Mycorrhizal Networks: How Fungi Link Trees Underground

What Are Mycorrhizal Fungi? Underground Partners Living in Plant Roots

Mycorrhiza: roots and fungi as one organ

When walking in a forest, we rarely imagine what is happening in the soil underfoot. Yet the tips of most trees' roots are entered by fungi (relatives of molds and mushrooms), forming a shared organ called a "mycorrhiza." The fungi that form it are mycorrhizal fungi.

Mycorrhizal fungi spread thin thread-like "hyphae" through the soil. Hyphae are even finer than root hairs and can slip between soil particles, gathering water and nutrients that roots alone cannot reach. In return, the plant gives the fungus sugars made by photosynthesis. A mycorrhiza is therefore a symbiosis in which a "nutrient-carrying fungus" and a "sugar-making plant" both benefit.

Kyoto University's Hiroaki Kadowaki, introduced by Science Portal (Japan Science and Technology Agency), likewise explains that mycorrhizal fungi receive sugars made by photosynthesis from trees and supply necessary nutrients such as nitrogen and phosphorus in return.

Two main types: ectomycorrhizal and arbuscular mycorrhizal fungi

There are several types of mycorrhizas, but two matter most for understanding forests.

Many edible mushrooms such as matsutake and shoro belong to the ectomycorrhizal group and are the visible "fruit" of the fungus. The body lies in the soil as a mass of hyphae, and the mushroom is only a part. Arbuscular mycorrhizal fungi, by contrast, grow inside roots and produce no mushrooms above ground, so they are easy to overlook.

Most plants have mycorrhizas

How many plants have mycorrhizas? A 2018 review in New Phytologist by Brundrett and Tedersoo organizes vascular plants as follows: about 72% form arbuscular mycorrhizas, about 2% form ectomycorrhizas, about 1.5% (such as the heath family) form ericoid types, about 10% (orchids) form orchid mycorrhizas, and about 8% form no mycorrhiza at all.

In other words, the image of plants living alone is the exception; most land plants live by borrowing a network of hyphae, a second "root" outside their own roots.

400 Million Years of Partnership: Mycorrhizas Have Supported Land Plants

The oldest fossils: the 400-million-year-old Rhynie chert

The history of mycorrhizas is surprisingly long. In the Rhynie chert of Scotland (a siliceous rock about 400 million years old, from the Early Devonian), structures that look like arbuscular mycorrhizas are preserved. Reports such as Taylor et al. (1995) identified "arbuscules," very similar to those of modern arbuscular mycorrhizal fungi, inside cells of fossilized plant roots.

When plants first moved from sea to land, their roots were still undeveloped. Fungi are thought to have taken over the job of drawing water and phosphorus from the soil. The view that the move onto land was itself supported by fungal symbiosis is one of the widely accepted hypotheses in plant science today.

Ectomycorrhizas arose in the Cretaceous

Brundrett and Tedersoo describe "three waves" in mycorrhizal evolution: first the establishment of the arbuscular type; second, the Cretaceous, when ectomycorrhizal, ericoid and orchid mycorrhizas newly arose; and third, the ongoing diversification and increasing complexity of roots in biodiversity-rich regions.

Ectomycorrhizas are tied to the leading trees of today's temperate and subarctic forests, such as the pine and beech families. The pine, oak and Castanopsis woods familiar from Japan's satoyama cannot be discussed without their ectomycorrhizal fungi.

The Birth of the "Wood Wide Web": The 1997 Experiment

Carbon moved back and forth between birch and fir

The idea that mycorrhizal fungi connect trees was spread widely by a field experiment by Suzanne Simard and colleagues, published in Nature in 1997. In a Canadian forest, seedlings of paper birch and Douglas-fir were planted side by side, one was given carbon dioxide containing carbon isotopes (¹³C and ¹⁴C), and researchers traced where the carbon moved.

The labeled carbon was reported to move in both directions between the two species, with a net movement toward the Douglas-fir in the second year. A third species with a different fungal type (western red cedar) served as a control to tell whether transport ran through hyphae.

The phrase "wood-wide web" on the cover of the same issue was a play on the "World Wide Web" and spread worldwide through popular books and documentaries.

Discovering the hub "mother trees"

In 2010, Beiler and colleagues in New Phytologist used genetic markers (microsatellite DNA) to identify individual fungi on Douglas-fir roots in a 30 m x 30 m plot of a British Columbia forest, Canada, and drew a "blueprint" of the fungal network. The Rhizopogon species R. vesiculosus and R. vinicolor each formed 13-14 genetic individuals (genets), and a single genet colonized up to 19 trees.

Tree size also had a strong positive relationship with the number of connections, giving a "scale-free" network in which larger old trees are linked to more trees. Young seedlings were growing within the fungal network formed by the old trees.

• The existence of "hubs": a few old trees with many connections
• Young trees were seen to "join" an existing fungal network as they grew
• However, this is evidence that trees are "connected," not that trees "deliberately help" one another

What Moves Through the Fungal Network?

Four flows: sugar, nitrogen, phosphorus and water

By tracking substances marked with isotopes, research on mycorrhizal networks has confirmed the flows below. How much moves, under what conditions, and how much it matters in a real forest differ from study to study.

Do parent trees really share with their young?

The most popular image in the wood wide web story is that "a large parent tree helps a shaded seedling by sharing sugars." This cannot, however, be said to be sufficiently demonstrated scientifically at present.

A 2023 paper by Karst and colleagues in Nature Ecology & Evolution examined three popular claims about fungal networks: (1) common mycorrhizal networks are widespread in forests, (2) seedlings grow better thanks to resources moving through the network, and (3) mature trees selectively send resources and defense signals to their offspring. They concluded that (1) and (2) have insufficient evidence because field results vary widely and have other possible explanations, and that no peer-reviewed support was found for (3). They also reported that poorly supported claims have increased over the past 25 years.

In response, in 2025 Simard and colleagues argued in Frontiers in Forests and Global Change that there were problems in how the literature was searched and papers were selected. Researchers are divided, and the scientific debate is ongoing.

Research in Japan's Forests: Ashiu Research Forest and the "Mycorrhizal Type Matching Effect"

Why do pine forests form naturally but cherry forests do not?

Research on mycorrhizal networks is advancing in Japan as well. Hiroaki Kadowaki of Kyoto University and colleagues measured seedling growth in large experiments that imitate forests and read DNA in the soil to show which microbes form the networks.

Earlier studies had shown that around parent trees with ectomycorrhizas (pine, oak, etc.), seedlings of the same species grow well (positive feedback), while around parent trees with arbuscular mycorrhizas (cherry, maple, etc.), seedlings of the same species grow poorly (negative feedback). DNA analysis also gave evidence that fungi are passed from parent trees to seedlings and shared.

This matches a familiar observation: pine forests form naturally with ease, whereas pure cherry stands rarely do.

Seedlings grow poorly when the mycorrhizal type differs

More interesting still, seedling growth drops sharply under trees with a different mycorrhizal type. Red pine seedlings planted under mountain cherry grew at a rate about 50% lower than those planted under red pine. Conversely, mountain cherry seedlings planted under red pine grew about 90% more slowly than those planted under mountain cherry.

The research team calls this the "mycorrhizal type matching effect." The idea is that seedlings thrive in soil rich in fungi of their own type and are at a disadvantage in soil of another type. Similar trends were seen in combinations such as red pine and Japanese maple, and Japanese hornbeam and mountain cherry.

Real forests are not so simple

However, when similar relationships were examined from tree density in census data (a survey of all trees above a certain size) from the Ashiu Research Forest in northern Kyoto Prefecture, they did not always match the matching effect seen in experiments. This is thought to be because many factors, such as light, water, seed dispersal by animals, pathogens and other microbes, interact in a real forest.

Kyoto University also announced in 2018 results suggesting that underground fungal networks may shape the stability and change of forests. The implication is that mycorrhizal fungi can drive not only the coexistence of many tree species but also succession, in which tree species replace one another.

Variations on Mycorrhizas: Orchids, Heaths and Pyrola

Orchid and heath mycorrhizas

Mycorrhizas are not only for forest trees. In Brundrett and Tedersoo's summary, orchids (about 10%) form orchid mycorrhizas, and the heath family and relatives (about 1.5%) form ericoid mycorrhizas. Orchid seeds are tiny and store almost no nutrients, so orchids are well known to depend on fungi from the germination stage.

Plants of the heath family are often found on poor, acidic soils and in wetlands, and ericoid mycorrhizas help them obtain nitrogen and other nutrients in such harsh environments. The type of mycorrhiza is deeply tied to the environment in which a plant lives.

Plants that do not "overdo" photosynthesis

In Pyrola (Ichiyakuso), widespread on Japanese forest floors, a study using stable carbon isotope analysis estimated that about 50% of its carbon comes from mycorrhizal fungi. Plants normally give sugars to fungi, but in this species living on the dim forest floor, it was suggested that part of its carbon is supplemented from the fungus.

The relationship between plants and fungi does not fit a one-way picture of "plant gives, fungus receives." It is thought to be a flexible trade in which the balance between giving and receiving can shift with conditions.

Mycorrhizal Fungi on a Global Scale: Carbon Cycling and Climate Change

Some of the carbon fixed by plants flows to fungi

Plants take in carbon dioxide through photosynthesis and hand part of it to fungi underground. A 2023 study by Hawkins and colleagues in Current Biology estimated how much carbon the world's mycorrhizal fungi receive for their underground mycelium. It suggests that about 13.1 billion tonnes of CO₂ equivalent per year of the carbon fixed by land plants flows, at least temporarily, to mycorrhizal mycelium, equal to about 36% of annual CO₂ emissions from fossil fuels.

Protecting carbon in the soil

Hyphae can leave carbon in the soil after they die, where it may remain for a long time as soil organic matter. Logging and conversion to farmland can cut fungal networks and cause soil carbon to be lost. As Kyoto University researchers point out, several studies show that even after trees are cut and replanted, underground microbes do not quickly return to their former state.

For climate action, attention is turning not only to the "number" of trees planted but to protecting the whole underground ecosystem, including fungi suited to that soil.

• Protecting forests means protecting the underground fungal network
• In reforestation, compatibility between the planted species and the soil's mycorrhizal type may affect success
• Development and farming that turn over soil sever underground networks

On a related note, for soil life as a whole, see also "Life in the Soil: Earthworms and Springtails Underground" to grasp how widely the underground ecosystem extends.

From Forest to Sea: Mycorrhizas Support Coastal Forests and Coastal Environments

Coastal black pine and "shoro"

Where land meets sea, coastal forests are where mycorrhizal fungi are most visibly at work. Sand is poor in water and nutrients, and exposed to sea wind and salt, yet black pine, the leading tree of Japan's coastal forests, survives by living in symbiosis with ectomycorrhizal fungi.

Research reports from Aichi and Shimane prefectures have tested seedlings inoculated with mycorrhizal fungi such as shoro (Rhizopogon) to address the low survival of black pine seedlings on coasts. Because mycorrhizal fungi are also explained to supplement water in the soil, research is advancing on them as a way to improve establishment in nutrient-poor, quickly drying sand. In a long-term survey on the Shimabara Peninsula in Nagasaki Prefecture, Scleroderma-type puffballs (kotsubutake) and Rhizopogon were inferred to be the dominant native mycorrhizal fungi, helping keep black pine healthy.

In experiments by Matsuda and colleagues at Mie University, higher sodium chloride concentrations tended to increase kotsubutake and shoro and decrease Suillus bovinus-type slippery jacks and Cenococcum geophilum. Choosing salt-tolerant fungi is thought to help coastal forest restoration.

Coastal forests are also connected to the marine environment

Coastal forests protect lives from tsunami, blowing sand and sea wind, and are also an entrance for water and nutrients flowing from land to sea. When the soil of forests and coastal forests is healthy, rainwater soaks in slowly, softening the sudden outflow of excess sediment and nutrients to the sea. For more, see "What Headwater Conservation Forests Do" and "Coastal Forest Restoration."

Tips for Reading the News: How to Take "Trees Talk"

The distance between headlines and papers

Expressions like "trees chatter underground" and "mother trees raise their young" captivate people but tend to be stronger than the actual research results. This is exactly what Karst and colleagues' 2023 paper pointed out: with each citation the story can grow a little, spreading as a stronger claim than the original study, a "citation bias."

Yet it is not accurate to take this as "the wood wide web was a lie." Results showing fungal connections and carbon movement, such as the 1997 experiment and the 2010 network analysis, have accumulated. The dispute is over how much the connections matter in a forest and whether trees can be said to "intentionally" send resources.

Three questions for judging how certain research is

1. Lab or field? Results from greenhouse seedlings do not necessarily apply to real forests.
2. "Moved" or "helped"? That a substance moved and that the receiver grew better are separate matters.
3. Are there other explanations? Check whether other routes, such as direct uptake from soil or roots, have been ruled out.

These three questions are useful when reading news about marine life or climate change, too. Simply remembering that "interesting" and "certain" are different things changes how you deal with information.

What We Can Do: Ways of Living That Protect the Underground Forest

An everyday action checklist

1. Do not compact the soil of forests and parks: staying on designated paths protects the soil around roots where hyphae gather.
2. Do not over-harvest mushrooms: a mushroom is the fungus's "fruit," but follow local rules so the mycelium in the soil is not damaged.
3. At tree-planting events, think about "where" as well as "what" to plant: choosing species that have long grown on the site is a shortcut to keeping compatibility with underground fungi.
4. Reconsider heavy use of chemical fertilizers and pesticides: excess phosphorus is said to reduce plants' need for fungi and weaken mycorrhizal function.
5. Join local coastal forest and satoyama conservation activities: they protect not only what is above ground but also the ecosystem below it.

Even without special skills, simply knowing that "there is a network of living things under the ground" changes a little how you step and how you relate to green spaces.

Observations to try at school or home

• Look for white thread-like hyphae under fallen leaves in a park or around a fallen log
• In a pine forest, gently dig the soil at the base of a mushroom and observe its position relative to pine roots (check with the site manager whether collecting is allowed)
• Compare how the plants at your feet differ under different tree species, such as pine, cherry and maple, in the same park