Walk into almost any established forest and you are standing on top of a network you cannot see. Beneath the soil, thread-like fungal structures called mycelium wind between and around tree roots, forming partnerships called mycorrhizae — a word that literally means "fungus-root." These fungi extend the reach of a tree's root system by orders of magnitude, foraging for water and mineral nutrients in soil the roots themselves could never access, in exchange for sugars the tree produces through photosynthesis. This partnership is ancient, essential, and not really in dispute. What has become contested is how much further the story goes.
In the 1990s, forest ecologist Suzanne Simard published research showing that carbon labelled with a radioactive isotope, injected into one tree, could be detected later in a neighbouring tree of a different species, apparently transferred through the shared mycorrhizal network connecting their roots. The finding, and the popular science writing that followed it, gave rise to the phrase "Wood Wide Web" and the idea of forests as a single, cooperating superorganism — with old "mother trees" recognised as hubs, allocating resources to help their offspring and even warning neighbouring trees of insect attacks through chemical signals sent through the fungal network.
What the Evidence Actually Shows
The underlying mycorrhizal networks are real and well documented. Fungal threads genuinely do connect the root systems of multiple trees, including trees of different species, across a forest. Nutrient and carbon transfer between connected trees has been measured in multiple experiments. That much is settled science.
What is more contested is the interpretation layered on top of it — the idea that this transfer represents deliberate cooperation, altruism, or communication in any meaningful sense, rather than a passive by-product of fungi moving resources to wherever concentration gradients or their own reproductive interests direct them. A major 2023 review by researchers including Justine Karst, examining the field's foundational studies, found that the evidence for large-scale, forest-spanning networks that meaningfully redistribute resources between trees was considerably weaker and more equivocal than popular accounts suggested. Some of the original field studies had smaller sample sizes and less definitive methodology than the scale of the claims built on top of them.
The Fungus's Perspective
An important reframe that has emerged from the more sceptical research: the fungi in these partnerships are not simply a passive wire carrying messages between trees for the trees' benefit. The fungi have their own reproductive and survival interests, and resource movement through the network may often serve the fungus more than any particular tree. A mycorrhizal fungus connected to multiple trees may direct resources toward whichever tree is providing it the most sugar in return, which could look like "helping" a struggling tree or could equally look like abandoning it, depending on the fungus's own incentives rather than any forest-level cooperative intention.
What Is Not in Dispute
Regardless of how the more dramatic claims settle, the practical ecological importance of mycorrhizal networks is not in question. Around 90 percent of land plant species rely on mycorrhizal partnerships to access adequate nutrients and water, particularly phosphorus, which moves poorly through soil and is difficult for roots to obtain without fungal assistance. Forests with intact, undisturbed fungal networks recover faster from disturbance and support more diverse plant communities than those where the soil fungal community has been damaged by heavy tillage, chemical use, or construction.
This matters directly for conservation and forestry practice, an area where organisations such as the Royal Botanic Gardens, Kew conduct ongoing research into soil fungal networks. Logging practices that preserve some mature trees and disturb soil less thoroughly tend to leave the fungal network more intact, which measurably improves the survival odds of replanted seedlings compared to clear-cut and replant approaches that destroy the existing network.
A More Modest, Still Remarkable Story
The most defensible version of the Wood Wide Web story is less dramatic than a forest of trees deliberately caring for each other, but still genuinely remarkable: an ancient, largely invisible fungal infrastructure that most plant life on Earth depends on, moving resources through pathways shaped by a mix of fungal self-interest and plant physiology, in ways that happen to make forests more resilient as a whole. That version does not need embellishment. A soil ecosystem this consequential, discovered this recently, and still this actively debated among the scientists who study it, is remarkable enough on its own terms.