The next time you walk through a forest, pause for a moment and look at the ground. Beneath your feet, in the first few centimeters of soil, stretches a network of dizzying complexity — a tangle of fungal filaments that links tree roots together over tens of meters, sometimes over kilometers. Biologists call it the mycorrhizal network. Some, using a convenient but slightly flattering image, have renamed it “the internet of forests”.

The image is not badly chosen. But like all metaphors, it simplifies what it tries to explain. And that is where things become interesting.

A symbiosis 450 million years old

Mycorrhizae — from the Greek mykes (fungus) and rhiza (root) — are symbiotic associations between plant roots and soil fungi. This relationship dates back about 450 million years, long before the first forests appeared. It probably played a decisive role in the colonization of dry land by plants.

The principle of the symbiosis is simple: the fungus infiltrates the tree's roots and extends its filaments (the hyphae) through the soil, far beyond what the roots could reach on their own. In exchange, it receives the sugars the tree produces through photosynthesis. It is estimated that about 30% of the sugars a tree makes are transferred in this way to its fungal partners — a considerable figure. Every day, a tree gives up nearly a third of the energy it draws from the sun to maintain this invisible network.

In return, the fungi provide their hosts with water, phosphorus, nitrogen and other minerals that roots alone would struggle to find. A fair market — if the notion of a contract has any meaning between a plant and a fungus.

What Suzanne Simard discovered in the forests of British Columbia

The name most often mentioned in discussions of this subject is Suzanne Simard, a Canadian biologist and professor at the University of British Columbia. In the 1990s, she carried out pioneering experiments in the forests of the Canadian northwest that would shake up the way we thought about the life of trees.

By injecting radioactively labeled carbon into Douglas firs and neighboring birches, she was able to track the path of that carbon through the seasons. The result: carbon moved between the two species through the fungal network that connected them. In summer, when fast-growing birches produce sugars in abundance, they transfer some of them to firs growing in the shade. In autumn, as the first cold weather approaches, the movement reverses.

Simard also identified what she called “mother trees” (mother trees): the oldest and largest individuals in a forest, which are also the most connected to the mycorrhizal network. These trees would be central nodes in the circulation of resources, able to support young seedlings growing in their shade.

The controversy: how far does “communication” go?

This is where caution is needed. Because while the scientific facts about resource transfers are solid and widely documented, the interpretation sometimes goes beyond what the data can truly support.

Speaking of “communication” between trees, of “solidarity” or “conscious cooperation” is part of the semantic drift that has made this subject popular — and also weakened its scientific credibility. Several researchers have been careful to qualify the point: transfers of carbon and nutrients through mycorrhizal networks do exist, but their real importance in forest life is still debated. The degree of cooperation between species remains poorly quantified. And the idea that a tree intends to “feed” its neighbors is, at this stage, not scientifically supported.

What we know for certain: forests function as interconnected systems, not as simple collections of competing individuals. What we still do not know precisely: the exact scale of these exchanges, their functional role in ecosystem resilience, and the fine mechanisms that regulate them.

Why this still changes the way we look at forests

Even with these reservations, discoveries about mycorrhizal networks profoundly change the way we can think about a tree.

For a long time, we viewed forests as arenas of competition: each tree fighting for light, water and minerals. Industrial forestry, with its single-species plantations and clear-cutting, was founded on this vision. Cutting down large trees to make room for young ones seemed logical within that framework.

Yet if old trees are the central nodes of mycorrhizal networks and if they do indeed support young seedlings during periods of stress, then removing them abruptly is not merely a loss of timber: it is an amputation of the support system of the entire forest. Studies of forests after clear-cutting show that soil mycorrhizal diversity can take decades to recover.

The forest as an invitation to humility

There is something almost dizzying in realizing that the planet's oldest ecosystems still contain dimensions that modern biology only began to map in the 1990s. The romantic image of trees that “talk” to one another has charmed the public, sometimes at the expense of rigor. But the heart of the matter may be even more fascinating than the metaphor: organized life, the circulation of resources and collective resilience can exist without a brain, without language and without intention.

When you return to walk in the forest, that network is there. You do not see it. You will probably never see it directly. But it is working — slowly, in the dark, a few centimeters beneath your soles.