You've probably walked through a forest and felt something—a sense of quiet connection, maybe, or just the peace of being among ancient things. Turns out, that feeling of connection isn't entirely in your head. Beneath your feet, trees are actually linked together through an underground network so vast and complex that scientists call it the "Wood Wide Web."

The Underground Internet of Trees

In 1997, Dr. Suzanne Simard published a paper in Nature that changed how we understand forests forever. She proved something that seemed almost magical: trees talk to each other.

Well, not talk exactly. They communicate through fungi.

These aren't the mushrooms you see sprouting after rain. Most of the action happens underground, where thread-like fungal filaments called hyphae weave through soil like impossibly fine spider silk. These filaments connect to tree roots and extend far beyond where any single root could reach. One tree's roots touch the fungal network, which touches another tree's roots, which connects to another, and another. Map it out and you'd see nearly every tree in a forest linked together in a living web.

The fungi involved are called mycorrhizal fungi. The name comes from Greek: myco for fungus, rhiza for root. They've been partnering with plants for about 400 million years—since plants first crawled onto land.

A Trade That Built Forests

This relationship works because each partner has what the other desperately needs.

Trees are solar-powered sugar factories. Through photosynthesis, they convert sunlight into carbohydrates. They're good at it. They produce more than they can use. Fungi, living in darkness underground, can't photosynthesize at all. They need those sugars to survive.

Meanwhile, fungi are exceptional miners. Their hyphae probe through tiny soil spaces that roots can't reach, extracting water, nitrogen, phosphorus, and other nutrients. Trees need these resources but can't access them efficiently on their own.

So they trade. Trees send carbohydrates down to the fungi. Fungi send nutrients up to the trees. Both thrive.

There are two main types of these partnerships in forests. Ectomycorrhizal fungi work with trees in temperate and boreal forests—pines, firs, oaks. They wrap around root tips like a sheath and slip between root cells. Arbuscular mycorrhizal fungi prefer tropical and subtropical trees. They actually penetrate root cells and form tiny tree-shaped structures called arbuscules.

Both types create networks. Both connect trees together.

When Trees Share Resources

Here's where it gets interesting. The fungal network doesn't just help individual trees. It becomes a distribution system for the whole forest.

Simard discovered this using radioactive carbon isotopes as tracers. She injected them into paper birch trees and tracked where they went. The carbon didn't stay put. It moved through the fungal network into Douglas fir trees growing nearby. Different species, sharing resources underground.

Later research found that up to 40% of the carbon in a tree's fine roots could come from other trees through the network.

The sharing isn't random. Older, larger trees—Simard calls them "mother trees"—act as hubs in the network. They're the most connected, with fungal links to dozens of other trees. And they actively support younger trees around them, sending carbon and nutrients to seedlings struggling in their shade.

Even more remarkable: mother trees can recognize their own offspring through the network. They show preference, sending more resources to their genetic kin than to unrelated seedlings of the same species. It's kin recognition, happening through fungal intermediaries underground.

The sharing even crosses species lines. In mixed forests, Douglas firs and paper birches exchange carbon seasonally. In summer, when birch leaves shade the firs, birch sends carbon to the struggling evergreens. In fall, when birches drop their leaves, firs return the favor. They take turns supporting each other.

Chemical Warnings and Forest Defense

Resource sharing is only part of the story. The network also carries warnings.

When a tree gets attacked by insects or infected by disease, it doesn't suffer in silence. It produces defensive chemicals to fight back. But it also sends signals through the mycorrhizal network to neighboring trees.

These warnings travel as chemical compounds through the fungal hyphae. Trees receiving the signal respond by ramping up their own defenses before the threat arrives. They produce compounds that make their leaves less palatable or toxic to insects. They strengthen their immune responses against pathogens.

Research on pine budworm infestations showed this clearly. Infested pines sent warning signals through the network. Nearby pines that hadn't been attacked yet started producing defensive compounds anyway. When the budworms reached them, they were already fortified.

Douglas firs do something similar. When researchers injured Douglas fir trees experimentally, the trees transferred carbon to neighboring ponderosa pine seedlings and increased defense enzymes in both species through the network.

Different species warning each other. Cooperation across the boundaries we usually think of as competitive.

Mapping the Wood Wide Web

Kevin Beiler, one of Simard's PhD students, decided to map these networks using DNA analysis. He wanted to see the actual structure.

What he found looked eerily like a neural network in a brain.

The biggest, oldest trees had the most connections—sometimes linked to dozens of other trees. Younger, smaller trees had fewer connections. But almost every tree was connected to the network somehow. Very few stood isolated.

Hub trees—the mother trees—acted as central nodes. Remove them and you'd fragment the network, leaving younger trees more vulnerable and less supported.

This architecture matters. It means forests aren't collections of individual trees competing for resources. They're cooperative systems where the success of one depends partly on the success of others.

What Happens When We Break the Network

Understanding mycorrhizal networks changes how we should think about logging.

Clear-cutting—removing every tree from an area—doesn't just take the trees. It destroys the underground network that took decades or centuries to develop. The fungi die without their tree partners. The soil structure collapses. When seedlings try to grow back, they're starting from scratch without the support system that would normally be there.

Old-growth forests have the most complex networks, built up over hundreds of years. They're exceptionally resilient partly because of this underground architecture. Young plantations lack it.

Simard and others now advocate for retention forestry: leaving mother trees and maintaining species diversity when harvesting timber. Keep the network intact and the forest can regenerate faster and stronger.

The Mother Tree Project, which Simard established at the University of British Columbia in 2015, works with First Nations in British Columbia to develop these regenerative practices. It's worth noting that Indigenous territories hold 80% of global biodiversity. Traditional Indigenous forest management often preserved old trees and diversity—practices that align with what the science now shows we should be doing.

Networks in a Changing Climate

Climate change threatens these networks in multiple ways.

Warming temperatures, drought, and pest outbreaks stress trees. Pine beetle infestations in western North America have killed millions of trees, disrupting networks across vast areas. Logging continues to fragment what remains.

But the networks are also surprisingly adaptive.

As climate zones shift, grassland ecosystems with arbuscular mycorrhizal fungi are moving upward into what used to be ectomycorrhizal forest zones. New fungal partnerships will form. Different species compositions will emerge.

Some research even suggests that native species being displaced by climate change may send resources and warning signals to incoming species through shared fungal partners. It's like the old residents giving the newcomers a head start—an ecological passing of the torch.

The networks will likely persist, just with different players.

What This Means for How We See Forests

Simard's work fundamentally changed forest ecology. The old view saw forests as battlegrounds where trees competed ruthlessly for light, water, and nutrients. The strongest survived. The weak died. Simple.

The reality is far more complex and, frankly, more interesting.

Forests are cooperative systems. Competition exists, certainly. But so does sharing, communication, and even something that looks like care—mother trees nurturing their young, old trees supporting seedlings, different species helping each other survive.

This doesn't mean forests are conscious or intentional in human terms. These are evolved systems, shaped by millions of years of natural selection that favored cooperation as much as competition.

But it does mean that when you walk through a forest, you're walking through a living network. Beneath every step, trees are linked together, sharing resources, sending warnings, supporting the next generation.

The Wood Wide Web isn't a metaphor. It's a physical reality, woven through the soil by billions of fungal threads, connecting the forest into something greater than the sum of its individual trees.

Next time you're among trees, remember: they're not standing alone. They never were.