Mycorrhizal networks

A mycorrhiza (from Greek: myco = fungus, rhiza = root) is a symbiotic association between a fungus and a plant root. The fungal partner sends out hyphae — threadlike filaments only a few micrometers wide — that radiate through the soil far beyond the root zone. These hyphae have an enormous surface area relative to their volume, making them vastly more efficient than roots at absorbing mineral nutrients, especially phosphorus and nitrogen.

When hyphae from a single fungal individual connect the roots of multiple trees, the result is a Common Mycorrhizal Network (CMN). Through these networks, carbon, nutrients, water, and signaling molecules can move between connected plants. The network is not passive plumbing — the fungus is a living organism with its own metabolic demands, actively controlling what flows where.

There are two major types of mycorrhizae. Arbuscular mycorrhizae (AM) penetrate root cells to form tree-shaped exchange structures called arbuscules; they partner with roughly 80% of plant species, including most crops and grasses. Ectomycorrhizae (ECM) wrap around root tips in a dense sheath and form a network between cells; they partner with most temperate and boreal forest trees — oaks, pines, birches, beeches. The mushrooms you see on a forest floor are often the fruiting bodies of ectomycorrhizal fungi, the visible tip of a network that extends meters underground.

Biologists once classified species interactions into tidy categories: mutualism (+/+), parasitism (+/−), commensalism (+/0). But mycorrhizal associations have forced a rethinking. The same fungus-plant pair can be mutualistic under some conditions and parasitic under others. The interaction is not a fixed category but a continuum that shifts with environmental context.

The key variable is nutrient availability. In nutrient-poor soil, trees cannot acquire enough phosphorus or nitrogen on their own. Mycorrhizal fungi provide this service, and the carbon cost the tree pays is a worthwhile investment — a connected tree grows faster, survives drought better, and reproduces more successfully than an unconnected one. The relationship is genuinely mutualistic: both partners benefit.

But in nutrient-rich soil, the calculus changes. The tree can absorb adequate nutrients through its own roots. The fungus, however, still colonizes the root and still extracts carbon. Now the tree is paying for a service it does not need. Experimental studies (notably by Nancy Collins Johnson and colleagues) have shown that under high soil fertility, mycorrhizal colonization can reduce plant growth rather than enhance it. The fungus has become, in functional terms, a parasite.

This is what the slider in the simulation controls. At the left (low nutrients), connected trees thrive. At the right (high nutrients), the fungal network becomes a carbon drain, and connected trees do worse than they would alone. The same network, the same species, the same biochemistry — but the ecological outcome reverses.

In 1997, Suzanne Simard published a landmark paper in Nature showing that carbon could move between paper birch and Douglas fir seedlings through shared mycorrhizal networks. The popular press ran with it: the “Wood Wide Web,” trees “talking” to each other, “Mother Trees” nurturing their offspring through underground fungal connections. Simard’s 2021 book Finding the Mother Tree presented these ideas to a wide audience.

Then came the pushback. In 2023, Justine Karst, Melanie Jones, and Jason Hoeksema published a systematic review and meta-analysis in Nature Ecology & Evolution that examined the empirical evidence for the most widely cited CMN claims. Their conclusions were sobering. They found that the claim that CMNs increase seedling survival was supported by only a handful of studies, most with small sample sizes and confounded designs. The idea that mature “Mother Trees” preferentially support their kin seedlings through the network had almost no rigorous field evidence. Many studies that were cited as supporting CMN benefits actually showed neutral or negative effects when examined carefully. The authors identified significant citation bias — positive results were amplified, null and negative results were ignored.

This does not mean CMNs do not exist or do not matter. Carbon does move through shared fungal networks — that much is well established. What is contested is the magnitude of the transfer, its ecological significance, and especially the narrative that trees are cooperating through these networks. The fungi are not altruistic infrastructure; they are organisms pursuing their own fitness. The “Wood Wide Web” metaphor may have done as much to obscure the biology as to illuminate it.

The popular narrative casts trees as the protagonists: they “send” carbon to their neighbors, they “warn” each other of pest attacks. But there is a growing body of work suggesting we have the agency backwards. The fungus may be the active agent, and the tree may be more like a host being managed.

Mycorrhizal fungi can preferentially allocate nutrients to hosts that provide more carbon — a form of biological market dynamics first described by Toby Kiers and colleagues. The fungus is not passively shuttling resources; it is trading, directing phosphorus toward the plant that pays the best price. When experimenters shade one tree (reducing its photosynthesis and thus its carbon supply), the fungal network reduces nutrient delivery to that tree and increases delivery to the better-lit neighbor. This looks less like tree cooperation and more like fungal profit-maximizing.

A 2025 study in PNAS by Yao and colleagues used isotope-labeled carbon and phosphorus to track resource flows in real time through ectomycorrhizal networks. They found that the fungal mycelium actively controlled the direction and magnitude of resource transfer based on its own nutritional status, not the needs of the plant. The fungus allocated resources to maximize its own growth and reproduction. Trees were not sending gifts; they were being taxed.

This reframing matters. If fungi are active agents pursuing their own fitness, then the mycorrhizal network is not a cooperative commons but a marketplace — and like all markets, it can produce outcomes that benefit both parties, or outcomes where one party exploits the other. The mutualism–parasitism continuum is not just an ecological curiosity; it is the expected behavior of a system where two organisms with different interests are locked in an ongoing negotiation.

Reading the display

Each circle is a tree. The green canopy (outer ring) shows health — larger and greener means the tree is thriving; small and brown means it is stressed. The inner brown circle represents the trunk. Lines between trees are fungal hyphae forming the Common Mycorrhizal Network. Animated dots flow along the hyphae: green dots are nutrients (phosphorus, nitrogen) flowing from the fungal network to the tree; gold dots are carbon (sugars) flowing from the tree to the fungus.

What to try

Start with the nutrient slider all the way to the left. Notice that connected trees are thriving — their canopies are large and green, and plenty of green nutrient dots are flowing inward. The relationship indicator reads “Mutualistic.” Now slowly drag the slider to the right, increasing soil nutrient availability. Watch what happens: the green nutrient flow diminishes (the trees need less help), but the gold carbon flow continues or even intensifies (the fungus still demands payment). Tree health begins to decline. The hyphae thicken as the fungus extracts more carbon relative to what it provides. The relationship indicator shifts toward “Parasitic.”

What is simplified

Real mycorrhizal networks are vastly more complex than this simulation. A single tree may be colonized by dozens of fungal species simultaneously. Different fungal species have different cost-benefit ratios. Nutrient transfer rates depend on soil chemistry, moisture, season, tree age, fungal age, and competitive interactions with other soil microbes. The simulation captures the central insight — that the same biological relationship shifts along a mutualism-parasitism continuum with environmental context — but it necessarily simplifies the mechanism. Think of it as a conceptual model, not a quantitative one.