Trees Talk Beneath Our Feet

In 1997, a University of British Columbia ecologist named Suzanne Simard injected radioactive carbon isotopes into paper birch and Douglas fir trees growing side by side in a forest. What she discovered upended decades of botanical thinking: the carbon moved between the trees. Not through the air. Not through touching branches. Through an underground network of fungal threads connecting their roots. Trees, it turned out, were talking to each other.

The Threads That Bind

The network Simard mapped exists because of mycorrhizal fungi—organisms that form partnerships with roughly 90% of plant species on Earth. These fungi produce mycelium, microscopic filaments finer than human hair that wrap around or penetrate tree roots. A single teaspoon of forest soil can contain miles of these threads. Individually, they're invisible. Collectively, they form what German forester Peter Wohlleben later dubbed the "Wood Wide Web."

The arrangement works like this: trees produce sugars through photosynthesis but struggle to extract certain nutrients from soil. Fungi excel at mining soil for nitrogen, phosphorus, and water but can't photosynthesize. So they trade. The fungi deliver nutrients to tree roots; the trees pay in sugar. Fungi typically retain about 30% of the sugar they collect as their fee.

But the network does more than facilitate individual partnerships. Because a single fungal network can connect to multiple trees—sometimes different species—it creates a living internet through which resources flow across the forest.

Two Fungal Philosophies

Not all mycorrhizal fungi operate the same way. Arbuscular mycorrhizal (AM) fungi actually penetrate root cells, forming tree-like structures inside the plant. These dominate tropical forests and promote rapid carbon cycling—nutrients move quickly through the system. Ectomycorrhizal (EM) fungi, by contrast, wrap around roots without penetrating them, forming a sheath. They're more common in temperate and boreal forests and tend to lock carbon away in soil rather than releasing it quickly.

This distinction matters more than it might seem. According to a 2019 global study published in Nature—one that analyzed 1.2 million forest plots across 70 countries—about 60% of trees worldwide partner with EM fungi. These fungi essentially act as carbon storage vaults. When forests shift from EM-dominated to AM-dominated systems, that stored carbon can be released back into the atmosphere.

Climate change is already forcing this shift. Models predict a 10% reduction in EM fungi by 2100 if emissions continue unchecked, with warming temperatures favoring the faster-cycling AM fungi. Dr. Martin Bidartondo at Kew Gardens warns that this could flip forest soils from carbon sinks to carbon sources, accelerating the very warming that triggered the change.

The Power of Mother Trees

Within these fungal networks, not all trees are equal. The biggest, oldest trees—what researchers call "mother trees"—function as hubs. Kevin Beiler's PhD research using DNA analysis revealed that these ancient trees maintain the most fungal connections, sometimes linking to hundreds of other trees. Their deeper root systems access water that younger trees can't reach, and they distribute it through the network.

Mother trees also appear to recognize their own offspring. A University of Reading study on Douglas firs found that trees can identify the root tips of relatives and preferentially send carbon and nutrients to their kin. When a mother tree is dying—whether from age, disease, or logging—it dumps its remaining carbon into the network, a final gift to its successors.

The network even transmits warnings. When insects attack a tree, it produces defense enzymes. Neighboring trees connected through the mycorrhizal network begin producing the same enzymes before they're attacked, as if the injured tree sent out an alert. Simard documented dying Douglas firs sending distress signals not just to their own species but to ponderosa pines, a different species entirely.

The Cooperation Problem

This evidence of cooperation created a problem for ecologists. For decades, the dominant framework for understanding plant communities emphasized competition: trees fighting for light, water, and nutrients in a zero-sum game. Simard's work suggested forests operated more like cooperatives, with established trees investing in the success of younger ones, even across species lines.

The resistance to this idea was fierce. Early reviewers of Simard's research questioned whether the carbon transfers were significant enough to matter, whether they were truly intentional, whether "communication" was too strong a word for what might be passive leakage. But subsequent research kept confirming the pattern. The transfers were bidirectional, flowing toward trees in need. Shaded saplings that couldn't photosynthesize adequately received sugar from taller trees. In summer, when Douglas firs were shaded by birch leaves, carbon flowed toward the firs; in fall, when birches lost their leaves, the flow reversed.

The debate revealed something about how science works. The competition model wasn't wrong—trees do compete. But it was incomplete. Forests are both competitive and cooperative, and the fungal network enables both strategies simultaneously.

When the Network Breaks

Clear-cutting a forest doesn't just remove trees. It severs the fungal network that took decades or centuries to develop. When foresters replant, the young trees must establish new fungal partnerships from scratch, without the support system that would have sustained them in an intact forest. They grow more slowly, if they survive at all.

This knowledge is now informing reforestation efforts, including the UN's trillion-tree campaign. The 2019 global mapping project helps determine which tree species to plant where based on their mycorrhizal associations. Planting an EM-associated tree species in AM-dominated soil, or vice versa, sets it up for failure.