The Underground Network: The Wood Wide WebThere’s an invisible world humming beneath our feet.
Under forests, a vast network of fungal filaments connects the roots of trees—a subterranean road system for resources.
This underground system, known as a mycorrhizal network, forms when the roots of two plants are colonized by the same fungus.
But will the fungi these trees depend on for survival be able to keep pace?
Trees supply fungi with sugars made through photosynthesis; fungi, in turn, improve trees’ access to water and minerals.
The Underground Network: The Wood Wide Web
There’s an invisible world humming beneath our feet. Under forests, a vast network of fungal filaments connects the roots of trees—a subterranean road system for resources. Isotopic labelling experiments have proven that carbon does travel from one tree to another along these fungal highways, even between species that, on paper, are fierce competitors. Some studies suggest these carbon flows ramp up when a tree is in decline.
This underground system, known as a mycorrhizal network, forms when the roots of two plants are colonized by the same fungus. The resulting mycorrhizae branch out in the earth, weaving a meshwork of hyphae—filaments that extend from the roots and sometimes connect entirely different species. Biologists have playfully dubbed this hidden life-support system the Wood Wide Web. The scale is dizzying: several hundred species of fungi can interact with a single tree, and a single fungus might connect twenty trees or more.
Mutual Exchanges, Not Altruism
Climate change is moving the goalposts for these relationships. As the planet warms, biodiversity is on the move—not just animals, but plants and entire forests. But will the fungi these trees depend on for survival be able to keep pace? Scientists are raising the alarm. As mycologist Marc-André Selosse of the Natural History Museum puts it, this relationship is about barter, not generosity. Trees supply fungi with sugars made through photosynthesis; fungi, in turn, improve trees’ access to water and minerals. Each partner gets something it desperately needs—this isn’t charity, it’s symbiosis.
The Canadian researcher Suzanne Simard put all this on the scientific map back in 1997, publishing in Nature a landmark study documenting carbon transfer between trees via mycorrhizal networks under natural conditions. Her work shifted the focus of forest biology, which had until then zeroed in on competition for light and water, toward the possibility of cooperation.
In the 1990s, isotope tracing experiments using enriched carbon dioxide on young birch and Douglas fir trees colonized by the same ectomycorrhizal fungus allowed researchers to quantify these exchanges. The measurements revealed two-way sharing, but with a net flow towards Douglas fir. The carbon received amounted to 10–25% of the fir tree’s photosynthetic output. In a similar experiment in Switzerland’s Jura forests, about 4% of the carbon compounds made by photosynthesis in one tree were transported to neighbors connected to the same ectomycorrhizal network.
Simard also observed a seasonal see-saw: in summer, sun-drenched birches gave carbon to shade-dwelling firs; in autumn, after the birches dropped their leaves, the flow reversed.
The Fate and Role of “Mother Trees”
Within this underground web, mature trees that are strongly connected—sometimes called ‘mother trees’—hold a central spot. An old oak or Douglas fir can be tied into the network with hundreds of neighbors. Simard’s research at the University of British Columbia found that carbon transfers intensify during a large tree’s final days, with a share of its reserves ending up in neighboring trees.
But the picture isn’t as simple as a selfless bequest. Several mechanisms have been proposed to explain these transfers, none of which involve intention on the part of the tree. The first explanation is physical: carbon moves much more efficiently through the fungal mycelium than it does via the surrounding raw soil, where it would otherwise be gobbled up and transformed by microbes. As a tree nears the end of its life, it gradually stops mobilizing its own reserves for growth. As its internal carbon sinks collapse, unused carbon follows the network’s gradients to still-active neighbors.
A 2025 study in Plant Diversity adds another layer of complexity. When researchers artificially reduced the carbon available to a loblolly pine (Pinus taeda), they saw a drop in root physiological activity, accompanied by a 110% increase in mycorrhizal colonization and a 340% increase in extramatrical hyphal length. In other words, when the tree struggled, its fungal partners ramped up their game. The upshot? This could suggest a fungal drive for survival more than any plant altruism. The fungus, keen to safeguard its own sugar supply from still-living trees, could be the middleman behind the so-called “final gift.” The practical effect on the ecosystem is the same, but the motive is strictly practical, not sentimental.
Scientific Debates and Practical Implications
The idea of trees selflessly supporting their kin is charming—but is it the full story? In 2023, Justine Karst, Melanie Jones, and Jason Hoeksema published an analysis in Nature Ecology & Evolution concluding that the real-world data is far thinner than public stories suggest, and that preliminary or heavily qualified results have often been cited as gospel truth. That same year, Nils Henriksson’s team cautioned in New Phytologist that isotope tracing methods, while fascinating, can sometimes lead to bold conclusions from tiny variations.
There is no disagreement about the existence of mycorrhizae and their critical role in protecting and nourishing roots, nor about carbon circulation between plants. The debate boils down to the scale of these flows, the precise function of the network, and the real benefit for young trees.
Still, these underground mechanisms help explain an old puzzle: seedlings often survive in deep forest shade, where photosynthesis alone ought to doom them. Something more is at work—a hidden helping hand from below.
Deforestation, intensive farming, and the widespread use of chemicals disrupt or destroy these networks. Clear-cutting not only removes trees—it also dismantles the very infrastructure that supports resource sharing.
A recent global map shines light on this immense hidden living network, revealing the dizzying scale of mycorrhizal fungi—plant allies that move billions of tons of carbon each year and could play a key role in climate regulation.
Forests house about 80% of the carbon stored in terrestrial ecosystems, and that stock goes well beyond the visible biomass. In 2025, an international team writing in Trends in Ecology & Evolution suggested a new framework for forest restoration: instead of judging a forest by its above-ground heft, we should pay close attention to mycorrhizal partnerships.
These plant-fungal alliances promote the formation of stable organic matter deep in the soil, allowing for long-term carbon storage and improved water retention. What happens below the roots might matter just as much as what grows above, especially in helping forests tolerate droughts and climate disruptions.
