If you think forests have underground networks allowing trees to feed their young or alert them to harmful attacks, you’ve sadly probably been misled by an ill-advised advertiser or an overly enthusiastic scientist. This is the conclusion of a study published in Nature Ecology & Evolution on February 13. It examines several much-publicized claims about mycorrhizal networks, the result of symbiotic associations between filamentous fungi and tree roots.
Justine Karst (University of Alberta, Edmonton), Melanie Jones (University of British Columbia) and Jason Hoeksema (University of Mississippi) did not question the existence of these mycorrhizae, which allow the fungi to benefit from the sugars produced by the trees in exchange for the water and minerals they help draw from the soil. What these researchers wanted to verify was the existence of other functions supposedly performed by these underground networks.
After reviewing the scientific literature, they found that the actual extent of these networks is poorly documented on the ground and that some of the roles they are thought to play, such as assisting seedlings, are “insufficiently substantiated” – the evidence may even be missing altogether. Justine Karst and her colleagues believe that over time scientists have taken an increasingly biased view of these studies, tending to “overestimate results and ignore confounding factors in order to promote the beneficial effects of these networks in forests.”
This idea that trees – even of different species – could communicate with each other and share resources underground is rooted in a study published by the journal Nature in 1997, and featured on the under the title “Wood-Wide-Web.” With the help of radioactive markers, Suzanne Simard, a professor of forest ecology at the University of British Columbia, showed for the first time in a controlled field study, bidirectional carbon transfers between birches and Douglas firs likely to benefit shrubs whose photosynthetic capacities were reduced by the shade of their elders.
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‘Rogues in the network’
Since then, the notion of cooperation between trees – which goes against the Darwinian vision of competition for light, water and nutrients – has enjoyed a large success. It has been echoed in bestsellers such as Peter Wohlleben’s The Hidden Life of Trees (Greystone Books, 2015), in a number of documentaries, or TED talks, such as the one by Simard. Titled “How trees talk to each other,” which has been viewed 5.5 million times.
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