It’s not everyday when ecology hits the main headlines. Just four years ago, Dr. Suzanne Simard published a book called Finding the Mother Tree, covering the prevalence of common mycorrhizal networks in natural forest communities worldwide. In her book, Dr. Simard argued that trees do not exist independently from one another and relay nutrients and carbon to one another through mycelial networks belowground. Subsequently, she deduced the presence of “mother trees” that serve as hubs for these complex networks to sustain the broader communities of trees around them.

Basics of mycorrhizal research

At its core, mycorrhizae is a category of fungi that has been known to associate with most plant species on Earth in one way or another. Trees benefit from the plant-mycorrhizal symbiosis as the fungi acquires nutrients from the soil to give to the plant, whereas the tree returns the favor by sharing with the fungi some of its sugars. The benefits of the plant-mycorrhizal symbiosis have been known to ecologists for generations and is generally agreed to be a key driver in several ecosystem processes such as nutrient cycling and plant growth and establishment.

For a long time, the plant-mycorrhizal symbiosis was assumed to be solely bipartite. However, more recent observations of plant-mycorrhizal interactions in the wild have challenged that notion to produce several very cool findings. For example, tripartite plant-fungal relationships have been found in several plants that have completely lost their ability to photosynthesize (read up on mycoheterotrophy if you are interested). Another recent development is the discovery of common mycorrhizal networks (CMNs), where the fungus attached to individual trees link up with one another to communicate and share resources.

Actually demonstrating the existence of CMNs is extremely challenging and often involves clever experimental design and inductive reasoning. For one, the hyphae that connects fungi together are often extremely thin (even thinner than your hair!), fragile and buried underground, making it difficult to observe directly. Even more challenging is to demonstrate the transfer of nutrients and carbon within these networks itself. Brilliant ecologists, many far smarter and well-equipped than I am, have overcome these hurdles through several approaches:

• Using radioactive isotopes (i.e. tracers) to track the movement of compounds from plant to plant (see here).
• Installing artificial barriers to restrict root growth towards other plants. Thus, any resource benefits reaped by the recipient partner plant compared to a control should be solely due to hyphae/microbial transfer (see here).

Several studies have successfully applied these techniques to validate the existence of CMNs that connect (and move resources between) two individuals, mostly in ex-situ settings to control for other external factors (such as nutrient release via leaf input). To the best of my knowledge, it was also Dr. Simard that pioneered the idea of inter-mycorrhizal connectivity in her landmark paper published in Nature back in 1997, although she did not term it the “mother tree hypothesis” (she used the term “mother tree” in her 2021 book to convey the imagery to laypeople).

Cool! What’s the issue?

For the layman ill-equipped with any ecological knowledge, this does sound like sexy science. Who knew trees could cooperate with one another!

So, what’s the issue?

What got ecologists and other experts rioting is something else in her book. Building on the idea of CMNs, she proposed her (in)famous mother tree hypothesis, where conduits of fungi connect trees together to share resources. The media quickly got excited. Since then, she has put out several talks and publications on the topic, which subsequently led to a surge in the media on inter-tree connectivity and communication (see here, here and here). However, some time in the past five years, a community of ecologists spearheaded by Jason Hoeksema, Melanie Jones and Justine Karst spoke out against her work, criticizing it for unsubstantiated evidence.

Very briefly, here is what I got out of the critique:

• Do CMNs even exist in natural environments (and if so, how widespread they are)? The evidence is already thin enough (my guess is <100 studies) and most (all?) studies have been conducted primarily in the temperate zone (or in greenhouses); never mind the tropics with thousands of coexisting species. Chalk yet another point to tropical ecology bias.
• Were hyphal networks demonstrably continuous? One of the more common ways to study CMNs is to utilize a setup that restricts root growth in the soil matrix, allowing only hyphae to proliferate between the soil zones of two neighboring plants. However, this still doesn’t allow one to see a continuous hyphal network, which will be important for the next question.
• Is there strong evidence for resource transfer through a continuous hyphal network? The positive effects seen in the recipient plants of many CMN experiments abovementioned could be explained as a single mycorrhizal fungus exuding organic compounds into the soil, which is then picked up by a different fungus attached to the recipient tree. To overcome this hurdle, one must prove beyond all doubt that the two mycorrhizal fungi abovementioned are one single, continuous entity (possibly through very precise genotyping).
• Even if the above points have been addressed, do plants derive ecologically significant physiological benefits by being interconnected within these fungal networks?
• Finally, proposing CMNs as a method of cooperation/altruism among coexisting tree species is a huge stretch, challenging a lot of what we know in evolutionary biology which is mostly geared towards competition and self-preservation. Any study that seeks to propose a contrary, novel mechanism must meet very high standards of evidence.

Currently, the debate between the two camps still rages on in the fungal ecology community. You can read it up in the following open-access papers (Dr. Karst’s criticism here; Dr. Simard’s response here).

Working thoughts

I have not followed the literature enough to ascertain the level of evidence for each of Dr. Simard’s and Dr. Karst’s arguments. Nonetheless, I do feel slightly vested in this debate given my experience in mycorrhizal research through my own work. Right now, I am slightly more inclined to Dr. Karst’s conservative approach before asserting a claim as radical as altruism among trees. But that’s just my opinion.

More interesting to me is how and why this topic got picked up by the mainstream media. Ecologists argue about all sorts of issues that appear alien to laymen, be it whether to homogenize soil samples for greenhouse experiments, or whether biodiversity enhances ecosystem functioning (see my previous post for a summary to BEF research). Why did this story among all things get so hyped up? I’m guessing people got really invested in the subject because of its anthropomorphic portrayal of trees exhibiting human tendencies like cooperation and altruism – probably a win in Dr. Simard’s book. However, when excitement takes over logic, it becomes really tempting to push out a story that sounds good on paper yet falls apart under scrutiny. Most of the time, ecology doesn’t offer a clean answer to what goes on in nature. Unfortunately, this time doesn’t seem to be an exception.

If there is a lesson to be learnt through this saga, it will probably be to exercise intellectual patience whenever one reads exciting, provocative headlines about anything in the media. Poor scientific communication coupled with a media team hungry to harvest more clicks can easily spiral into tabloid science articles. Stuff like: “Motherless babies”, or “Planting a trillion trees to fight climate change” or “Perfect preservation of old religious texts” often sound intriguing or agreeable to the reader but paints a vastly different picture past the headlines. Hold the phone!