Do trees really support each other through a network of fungi?

Do trees really support each other through a network of fungi?

  • Science
  • February 13, 2023
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The tips of tree roots are intertwined with fungal filaments, creating a hidden underground network that both organisms appear to benefit from: the filaments, known as hyphae, mine minerals from the soil that trees can then absorb into their roots, while the fungus gains a steady stream Source of sugar from the trees.

Even more poetically, research has suggested that these connections — known as mycorrhizal networks — can stretch between trees, allowing one tree to transfer resources to another underground. Some researchers even argue that trees cooperate, with older trees passing on resources to seedlings and nurturing them like a parent.

This idea of ​​forests as cooperative, caring places has permeated both scholarly literature and popular culture, most notably in the 2022 book Finding the Mother Tree: Discovering the Wisdom of the Forest by forest ecologist Suzanne Simard of the University of British Columbia . There’s even a funny name for the phenomenon: the “Wood Wide Web”.

However, a new analysis published in Nature Ecology & Evolution argues that the evidence that mycorrhizal networks facilitate tree cooperation is not as strong as popular history suggests. It’s not that there aren’t relationships between trees and fungi, says co-author Justine Karst, an ecologist who studies mycorrhizal networks at the University of Alberta. Rather, in many cases, suggestive evidence or studies have been viewed, with many reservations, as more definitive than they really are. “We don’t want to take away anyone’s joy or curiosity or wonder about the forest, but we just want to stop some of the misinformation,” says Karst.

The problem with studying mycorrhizal networks is that they are very delicate: dig up a root and you’ve destroyed the very web of fungi and wood you wanted to study. That makes it hard to tell if a given fungus really connects two trees. The best way around the problem is to take mushroom samples from different locations, sequence their genetic information, and create a map of where genetically identical mushrooms grow. This is a tremendous amount of work, Karst says, and she and her coauthors were only able to find five such studies on just two forest types, encompassing just two species of trees and three species of fungi.

What makes these studies even more difficult is the ephemeral nature of fungal networks. Fungi can grow as individuals after fission, says Melanie Jones, a plant biologist at the University of British Columbia and co-author of the new analysis. Even genetic samples only provide a snapshot and cannot reveal whether the mushroom pieces collected on two different trees are actually still connected. They may have been severed by part of the fungus dying off or by something taking out a bite. “It’s a very touchy subject,” Jones says.

These limitations raise questions about how widespread mycorrhizal networks are and how long they have existed.

It is clear that substances from one tree can end up being taken up by a neighboring tree in the forest. Researchers can test this by tagging a tree with a chemical compound labeled with a specific marker. In a 2016 study in a Swiss forest, researchers sprayed the leaves of some trees with a specific isotope of carbon and found that isotopes appeared in unsprayed neighbors. However, it’s not clear that fungi are necessarily responsible for this transmission, Jones says. Resources can also move directly from root to root and through the ground, and it’s very difficult to separate these paths in a real forest. Researchers try to erect barriers between trees so fungal hyphae and roots can’t connect them, leaving only the soil route as a possible route of transmission. But these barriers themselves (usually made of fine mesh) can interfere with tree growth and complicate the picture.

In order to test the effect of mycorrhizal networks, researchers often set up wide-meshed barriers that let fungi through but not tree roots. But Karst and Jones claim that in such cases, some researchers have rarely checked whether an interconnected mycorrhizal network had actually formed. The strongest evidence that trees send resources via fungal pathways versus roots or soil comes from a 2008 study that used a mesh to allow only fungi, but not roots, to ponderosa pine seedlings with older ones Joining pine trees in a real forest say Karst and Jones. The researchers then cut down several older pine trees and treated the cut trunks with colored water. The dye showed up in the seedlings despite a lack of connections between the roots, suggesting that fungal hyphae had carried out the transfer.

That suggests trees are transferring water, Jones says, but it still leaves the question: Does it matter to the seedlings? If mycorrhizal networks have evolved to allow older trees to help their younger relatives survive, resource transfer must improve seedling survival. Again, Karst and Jones claim some of the evidence is shaky. “In the really well-controlled experiments, less than 20 percent show that the seedlings did better,” says Jones. In the remaining 80 percent, she adds, the seedlings associated with hyphae performed either equal to or worse than those cut off from the fungal network.

The idea that trees send each other underground warnings of herbivorous insects or other dangers is based on a single greenhouse study in which a Douglas fir and a ponderosa pine were connected only by fungal networks. When researchers stressed the Douglas fir by exposing it to insects, the ponderosa pine also began emitting defensive chemicals. However, the effect disappeared when the firs and pines were connected by both roots and fungi, which occurs in nature. “The main message is that this was not tested in a forest,” says Karst. “When you see these images of ancient forests and big trees and they’re passing signals to each other, it just hasn’t been tested.”

The idea that forests are cooperative rather than competitive also contradicts the fundamentals of natural selection, says Kathryn Flinn, a plant community ecologist at Baldwin Wallace University in Ohio, who was not involved in the new analysis. The case for the collaboration is that trees in a healthy forest survive better than trees in a diseased one, but such instances of natural group selection are rare in the wild, Flinn says. And in forests, individual selection encourages competition, with certain trees vying for resources in ways that would preclude any group benefit. “I find this whole controversy really interesting because it’s an example of people wanting to project their own values ​​onto nature and/or want to see nature as a model for human behavior,” says Flinn.

Simard, whose research on forests has provided much of the basis for arguments that trees cooperate, declined to answer specific questions about the new analysis but said in a statement that she stands by her research. “Forests provide critical support for life on our planet. Reducing ecosystems to their component parts prevents us from understanding and appreciating the emerging relationships and behaviors that allow these complex ecosystems to thrive,” she says. “For decades, a delimited approach has prevented us from better understanding why forests help regulate global climate and are home to such rich biodiversity. The application of reductionist science to complex systems is accelerating the exploitation and degradation of forests worldwide.”

Karst, Jones and their co-author Jason Hoeksema of the University of Mississippi agreed that a reductionist view of the forest – testing individual parts of the network individually rather than in context – is not the only way to study ecology. However, these reductionist studies were used to make big claims about mycorrhizal networks, they said, adding that they wanted to focus their analysis on what the results really showed. They limited their analysis to studies conducted in real forests, they said, because those are the most relevant to the real world.

Karst says that she and her colleagues don’t intend to halt research in this area, instead they are moving into new forest types and investigating the most promising areas, such as B. the water transfer between trees want to promote. For her part, Karst believes that the idea that mycorrhizal networks are involved in at least some tree-to-tree networks may still be true, and better-designed experiments could unravel that truth. “I want to try again,” says Karst.

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