The possibility that communication networks of fungi exist connecting forest ecosystems in a ‘wood-wide web‘ has increasingly gained attention among researchers in recent decades.
Yet it might be more hype than hyphae, according to a perspective recently published in Nature Ecology & Evolution.
Three biologists from the University of Alberta and University of British Columbia in Canada, and the University of Mississippi in the US, argue that other scientists have unwittingly exaggerated the evidence supporting models of highly complex mycorrhizal networks by overlooking the limitations of previous studies.
The biologists analyzed more than 1,500 scientific papers and tallied the number of claims based on weak or missing evidence. They found that the percentage of unsupported claims doubled over the past 25 years, with a bias towards citing positive effects of fungal networks.
“Some of the unsupported citations came from earlier publications of our own,” biologists Jason Hoeksema, Melanie Jones, and Justine Karst write.
“These types of unsupported statements are an issue because we scientists, probably without intent, have become vectors for unsubstantiated claims.”
Models of ‘wood-wide webs’ argue symbiotic networks of fungi and trees provide resources such as water and nitrogen to other plants, while also “whispering” warnings to one another and their offspring about insect predators.
The fact that fungi form interdependent relationships with trees by living inside plant roots (known as mycorrhiza) or by living in the soil close by (known as ectomycorrhiza) isn’t contested. However, the architecture and complexity of vast, resource-sharing systems known as common mycorrhizal networks (CMN) is much harder to pin down. And the evidence for anything vast and complex just isn’t there yet, the biologists say.
“Arguments are now being made to change forest management and policy based on this information,” write Hoeksema, Jones, and Karst.
“[Scientists] may be shaping the public narrative with an increasingly inaccurate characterization.”
Mapping out the fungi and trees in a forest is an arduous task and only five studies have been performed across two forest types; only two species of tree out of an estimated 73,300 worldwide.
These studies can’t show that the fungal connections are permanent, either.
“Hyphae and mycorrhizal roots turn over quickly and are grazed – processes that break connections,” the biologists write.
Experiments have been done using potted plants with various mesh arrangements to prevent roots or fungi (or both) from growing in certain zones. This has made it possible for scientists to examine the impact of denying a plant access to the fungi network.
However, even when these experiments produce positive results (perhaps by observing a plant’s stunted growth due to isolation from the CMN), it’s hard to rule out alternative explanations that could also explain the results. For instance, adding a mesh around a plant root system is quite an artificial intervention and it could change the makeup of the pathogens or fungi in the soil, which could influence the growth of a plant.
These potential confounding factors are not properly controlled in many experiments, the researchers argue. And, even when experimental limitations are pointed out by the authors, they are often overlooked by researchers citing the original studies. This gives the impression that the evidence to support fungal networks is much stronger than it really is.
Hoeksema, Jones, and Karst make several recommendations for experiments that could pin down the existence of fungal networks, including mapping the fungi in a wide range of forests worldwide, using dyes to trace water flowing through the network, and collecting additional data on possible confounding factors.
“Let us devise new experiments, demand better evidence, think critically about alternative explanations for results and become more selective with the claims we disseminate,” the trio urges.
“If not, we risk turning the wood-wide web into a fantasy beneath our feet.”
This perspective article was published in Nature Ecology & Evolution.