by Bella Brown
Bella Brown is an environmental science major and a biology minor from Glastonbury, CT. Bella says that she is “super passionate about nature and finding new ways to preserve and understand it” and when she was presented with the task of writing a research paper she knew that she wanted to further explore a podcast episode about trees communicating through fungi, or “talking trees.” She says it “seemed like a huge breakthrough on understanding how our forests function as complex ecosystems.” Bella also notes that one thing she loved about writing this paper was the freedom to choose whatever topic she wanted. “Because I was so interested in the topic I chose, I found myself devoting hours to research, pouring through countless scientific journals and staying up many late nights. I found myself mapping out each part of the system on white boards, drawing trees for my friends, and explaining the fungal networks that connected them beneath the ground.” Bella believes that getting the general public to perhaps find emotional worth in trees can be a large step in helping to preserve our forests. She is also thankful for the opportunity to “connect my passions of the arts and science together, and focus on researching something that real environmental scientists and biologists in the field are still actively studying.” Bella’s goal with this essay was to present science in a way that made sense to those who haven’t studied it. She hopes to continue researching and finding ways to protect our environment.
A few months ago, I was on my usual morning podcast walk– a regiment I had established for myself that summer. In search of something new to listen to, I stumbled upon Vox’s Unexplainable – a podcast dedicated to all the debates ongoing in science currently. As an environmental science major and self-proclaimed tree-hugger, the episode “Talking Trees” immediately caught my eye. My first thought was that it was something like Grandmother Willow from Pocahontas, but little did I know there was an entire hidden world underneath the forest. Networks of fungi connect the trees underground, forming something called the Wood Wide Web. The host, Mandy Nguyen explains: “the idea is that trees communicate with each other. Somehow, they use fungi like telephone wires, linking the forest up in a big social network” (Nguyen) According to Nguyen, trees exchange nutrients with each other, send warnings, and help each other grow. Immediately, I was hooked; this could completely change our perspectives on how forests work, and therefore, how we should go about acquiring lumber, planning parks, and neighborhoods. The idea is still widely contested by scientists, for a multitude of reasons which I will get into. So, therein lies the question: Can trees communicate with each other through underground fungal networks? And if so, how?
Before I delve into the issue itself, it’s important to have a basic knowledge of the structure of the Wood Wide Web, and its parts. First the star of the show– the fungus. Deep in the soil of forests exist a tangle of long white fibers, known as “ectomycorrhizal fungi.” The term “ectomycorrhizal” simply means that the fungus encloses the root cells of the trees they connect, rather than penetrate through them, as vesicular–arbuscular mycorrhizal fungus does (Simard et al. 580). These two types of fungi are categories of mycelium– fungi that take on a root-like structure with many threads. The individual branches of the fungus are known as “hypha,” so imagine what bare tree branches look like intertwined in the winter sky, yet underground (Simard et al. 580). These ectomycorrhizal fungi (we’ll call them ECMs for future reference) are in a mutualistic relationship with the trees of the forest. This means that both organisms benefit from their interactions. The idea that “the mycelia act as microbial extensions of tree root systems and improve access to water and nutrients while the fungi in return receive carbon (C) from tree photosynthesis’” is widely accepted as fact in the scientific community (Henriksson et al. 19). Essentially, the trees provide the ECMs with carbon, and in return, the ECMs act as an extension of the tree’s roots and improve their access to nutrients and water.
However, the details of the Wood Wide Web hypothesis are widely debated. Some scientists believe there isn’t enough concrete evidence, but others believe there is just enough. The very first studies were done related to the transfer of carbon through these fungal networks. First done in a lab, scientists labeled a CO2 molecule, in one sapling connected to another by the fungus, and found the same molecule in the other tree (Nguyen). This, however, did not prove the process occurs in the wild, or out in forests naturally. So, Suzanne Simard, in one of the most famous studies surrounding the Wood Wide Web to date, set out to do just that.
Simard, a Canadian environmental scientist and professor, published her 1997 experiment, “Net Transfer of Carbon Between Ectomycorrhizal Tree Species in the Field” in the Nature Journal, and sparked a huge conversation in the scientific community. Seeing as this is the most important study done about the hypothesis, I will explain her process. Simard was testing for two things: to see if the carbon could be transferred through the ECMs in both directions, and to see what affected the magnitude (amount) of carbon transferred. In order to do so, she planted three types of saplings, Paper Birch (Betula Papyrifera) and Douglas Fir (Pseudotsuga Menziesii) which were connected by ECMs. Then she planted Western Red Cedars (Thuja Plicata) which were not, and these acted as a control (Simard et al. 580). Simard labeled molecules of two specific isotopes of carbon and looked to see if these specific molecules were showing up in the trees. The Paper Birches became the “donor trees,” where she input the carbon, and the Douglas Firs the “receiver trees,” who did not have these isotopes of carbon in them originally. She also exposed the Douglas Firs to three different levels of light, to see if the shading of the plant would affect how much carbon was transferred. She repeated this process at two different growth stages– 2 years, and 3 years (Simard et al. 580).
This study brings up one of the big questions surrounding trees communicating through these ECMs: can trees transfer nutrients with each other? And if another tree is struggling, will trees support it and give it extra help? What Simard found was incredible. Not only did the trees transfer carbon with each other in both directions, but when the Douglas Fir was more shaded, the Paper Birch transferred higher amounts of carbon! This “indicat[ed] that source–sink relationships regulate such carbon transfer under field conditions” (Simard et al. 580). A source–sink relationship means that the amount of nutrients given by the source (Paper Birch) are dependent on the ability of the receiver (Douglas Fir) to utilize said nutrients and photosynthesize. A higher sink strength means that the receiver has a higher need for nutrients. So, as the Douglas Firs got less light, they needed more nutrients to be able to survive, and the sink strength increased (Simard et al. 580). This effectively proves that the trees connected by ECMs in the forest work together as a group to survive, and they are able to communicate distress to each other in some way, so that one tree knows how to send nutrients to another!
Simard was sure that this carbon transfer was through the fungal networks, as the “Thuja Plicata seedlings lacking ectomycorrhizal absorb[ed] small amounts of isotope, suggesting that carbon transfer between B. papyrifera and P. menziesii is primarily through the direct hyphal pathway” (Simard et al. 579). The amount of the carbon isotope found in the Western Red Cedar was far less, almost 1% of the carbon transfer between the Birch and Fir trees, and the only difference was that the Redcedars were not connected to the other trees by ECMs (Simard et al. 580). This accounted for nutrient transfer through soil/water, which many scientists believed to be the only way nutrients could move tree to tree. Thus, the majority of the carbon transfer was through the ECMs.
So yes, Simard seemingly proved that the trees in a forest work together, explaining that “a more even distribution of carbon among plants as a result of below-ground transfer may have implications for local interspecific interactions, maintenance of biodiversity and therefore for ecosystem productivity, stability and sustainability” (Simard et al. 581). The transfer of carbon amongst trees balances out the distribution of carbon in the forest and allows for a stable ecosystem. However, all of her experiments were on saplings, or new growth forests. Thus, her findings weren’t enough to convince me that this was true for all forests. But Gabriel Popkins, environmental journalist, agrees with Simard’s claims, providing evidence that this process also occurs in old growth forests. In an interview with Nguyen, Popkins explains that “a scientist in Switzerland…[took] advantage of an experiment that was already being done in a much older, kind of mature forest of several different types of trees,” and showed that “carbon dioxide would be fed to one type of tree. He was able to show that it later showed up in other species of tree” (Popkins qtd. in Nguyen). And since this carbon transfer was proven to occur in both new growth and old growth forests, we can concur that trees can in fact share carbon with each other through these ECMs, and that the trees are in mutualism rather than competition.
This has huge implications. The typical view of forests was that trees were stuck in a Darwinian struggle with each other, where “individual trees compete for light, water, [and] nutrients in the soil, and the ones that survive, reproduce” (Nguyen). But with this new information, we see that trees are acting more like a flock of birds as they help each other to survive. What is so incredible is that it isn’t solely between trees of the same species– it’s all the trees that are connected by this fungus. Therefore, I think scientists who believe in the Wood Wide Web should be giving the fungus itself more credit. In most scientific papers, it is simply viewed as a pathway, rather than a living organism. There must be a reason why the fungus is allowing for this transfer of carbon from tree to tree, but what is it?
Nils Henriksson, forest ecophysiologist, sees that fact that “the evolutionary incentive for a mycorrhizal fungus to redistribute C toward the seedlings has not been addressed,” as proof that the whole process does not exist (21). Essentially, since there hasn’t been a study regarding why the fungi aid in this process, Henriksson believes they simply don’t. However, I think this just means there is more research to be done. There are in fact some hypotheses offered which make logical sense. Popkins offers the idea that:
It may just be that tree one exchanges some sugar with a fungus in exchange for some nutrients. And then the fungus, may say, hey, I’ve got this sugar, now I’m gonna go take it over here and, and give it to this other tree. Because I know that if I sort of strengthen my relationship with the second tree, then this tree will provide me sugar in the future. (Popkins qtd. in Nguyen)
Basically, he’s saying that the fungus randomly distributes the nutrients to other surrounding trees, to create more relationships. However, this idea does not take into account the evidence of the trees’ source-sink relationship with one another. If it was truly random, and the fungus was just spreading sugar to another tree to form another relationship, then the magnitude of carbon transferred would not have increased with the shading of the Douglas Fir.
Kathryn Flinn, plant ecologist, brings the idea of altruism into the mix, possibly filling this hole in Popkin’s claim regarding the ECMs’s motivations. Altruism is when one organism does something to benefit another at a personal cost. There are many reasons why this could occur, but one is that “altruism can arise if a recipient is likely to reciprocate, ultimately benefiting the donor” (Flinn). So, if the fungus knows that the tree it’s being told to transfer the donor tree’s nutrients to will eventually give it some of that carbon, it will transfer the nutrients to it without immediately getting reciprocation. Unfortunately, there has not yet been much research done about this, so we can only speculate.
While Simard has not yet commented on the ECMs’s motivations, she does have another very intriguing idea– the mother tree hypothesis. In her book, Finding the Mother Tree, she explains that “the old trees nurture the young ones and provide them food and water just as we do with our own children” (34). These old trees are known as overstory trees, or those which grow taller than the whole canopy (Henriksson 19). Simard came to this conclusion after a study where she and her team traced the ECMs’s pathways underground, seeing which trees were connected and their relative distances. They found that “although trees from all [age] cohorts were linked, large mature trees acted as hubs with a higher degree of connectivity and a more central position in the [mycorrhizal network]” (Beiler 548). Essentially, in looking at a map of the fungal network, most of the trees are connected to these older trees, they act as grand central stations for the nutrients. It makes sense as to why Simard would think that therefore, these trees act as “mothers” and help the younger trees connected to them grow. However, I offer a completely different take: what if these trees were actually doing the opposite through all these fungal connections and depleting the surrounding trees of their nutrients?
Logistically, it makes sense. How do these trees come to tower so much higher over the forest canopy? Certainly, they couldn’t be giving away a lot of nutrients, they would instead need to be gaining more. And while they do have age on their side, perhaps it is the years of taking that has allowed them to grow so tall. There have actually been a lot of studies showing that seedlings don’t do well growing next to these larger trees, dating way back. As a matter of fact, “as early as 1926, a Finnish field study showed that below ground competition hampered seedling establishment in extremely nutrient-poor Scandinavian pine heaths” when they were surrounded by older trees (Henriksson et al. 20). So rather than sharing their nutrients with the seedlings to help them grow in an area with depleted soil, the older trees outcompeted them for what little nutrients existed in the ground.
At the time, scientists did not know of the existence of ECMs, but using the knowledge we now have, it would make sense if these older trees were taking the carbon from the seedlings without returning any, in order to survive in such a nutrient poor area. A similar, more recent study done in British Columbia, also showed that “soil moisture content and [Nitrogen] mineralization rates were also lower in the presence of overstory trees (Walters et al. 2006), indicating higher competition for these resources, rather than facilitation” (Henriksson et. al 21). Basically, the soil surrounding the older trees was very depleted of nutrients, indicating that the trees were competing for these resources instead of sharing. Thus, proving that these overstory trees were taking the resources from the soil before the seedlings could get to them. In addition to these studies, we can simply look at the way trees reproduce. Kathryn Flinn brings up a great point that “a mature maple tree produces millions of seeds, and on average only one will grow to reach the canopy. The rest will die, with or without help from mom.” These seedlings start to grow all around the older maple tree– millions– yet only one survives. If the mother tree hypothesis was really true, many more would survive to grow up and join their “mother.” Thus, I deduce that overstory trees are really using these ECMs as a way to take nutrients from seedlings, rather than share with them. It is more of a parasitic relationship than mutualistic.
Another big problem scientists have with this idea of the mother tree hypothesis and the Wood Wide Web, is that the general public is comparing them to humans. This is anthropomorphism because people are attaching human traits and emotions to the trees, and whether or not the science proves these hypotheses, they want to believe them. Every time I’ve explained what I’m writing this essay about to friends, I usually get the response that it’s “so cute” that trees are sharing with each other. Immediately, people want to believe everything that’s told to them, because it’s “cute” and kind of like the trees are “friends.” Popkins expresses his concern for this as “we really do wanna be careful about anthropomorphizing trees because trees are, um, very, very different from us in a lot of really important ways…it doesn’t mean that there isn’t complicated, profound, nuanced stuff going on in the forest. It just means that we can’t sort of map our own concepts onto that. We need to really try to understand trees and forests on their own terms” (Popkins qtd. in Nguyen). However, Simard brings up a valuable point that perhaps anthropomorphizing science is a useful tool to get the general public to care about these environmental issues. As a matter of fact, “in interviews, Simard has said that she purposely uses anthropomorphism and culturally weighted words like ‘mother’ – even though the trees in question are male as well as female – so that people can relate to trees better, because ‘if we can relate to it, then we’re going to care about it more’” (Flinn). It makes sense– it’s simple psychology that we care more about things we can see ourselves in. So, when it comes to issues like forests, and climate change, perhaps anthropomorphism can be a valuable tool. If the general public cares, then it reaches up to the government, and if the government cares, then they can pass legislation to protect our environment.
All in all, more research must be done. There is still work to be done looking at the transfer of other nutrients through these fungi. Simard and Popkins only discuss the transfer of carbon yet generalize it to the transfer of “all nutrients,” which is a very big step to take. Carbon, nitrogen, phosphorus, and water are all very different compounds. We also must do more research regarding the fungi’s motivations. The interactions between organisms in nature are much more complicated than just good and bad; ecosystems are an intricate network of relationships that we do not fully understand. However, one thing remains true, ectomycorrhizal fungus is an organism. Organisms don’t just do things for no reason. Every interaction in an ecosystem has a purpose; they all meld together to prevent collapse. But there has not been a single study finding the reason, and that is what needs to happen next. And while my claim that overstory trees use these fungal networks to take rather than give does have evidence backing it, there has not been a study done on this exact idea yet. I may be an environmental science major, but I am also a freshman, who has not studied this in the field before.
But there are a few things we know for sure. Trees can talk to each other. The mutualistic relationship between the fungi and the trees creates another intricate relationship between the trees themselves. And since we know that trees share carbon with each other to survive, it brings up the idea that “if I cut down one tree the rest of the rest will be fine.” Because now we know, it probably won’t be! This should be taken into consideration in any situation where trees are being cleared. Tree preservation is imperative as they are our main source of oxygen. If we cut down one tree in the forest, and the others are depleted of nutrients, then they may die. When they die, we lose a source of oxygen which adds up to the thousands of forests being clear-cut. And FYI, we need oxygen to survive. Trees are also habitats for thousands of organisms and if you take them away, the ecosystem collapses. Ecosystem collapse has giant societal effects, as we rely on animals and organisms for our culture and livelihoods. So, if not to preserve the forests just for the sake of protecting them, then we should work to preserve us, the human race, and our culture. The earth is our only home. If it dies, we die.
Works Cited
Beiler, Kevin J., et al. “Architecture of the Wood-Wide Web: Rhizopogon spp. Genets Link Multiple Douglas-fir Cohorts.” New Phytologist, vol. 185, no. 2, Jan. 2010, pp. 543-553.
Flinn, Kathryn. “The Idea That Trees Talk to Cooperate Is Misleading.” Scientific American, 19 July 2021.
Henriksson, Nils, et al. “Re‐Examining the Evidence for the Mother Tree Hypothesis – Resource Sharing among Trees via Ectomycorrhizal Networks.” New Phytologist, vol. 239, no. 1, 7 May 2023, pp. 19-28.
Nguyen, Mandy, host. “Talking Trees.” Unexplainable, Episode 119, Vox, 12 April 2023.
Simard, Suzanne. Finding the Mother Tree. New York, Alfred A. Knopf, 2021.
Simard, Suzanne W., et al. “Net Transfer of Carbon Between Ectomycorrhizal Tree Species in the Field.” Nature, vol. 388, no. 6642, Aug. 1997, pp. 579–582.