Fungus filaments and the tips of tree roots become intertwined to form a secret underground network that appears to benefit both organisms: The fungus gets a steady supply of sugar from the trees while the filaments, or hyphae, break down soil minerals that trees can then take into their roots.
To put it more poetically, studies have suggested that these connections, which are referred to as mycorrhizal networks, may extend between trees, making it possible for one tree to transfer resources belowground to another. Some researchers even contend that older trees are transferring resources to seedlings and nurturing them in the same manner as a parent would.
This idea that forests are places that work together and care for each other has gained traction in both scientific literature and popular culture, most notably in the book Finding the Mother Tree: Forest ecologist Suzanne Simard of the University of British Columbia wrote the book Discovering the Wisdom of the Forest. The phenomenon even has a humorously used name: the "wide web of wood."
Insufficient Supporting Evidence
However, a new analysis that was published in Nature Ecology & Evolution contends that the popular narrative is not supported by sufficient evidence for mycorrhizal networks facilitating tree cooperation. According to co-author Justine Karst, an ecologist at the University of Alberta who studies mycorrhizal networks, there are relationships between fungi and trees. Instead, studies with numerous caveats or suggestive evidence have frequently been interpreted as more conclusive than they are. Karst states that the team just desires to tamp down on some of the misinformation. They don't want to slay anyone's joy, interest, or wonder concerning the forest trees.
The fact that mycorrhizal networks are so delicate presents a challenge in research: If you dig up a root, the very web of fungi and wood that you wanted to study has been destroyed. Because of this, it is challenging to determine whether or not a particular fungus binds two trees together. Sample fungi from various locations, sequencing their genetic information, and creating a map of the locations where genetically identical fungi are growing is the most effective solution to the issue. Karst says that this is a lot of work, and she and her co-authors found only five studies of this kind that covered just two types of forests, two tree species, and three fungi, as per Scientific American.
The ephemeral nature of fungi networks makes these investigations even more difficult. Melanie Jones, a plant biologist at the University of British Columbia who is also a co-author of the new study, asserts that fungi can form new individuals after being split. Even genetic samples only provide a snapshot and cannot demonstrate whether the fungi fragments collected from two distinct trees are still connected. They could have been cut off by something eating away at the fungus or by a piece of it dying. Jones states, "It's a very controversial matter." Due to these limitations, it is unclear how widespread and how long mycorrhizal networks are.
Connection of Network of Fungi
It is abundantly clear that substances from one tree may ultimately be absorbed by another nearby tree in the forest. By providing one tree with a chemical compound that has been marked with a particular marker, researchers can test this. In a 2016 study in a Swiss forest, isotopes of carbon were sprayed onto the leaves of some trees and found to be present in their neighbors who had not been sprayed. Jones asserts that fungi aren't necessarily to blame for this transfer, but it's not clear. In a real forest, it is extremely challenging to distinguish these pathways because resources can also move directly through the soil and from root to root.
Researchers are attempting to construct barriers between trees so that fungal hyphae and roots cannot connect them, leaving only the path through the soil as a potential transmission route. However, tree growth can be affected by these barriers, which are typically made of fine mesh, making the situation more complicated.
NewScientist mentioned that the wide mesh barriers that allow fungi but not tree roots to pass through are also frequently used by researchers to test the effect of mycorrhizal networks. However, Karst and Jones argue that, in such instances, few researchers have verified the existence of a connected mycorrhizal network. According to Karst and Jones, a 2008 study in which mesh was used to allow only fungus, not roots, to connect Ponderosa pine seedlings to older pines in a real forest provided the strongest evidence that trees send resources via fungal pathways rather than roots or soil. After that, the researchers cut several older pines and used dyed water to treat the cut trunks. Despite the absence of roots connecting the seedlings, the dye was present, indicating that fungal hyphae were responsible for the transfer.
Evolved Mycorrhizal Networks
Jones claims that this suggests that trees move water, but it leaves open the question: Is anything of this relevant to the seedlings? The resource transfer must improve seedling survival if mycorrhizal networks have evolved to allow older trees to help their younger relatives survive. Additionally, Karst and Jones assert that some of the evidence is questionable. She goes on to say that 80%, of the seedlings with hyphae connections performed either as well or worse than those without the fungal network.
In the meantime, a single greenhouse study in which a Douglas fir and a Ponderosa pine were only connected by fungal networks supports the hypothesis that trees communicate with one another underground about dangers like herbivorous insects or other threats. The Ponderosa pine began producing defense chemicals as well when researchers subjected the Douglas fir to stress by exposing it to insects. However, the effect vanished when the firs and pines were joined by fungi and roots, as is the case in nature.
Kathryn Flinn, a plant community ecologist at Baldwin Wallace University in Ohio who was not involved in the new analysis, asserts that the fundamentals of natural selection also conflict with the idea that forests are cooperative rather than competitive. According to Flinn, such instances of group natural selection are uncommon in the wild. The argument for cooperation is that trees in a healthy forest survive better than trees in a sickly one. Additionally, competition is favored by individual selection in forests, with distinct trees competing for resources in a manner that prevents group benefits.
The Tree is Still a Mystery
In response to specific inquiries regarding the new analysis, Simard, whose research on forests has provided much of the foundation for the arguments that trees cooperate, stated that she stands by her research. Forests are essential to the survival of life on our planet. She asserts that declining ecosystems to their parts hinder their comprehension and preference of the emergent connections and behaviors that enable these complicated ecological systems to flourish. A compartmentalized approach has prevented us from better comprehending why forests contribute to climate regulation and support such diverse biodiversity for decades. The global exploitation and degradation of forests are accelerated when reductionist science is applied to complex systems.
Karst, Jones, and the University of Mississippi's Jason Hoeksema, who was a co-author of their study, agreed that the only way to study ecology is not through a reductionist perspective of the forest, in which each component of the network is tested on its own rather than in context. But these reductionist studies have been used to make big claims about mycorrhizal networks, they said, adding that they wanted to focus on what the actual results showed in their analysis. They claimed that because they are most applicable to the real world, they restricted their analysis to studies carried out in actual forests.
According to Karst and her coworkers, they do not intend to halt research in this area; rather, they intend to expand it into new types of forests and encourage research into the most promising areas, such as the transfer of water between trees. Karst, on the other hand, believes that better-designed experiments might be able to discover the truth about the hypothesis that mycorrhizal networks are involved in at least some tree-to-tree networking. Karst says they want to do it again.
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