Today's Editorial

Today's Editorial - 10 March 2023

Do trees really talk to each other?
Source: By SANDHYA RAMESH: The Print
Bengaluru: If humans have the world wide web, trees too are connected on the ‘wood wide web’. Or so popular science claims. But this may be more fiction than fact.
In the past few years, there’s been a lot of discussion about how fungal networks attached to roots of trees in forests can transmit molecules and signals between plants. Many news articles, documentaries, podcasts, TED Talks, and other popular science media have talked about how fungal root networks enable trees to ‘talk’ to each other, send alarm signals, share resources, and more.
Now, a team of experts from Canadian and US universities who study these networks have claimed that citation bias and overinterpretation of results could be leading to a misunderstanding of how these networks work. In their review of existing literature and citation analysis of existing studies and early influential papers, they found that common claims made about these networks were cited from unsupported statements. In one study, 84 per cent of citations were unsupported.
The peer-reviewed perspective is a scholarly review of primary research literature and was published in the journal Nature Ecology & Evolution 13 February 2023. It concludes that there is no evidence for some popular claims and that further studies are needed to understand how the networks work.
Underground symbiosis
Some fungi in the soil interact with root systems and colonise root tissues, and then start to regulate plant nutrition and soil health.
The relationship between plants and these fungi is typically symbiotic — the plants produce carbohydrate molecules through photosynthesis and supply energy to the fungi, while the fungi break down minerals in the soil and supply them to the roots, along with water. This relationship is called mycorrhiza.
An underground network of such interconnected micorrhizal fungi and roots is called a common mycorrhizal network or CMN. Such a network physically connects at least two different plants’ roots through the same genetic individual fungus. Such mycorrhizal fungi have the potential to facilitate plant-to-plant interactions by transferring molecules — including the essential carbon, nitrogen, and phosphorus — from one plant to another through roots.
Among the many claims made by early studies on CMN, the researchers dispute three big ones — that CMNs are widespread in forests that resources are transferred through CMNs to help seedling growth, and that plants send warning signals through these networks to offspring.
Are CMNs widespread?
Since tree seedlings grow close to trees, it is believed that CMNs exist connecting trees and their seedlings. However, studying the extent of this network is not feasible with current technology, and sampling would fragment the network.
Instead, scientists obtain samples of roots, and identify the fungal and plant DNA, subsequently mapping the types of fungi to their trees in forests. These fungi exist as ‘genets’ or colonies of genetically identical individuals. Mapping these genets with trees has provided strong evidence for the fungi linking roots of trees that are in close proximity.
Upon analysing the existing research, the team discovered that there have been only five studies that have mapped genets and trees across only two types of forests. Just two tree species of the 73,300 worldwide, and only three species of fungi, have been mapped to understand CMNs.
Additionally, there isn’t any data available about the permanence and extent of fungal and root connections. Many roots are destroyed by animals or simply die, killing fungal connections with them and disconnecting from the network.
Alternatively, different trees could have been colonised by different genets of the same fungus, indicating that a CMN would not exist even if individual roots have genetically identical fungi.
It is often assumed that when plants of the same mycorrhizal type grow together, a CMN would be present. The scientists, however, contest these claims saying, there isn’t enough evidence to support widespread CMN networks or continuous fungal links between trees.
Can trees transfer resources to seedlings?
The first study that could be replicated and which confirmed resource transfer in the field was published in 1997. However, the authors claim the results from this study have been popularly interpreted as CMNs being able to equalise resources within a plant community.
Despite showing evidence that these fungi could transfer molecules between plants, no studies have shown that this phenomenon helps in improving plant performance.
The scientists evaluated 26 field studies of CMNs’ resource transfer between plants and of seedling performance (growth, survival, or physiology).
Resource transfer can be measured by looking at a physical barrier in the flow of the molecules. Since CMNs cannot form between trees of different mycorrhizal types, any resource transfer between two trees with different fungi will have to pass through the soil. This soil acts as a natural physical barrier and enables scientists to detect resource transfer, albeit not entirely accurately. Additionally, scientists have also simulated a natural barrier using mesh bags, which restricted fungal access to roots.
However, studying natural barriers cannot provide data about seedling performance. Additionally, it is not possible to establish continuous connection between plants of the same fungi types as any disturbance will fragment the network.
Furthermore, studies that estimated seedling performance between the same mycorrhizal types were not able to establish transfer through the soil. “From our review of field studies in forests, we conclude that, for every study interpreting CMNs as mediating interplant resource transfer or benefiting seedling performance, the results can be explained without invoking CMNs,” write the authors.
Do parent trees talk to offspring?
An increasingly popular idea today is that trees preferentially communicate with offspring by sending resources or signals as warnings of damage (eg. insect bites). Many experts say studies have claimed that injured trees send infochemicals as alerts to other trees, even going back to the early 80s. But the authors found no evidence from any peer-reviewed published studies to support this claim.
There is only a single peer-reviewed study that addressed signalling after an insect bite, but it was conducted in a greenhouse. Signalling stopped as soon as root interactions between neighbouring seedlings were established.
In another study where carbon transfer was established, it was through a soil solution and not fungi. In one study where trees were attacked by insects, seedling performance was evaluated years after the attack without any data on signalling. “In summary, there is no current evidence from peer-reviewed, published field studies to support this claim,” write the authors, adding, “.. the results from these studies either do not support or actually run counter to the claim.”
Citation bias and future studies
The authors found 18 influential field studies that had at least 50 citations each, ie. the results of a study were used as a reference for at least 50 other studies. In total, they evaluated 593 papers citing seven such influential studies on CMN structure and 1,083 papers citing 11 influential studies on CMN function. Citations that were supported by evidence were marked as “supported” and the rest as “unsupported”.
For both sets of studies, they found that the rate of unsupported citations climbed over time as studies built upon one another. Ultimately, currently, a quarter of studies on CMN structure and nearly half of all citations about CMN function are unsupported.
For each year of the publication record for studies on CMN structure, 1,047 times more unsupported citations were observed, and for each year of CMN function studies, 1,032 additional unsupported citations were present. These unsupported citations and statements tend to overstate results, explain the authors, which cause subsequent studies to ignore alternative explanations.
Overinterpretation is the most common reason for unsupported citations. For example, one study concluded that CMNs had the potential to form in a region, and further studies cited it as evidence for the presence of CMNs.
“Indeed, some of the unsupported citations came from earlier publications of our own,” admit the researchers. They say that studies of CMNs should be wary of gaps in knowledge and the uncertainty that “popular science has overlooked”.
They make a series of recommendations for future experiments, including mapping genotypes of trees and fungi across the world in a variety of forests, testing the true impact of CMN on tree growth, designing experiments to observe interplant resource transfer, using dye tracers for tracking water flow and understanding the resilience of CMNs.
“Let us devise new experiments, demand better evidencethink critically about alternative explanations for results and become more selective with the claims we disseminate,” said the authors in their concluding statement. “If not, we risk turning the wood wide web into a fantasy beneath our feet.”