Dig beneath a forest and the roots are not alone. Wrapped around them, penetrating them and extending far beyond them are microscopic fungal threads called hyphae. Together those threads form mycelium, an underground biological system capable of exploring volumes of soil that a tree's roots could never efficiently reach by themselves.
In exchange for sugars produced by photosynthesis, mycorrhizal fungi can supply plants with mineral nutrients and water. A single fungal individual may associate with more than one plant, potentially linking neighboring root systems into what ecologists call a common mycorrhizal network.
It is an extraordinary relationship, and it has inspired one of modern ecology's most memorable metaphors: the “wood wide web.”
The popular version goes much further. Forests, we are told, function like social networks. Old “mother trees” feed shaded seedlings through fungal cables. Trees under attack send chemical warnings to their neighbors. The fungal network becomes a botanical internet through which trees cooperate for the good of the forest.
Some observations behind that story are real. But the most human-sounding claims remain scientifically controversial, and several prominent forest ecologists argue that the evidence has been repeatedly stretched beyond what experiments actually demonstrate.
The underground world is fascinating enough without turning it into Facebook for trees.
What a mycorrhiza actually is
The word mycorrhiza comes from Greek roots meaning fungus and root. It describes a symbiotic association between certain fungi and plant roots, a partnership that is ancient, widespread and fundamental to terrestrial ecosystems.
There are several types. Arbuscular mycorrhizal fungi penetrate root cells and form highly branched exchange structures called arbuscules. Ectomycorrhizal fungi, common partners of many forest trees such as pines, oaks, beeches and birches, typically form a sheath around fine roots and grow between root cells rather than penetrating them in the same way.
The economic logic of the partnership is relatively straightforward. Plants are exceptionally good at harvesting solar energy and converting atmospheric carbon dioxide into carbon-rich compounds. Fungal hyphae are exceptionally good at exploring soil.
A tree therefore supplies carbon compounds to its fungal partners. In return, the fungi can help acquire nutrients such as phosphorus and nitrogen, as well as water. The exchange can strongly influence plant growth, nutrient cycling, soil carbon and the structure of entire ecosystems.
This is not a fringe hypothesis. Mycorrhizal symbiosis is one of the central facts of plant ecology.
The debate begins when a fungal partner connects two plants.
Can one fungus really connect different trees?
Yes. Common mycorrhizal networks are biologically possible and have been demonstrated in experimental systems. Compatible plants can associate with the same fungal mycelium, creating a continuous fungal pathway between their root systems.
In 1997, Suzanne Simard and colleagues published an influential field experiment in Nature involving paper birch and Douglas fir seedlings. By labeling plants with different carbon isotopes, the researchers detected bidirectional movement of carbon between the two species. Douglas fir seedlings showed a net carbon gain, and the measured transfer varied with shading.
The paper did not describe trees as conscious collaborators, nor did it use “wood wide web” in its text. But the phrase appeared prominently around the publication and proved irresistible.
Later studies have continued tracing belowground carbon. A 2022 experiment involving Aleppo pine and Palestine oak saplings growing in forest soil used carbon-13 labeling and DNA techniques to identify fungal species associated with carbon moving along tree–fungus–tree pathways. The researchers found labeled carbon in neighboring trees and identified particular ectomycorrhizal fungi involved in the system.
So carbon can move belowground between plants associated with mycorrhizal fungi.
That sentence, however, is not identical to saying that an adult tree deliberately “feeds” another tree.
Carbon movement is not necessarily tree generosity
This distinction is at the center of the modern argument.
Finding an isotope that began in Tree A inside Tree B tells scientists that carbon moved. It does not automatically reveal why it moved, whether the fungus or the plants controlled the flow, whether Tree B gained a meaningful nutritional benefit, or whether the same transfer happens frequently in mature natural forests.
There are also alternative routes. Carbon compounds can move through soil, root exudates and microorganisms. Roots can interact directly. Fungal networks are difficult to manipulate without simultaneously altering soil structure and other biological processes.
Most importantly, the fungus is not an inert cable.
A mycorrhizal fungus is a living organism with its own evolutionary interests. It obtains carbon from plants and invests resources in places that improve its own survival and reproduction. Describing it as a wire through which one tree sends a gift to another can erase the most important participant in the transaction.
In 2023, forest ecologists Justine Karst, Melanie Jones and Jason Hoeksema published a critical review in Nature Ecology & Evolution examining some of the best-known claims about common mycorrhizal networks. They concluded that evidence was insufficient to support broad statements that these networks are ubiquitous in forests or that resource transfer through them generally improves seedling performance.
They also identified what they described as positive citation bias: uncertain or narrowly demonstrated findings had sometimes been cited later as if they were established facts.
The forest had acquired a compelling story faster than the evidence could keep up.
What about the famous “mother trees”?
The mother-tree hypothesis proposes that large, established trees occupy central positions in belowground networks and can support nearby seedlings, particularly their own offspring, by transferring carbon or other resources.
It is a powerful ecological image. It has influenced books, documentaries, conservation discussions and even the way many people emotionally perceive forests.
But the strongest version — mature trees preferentially sending resources to their offspring through mycorrhizal networks — has not been demonstrated by peer-reviewed field evidence to the degree often implied in popular accounts.
The 2023 critical review found no published peer-reviewed evidence establishing preferential resource transfer from mature trees to related seedlings through common mycorrhizal networks. A separate re-examination of the mother-tree hypothesis similarly argued that many claims about resource sharing exceed what current experiments can establish.
That does not mean large trees are ecologically unimportant. They profoundly influence forest microclimate, seed production, habitat, soil conditions, fungal communities and carbon storage. Nor does it mean seedlings never receive carbon originating from other plants.
It means that “an isotope moved through an ecosystem” and “a mother tree fed its child” are scientifically different statements.
Do attacked trees warn their neighbors?
Plants absolutely respond to attack. When insects feed on leaves or pathogens invade tissues, plants can alter defensive chemistry and release volatile organic compounds. Neighboring plants can sometimes detect airborne chemicals and modify their own defenses.
Researchers have also asked whether warning signals can travel underground.
Controlled experiments have produced intriguing results. In some systems, plants connected through fungal pathways show defensive responses after a neighboring plant is attacked by aphids or pathogens. Such experiments suggest that mycorrhizal connections could participate in belowground signaling under certain conditions.
But extrapolating this to mature trees “warning” one another across natural forests is difficult.
The 2023 review found no peer-reviewed evidence supporting the specific popular claim that mature trees send defence signals to their offspring through common mycorrhizal networks. Experiments that isolate fungal pathways often necessarily simplify roots, soils and microbial communities, making it difficult to know how important the same mechanism is in a real forest.
And again, language matters. A chemical change traveling through a shared biological system does not require intention. Plants can alter one another's physiology without possessing anything resembling a conscious message to send.
Why the Wood Wide Web metaphor can mislead
The internet analogy works because it makes an invisible system easy to imagine. Roots are computers, fungal hyphae are fiber-optic cables and chemicals are messages.
Biology is messier.
The fungal network eats, grows, competes, reproduces and trades. Connections can form and disappear. Different fungal species associate with different plants. Some relationships are beneficial under one environmental condition and costly under another. Fungi compete with one another, plants compete with neighboring plants, and the same organisms can simultaneously cooperate and compete over different resources.
A forest is not a single superorganism working toward a common objective.
Evolution generally favors traits that improve the reproductive success of organisms and their genes, not behavior designed to maintain an ecosystem for everyone's benefit. Mutualisms arise because each partner can gain something from the interaction. Those mutualisms can still produce consequences that benefit entire communities, but cooperation does not have to mean altruism.
Recent research reinforces how important the partnerships themselves are. Large-scale studies have linked mycorrhizal strategies to forest productivity, tree recruitment, biodiversity and carbon storage. A 2024 review of plant–arbuscular mycorrhizal fungus–bacterium interactions describes complex exchanges in which plant carbon moves downward while mineral nutrients such as phosphorus and nitrogen move through fungal and bacterial partners toward plants.
The underground ecosystem is therefore more complicated than a tree-to-tree network. It is a multi-kingdom marketplace.
The real forest is stranger than the metaphor
The scientific pushback against the Wood Wide Web story is sometimes interpreted as an argument that underground fungal networks do not exist. That would be the wrong conclusion.
Mycorrhizal fungi exist. Their hyphae can connect plants. They transform how plants acquire nutrients. Carbon can move between plants and fungi and, under documented conditions, appear in neighboring plants. These relationships influence forest ecology at enormous scales.
What remains unsettled is how often common networks directly transfer ecologically important quantities of resources from one tree to another in natural forests, who controls those transfers, whether seedlings consistently benefit and whether trees use these pathways as purposeful communication channels.
Those questions are difficult precisely because a forest floor contains so many interacting pathways at once.
Calling the system a “Wood Wide Web” is useful if it reminds us that a tree is not an isolated organism. It becomes misleading when the metaphor quietly turns fungi into cables and trees into benevolent social beings sending food and warnings to friends and family.
The better picture is less sentimental and more interesting. Beneath every step in a forest lies an active economy of roots, fungi, bacteria, nutrients, carbon and chemical signals. Resources move. Organisms bargain through biology. Partnerships form because both sides can benefit, while competition never disappears.
Trees may not “talk” the way the internet metaphor suggests. But the soil beneath them is anything but silent.
