Mycelial Networks

The primary body of a fungus is not the mushroom but the mycelium — a branching network of hyphae that grows through soil, wood, leaf litter, or any viable substrate. The mushroom is a fruiting body, a temporary reproductive structure. The organism itself is the network.

Scale

Individual mycelial networks can be enormous. The most cited example is Armillaria ostoyae in Oregon’s Blue Mountains, estimated at roughly 965 hectares (2,385 acres) and several thousand years old. Its extent was determined by sampling Armillaria isolates from across the area and testing somatic compatibility — when isolates from different locations fuse and grow together without rejection, they are considered the same genetic individual. DNA fingerprinting (using microsatellite markers) confirmed that the isolates are clonal, supporting the single-individual interpretation. Whether this organism is truly “one individual” in a biologically meaningful sense — or a network of somatically compatible clones that share resources — is debated, but its genetic uniformity across that area is well established.

Network architecture

Mycelial networks are not random tangles. Mark Fricker, Lynne Boddy, and colleagues at the University of Oxford have analyzed the topology of fungal networks using graph theory and found that foraging mycelia of cord-forming Basidiomycota (such as Phanerochaete velutina) form networks with properties resembling engineered transport systems:

• Short path lengths. The average number of hyphal connections between any two points in the network is small relative to network size, characteristic of small-world networks.
• High redundancy. Multiple alternative pathways connect major nodes, so that severing a single connection does not disconnect the network. This fault tolerance arises from anastomosis — the fusion of hyphal branches to create cross-connections.
• Preferential reinforcement. When a nutrient source is discovered, hyphae connecting the source to the rest of the network thicken and increase their transport capacity, while unproductive peripheral hyphae thin and may be withdrawn. This remodeling reallocates biomass from exploration to exploitation.

Bebber et al. (2007) compared fungal network topologies to engineered transport networks and found that mycelia achieve similar cost-efficiency trade-offs — minimizing total network length while maintaining short path lengths between nodes. These architectural properties emerge from local rules — individual hyphae respond to chemical gradients, mechanical contact, and cytoplasmic flow rates — without central coordination. The resulting network topology is an emergent property of distributed growth and remodeling.

Anastomosis and self/non-self recognition

Anastomosis — the fusion of two hyphae — is what converts a branching tree into a true network with loops and redundant pathways. Hyphal fusion involves recognition, adhesion, cell wall dissolution, membrane merger, and cytoplasmic mixing.

Fungi distinguish self from non-self through vegetative compatibility (vic/het) genes. Neurospora crassa, the best-studied model, has at least 11 het loci. When two hyphae from the same genetic individual meet, they fuse freely. Work by N. Louise Glass and André Fleissner on Neurospora revealed that approaching hyphae engage in a “ping-pong” signaling oscillation — the two tips alternate between signal-sending and signal-receiving states every 8-12 minutes, using the MAK-2 kinase and the SO protein, until they make contact and fuse.

When hyphae from genetically different individuals meet, the outcome depends on their het genotypes. If they match at all relevant loci, they fuse. If incompatible at even one het locus, the fused cells undergo programmed cell death (sometimes involving prion-like mechanisms, as with the HET-s system in Podospora anserina), forming a visible barrage zone that prevents cytoplasmic mixing. This system limits resource sharing to genetically identical or closely related networks.

Common mycorrhizal networks (CMNs)

When mycorrhizal fungi associate with multiple plant root systems simultaneously, they create common mycorrhizal networks — shared fungal connections between plants. A single ectomycorrhizal network can connect trees of different species, and arbuscular mycorrhizal networks routinely link dozens of plants.

What moves through CMNs

Carbon (as sugars and lipids), phosphorus, nitrogen, water, and signaling molecules can all move through CMNs. Documented transfers include:

• Carbon. Isotope labeling experiments (using 13C and 14C) have demonstrated that carbon moves from one tree to another through shared mycorrhizal fungi. Suzanne Simard and colleagues showed that Douglas fir and paper birch exchange carbon bidirectionally through ectomycorrhizal networks, with net flow direction shifting seasonally — birch shaded by fir canopy in summer receives carbon, while fir receives carbon from birch in spring before its needles fully emerge.
• Defense signals. Babikova et al. (2013, Ecology Letters) showed that bean plants connected by arbuscular mycorrhizal networks transmitted defense signals when one plant was infested with aphids, causing uninfested connected plants to upregulate defense chemistry.
• Water. Hydraulic redistribution through mycorrhizal hyphae can move water from moist to dry soil zones, benefiting connected plants.

The “wood wide web” debate

The popular narrative of the “wood wide web” — in which mother trees nurture seedlings and dying trees dump their carbon into the network — has faced scientific pushback. Karst et al. (2023, Nature Ecology & Evolution) conducted a meta-analysis and systematic review of CMN studies and concluded that:

1. Evidence for large, ecologically significant carbon transfers between adult trees through CMNs is weaker than widely claimed. Most documented transfers are small relative to the trees’ total carbon budgets.
2. Many studies have methodological limitations — difficulty distinguishing CMN-mediated transfer from soil diffusion, confounds from root grafting, and limited replication.
3. The evidence for CMN-mediated defense signaling is more robust but still limited in scale and species coverage.
4. The concept of “mother trees” preferentially nurturing offspring through CMNs is supported by some experiments but has not been demonstrated at the scale or with the consistency suggested by popular accounts.

The current scientific picture is that CMNs exist, can mediate nutrient and signal transfer, and are ecologically significant in some contexts — but the extent, magnitude, and specificity of these transfers varies greatly by fungal species, plant species, soil conditions, and ecosystem type. The field is moving toward more rigorous, replicated field experiments to resolve these questions.

Transport mechanisms

Nutrients and signaling molecules move through hyphae by multiple mechanisms:

• Cytoplasmic streaming — motor proteins (myosin, kinesin) drive bulk flow of cytoplasm along actin filaments and microtubules, carrying dissolved nutrients, organelles, and signaling molecules. Flow rates of 1-5 micrometers per second have been measured in individual hyphae.
• Turgor-driven bulk flow — pressure gradients between hyphal regions drive mass flow of cytoplasm through the hyphal lumen, analogous to phloem transport in plants.
• Vacuolar transport — the vacuolar tubular network provides a second long-distance transport pathway, particularly for phosphorus (stored as polyphosphate granules) and amino acids.

In septate fungi, transport between hyphal compartments occurs through septal pores, which can be opened or closed by Woronin bodies (in Ascomycota) or dolipore septa (in Basidiomycota), allowing the fungus to regulate flow and compartmentalize damaged regions.

Carbon flow through mycorrhizal networks

Hawkins et al. (2023) estimated that roughly 3.93 Gt CO2-equivalent of carbon per year flows from plants to mycorrhizal fungi globally — representing approximately 13% of annual plant photosynthetic fixation. This makes mycorrhizal fungi one of the largest terrestrial carbon sinks, though the fate of this carbon (whether it is respired back to the atmosphere, incorporated into fungal biomass, or stabilized in soil) varies by ecosystem and remains a major question in carbon cycle research.

Conservation

The Society for the Protection of Underground Networks (SPUN), founded in 2021 by Toby Kiers and Jeremy Grantham, is conducting global surveys to map mycorrhizal network distribution and diversity, with the goal of incorporating underground fungal networks into conservation planning. Their initial mapping effort, published in collaboration with researchers at ETH Zurich, used machine learning to predict global mycorrhizal fungal diversity from soil, climate, and vegetation data, identifying priority regions for conservation — particularly tropical forests and high-latitude ecosystems where mycorrhizal diversity is high but under-sampled.

Related concepts

• Fungal Symbiosis — the symbiotic associations that mycorrhizal networks establish
• Fungal Chemical Ecology — chemical signaling that mediates network behavior
• Fungal Intelligence — distributed optimization and adaptive remodeling in mycelial networks
• Mycelium — the structural basis of the network
• Hyphae — the individual filaments composing the network
• Anastomosis — hyphal fusion, the process that creates network topology
• Holobiont — multi-species assemblages connected by mycorrhizal networks
• Niche Construction — how mycelial networks reshape the environments they inhabit