The Day I Learned Trees Talk: How Mycorrhizal Fungi Connect Forests Underground
- Carlos
- Aug 3
- 8 min read
Audio Overview
I was hiking in the Ozark mountains over ten years ago when I stumbled across something that didn’t look all that special at first. I saw a fallen log blanketed in moss, its bark crumbling, surrounded by a circle of young saplings. It was a cool, damp day, and the trail was empty. I paused, took a deep breath, and stared at that decaying trunk with renewed growth.
The longer I looked, the more questions I had. Why were all these young trees clustered around this dead log? Why did it seem like life was springing from decay?
Later I’d learn that the log was called a “nurse log,” and that it wasn’t just sheltering the new generation. It might be feeding them, too.
That moment sent me down a rabbit hole that would change the way I see forests forever. What I learned blew my mind: trees can communicate, share resources, and support each other, not through their leaves, but through a vast underground fungal network.
Welcome to the Wood-Wide Web
There’s a term scientists and science writers have been using for a few decades now: the wood-wide web. It’s not a metaphor anymore. It’s a scientific reality.
Beneath nearly every forest lies a network of mycorrhizal fungi. These are microscopic, threadlike organisms that form mutualistic relationships with tree roots. The fungi trade nutrients and water for carbon-rich sugars, effectively forming a natural barter system between species. These mycorrhizal networks connect not just a single tree and its fungal partner but entire ecosystems of trees.
This system allows for something astonishing: resource transfer and chemical communication between trees. In other words, trees are talking and they’re doing it through fungi.
The Science Behind Tree Communication
One of the most cited studies in this space was conducted by forest ecologist Suzanne Simard, who famously demonstrated that carbon can move from one tree to another via mycorrhizal networks. In a 1997 Nature study, Simard used radioactive carbon isotopes to track how birch trees transferred carbon to Douglas firs, especially when the firs were shaded and struggling. This wasn’t just chemical drift in the soil. It was a deliberate transfer of resources across species, through the underground network of fungi.
More recently, a 2022 study published in ISME Journal reinforced these findings using modern tools. Researchers grew oak and pine saplings together, exposed one to carbon-13-labeled CO₂, and traced how the labeled sugars moved through shared fungal partners, specifically, ectomycorrhizal fungi like Tomentella. They found that within just four days, carbon had traveled from one tree into the roots of the other, even across different species.
The implications are profound: trees can share carbon, communicate stress signals, and even bolster one another’s immune systems. And fungi are the messengers that make this all possible.
Seedling Survival and the “Nurse Log” Effect
That fallen log I saw on my hike? It turns out it’s not just sheltering new seedlings—it may be supporting them through this very network.
In a 2009 study in Ecology, researchers found that seedlings with access to established mycorrhizal networks had significantly higher survival and growth rates, particularly under stress. Trees that tapped into the fungal networks of nearby “parent” or “nurse” trees were more resilient, receiving water, nitrogen, and even carbon from more mature neighbors.
This explains the cluster I saw in the forest. The decaying log wasn’t simply decomposing; it was acting as an underground lifeline for new life. It’s the forest’s version of intergenerational care.
The “Mother Tree” Metaphor Is Useful, but Imperfect
Enter the Mother Tree metaphor. It’s a term popularized by Simard and widely shared in both scientific circles and the mainstream press. The image is powerful: wise old trees nurturing younger ones, connected by living fungal threads. Some studies even show that these older trees can recognize and prioritize their own kin, sending more resources to genetically related seedlings.
But not everyone agrees with this interpretation.
In a 2023 perspective article published in Nature Ecology & Evolution, researchers Karst, Jones, and Hoeksema argued that the “Mother Tree” narrative may overstate the evidence. They warn that positive citation bias (the tendency to cite supportive findings and ignore ambiguous ones) has shaped public and even academic perceptions. Their analysis showed that not all forests exhibit strong carbon transfer, and not all seedlings benefit consistently from mycorrhizal connections.
Still, their conclusion isn’t that these networks are myths. It’s that they’re more complex and variable than we’ve been led to believe. Some trees share. Some compete. Some fungal species facilitate transfer. Others don’t. The reality is far messier than a tidy metaphor suggests.
And maybe that’s the point. Forests aren’t fairytales. They’re living systems, shaped by cooperation, competition, and chance.
Mapping the Underground Forest
Want more proof that these networks are real? In 2010, scientists mapped an actual fungal network in a 30x30 meter plot of Douglas fir forest. They found that a single individual fungus (or genet) of Rhizopogon connected nearly 20 trees. These trees, young and old, weren’t isolated. They were sharing the same fungal infrastructure.
In fact, over 60% of the forest’s older trees were connected to seedlings. It was like discovering an invisible subway system beneath the soil, with some trees acting as major hubs and others as passengers along for the ride.
A New Way of Seeing
These days, when I walk through the forest, I don’t just see trees. I see relationships. I imagine threads of fungi (microscopic, silent, ancient) passing signals, sugars, and water back and forth like secret notes between neighbors.
I still think about that nurse log a lot. I’ve gone back to that trail more than once, just to check in. The saplings are taller now. Greener. Stronger. I like to think the log had something to do with it, not just because of what it was, but because of what it was still connected to.
It’s easy to think of forests as collections of individuals. But what if they’re more like communities? What if resilience comes not from standing alone, but from being intertwined?
Final Reflection: What Are Your Hidden Networks?
Forests might not speak in words, but they do communicate. They respond. They share. They adapt. And they do it through invisible alliances we’re only just beginning to understand.
So here’s my question for you:
What unseen networks are helping you survive, grow, or thrive? Who are the fungi in your life? Who are the quiet connectors, the ones linking you to support without asking for attention?
The next time you walk through the woods, take a moment to look down. You may not see them but the threads are there.
Debating the Wood-Wide-Web
The concept of the "wood-wide web," referring to common mycorrhizal networks (CMNs) formed by ectomycorrhizal fungi (EMF) connecting the roots of multiple plants, has gained significant public interest. While research over the past five decades has provided evidence for the existence and function of these networks, their interpretations are currently subject to debate and scrutiny.
Here are the main debates and limitations highlighted in the sources:
Accuracy of Claims and Citation Bias
◦ Criticism: Justine Karst, Melanie D. Jones, and Jason D. Hoeksema argue that recent claims in popular media about CMNs are disconnected from scientific evidence, and that a bias towards citing positive effects has developed in the scientific literature.
They claim that unsupported assertions about CMNs have doubled over the past 25 years, potentially obscuring a clear understanding of their structure and function. They conclude that current knowledge is too sparse and unsettled to inform forest management.
◦ Response: Suzanne W. Simard, Teresa L. Ryan, and David A. Perry refute this, stating that Karst et al.'s analysis of CMN research is "narrow and incomplete" and uses "unclear criteria". They argue that the presence of a citation bias does not negate the demonstrated reality of CMN-dependent facilitation and carbon transfer.
Widespread Nature and Significance of Resource Transfer
◦ Criticism: Karst et al. contend that the claims of CMNs being widespread in forests and that resource transfer significantly increases seedling performance are insufficiently supported. They argue that field study results vary widely, have alternative explanations, or are too limited for generalization. They suggest that measured carbon transfer would be "insignificant" to plants and that isotopes in some studies did not enter recipient shoots.
They also imply that CMN research overlooks other belowground transfer pathways, like through soil or disconnected networks.
◦ Response: Simard et al. maintain that CMNs have been shown to exist and connect trees, seedlings, shrubs, and mycoheterotrophic herbs. They assert that their studies, including Simard et al. (1997) and Cahanovitc et al. (2022), have repeatedly demonstrated significant carbon transfer into both shoots and roots of recipient trees and seedlings.
For example, 13C from Pinus halepensis and Quercus calliprinos was assimilated into roots within four days, transferred to various EMF species, and then further transferred to neighboring trees of both similar and distinct phylogenies. They clarify that carbon subsidies to roots and associated mycorrhizae enhance nutrient gathering for photosynthesis. They also emphasize that their research has consistently shown multiple belowground pathways functioning simultaneously, including CMNs, mycorrhizal roots, and soil, contrary to claims that they ignore alternatives.
The "Mother Tree" Hypothesis and Kin Recognition
◦ Criticism: Karst et al. specifically state that there is "no peer-reviewed, published evidence that mature trees preferentially send resources and defence signals to offspring through CMNs". They also dismiss the discovery of kin recognition in trees because the precise belowground mechanism remains elusive. Robinson et al. (2023) refer to "the mother tree hypothesis" as harmful and misinterpret the metaphor used in Simard's memoir.
◦ Response: Simard et al. clarify that the "mother tree" metaphor was used in a memoir (Simard, 2021) to convey scientific meaning to the public about the regenerative nature of forests, not as a strict scientific hypothesis of preferential gifting. They state that the discovery of kin recognition is novel, even if the exact mechanism is still being investigated.
Generalizability of Findings
◦ Criticism: Henriksson et al. (2023), Karst et al. (2023), and Robinson et al. (2023) suggest that findings from temperate Douglas-fir forests are inappropriately generalized to forests worldwide. Henriksson et al. (2023) specifically compare Simard's work to boreal pine forests in Europe, suggesting different roles for CMNs.
◦ Response: Simard et al. assert that their results are place-based and context-dependent, applicable to the specific temperate forests where their studies were conducted. They acknowledge that different species in various ecosystems behave differently and that mycorrhizal types vary globally, requiring further hypothesis testing to clarify the role of CMNs in diverse biomes.
Competition vs. Cooperation in Forest Dynamics
◦ Criticism: Karst et al. argue that CMN research ignores the important role of competition in forest dynamics, possibly due to interpretations emphasizing cooperative relationships.
◦ Response: Simard et al. state that their articles do not negate the process of competition in forests. Instead, they repeatedly discuss that multiple types of species interactions, including competition, occur simultaneously, and understanding forest dynamics requires accounting for this complexity.
Methodological Limitations in Complex Field Studies
◦ Criticism: Karst et al. propose a "strict set of requirements" for CMN research to be considered valid, which Simard et al. deem unrealistic to satisfy in the field given the complex, variable, and dynamic nature of forests.
◦ Response: Simard et al. argue that critics fail to provide alternative explanations with greater probability or new information that would warrant discounting peer-reviewed research. They affirm that their research, despite inherent limitations of studying complex natural systems, has contributed to a paradigm shift in understanding forests as connected systems rather than just collections of competing individuals.
In essence, the debate revolves around the interpretation and generalization of existing evidence for CMNs. While there is agreement on the existence of fungal networks and belowground carbon transfer, critics emphasize the variability of results, the need for more robust evidence for certain claims (like preferential gifting), and the potential for overstatement or misinterpretation in popular narratives. Proponents defend the scientific rigor of their work and its contribution to a more holistic understanding of forest ecosystems.
Comments