 study that mapped the distribution of mycorrhizal fungi is now routinely cited as evidence of nutrient transfer between trees, despite not having investigated nutrient transfer at all.
They also noted that alternative explanations offered by the original authors were typically dropped by later papers citing them, and that the literature showed a citation bias toward positive results.
The authors described the process as resembling a game of telephone, with each retelling drifting further from what the original studies actually showed. Their warning was blunt: without better evidence, the wood wide web risks becoming a fantasy beneath our feet.
The Reply
This is a live scientific argument rather than a settled debunking, and the other side has answered.
Suzanne Simard, the forest ecologist at the University of British Columbia whose work made the idea famous, has pushed back, arguing that the critiques focus narrowly on particular claims while missing the broader picture, and she continues a long-term research programme investigating these questions.
Other researchers have argued from a different direction. Certain plants — mycoheterotrophs, which lack chlorophyll and cannot photosynthesise — survive entirely on carbon obtained through fungi connected to green plants. Their existence is difficult to explain unless resource transfer through shared fungal networks truly occurs, which some argue is overlooked evidence for the phenomenon.
And some experimental work continues to report transfer between connected plants under specific conditions.
What most researchers appear to agree on is roughly this: mycorrhizal networks exist, they can connect plants, and resources can move through them. The dispute concerns scale, prevalence, direction, magnitude, and whether any of it constitutes anything resembling intent.
The methodological difficulty is real and worth stating. Fungal threads are fine, they break when disturbed, and establishing that a particular filament connects two particular trees is extremely hard even with genetic sampling. Alternative routes for nutrients — through soil, through overlapping roots — are difficult to exclude.
What Fungi Do Regardless
Even setting the network claims aside, the fungal side of this relationship is remarkable in ways that get less attention than the sharing story.
Mycorrhizal fungi vastly extend the volume of soil a plant can draw on. Root hairs are limited by how far a plant can afford to grow them; fungal threads are far finer, far cheaper to produce, and reach into pores roots cannot enter. The effective absorbing surface of a tree is increased enormously.
They are particularly important for phosphorus, which moves through soil very slowly and is quickly depleted immediately around a root. A plant relying on roots alone exhausts the phosphorus in its immediate vicinity and then struggles; a plant with fungal partners keeps drawing from further out.
The fungi also protect. They form a physical sheath around root tips in some associations, and there is evidence they help plants tolerate drought, heavy metals and certain pathogens.
In exchange, the plant hands over a substantial share of the sugars it makes — estimates commonly run to a fifth or more of everything it photosynthesises, which is an enormous ongoing payment.
That is the relationship that is not in dispute, and it is a really extraordinary piece of biology. Most plants on land are, in effect, running a permanent joint venture with an organism from an entirely different kingdom, and have been for hundreds of millions of years.
Why the Story Spread So Well
The interesting question is not only what is true but why the strong version travelled so effectively, and the answer is not flattering to any of us.
The account replaces competition with cooperation, which many people find more appealing as a picture of nature. One ecologist quoted on the subject noted that the idea of a mutualistic, sharing fungal network appeals to a sense of nature as a harmonious web.
It also anthropomorphises, and anthropomorphic stories are memorable. Mother trees, communication, warnings, nurturing — this is narrative vocabulary, and it lodges in a way that terms like carbon flux do not.
And it arrived through books and films rather than through journals, which means the popular version was never subject to the qualification that accompanies a paper.
There is a further wrinkle the critics raise that is worth taking seriously. Overstated claims about cooperative forests can influence land management, encouraging a hands-off approach where active, evidence-based intervention might do more good against fire, pests and drought. On that view, deflating the hype is not anti-forest; it is conservation done carefully.
What to Believe on a Walk in the Woods
The reasonable position is neither the enchanted version nor a dismissive one.
Under your feet in most forests there is an enormous fungal presence, physically integrated with tree roots, without which the trees would struggle. That relationship is ancient, essential and extraordinary.
Whether the forest around you is a communicating society in which elders feed their young is not established, and the specific evidence for the most-repeated claims is thinner than almost anyone realises.
The honest summary is that a real and important biological relationship acquired a set of appealing narrative claims, those claims outran their evidence, and researchers in the field are now doing the unglamorous work of establishing which parts hold.
That is science functioning correctly rather than failing, even though the process has produced a less charming forest than the one many people had been picturing.
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