In 1997, a forest ecologist named Suzanne Simard sealed paper birch and Douglas fir seedlings inside plastic chambers in the soil of British Columbia, fed one species carbon-14 dioxide and the other carbon-13 dioxide, then waited nine days and dug them up. When she and her co-authors analyzed the tissue, carbon that had started out in the air around one species turned up inside the other. A third species, western red cedar, was planted alongside as a check, since cedar doesn’t share the same type of fungal partner and shouldn’t have picked up labeled carbon through that specific route. It largely didn’t. The experiment, published in Nature that August, was a narrow, carefully controlled test of one question: can carbon cross between two tree species through fungi connected to both of their roots.

Nature’s own editors, not Simard, are the ones who attached a catchy name to the finding. The phrase “wood-wide web” appeared in the same August 1997 issue, on the cover.

What the original paper actually showed

The finding itself held up as real and interesting: carbon moved in both directions between paper birch and Douglas fir, with a net gain to the fir in the second year of the field experiment. This is a measurable, replicated-in-spirit result about a physical pathway, fungal hyphae connecting the roots of two plants, that clearly exists in at least some forests. It is not, on its own, a study of intention, communication, or care between trees. Nothing in the 1997 paper claims that.

How the story grew from there

Over the following two decades, the finding travelled a long way from the original experiment. Simard’s 2016 TED talk on how trees “talk” to each other has been viewed millions of times. Her 2021 memoir, “Finding the Mother Tree,” extended the idea further, describing older trees as recognizing and preferentially supporting their own offspring through these fungal connections. Peter Wohlleben’s bestselling “The Hidden Life of Trees” carried a similar framing to a much wider popular audience, and Richard Powers’ novel “The Overstory,” which won the Pulitzer Prize for fiction in 2019, folded related imagery of interconnected forests into literary fiction. None of this is unusual for a striking scientific finding, but the distance between the narrow 1997 result and the popular image of a forest as a caring, communicating family had grown considerable by the early 2020s.

The stakes aren’t purely academic. Forest management decisions, including how aggressively a site is logged and whether older “legacy” trees are deliberately left standing, have sometimes been justified partly by appeals to mother tree facilitation. If that specific mechanism turns out to be unsupported, the justification for those particular practices would need to rest on other grounds instead, even where the practices themselves might still be reasonable for other, better-established reasons.

A 2023 review pushed back hard

In February 2023, Justine Karst, Melanie Jones, and Jason Hoeksema published a review in Nature Ecology & Evolution examining three specific claims that had become common in both scientific and popular writing about these fungal networks, sometimes called common mycorrhizal networks or CMNs. They found that the claim that CMNs are widespread in forests was insufficiently supported, since field evidence came from a small number of forest types and often admitted alternative explanations. They found the claim that resource transfer through CMNs measurably improves seedling performance was similarly thin. And for the most specific and most widely repeated claim, that mature trees preferentially send resources and defense signals to their own offspring through these networks, they reported finding no peer-reviewed published evidence at all. Two other research teams published broadly similar critiques around the same time, raising related concerns about how confidently the field’s language had outrun its data.

The review’s title named a specific mechanism for how this happens: positive citation bias, where later papers cite earlier ones in ways that state a finding more strongly than the original data supported, and that inflation compounds as each new paper cites the last.

Simard’s side of it

Simard has disputed the review’s methodology rather than conceding the point. Speaking to CBC News shortly after the review appeared, she said she thought critics were reacting to a claim she hadn’t actually made, that mycorrhizal networks are the only pathway operating in a forest, when her own papers acknowledge other pathways exist alongside them. In a formal response published in Frontiers in Forests and Global Change in 2025, Simard and two co-authors argued that Karst and her colleagues had searched the primary literature narrowly and used subjective, unclear criteria to select which 18 studies counted toward their conclusion.

Where that leaves things

I wrote recently about how “tree” itself isn’t a strict biological category, just a shape that unrelated plant lineages have separately evolved into again and again. The mycorrhizal network story has a similar shape to it: a real, specific, well-documented finding sitting underneath a much broader popular claim that grew well past what that finding, on its own, established.

The physical structure isn’t in serious dispute. Fungal hyphae connecting the roots of different trees are real, and carbon can move across them, as the 1997 experiment showed under controlled field conditions. What remains genuinely unsettled is how common these connected networks are across forest types worldwide, how much they actually matter for a seedling’s odds of survival, and whether anything resembling deliberate, kin-directed care is happening at all. On that last, most evocative claim, the one that made it into a TED talk, a memoir, and a great many magazine features, the specific peer-reviewed evidence still isn’t there.