Full Practice — Academic Passage

A walk through a mature forest gives the impression of a slow, silent competition. Each tree pushes its crown a little higher than its neighbour's, racing for light, and the losers are shaded out. For most of the twentieth century that was how forest ecologists described what they saw. The picture below the soil, it turns out, is far less lonely. Beneath a single footstep can lie hundreds of kilometres of fungal thread, knitting the roots of different trees — and often different species — into a shared network. Researchers have taken to calling it the "wood wide web".

The threads belong to mycorrhizal fungi, which wrap around or grow inside tree roots. The relationship is a trade. The fungus cannot photosynthesise, so it draws sugars from the tree; in return, its vast, hair-thin filaments reach into pockets of soil that roots are too coarse to enter, harvesting water and mineral nutrients such as phosphorus and nitrogen and passing them back. A teaspoon of forest soil may hold several kilometres of these filaments. Because one fungus can connect to many trees at once, and one tree to many fungi, the result is not a set of private partnerships but a genuine web.

That much has been accepted for decades. The bolder claim — that trees use the network to move resources to one another — is more recent, and it is largely associated with the Canadian ecologist Suzanne Simard. In a series of field experiments beginning in the 1990s, Simard fed paired seedlings carbon dioxide made with distinguishable carbon isotopes, one heavier than the other, and then tracked where the carbon ended up. She found that carbon fixed by one tree could later be detected in a neighbour connected through the same fungal network. Crucially, the flow was not random: more carbon moved toward seedlings growing in shade, which needed it most, than toward those in full sun.

Simard went further, describing large, old trees as "mother trees" or hub trees — nodes with unusually many connections that appear to channel carbon and nitrogen toward nearby seedlings, including their own offspring. In one striking result, a dying tree was recorded shunting a pulse of its stored carbon into the network shortly before it expired, as though emptying its account before closing it. Other work has suggested that a tree under insect attack can release chemical warning signals that travel through fungal connections, prompting neighbours to raise their own chemical defences before the pest arrives.

The findings caught the public imagination, and the language did too. Talk of trees that "nurture" their young or "warn" their neighbours makes for good headlines. It also makes many scientists uneasy. A 2023 review by a group of forest researchers argued that popular accounts had raced ahead of the evidence. Yes, they agreed, mycorrhizal networks exist and carbon can move through soil — but proving that a mother tree deliberately favours its kin, in a real forest rather than a controlled pot, is much harder. Carbon detected in a neighbour might have travelled through the fungus for the fungus's own purposes, or leaked into the soil and been taken up separately, rather than being a gift from one tree to another. The critics were careful to say the phenomenon is real; their complaint was about how confidently its purpose has been described.

Distinguishing between these explanations matters, because the network has become an argument in how forests should be managed. If old hub trees really do sustain the seedlings around them, then clear-cutting a stand and leaving a few scattered survivors may sever connections that a young forest depends on to establish itself. Some foresters now argue for retaining clusters of mature trees precisely to keep the underground network intact, and a handful of forestry companies in British Columbia have begun trialling exactly this approach on working land. That is a reasonable precaution even under the cautious reading of the evidence, since mature trees plainly benefit a forest in other ways — stabilising soil, storing carbon and sheltering wildlife. The counter-argument is cost: leaving valuable timber standing is money left uncut, and a firm needs a solid reason to accept the loss.

What is not in dispute is that the tidy image of trees as lone competitors was too simple. Whether the fungi are generous messengers or self-interested traders running their own economy, the forest floor is a marketplace of astonishing reach, and the trees above it are quietly plugged into it. The debate now is not whether the wood wide web exists, but how much of the story we have been telling about it the evidence can actually bear.