What Is a Mycorrhizal Network? Fungi Trading With Roots Underground

A mycorrhiza is an association between a fungus and a plant root, and it comes in two main forms. Arbuscular mycorrhizal fungi penetrate root cells and form branching structures inside them, and this type associates with the large majority of plant species including most crops and is the older arrangement, with fossil evidence indicating it accompanied plants onto land over four hundred million years ago. Ectomycorrhizal fungi sheath the root without entering the cells and dominate in temperate and boreal forests, associating with pines, oaks, birches and beeches, and they include most of the familiar mushroom-forming species. The exchange in both cases is straightforward: the plant supplies carbon compounds from photosynthesis, typically a substantial share of what it fixes, and the fungus supplies phosphorus above all, along with nitrogen, water and trace elements, using hyphae far finer than roots that reach into pores roots cannot enter and cover a soil volume orders of magnitude larger.

The economics of the trade favour both partners for specific reasons:

• •Phosphorus moves extremely slowly through soil and is depleted in a zone immediately around a root, so reaching beyond that zone is exactly what a plant cannot do alone
• •Fungal hyphae are a fraction of the diameter of the finest roots, so they access small pores and cost far less carbon per unit of soil explored
• •Fungi release enzymes and acids that liberate nutrients bound in organic matter and minerals, which most plants cannot do
• •The association improves drought tolerance, since hyphae maintain water uptake as soil dries
• •Colonised plants show increased resistance to some root pathogens, partly through physical occupation and partly by priming the plant's defences
• •The relationship is regulated rather than automatic, with evidence that plants preferentially supply carbon to fungal partners delivering more nutrients, and that fungi reciprocate, which looks like a biological market with sanctions against poor partners

Because one fungal individual can colonise several plants, roots of different trees are physically connected through shared fungal tissue, and experiments have shown that labelled carbon can move from one tree into another through those connections. That observation grew into a widely repeated claim that forests are cooperative networks in which mature mother trees deliberately feed seedlings and send warning signals to neighbours. The underlying measurements are real and the interpretation has been challenged forcefully. A prominent review published in 2023 by Justine Karst, Melanie Jones and Jason Hoeksema examined the evidence and found that common networks are less well demonstrated in field conditions than commonly stated, that the amount of carbon transferred is frequently small and may not benefit the recipient, that transfers may pass through soil rather than through fungal connections, and that the most cited claims had been amplified through repetition rather than through accumulating evidence. Their argument is not that networks do not exist but that the popular account outran what has been shown, and the exchange since has been a useful example of a field correcting an appealing story.

Agriculture disrupts these associations comprehensively. Ploughing breaks up hyphal networks physically, high phosphorus fertiliser removes the plant's incentive to supply carbon so colonisation falls, fumigants and some fungicides kill the fungi directly, and bare fallow periods starve them since they cannot survive without a living host. That matters because the same fertiliser that suppresses the partnership is the one causing runoff problems, so restoring the biology is a route to using less of it. Practices that support mycorrhizal fungi include reduced tillage, continuous living cover through cover crops, diverse rotations and lower phosphorus inputs. Commercial inoculant products exist and their field performance is inconsistent, since native fungi are usually already present and locally adapted. In forestry and restoration the associations are more clearly decisive, since many tree species establish poorly without the right fungal partners, which is why soil from an established woodland is sometimes transplanted to a restoration site deliberately.

Most land plants trade sugar for phosphorus, nitrogen and water with fungi in or on their roots, an association over four hundred million years old, because fungal hyphae are far finer than roots and reach soil the plant cannot. Evidence suggests both partners reward better-performing partners. The popular claim that forests form cooperative networks in which mother trees feed seedlings has been substantially challenged, with a 2023 review finding the field evidence weaker than the repetition suggests.