When Justine Karst's son told her that trees could communicate through underground networks, she worried that things had gone too far.

When watching an episode of "Ted Lasso" in which a soccer coach told another that trees in a forest cooperated rather than competing for resources, her colleague had the same feeling.

Few recent scientific discoveries have captured the public's imagination quite like the wood-wide web, which is a network of fungus that helps forests thrive. Studies in the late 1990s showed that sugars can flow underground. In a few forests, researchers have found that mycelial threads could be connecting trees.

The conventional view of forests is that they are just a population of trees. Forests wouldn't exist without both.

The conclusions drawn from this research have been drawn by both scientists and non scientists. They have said that forests are made fundamentally cooperative places, with trees and fungi united in common purpose, and that they help trees talk to each other. The concept has been featured in a number of media reports, TV shows and best-selling books. The highest grossing movie of all time has it.

The theory could be influencing what happens in the forest. Some scientists think that forests should be managed to protect the networks of fungi.

Justine Karst, a mycologist at the University of Alberta, during a visit to Bunchberry Meadows near Edmonton. She was worried when her son came from 8th grade and told her trees talk underground.Credit...Todd Korol for The New York Times

The wood-wide web has caused a backlash among scientists. In a recent review, Dr. Karst, Dr. Hoeksema and Melanie Jones found no evidence that trees can communicate, swap resources or thrive. Scientists haven't shown that the webs are widespread or important in the forest.

Some of their peers have been waiting for a reality check. A presentation Dr. Karst recently gave is very timely according to a mycologist at the university. He wanted it to re-orient the field.

Others maintain that the wood-wide web is on firm ground and are confident that further research will confirm many of the hypotheses proffered. The evidence Dr. Karst marshaled is impressive according to Colin Averill. He said that the way he interprets the evidence is different.

Mycorrhizal fungi form one of Earth's most widespread symbioses. The fungi swap some of their treasures with plants in exchange for sugars and other carbon-containing molecule.

David Read showed in a 1984 paper that compounds labeled with a radioactive form of carbon can be used to grow plants. Suzanne Simard was an ecologist with the British Columbia Ministry of Forests when she demonstrated two-way carbon transfer in a forest. The absorption of radioactive carbon spiked when Dr. Simard and her colleagues shadedDouglas firs to reduce how much they photosynthesized.

The discovery of the "wood-wide web" was published in 1997 in the journal Nature. The study had methodological flaws that made it hard to understand the results. She and her colleagues designed more studies to address the criticisms.

The wood-wide web gained followers over time. Dr. Simard's 1997 paper has received almost 1,000 citations and her 2016 TED Talk "How trees talk to each other" has been viewed more than five million times.

The book "The Hidden Life of Trees" has sold more than 2 million copies and was written by a German forester.

Scenes from Rowan Oak in Oxford, Miss. Advocates of the wood-wide web theory believe evidence will mount in its favor. “If you ask me if in the future, we will be showing that trees actually can communicate,” one said.Credit...Robert Wayne Lewis for The New York Times

There has been continued growth in forest research. The Weizmann Institute of Science in Israel extended Dr. Simard's research into a mature Swiss forest of pine, larch, and beech trees. In an experimental forest plot, his team tracked carbon isotope from one tree to the roots of other trees. Most of the carbon movement was attributed to mycorrhizal fungi, but they didn't prove it.

Dr. 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. She supports the idea that trees communicate via mycorrhizal networks and against the idea that competition between trees is the main force shaping forests. She referred to trees as "super-cooperators" in her talk.

The popularity of the wood-wide web has led to a skeptical reaction. Dr. Simard and others exaggerated the degree of cooperation among trees in forests according to an argument made by an ecologist last year. According to most experts, groups of organisms whose members sacrifice their own interests on behalf of the community rarely evolve.

She believes that fungi distribute carbon according to their own interests. She said in an interview that it seemed like the easiest explanation.

Dr. Jones was a co-author of a paper in 1997 by Suzanne Simard that started the idea of the wood-wide web. Credit...Jennilee Marigomen for The New York Times

Some people who used to promote the idea of shared networks are rethinking it. Dr. Jones regrets that she and her colleagues wrote in the paper that they had evidence for tree-to- tree correlations. They didn't look at whether the carbon flows were caused by fungi.

The literature review was done by Dr. Karst, Dr. Hoeksema and Dr. Jones Field studies in forests were the focus of the research.

The wood-wide web hypothesis could have other explanations according to Dr. Karst in an August presentation. Scientists assumed that if a particular fungus was found on multiple tree roots or resources were moved between trees, it must be linked. Resources can travel part of the way through the soil.

Some experimenters, including Dr. Karst and her colleagues, have installed fine meshes and added trenches or air gaps between the plants to disrupt the growth of the fungus. Dr. Karst said that those tactics reduce how much soil a seedling can directly gather, or they alter the mix of fungi growing inside the meshes, making it difficult to determine the effects of a network.

There are a growing number of statements in the scientific literature that say that trees are helped by a network of fungi. Dr. Karst and colleagues found that the original work that was left out of the newer studies still provided proof of networks in forests.

In her presentation, Dr. Karst said that scientists have become a breeding ground for false claims. Changes in how forests are managed have been called for by several recent papers.

Jason Hoeksema of the University of Mississippi said a reference to the wood-wide web on "Ted Lasso" motivated him to join a challenge to the idea. He says studies don't prove trees benefit from fungal networks.Credit...Robert Wayne Lewis for The New York Times

According to Dr. Karst, it is highly likely that there are shared fungal networks in the forest. In a 2012 study, Dr. Simard and his team found the same genetic material on the Douglas fir trees. The researchers found the same repeating DNA segments in the soil between the trees as they did in the root zone. The study did not look at what resources were flowing through the network.

Dr. Karst and her colleagues say common claims about those networks don't hold up In many studies, the networks appeared to have no effect at all. Dr. Hoeksema said that no one has shown that the distribution of resources among trees increases their fitness. The wood-wide web has been described as benefiting trees almost all the time.

Time will vindicate the wood-wide web according to others.

The title of Dr. Karst's presentation suggests that the concept of the wood-wide web is flawed. He hopes scientists will look for networks in more forests to find more clues. Some of the most compelling evidence for the wood-wide web has been created by members of Dr. Karst's team.

He said that there are trees in some forests that are connected by fungi.

A 2020 follow-up study of the same Swiss forest and a lab study using forest soil is what Dr. Klein's team has placed its speculation about a network on. According to Dr. Karst and her colleagues, the studies didn't really map the networks in the forest.

Dr. Klein is more hopeful than the Karst team that some of the bolder claims will be born out.

He said that he would not be surprised if in the future we showed that trees can communicate.

Dr. Simard, the University of British Columbia scientist who has studied the wood-wide web, says that mapping fungal networks in forests is challenging, but other methods convinced her they are common.Credit...Jennilee Marigomen for The New York Times

Dr. Simard said that few real-world fungal networks have been mapped because of the difficulty in doing such studies. Kevin was the graduate student who led the field work for the study. It takes very long.

She said that studies published on other forests using other methods convinced her that shared fungal networks are not uncommon.

The field of mycorrhizal networks has been plagued by having to redo experiments. You have to go to the next step.

Dr. Simard said that field studies of the type Dr. Hoeksema wants would be difficult to complete. She said that none of the studies can do everything at once. You have to make sense of it.

Dr. Simard said she wasn't aware of any forest being managed solely on the basis of the findings she had made. The man was not Mr. Wohlleben.

An independent mycologist whose book "Entangled Life" was referenced in the "Ted Lasso" episode said the new critique is the latest flare-up in a decades-old debate. The invisible underground realm has long been difficult for scientists to understand.

A paper has been submitted to a peer-reviewed journal since Dr. Karst gave her talk. Even if it turns out that trees aren't whispering secrets to each other, there's still plenty of intrigue.

Dr. Karst said that the true story is very interesting. The forest is still a mystery.