The term “Mother Tree” was coined by Canadian Professor of Forest Ecology at the University of British Columbia, Suzanne Simard. Her research, from the 1990s through today, indicates that trees in a forest, rather than simply competing with each other for survival, form communities that include trees and fungi that share their nutrients to strengthen overall community survival. Further, her research suggests that the largest trees share their photosynthesized carbon-based sugars with other tree species in their community. Dr Simard called them Mother Trees because they appear to favor the needs of their offspring compared to unrelated trees. She maintains that mycorrhizal networks link trees of multiple species, sharing sugars from trees that produce more than they need with those who don’t have enough, and the fungi who are not able to photosynthesize. In return, the fungi absorb water and nutrients from the soil and pass it on to trees in need in the community.

This idea of cooperative forest communities runs counter to the historical belief that nature is simply a survival of the fittest competition between plants and animals and conflicts with the conventional lumber-industry-based forest management practices of clear-cutting forest sections and replanting monocultures. In Simard’s view, this disregards the evidence and ignores the potential benefits of diverse and cooperative community relationships. From a commercial standpoint, her data appears to show that the clear-cutting/monocultural replacement practice reduces forest health and ultimately lumber production.

Simard’s History in the Forest

Suzanne Simard is the daughter and granddaughter of traditional foresters who cut trees selectively and used horses to transport them. Growing up in the forest, she came to love the apparent relationships she observed between the forest dwellers. Her love of nature, and forests in particular, led her to summer forestry jobs during graduate school and full-time work as a Research Scientist for the BC Ministry of Forests. During her early work she saw that forests that were clear cut for lumber were slower to regrow when row-planted with a single tree species, than trees of multiple species ranging from young to mature, naturally spaced and selectively harvested.

Trying to understand why this was true led her and her team to perform decades of research. Early evaluations demonstrated that the healthy trees in the natural forests had root systems that were tied extensively to networks of the mycelia of mycorrhizal fungi. With further testing, she found that the trees were sending a portion of the photosynthetic sugars they generated to the fungi, in exchange for nutrients that they needed. This was demonstrated by inserting radioactive carbon isotopes into multiple trees and measuring whether and what nutrients were transmitted between the trees through the fungal mycelia. The measurements showed that in fact, sugars were transmitted both for fungal use and for passing on to multiple tree species in the community. At the same time, soil-based nutrients were transmitted from fungi to trees that were part of the same mycorrhizal network. Later study showed that the fungi passed some of the sugars on to trees of both the same and different species, running counter to the historical beliefs of pure competition between plants and apparently indicating that the mature trees influence which trees-in-need receive the nutrients. While more research is needed to establish how much nutrient is transferred, one study found that an average of 6% of required sugars were received through the network by the receivers from the senders.

Further work demonstrated that the oldest, largest trees in the forest area, those with the broadest crowns, were the largest donators of the sugars, and that the largest donations went to their direct offspring. Simard began calling these large older trees “Mother Trees” to emphasize the observation of parental support for offsprings and to present the findings in terms that would be understandable by the public. Other researchers, who support her work but object to using human terms to characterize the plant/fungal relationships, prefer the term “Hub Tree”.

To Simard, these findings were exciting, if not unexpected, given her childhood preconceptions. But when she attempted to promote the concepts and benefits of building forest communities, she confronted resistance from the historical establishment in the forestry industry whose belief was that clear cutting and regrowth of row planted monocultures produced the most lumber at the lowest cost. The idea of leaving the oldest, largest trees in the forest while selectively removing younger, smaller trees held little appeal.

Her original experiments and reports took place during the 1990s. She has spent the decades since then expanding the scope of her research while building understanding and acceptance of her findings in the forestry industry and beyond.

What the Skeptics Say

While there is growing acceptance of Simard’s claims, many researchers and forestry management professionals only partially accept and in some cases reject her findings.

Forestry organizations remain largely wedded to clear cutting and replacing the cut trees with seedlings of a single species, widely-spaced and fungus-free. Leaving the largest trees in place, taking up space in the forest, in their minds reducing lumber production and increasing the cost of harvesting, is hard for the industry to accept. They are more inclined to find fault with Simard’s research techniques.

While many forest scientists support Simard’s conclusions, others feel that her assertions are overstated. The objectors agree that fungal networks exist in the forest, but they believe that plants are working more in their self-interest than as a cooperative community. For example, the mycorrhizae can penetrate tree roots to both deliver and extract nutrients from their hosts. Many doubters believe that the fungi trade materials with trees because it serves their own needs, not necessarily to also support the needs of others.

Simard’s research has measured cooperation mainly between Douglas Firs and Paper Birch trees. Other research done in forests and labs around the world, monitoring different species, appears to show inconsistent evidence of her claimed cooperation and nutrient trading between trees through fungal networks. Similarly, indications of favoring offspring over other trees of the same and related species are not conclusive.

Acceptance of fungal networks and transfer of sugars and soil nutrients within the network communities, is generally widespread. However, researchers point out that Simard’s work has been done in the forests of Western Canada, on a small variety of tree species, affected by the region’s specific local environmental and soil conditions. Research from other geographic areas shows less consistent and at times conflicting results.

Questioners also point out the existence of parasitic relationships between trees and fungi, and that the allelopathic emissions by some trees (black walnut for example) demonstrate the competitive aspects of forest relationships.

So, the argument is generally not so much about whether networks exist or that trading of nutrients between trees and fungi occurs. Rather it challenges the concept of systematic sharing of nutrients between different species and prioritization of supporting offspring ahead of non-related trees. The conflicting data indicates that additional research is needed to determine the validity and breadth of Simard’s conclusions.

Future Direction

Simard has been working in the field for over 30 years and her team’s experiments have been detailed and thorough. Research by others around the world, while not always reaching the same conclusions, has been building agreement with many of her basic premises. Continuing research, including more species, different environments, and varying soil conditions, is needed to fully test her basic claims and add credibility and clarity to her conclusions. A growing base of such research is testing her hypotheses.

Climate change is adding significance to the investigations. The importance of pulling carbon from the atmosphere and utilizing live trees and soil to store it sustainably, has never been more important. The Mother Tree concept of photosynthesizing carbon from the atmosphere and using it to grow more trees faster via mycorrhizal networks has strong appeal. But continuing research covering a range of species and environments is clearly justified.

In a broader sense, climate change has also illustrated the importance of biodiversity and in building healthy communities that include plant, animal (including humans), and microbial life. While the Mother Tree concept, and more broadly, the idea that communities are formed by trees and fungi, are a relatively narrow portion of the larger diversity picture, their relationships are certainly a part of it. There is little doubt that the study of these relationships is important and deserves continuing research as the criticality of the planet’s declining tree population in the face of climate change, a growing human population and a reduction of species around the world, becomes more obvious. Stay tuned…

Featured Photo:  Ralph Morini

SOURCES:

Finding the Mother Tree, Suzanne Simard, Alfred A. Knopf, 2021, 348 pages.

The Hidden Life of Trees, Peter Wohlleben, Greystone Books Ltd., 2016, 272 pages.

Entangled Life, Merlin Sheldrake, Random House Books, 2020, 352 pages. (Note Chapter 6 titled: Wood Wide Webs).

“About Mother Trees in the Forest,”  The Mother Tree Project,  https://mothertreeproject.org/about-mother-trees-in-the-forest/

“Mother Trees are Intelligent: They Learn and Remember,”  Scientific American, May 4, 2021, https://www.scientificamerican.com/article/mother-trees-are-intelligent-they-learn-and-remember/

“Exploring The Underground Network of Trees – The Nervous System of the Forest,” Valentina Lagomarsino, with figures by Hannah Zucker, Science in the News, Harvard University (2019)

“Are Trees Talking Underground? For Scientists it is in Dispute,” NY Times, June 20, 2023, https://www.nytimes.com/2022/11/07/science/trees-fungi-talking.html

“Can Plants Talk to Each Other,” Katie Sylvester, Biomechanics in the Wild, Katie Sylvester, 2019: https://sites.nd.edu/biomechanics-in-the-wild/2019/03/06/can-plants-talk-to-each-other/

Net transfer of carbon between ectomycorrhizal tree species in the field, Simard et al, Nature, August 7, 1997: https://www.nature.com/articles/41557

“How do Trees Collaborate,” NPR/Ted Radio Hour, January 13, 2017: https://www.npr.org/transcripts/509350471