Recent scientific discoveries have captured the public’s imagination quite like the wood-wide web — a wispy network of fungal filaments hypothesised to shuttle nutrients and information through the soil and to help forests thrive. The idea sprouted in the late 1990s from studies showing that sugars and nutrients can flow underground between trees. In a few forests, researchers have traced fungi from the roots of one tree to those of others, suggesting that mycelial threads could be providing conduits between trees.

These findings have challenged the conventional view of forests as a mere population of trees: Trees and fungi are, in fact, coequal players on the ecological stage, scientists say. Without both, forests as we know them wouldn’t exist.

Scientists and nonscientists alike have drawn grand and sweeping conclusions from this research. They have posited that shared fungal networks are ubiquitous in forests around the world, that they help trees talk to each other and, as Coach Beard on Ted Lasso articulated, that they make forests fundamentally cooperative places, with trees and fungi united in common purpose — a dramatic departure from the usual Darwinian picture of interspecies competition.

The concept has been featured in numerous media reports, TV shows and bestselling books, including a Pulitzer Prize winner. It even shows up in Avatar, the highest-grossing movie of all time.

And the theory could be starting to influence what happens in real forests. Some scientists, for example, have suggested managing forests explicitly to protect fungal networks.

But as the wood-wide web has gained fame, it has also inspired a backlash among scientists. In a recent review of published research, Karst, Hoeksema and Melanie Jones, a biologist at the University of British Columbia, Okanagan, found little evidence that shared fungal networks help trees to communicate, swap resources or thrive. Indeed, the trio said, scientists have yet to show that these webs are widespread or ecologically significant in forests.

For some of their peers, such a reality check is long overdue. “I think this is a very timely talk,” said Kabir Peay, a mycologist at Stanford University, about a presentation Karst recently gave. He hoped it could “reorient the field.”

Others, however, maintain that the wood-wide web is on firm ground and are confident that further research will confirm many of the hypotheses proffered about fungi in forests. Colin Averill, a mycologist at ETH Zurich, said that the evidence Karst marshaled is impressive. But, he added, “the way I interpret the totality of that evidence is completely different.”

Most plant roots are colonised by mycorrhizal fungi, forming one of Earth’s most widespread symbioses. The fungi gather water and nutrients from the soil; they then swap some of these treasures with plants in exchange for sugars and other carbon-containing molecules.

David Read, a botanist then at the University of Sheffield, showed in a 1984 paper that compounds labeled with a radioactive form of carbon could flow via fungi between lab-grown plants. Years later, Suzanne Simard, then an ecologist with the British Columbia Ministry of Forests, demonstrated two-way carbon transfer in a forest between young Douglas fir and paper birch trees.

When Simard and her colleagues shaded Douglas firs to reduce how much they photosynthesised, the trees’ absorption of radioactive carbon spiked, suggesting that underground carbon flow could boost young trees’ growth in the shady understory.

Simard and colleagues published their results in 1997 in the journal Nature, which splashed it on the cover and christened the discovery the ‘wood-wide web.’ Soon after, a group of senior researchers criticised the study, saying it had methodological flaws that confounded the results. Simard responded to the critiques, and she and her colleagues designed additional studies to address them.

Over time, the criticisms faded, and the wood-wide web gained adherents. Simard’s 1997 paper has garnered almost 1,000 citations and her 2016 TED Talk, “How trees talk to each other,” has been viewed more than 5 million times.

In his book The Hidden Life of Trees, which has sold more than 2 million copies, Peter Wohlleben, a German forester, cited Simard when describing forests as social networks and mycorrhizal fungi as “fiber-optic internet cables” that help trees inform each other about dangers such as insects and drought.

Subterranean forest research has continued to grow, too. In 2016, Tamir Klein, a plant ecophysiologist then at the University of Basel and now at the Weizmann Institute of Science in Israel, extended Simard’s research into a mature Swiss forest of spruce, pine, larch and beech trees. His team tracked carbon isotopes from one tree to the roots of other nearby trees, including different species, in an experimental forest plot. The researchers attributed most of the carbon movement to mycorrhizal fungi but acknowledged they had not proven it.

Simard, who has been at the University of British Columbia since 2002, has led further studies showing that large, old “mother” trees are hubs of forest networks and can send carbon underground to younger seedlings. She has argued in favour of the view that trees communicate via mycorrhizal networks and against a long-held idea that competition between trees is the dominant force shaping forests. In her TED Talk, she called trees “super-cooperators.”