Do trees talk underground? Scientists differ

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Justine Karst, a mycologist at the University of Alberta (Canada), feared things had gone too far when her eighth-grade son came home and told her he had learned that trees could communicate through underground networks.

His colleague Jason Hoeksema of the University of Mississippi (USA) had a similar feeling when he watched an episode of “Ted Lasso”, in which one football coach tells another that the trees in a forest cooperated, rather than competed, by resources.

Few recent scientific discoveries have captured the public imagination like the network of trees — a delicate web of fungal filaments that is hypothesized to transport nutrients and information through the soil and help forests thrive.

The idea emerged in the late 1990s from studies showing that sugars and nutrients can flow underground between trees.

In some forests, researchers have traced fungi from the roots of one tree to the roots of another, suggesting that strands of mycelium may be providing conduit channels between the trees.

These findings challenged the conventional view of forests as a mere population of trees: trees and fungi are, in fact, equivalent actors in the ecological landscape, scientists say. Without both, forests as we know them would not exist.

Scientists and non-scientists alike have drawn great and far-reaching conclusions from this research. They postulated that shared networks of fungi are ubiquitous in forests around the world, that they help trees “talk” to each other and, as Coach Beard articulated in “Ted Lasso”, make forests fundamentally cooperative places, with trees and fungi united in common purpose—a drastic departure from the usual Darwinian image of competition between species.

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

And the theory may 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 “great tree network” gained fame, it also inspired a reaction among scientists.

In a recent review of published research, Karst, Hoeksema and Melanie Jones, a biologist at the University of British Columbia at Okanagan (Canada), found little evidence that shared fungal networks help trees communicate, exchange 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, this reality check is long overdue. “I think this is a very timely conversation,” Kabir Peay, a mycologist at Stanford University, said of a presentation Karst gave recently. He hoped he could “reorient the field”.

Others, however, maintain that the tree web is on solid ground and are confident that further research will confirm many of the hypotheses put forward about fungi in forests. Colin Averill, a mycologist at ETH Zurich (Switzerland), said the evidence Karst has gathered is impressive. But, he added, “the way I interpret the totality of this evidence is completely different.”

Most plant roots are colonized by mycorrhizal fungi, forming one of the most widespread symbioses on Earth. Fungi collect water and nutrients from the soil; they then pass on some of these treasures to plants in exchange for sugars and other carbon-containing molecules.

David Read, a botanist at the University of Sheffield (UK), showed in a 1984 paper that compounds labeled with a radioactive form of carbon could flow through fungi between plants grown in the laboratory.

Years later, Suzanne Simard, then an ecologist at the British Columbia Ministry of Forestry, demonstrated bidirectional carbon transfer in a forest between young Douglas firs and paper birches.

When Simard and his colleagues shaded the spruces to reduce their ability to photosynthesise, the uptake of radioactive carbon by the trees increased, suggesting that the underground flow of carbon could support the growth of young trees in the shady understory.

Simard and his colleagues published their results in 1997 in the journal Nature, which put the work on the cover and dubbed the discovery the “wood wide web”. Soon after, a group of senior researchers criticized the study, saying there were methodological flaws that confounded the results. Simard responded to the criticisms, and she and her colleagues designed additional studies to address them.

Over time, the criticism dissipated and the tree network gained adherents. Simard’s 1997 paper received nearly 1,000 citations and his 2016 TED Talk, “How Trees Talk to Each Other,” has been viewed over 5 million times.

In his book “The Hidden Life of Trees” [A vida oculta das árvores]which has sold more than 2 million copies, Peter Wohlleben, a German forestry engineer, quoted Simard when describing forests as social networks and mycorrhizal fungi as “fiber-optic internet cables” that help trees inform each other about hazards such as insects and drought.

Underground research in forests has also continued to grow. In 2016, Tamir Klein, a plant ecophysiologist at the University of Basel (Switzerland) and now at the Weizmann Institute of Science in Israel, extended Simard’s research to a mature Swiss forest of spruce, pine, larch and beech. Her 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 movement of carbon to mycorrhizal fungi, but acknowledged that they did not prove it.

Simard, who has been at the University of British Columbia since 2002, has led other studies showing that large, ancient “mother” trees are centers of forest networks and can send carbon underground to younger plants. She championed the idea that trees communicate through mycorrhizal networks and challenged an old idea that competition between trees is the dominant force shaping forests. In her TED talk, she called trees “super cooperative.”

But as the Wood Wide Web’s popularity grew, both inside and outside scientific circles, a skeptical reaction evolved. Last year, Kathryn Flinn, an ecologist at Baldwin Wallace University in Ohio, argued in Scientific American that Simard and others exaggerated the degree of cooperation between trees in forests. Most experts, Flinn wrote, believe that groups of organisms whose members sacrifice their own interests for the sake of the community rarely evolve as a result of the powerful force of natural selection among competing individuals.

Instead, she suspects that fungi are likely to distribute carbon in their own interests, not the trees’. “That seems to me the simplest explanation,” she said in an interview.

Even some who once promoted the idea of ​​shared fungal networks are rethinking the hypothesis. Jones, one of Simard’s 1997 co-authors, says she regrets that she and her colleagues wrote in the paper that they had evidence of fungal connections between the trees. In fact, says Jones, they did not examine whether fungi mediated carbon fluxes.

For their recent review of the literature, Karst, Hoeksema and Jones pooled all the studies they found with claims about the structure or function of such subterranean fungal networks. The researchers focused on field studies in forests, not laboratory or greenhouse experiments.

In an August presentation based on the review at the International Mycorrhiza Society conference in Beijing, Karst argued that much of the evidence used to support the Wood Wide Web hypothesis could have other explanations.

For example, in many papers, scientists believed that if they found a specific fungus on multiple tree roots or that resources moved between trees, they must be directly linked. But few studies have ruled out alternative possibilities — for example, that resources might travel part way through the ground.

The researchers also found a growing number of claims not supported by the scientific literature about fungal networks connecting and helping trees.

Papers like Klein’s are often cited by others as evidence of networks in forests, as Karst and colleagues found, with caveats that were in the original work left out of the more recent studies.

“Scientists,” Karst concluded in his presentation, “have become vectors of unfounded claims.” Several recent articles, she notes, have called for changes in the way forests are managed, based on the “wood wide web” concept.

Karst said “it is highly likely” that there are shared fungal networks in the forests. In a 2012 study, Simard’s team found identical fungal DNA in the roots of nearby Douglas pine trees. The researchers then sampled the soil between the trees in thin slices and found the same repeating segments of DNA known as “microsatellites” in each slice, confirming that the fungus filled in the gap between the roots. But that study did not examine what resources, if any, were flowing through the network, and few other scientists have mapped fungal networks with such rigor.

Even if fungal networks exist between trees, Karst and his colleagues say common claims about such networks do not hold up. For example, in many studies, putative networks appeared to either stunt the growth of trees or have no effect.

No one has shown that fungi distribute significant amounts of resources among trees in a way that increases the fitness of recipient trees, Hoeksema said. However, almost all discussions of the “Wood Wide Web”, scientific or popular, describe it as beneficial to trees.

Others, however, remain convinced that time will prove the tree network.

While the ubiquity of shared fungal networks and their importance to tree growth remain open questions, ETH Zurich’s Averill said that the title of Karst’s presentation — “The Decay of the Tree Network?” incorrectly suggests that the concept itself is flawed. Instead, he hopes that scientists will take advantage of the tantalizing clues collected so far, looking for nets in more forests. Indeed, members of Karst’s team have generated what Averill considers some of the most compelling evidence for the tree network.

“It’s very clear that in some forests in some different places trees are absolutely connected by fungi,” he said.

Simard agreed that few real-world fungal networks have been mapped using DNA microsatellites because of the difficulty of doing such studies. Kevin Beiler, a graduate student who led the fieldwork for the 2012 study with Simard, “spent five years of his life mapping these networks,” Simard said. “It takes a long time.”

Despite these challenges, she said, studies published in other forests using other methods have convinced her that shared networks of fungi are common.

“The field of mycorrhizal networks has been hampered by having to keep going back and redoing these experiments,” Simard said. “At some point you have to move on to the next step.”

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