This passage, written for this guide, discusses the fungal networks that connect the trees of a forest beneath the ground.
Walk into an old forest and it is easy to read the scene as a contest. Trees crowd one another for light, throwing their crowns as wide as they can and racing upward to shade out their rivals. For a long time this was more or less the story that biologists told as well: a forest was a collection of individuals, each rooted in its own patch of soil, each fending for itself. Over the past few decades, though, research on the fungi that live among tree roots has begun to complicate that tidy picture. Beneath the leaf litter runs a web of connections so pervasive that some scientists have taken to calling it the "wood-wide web."
The threads of that web are fungi. Most trees form a partnership with certain soil fungi known as mycorrhizae, a word built from the Greek for "fungus" and "root." The fungus pushes its microscopically thin filaments, called hyphae, out through the soil and into the tree's finest roots; a single handful of forest soil can hold a startling length of these threads. There the two organisms strike a bargain. The fungal filaments reach into pockets of soil far too small for any root to enter, drawing up water and mineral nutrients—phosphorus and nitrogen above all—and passing a share to the tree. In return, the tree delivers sugars that it manufactures in its leaves by photosynthesis, sugars the fungus cannot make for itself. Neither partner could gather nearly so much alone.
Because a single fungus can be joined to more than one tree at a time, the bargain rarely stays private. The hyphae of one fungal individual may sheathe the roots of dozens of trees, knitting them into a shared network. Through such a network, materials need not simply flow from soil to tree; they can pass from tree to tree. A seedling struggling in deep shade on the forest floor, too starved of light to photosynthesize much of anything, may take in a subsidy of carbon drawn from a taller neighbor whose crown reaches the sun. The flow need not run only one way, either; over the course of a season it can reverse, moving toward whichever partner is most in need.
The clearest evidence for that kind of transfer comes from experiments with labeled carbon. Researchers—among them the Canadian ecologist Suzanne Simard, whose studies did much to bring the idea to a wide audience—have fed trees carbon dioxide built from distinctive forms, or isotopes, of carbon. When that tagged carbon later shows up in the tissues of a neighboring tree of a different species, one sharing the same fungal partners, the most economical explanation is that it traveled underground through the fungi. In Simard's well-known work in the forests of British Columbia, carbon moved between paper birch and Douglas fir in just this way.
The largest and oldest trees appear to sit near the center of these networks. Having had decades to spread, their root systems are the most widely linked, tied to many younger trees around them. Simard has called such trees "mother trees," and the phrase captures something real: a big hub tree can be connected to a great many seedlings, some of them its own offspring, and can act as a reservoir of carbon and nutrients on which the network draws. When a hub tree is felled, the seedlings it once supplied often fare noticeably worse.
The traffic is not limited to food. There is evidence that chemical warning signals travel the same routes. When a tree comes under attack—by insects, say, or by drought—compounds tied to its defenses can appear in the tissues of connected neighbors, which then behave as though bracing for the same trouble. A message sent by one tree, in other words, can leave another better prepared.
None of this makes a forest a peaceable kingdom free of competition. Trees still shade one another out, and the fungi exact a real price for their services; some researchers argue that these relationships are better read as a marketplace, full of hard bargaining, than as simple generosity. But the older image of the forest as a mere crowd of isolated competitors no longer fits what lies beneath the surface. A forest begins to look less like a collection of separate individuals and more like a community whose members are bound together—feeding, signaling, and in a sense provisioning one another through a fabric of fungi. That the wood is connected at all is by now the settled discovery; how much the trees truly "cooperate" is the argument still being worked out. Either way, the ground beneath a forest is far busier, and far more social, than it looks.
