In 1997, a Canadian forest ecologist named Suzanne Simard injected radioactive carbon into paper birch trees and watched it flow through the soil into neighboring Douglas firs. The scientific establishment was skeptical—trees were supposed to be solitary competitors, not collaborators. But Simard's radioactive tracers told a different story: the trees were connected by an underground network of fungal threads, and they were actively trading resources.
The Internet Beneath Our Feet
The network Simard discovered wasn't new. Plants and mycorrhizal fungi have been business partners for 475 million years, long before the first dinosaurs. What was new was understanding how the partnership worked—and how sophisticated it had become.
Mycorrhizal fungi spread through soil as mycelium, microscopic threads that branch and reconnect in patterns that resemble neural networks or fiber-optic cables. A single gram of healthy soil contains up to 90 meters of these threads. Scale that up to the top ten centimeters of Earth's forest soils, and you're looking at roughly 450 quadrillion kilometers of living network—about half the width of our galaxy.
Between 80 and 90 percent of plant species plug into this system. The fungi colonize plant roots and extend far into the surrounding soil, accessing water and minerals the plant could never reach on its own. In exchange, the plant feeds the fungus sugars produced through photosynthesis. The fungi take about 30 percent of everything a tree makes. It's expensive, but the plant gets a better deal than going it alone.
More Than Simple Exchange
The relationship goes well beyond a one-to-one transaction. Individual fungal networks connect multiple plants—sometimes hundreds of them, across different species. PhD student Kevin Beiler mapped these connections using DNA analysis and found that every tree in a forest patch was linked together through shared fungal partners. The biggest, oldest trees—what Simard calls "mother trees"—had the most connections, serving as central hubs.
This creates opportunities for resource transfer between plants. When Douglas fir trees are attacked by insects, they dump carbon into the fungal network. Neighboring trees, including different species like ponderosa pine, receive this carbon and simultaneously up-regulate their defense enzymes. The attacked tree is essentially broadcasting a warning, and its neighbors are responding.
Trees also recognize their relatives through these networks. Simard's research at the University of British Columbia, later confirmed at England's University of Reading, showed that mother trees identify the root tips of their offspring and preferentially send them carbon and nutrients. In deeply shaded parts of forests where saplings receive less than two percent of full sunlight—not enough to photosynthesize adequately—these young trees survive because their parents and other adults pump sugar directly into their roots through fungal intermediaries.
The most striking example Simard found was a 400-to-500-year-old beech stump with no branches, no leaves, and no way to photosynthesize. It was still alive, its interior tissue still green with chlorophyll. Neighboring trees had kept it on life support for centuries through the mycorrhizal network.
The Fungi Aren't Passive Pipes
Describing these networks as plant communication systems misses half the story. The fungi aren't neutral infrastructure—they're active participants with their own interests and strategies.
Mycorrhizal fungi discriminate between plant partners. They exchange more resources with plants that provide more carbon, and they can move nutrients like phosphorus to areas where it's scarce and therefore commands a higher "price" in sugar. Some fungi hoard resources when market conditions are unfavorable, waiting for better terms. The network operates less like a socialist commune and more like a commodities exchange, with the fungi as shrewd traders.
Different fungal groups have evolved distinct strategies. Arbuscular mycorrhizal fungi, which partner with about 70 percent of global plant biomass including all major crops, form tree-like structures called arbuscules inside plant root cells where trading happens. Ectomycorrhizal fungi, which associate with most temperate forest trees, wrap around root tips without penetrating cells. This ectomycorrhizal lifestyle has evolved independently at least 70 times from decomposer fungi—evidence that the arrangement is highly profitable.
The fungi also provide services beyond nutrient delivery: pathogen protection, heavy metal tolerance, enhanced drought resistance. Some orchid species have become so dependent on their fungal partners that 250 of them have lost the ability to photosynthesize entirely, living as parasites on the network.
When Forests Change Hands
These underground connections become especially important during ecological transitions. As climate shifts, forests are changing composition—Douglas fir declining in some regions while ponderosa pine expands. Simard found that dying Douglas firs transfer carbon to incoming ponderosa pine seedlings and send defense signals that help the newcomers establish themselves. The network facilitates not just the survival of individual species but the transformation of entire ecosystems.
This challenges the Darwinian assumption that competition drives natural selection in forests. Competition certainly exists—trees shade each other, roots fight for space, and fungal networks can transmit pathogens as easily as nutrients. But cooperation, mediated by fungal partners pursuing their own interests, appears equally important to forest function. Trees that share resources through mycorrhizal networks may outcompete forests where individuals go it alone.
The implications extend beyond ecology. These fungal networks, formed by that ancient 475-million-year-old partnership, correspond with a 90 percent reduction in atmospheric CO2 during Earth's history. They literally changed the planet's atmosphere. Understanding how they work today—how they move carbon, support biodiversity, and respond to stress—matters for predicting how forests will handle the rapid climate change now underway.
The wood-wide web will persist, but which fungi will dominate and which plants they'll favor remains an open question. The network adapts. It always has.