The idea that trees communicate and share resources with each other via an underground network of fungi, sometimes called the “wood wide web”, has little evidence to back it up, say researchers who have reviewed the science.
Mycorrhizal fungi live on plants’ roots. They are thought to be beneficial to trees and to grow into vast networks beneath the forest floor. In recent years, books and documentaries have popularised the idea that these fungal networks help trees recognise related individuals – allowing them to shuttle resources to feed their own seedlings – and even send alarm calls to their kin when under threat.
The wood wide web, with its evocation of nature as a system of harmony and cooperation, has captured the popular imagination, even inspiring Richard Powers’s Pulitzer prize-winning novel The Overstory and the blockbuster movie Avatar.
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The problem is there is little evidence that any of this is true, says at the University of Alberta in Canada.
Karst and her colleagues reviewed 1676 studies on the structure and function of mycorrhizal networks and were surprised to find that three major claims about fungi’s partnership with trees are based on only a few studies that have been cited repeatedly.
Firstly, the authors say more studies are needed to establish whether mycorrhizal networks really are widespread in forests.
Researchers have mapped mycorrhizal networks by analysing fungal and plant DNA at their roots, but this has only been conducted on two of the world’s 73,300 tree species.
“There’s some evidence and we do think it’s possible that these trees are connected, but as for the structure of the mycorrhizal networks, we don’t have a good handle on that,” says Karst. It is also unclear how long those networks last, she adds.
Mutual aid
Secondly, when the researchers looked at studies testing whether the trees could help their seedlings by sending them nutrients, they found less evidence.
The most-cited studies compare two sets of tree seedlings: one planted inside mesh bags and another planted freely in the soil. The mesh lets water and nutrients in and out, but blocks fungi, preventing the mycorrhizal networks from forming.
Those studies show that tree seedlings without the mesh fare better, suggesting that they prosper as mycorrhizal networks connect trees to one another.
But, by restricting the fungus, the experiments may do more than just prevent fungi from communicating. The mesh could also restrict the amount of soil the fungus connected to the seedling can tap into, impairing other functions beneficial to the seedling, such as taking up water and nutrients.
“It has nothing to do with connections to mycorrhizal networks. It has everything to do with the amount of resources that these fungi can tap into, so it’s a confounding effect,” says Karst.
Putting aside the limitations of the methodologies used in the most-cited studies, the authors say they found little evidence for the seedling idea. Out of 28 field studies using mesh, only five found that seedlings did better in survival or growth when connected through a mycorrhizal network and just as many studies found negative effects, the review found.
“Some of these experiments were highly replicated and even then still found either no significant effect or found a negative effect of mycorrhizal networks on seedling performance,” says at the University of Mississippi, a co-author of the review.
Family ties
Thirdly, the team also found little evidence that the trees could recognise their kin or send alarm calls when threatened.
The only peer-reviewed study suggesting that trees can communicate with one another through a mycorrhizal network was conducted in a greenhouse. The level of carbon passing through the fungus and the activity of defence enzymes in recipient plants increased when the trees , suggesting they were communicating warning signals. For some reason, however, that effect ceased when the roots of the trees were allowed to intermingle as they would in a forest.
Some studies suggest that carbon can be transmitted between trees through the mycorrhizal network; however it’s possible they could have been transmitted through soil, not the fungi.
“The evidence is just so scant, and even in some cases contrary, so to build a narrative that trees are [recognising kin or sharing resources] is unfounded,” says Hoeksema.
Karst and her colleagues call for more mapping of mycorrhizal networks to understand their structure and for more measurements to be taken to establish their activity.
Hoeksema says studies that have found different outcomes for trees depending on whether they are connected by mycorrhizal networks or not suggest that they are having some impact.
“​​I suspect that, under certain circumstances, these mycorrhizal networks have important effects on the physiology and the growth of trees, especially young trees,” he says.
Hoeksema says what is clear is that fungi play a significant role in the life of trees, passing them nitrogen, phosphorus and other nutrients from the soil, while the trees give the fungi sugars.
“There’s actually plenty of strong evidence for all kinds of cool, amazing aspects of forest-fungal interactions that we could equally tell great stories about that are based on solid science,” he says.
at the University of British Columbia in Canada, who has written books on how trees communicate through fungi, says she stands by her research.
“Reducing ecosystems to their individual parts hinders us from understanding and appreciating the emergent relationships and behaviours that make these complex ecological systems thrive,” she says. “For decades, a compartmentalised approach has hindered us from better understanding why forests help regulate global climate and harbour such rich biodiversity. It is my hope that, in time, forest scientists will come to understand ecosystems as complex systems.”
Nature
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