In 1997, forest ecologist Suzanne Simard published a paper that would reshape how we think about forests. She and her colleagues had injected carbon isotopes into paper birch trees and tracked where they went. The carbon didn't stay put. It moved through underground fungal networks into neighboring Douglas firs, even crossing species lines. The finding suggested that trees weren't just competing individuals but members of an interconnected community, trading resources through a hidden web beneath our feet.

Twenty-nine years later, that vision of forests as cooperative networks has captured the public imagination—perhaps too well. The reality of mycelial nutrient trading is both more complex and more contentious than the popular story suggests.

The Underground Economy

The basic mechanics are real enough. Most plants—over 83% of species—form partnerships with mycorrhizal fungi, organisms that colonize their roots and extend thread-like hyphae into the surrounding soil. The deal is simple: plants provide fungi with carbon from photosynthesis, and fungi provide plants with minerals and water their roots can't easily access alone.

These fungal threads are astonishingly dense. A single gram of boreal forest soil contains up to 650 meters of fungal hyphae, creating what amounts to an auxiliary root system orders of magnitude more extensive than the plant could build itself. One fungal individual can span 90 square meters. The largest known organism on Earth is a fungus in eastern Oregon whose mycelium spreads across hundreds of square miles.

When multiple plants connect to the same fungal network—what researchers call a common mycorrhizal network, or CMN—the potential for nutrient trading emerges. Fungi can theoretically shuttle carbon, nitrogen, phosphorus, and water between plants, protecting these resources from soil microbes and chemical binding during transport.

The question isn't whether this can happen. It's whether it matters.

What the Studies Actually Show

The evidence for nutrient transfer through CMNs is solid in controlled settings. Lab experiments have demonstrated movement of isotopically labeled carbon and nutrients through fungal networks since 1969. Simard's 1997 field study confirmed it could happen in real forests.

But demonstrating that something can occur and proving it has ecological significance are different propositions. When researchers examined 28 experiments testing whether seedlings benefit from CMN connections to mature trees, the results were all over the map. Species, timing, location, and soil type all influenced outcomes. No clear pattern emerged.

More troubling, when studies allowed natural root intermingling—the actual condition in forests—only 18% showed positive CMN effects strong enough to overcome the negative effects of roots competing for the same resources. In most cases, being connected to a network didn't help seedlings enough to offset the cost of competing with established trees.

The amounts of carbon transferred are typically small compared to what plants produce through photosynthesis. This makes direct growth benefits unlikely, since most plants are nutrient-limited rather than carbon-limited. Why would a seedling struggling for nitrogen benefit from receiving extra carbon?

The Replication Crisis Reaches the Forest Floor

In 2022, researchers analyzed peer-reviewed papers about CMNs and found something disturbing: fewer than half the statements made about original field studies were accurate. The literature showed significant citation bias, with positive findings amplified and null results quietly ignored.

One of the authors involved in that landmark 1997 study, Melanie Jones, later admitted to confirmation bias. Alternative explanations for the observed carbon transfer—respiration, root exudation, turnover, redistribution by soil organisms—had been downplayed in favor of the network hypothesis.

The problem isn't that the research was fraudulent. It's that studying CMNs is extraordinarily difficult. Carbon transfer is elusive because labeling intensity is often low, and the spatial and temporal complexity of soil makes it nearly impossible to sample the right root connected to the right fungus at the exact moment of transfer. Researchers face enormous pressure to find positive results, and the cooperative forest narrative is compelling enough that ambiguous data gets interpreted favorably.

Two Types of Partnership, Two Different Stories

Part of the confusion stems from lumping different mycorrhizal types together. The two main categories—ectomycorrhizal (EcM) and arbuscular mycorrhizal (AM) fungi—function quite differently.

AM fungi colonize over 80% of plant species but form relatively small networks, with individual fungi spanning just 20 centimeters. EcM fungi colonize only 3% of species, but those species include most temperate and boreal forest trees. Every member of the pine family depends entirely on EcM partners—they're obligate mycotrophs that cannot survive without fungi.

EcM fungi also have capabilities AM fungi lack. They share ancestry with decomposer fungi and can break down organic matter directly, potentially transferring those nutrients through the network. EcM fungi dominate in mature, late-successional forests, suggesting they play a role in forest development.

The ecological significance of CMNs might be greatest for these EcM-dominated systems, particularly as forests face climate stress. Research from the Mother Tree Project, established by Simard in 2015, shows that seedlings in drier climates depend more heavily on mycorrhizal networks for establishment and survival. The benefits might not be direct carbon transfer for growth but something more subtle: carbon for osmoregulation during drought, or defensive compounds shared between plants.

Networks Without the Narrative

The mycelial networks are real. So is nutrient transfer. What remains uncertain is how much this underground trading actually shapes forest ecology compared to other processes—direct root competition, soil microbial activity, hydraulic redistribution through roots themselves.

The popular narrative of forests as cooperative superorganisms connected by "wood wide webs" has inspired people to care about forest conservation, which matters. But it's gotten ahead of the science. The networks exist, but we're still figuring out what they do, and the answer appears to be "it depends" more often than we'd like.

The mycorrhizal symbiosis itself, though, is beyond question. It helped plants colonize land 500 million years ago, and it remains essential for most terrestrial ecosystems today. Whether nutrients flow primarily within individual plant-fungus partnerships or across networked communities, the partnership itself is what matters. Trees and fungi need each other. The rest is details we're still working out.