Do Trees Really Support Each Other through a Network of Fungi?

There is a common belief that trees collaborate and communicate with one another through a fungal web. But, not everyone is persuaded.

The tips of tree roots are entangled with fungus filaments, creating a secret underground network that appears to be advantageous to both organisms. The filaments, known as hyphae, break down minerals from the soil that trees can then take up into their roots, while the fungus receives a consistent source of sugar from the trees.

Research has more eloquently suggested that these linkages, known as mycorrhizal networks, may stretch between trees, allowing one tree to move resources belowground to another. Some scientists even contend that elder trees are helping younger ones by giving them resources and caring for them like a parent would.

In particular, the book Finding the Mother Tree: Discovering the Wisdom of the Forest, by forest ecologist Suzanne Simard of the University of British Columbia, published in 2022, is a notable example of how this idea of forests as cooperative, caring places has gained traction in both scientific literature and popular culture. The phenomenon is also known by the punny moniker "wood-wide web."

Nevertheless, a recent research asserts that the evidence for mycorrhizal networks promoting tree cooperation is not as robust as the popular tale would imply. It was published in Nature Ecology & Evolution. According to co-author Justine Karst, an ecologist at the University of Alberta who specializes in mycorrhizal networks, it's not that there aren't any linkages between trees and fungus. Instead, suggestive findings or studies with several limitations have frequently been interpreted as being more conclusive than they actually are. Karst adds, "We just want to tamp down on some of the misconceptions. We don't want to extinguish anyone's enthusiasm, interest, or amazement about the forest.

Mycorrhizal networks are difficult to examine because they are so delicate: remove a root, and you've just ruined the web of fungus and wood you were trying to figure out. This makes it difficult to determine if a certain fungus is actually joining any two trees. The ideal solution is to collect samples of fungus from various regions, sequence their genetic makeup, and create a map of the areas where genetically similar fungi are present. This is a lot of work, according to Karst, and she and her co-authors could only locate five such studies in just two different types of forests with just two tree species and three different forms of mushrooms.

The transient nature of fungal networks makes these research much more difficult. Melanie Jones, a plant scientist at the University of British Columbia and a co-author of the new investigation, claims that fungi may thrive as individuals after being divided. Even genetic tests only offer a snapshot and are unable to determine if the fungus fragments found on two distinct trees are still in fact related. They could have been severed when a portion of the fungus died or when something chewed into it. It's a pretty complicated problem, adds Jones.

These restrictions make it difficult to determine how broad and how long-lasting mycorrhizal networks are.

It is obvious that chemicals from one tree in the forest may wind up being absorbed by a neighboring tree. By giving one tree a chemical component marked with a certain marker, researchers may test this. In a 2016 investigation in a forest in Switzerland, scientists sprayed the leaves of certain trees with a specific carbon isotope and discovered that other trees that weren't sprayed also had that isotope. Jones notes that it's not certain whether fungi are actually to blame for this translocation. It may be quite challenging to distinguish between resources moving directly from root to root and through the soil in a real forest. To prevent fungal hyphae and roots from connecting trees, researchers strive to erect barriers between them, leaving just the soil passage as a potential route of transmission. Nevertheless, the barriers themselves—which are often formed of fine mesh—can impact tree development, further aggravating the situation.

Very often used by researchers to assess the impact of mycorrhizal networks are wide-mesh barriers that let fungi to pass but not tree roots. Yet, according to Karst and Jones, few studies have really verified that a linked mycorrhizal network has truly evolved in these circumstances. According to Karst and Jones, a 2008 research in which mesh was used to enable only fungus, not roots, to connect Ponderosa pine seedlings to older pines in a real forest provided the greatest evidence for trees transmitting resources via fungal routes as opposed to roots or soil. The scientists next chopped a few elder pine trees, treating the severed trunks with colored water. Despite the absence of linkages between roots, the dye appeared in the seedlings, proving that fungus hyphae were responsible for the transfer.

According to Jones, that is suggestive of trees moving water, but it is yet unclear whether or not this is important for the seedlings. The resource transfer must increase seedling survival if mycorrhizal networks have developed to allow older plants to assist their younger relatives in surviving. Karst and Jones assert that some of the evidence is suspect there as well. Less than 20% of the really well-controlled tests, according to Jones, demonstrate that the seedlings outperformed the control group. She continues by saying that in the remaining 80%, the hyphae-connected seedlings either performed as well or worse than the ones that were cut off from the fungus network.

The theory that trees warn one another about herbivorous insects or other risks through subterranean networks is based on a single greenhouse experiment in which a Douglas fir and a Ponderosa pine were solely connected by fungus networks. The Ponderosa pine also started producing defensive compounds when researchers put the Douglas fir under stress by exposing it to insects. The illusion vanished, however, when the firs and pines were joined together by roots and fungus, as is the case in the wild. The crucial point is that this hasn't been put to the test in a forest, according to Karst. "It just hasn't been tested when you see those photographs of old woods, gigantic trees, and they're exchanging messages to one other."

According to Kathryn Flinn, a plant community ecologist at Baldwin Wallace University in Ohio who was not involved in the current work, the assumption that forests are cooperative rather than competitive also runs counter to the foundations of natural selection. The justification for cooperation is that trees in a healthy forest outlive those in a diseased one, but according to Flinn, such examples of group natural selection are uncommon in the environment. Individual selection also encourages competition in forests, where some trees compete with one another for resources in a way that prevents any group advantages. The entire dispute, according to Flinn, "is incredibly fascinating to me because it's an example of people wanting to project their own values onto nature or wanting to see in nature a model for human conduct."

Simard declined to respond to specific questions regarding the new study but stated in a statement that she stands by her findings. Simard's research on forests has served as a major source of inspiration for the assertions that trees collaborate. "Life on our planet depends critically on forests for support. We are prevented from understanding and appreciating the emerging linkages and behaviors that enable these intricate ecological systems to function if we break ecosystems down into their component elements, according to her. "For decades, a segmented approach prevented us from comprehending why forests contribute to climate regulation and support such a wealth of species. The exploitation and destruction of forests throughout the world is accelerated by the application of reductionist science to complex systems.

Karst, Jones, and Jason Hoeksema, a University of Mississippi researcher and study co-author, concurred that studying ecology is not limited to a reductionist perspective of the forest, which tests each component of the network separately rather than in the context of the whole. They continued by stating that large assertions regarding mycorrhizal networks have, however, been made using these reductionist research and that they wished to concentrate their investigation on what the actual findings revealed. They said that because these research were the most applicable to the real world, they only included studies done in actual forests in their study.

According to Karst and her colleagues, their goal is not to stifle research in this field but rather to expand it into novel forest types and promote examination of the most promising regions, such as water transport between trees. Karst thinks that with better-designed tests, it may still be possible to prove the hypothesis that mycorrhizal networks play a role in at least some tree-to-tree networking. Karst states, "I want to give it another shot."