Can trees “talk” through fungal networks?

Darwin saw life in a forest as a constant struggle between trees that needed to compete for access to sunlight, space, water and nutrients. The winners in the competition were the fittest individuals, and they would grow tall at the expense of others. But recently there has been a change in understanding and people have begun to see forests a cooperative systems. They talk of trees sharing chemical information, sending resources to their offspring and signaling when they’re injured or diseased – all by means of a network of fungal threads that appear to connect them underground. 

The concept of trees “talking” to one another via fungal threads that spread out from the roots is so intriguing, it’s been promoted in popular media – even being raised in the Apple TV show Ted Lasso – and been featured in recent literature, both fictional and non-fictional. The network of fine fungal threads that branches out between tree roots in forests has even been dubbed the “wood-wide web,” but the science behind these ideas is unproven, cautions University of Alberta expert Justine Karst.

In collaboration with Melanie Jones of the University of British Columbia and Jason Hoeksema of the University of Mississippi, Karst has published a peer-reviewed article in Nature Ecology & Evolution that reviews the current scientific literature about the topic. The researchers challenge three popular claims about the capabilities of underground fungal threads, known as common mycorrhizal networks (CMNs) that connect the roots of multiple plants. 

“It’s great that CMN research has sparked interest in forest fungi, but it’s important for the public to understand that many popular ideas are ahead of the science,” said Karst, associate professor in the U of A’s Faculty of Agricultural, Life & Environmental Sciences.

Fungi are living organisms such as molds, yeast and mushrooms, some of which form threads as they grow underground. They cannot make their own food through photosynthesis, and have a special symbiotic relationship with trees and other green plants whereby they acquire food products in exchange for providing nutrients and water to the plants. So, while the existence of CMNs has been scientifically proven, some of the claims about the benefits they offer to trees and their seedlings have yet to be substantiated.

The researchers reviewed published evidence from existing field studies in order to evaluate the popular claims. They found that one of the claims, that CMNs are widespread in forests, isn’t supported by enough scientific evidence. Not enough is known about CMN structure and its function in the field, as there are “too few forests mapped.”

The second claim, that resources such as nutrients are transferred through CMNs from adult trees to seedlings, and that they boost the survival and growth rates of the seedlings, was also found to be questionable. A review of 26 studies, including one in which Karst is a co-author, established that while resources can be transferred underground by trees, CMNs don’t necessarily bring about that flow. In addition,  seedlings typically don’t benefit from CMN access. Overall, the review revealed roughly equal evidence that connecting to a CMN would improve or hamper seedling growth or survival, with neutral effects most commonly reported.

The third claim, that adult trees preferentially send chemical “warning signals” of insect damage to young trees through CMNs, is not backed up by a single peer-reviewed, published field study, noted Professor Karst and her co-authors.

The researchers say unsubstantiated information can shape and distort the public narrative about CMNs, and that could, in turn, affect how forests are managed. It is important, therefore, to back up these claims with solid empirical evidence before action is planned.

“Distorting science on CMNs in forests is a problem because sound science is critical for making decisions on how forests are managed. It’s premature to base forest practices and policies on CMNs per se, without further evidence. And failing to identify misinformation can erode public trust in science,” concluded the researchers.

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By Alison Bosman, Earth.com Staff Writer

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