The mother tree hypothesis suggests that trees form networks to share resources and communicate using fungi growing in their roots. Find out why a new study questions the evidence for this idea and calls for better experiments to uncover the truth.
The more time I spend in nature, the more I’m convinced it’s an interconnected web of cooperation. That’s what led me to pick up Professor Suzanne Simard’s bestselling book Finding the Mother Tree.
Part autobiography, part science communication, and lavishly illustrated, her book tells the story of how Professor Simard grew up in a logging family, and went on to develop a theory that trees communicate. Her findings show that trees use the fungi that grow in their roots to share resources and send signals below the ground.
Professor Simard is remarkably candid about the biases she encountered from the forest industry and the scientific community about her research. She describes her emotions in stark detail as she presents her findings to skeptical and even hostile audiences of her peers.
Finding the Mother Tree is a Painfully Personal Book
Finding the Mother Tree is a painfully personal book, outlining in detail the arduous odyssey that led to its author’s discoveries. This includes personal details about her family struggles and her ordeal with breast cancer.
Botanists call the organic root connections underlying Professor Simard’s discoveries mycorrhizal fungi, and they emanate out from what Professor Simard calls a “mother tree.” Ecologists call these structures “common mycorrhizal networks” (CMNs) or more affectionately, the “wood wide web.”
According to Professor Simard’s research, birch trees, which loggers often view as weeds, and fir trees exchange carbon through their root systems. “Paper birch and Douglas fir were trading photosynthetic carbon back and forth through the network,” she writes.
‘Birch Tree Generously Giving Fir Tree Resources’
“Even more stunning, Douglas fir received far more carbon from paper birch than it donated in return,” her book continues. “Far from birch being the ‘demon weed,’ it was generously giving fir resources.”
Further research revealed that the trees exchange other nutrients like sugars and communicate with each other about threats and pests. “Trees and plants have agency,” Professor Simard writes, “They cooperate, make decisions, learn and remember.”
More recently, Professor Simard has arrived at some even more remarkable conclusions. She suggests that mother trees send more nutrients and signals to their offspring than to genetically unrelated trees.
Findings Challenge Survival of the Fittest
These findings challenge the traditional, survival-of-the-fittest view of evolution and ecology. The conventional wisdom about Darwin’s theory of natural selection is that competition drives the natural world.
Professor Simard challenges this interpretation of Darwin. “Darwin also understood the importance of cooperation,” she told Scientific American. “It’s just that it never got the same traction as his natural-selection-based-on-competition theory.
Justine Karst is an associate professor in the Faculty of Agricultural, Life and Environmental Sciences at the University of Alberta. She researches mycorrhizal fungi, ecological disturbance and restoration ecology, mainly in the boreal forest, and she was her faculty’s professor of the year in 2020.
Study Argues Evidence for ‘Wood Wide Web’ is Overstated
Professor Karst and her co-authors, Professors Melanie Jones and Jason Hoeksema, published a study this week in the journal Nature Ecology and Evolution. In it, they argue the evidence for Professor Simard’s “wood wide web” is overstated.
The researchers evaluated the evidence for the mother tree hypothesis advanced by Professor Simard and other like-minded scientists that:
- CMNs are widespread
- Resources are transferred through CMNs
- Mature trees preferentially send resources and defence signals to offspring through CMNs
The team concluded that the field study results vary too widely to support the first two claims. As for the third claim, they didn’t find any peer-reviewed, published evidence behind it at all.
Question Fungus Transfer and Network Formation
The new study’s authors don’t dispute that trees can take up substances from one another. However, they question that fungi are the transfer medium and that complex networks can form this way.
The resources could just as easily be moving from root to root or through the forest floor. Also, most researchers don’t take the time to confirm that a CMN has actually formed in the forest between the trees they study.
“The main message is that this hasn’t been tested in a forest,” Professor Karst told Scientific American. “When you see those pictures of ancient forests, big trees and they’re passing signals to each other, it just hasn’t been tested.”
Objects to Studying Ecosystems as Sets of Individual Parts
In a written statement, Professor Simard insisted that she stands by her mother tree research. She also objects to the approach of studying ecosystems as sets of individual parts.
“For decades, a compartmentalized approach has hindered us from better understanding why forests help regulate global climate and harbor such rich biodiversity,” Professor Simard wrote. “Applying reductionist science to complex systems accelerates the exploitation and degradation of forests worldwide.”
Professors Karst, Jones and Hoeksema don’t disagree about reductionist science. Even so, they point out that much of the evidence supporting the hypothesis that trees use CMNs to communicate actually comes from reductionist studies.
Researchers Not Trying to Shut Topic Down
To be clear, Professor Karst and her colleagues aren’t trying to shut this topic down. In principle, they’re still open to the idea of the wood wide web.
Of course, I’m in no position to choose a side in this mother tree debate. However, I do agree with science author Guy Murchie.
“There is no end to interrelations anywhere,” he wrote. “Both vegetables and animals are turned into geological forces by such action as jungles that stem erosion and build soil, polyps that raise coral islands, or beavers that change the shapes of rivers.”
And Another Thing…
I think most readers have also experienced the intuition that all living things are interrelated in nature’s web. Even if Professor Simard has overstated her case on the specific example of mother trees and CMNs, her holistic approach to ecology is popular because it resonates with humanity.
For her part, Professor Karst explained that her study simply calls for improved experiments that can uncover the truth. She wrapped things up with Scientific American, saying, “I want to have another go at it.”
We always have more to learn if we dare to know.
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