#see also: Peter Wohlleben's anthropomorphism of trees

[probablyasocialecologist]

The three of us have studied forest fungi for our whole careers, and even we were surprised by some of the more extraordinary claims surfacing in the media about the wood-wide web. Thinking we had missed something, we thoroughly reviewed 26 field studies, including several of our own, that looked at the role fungal networks play in resource transfer in forests. What we found shows how easily confirmation bias, unchecked claims, and credulous news reporting can, over time, distort research findings beyond recognition. It should serve as a cautionary tale for scientists and journalists alike.

First, let’s be clear: Fungi do grow inside and on tree roots, forming a symbiosis called a mycorrhiza, or fungus-root. Mycorrhizae are essential for the normal growth of trees. Among other things, the fungi can take up from the soil, and transfer to the tree, nutrients that roots could not otherwise access. In return, fungi receive from the roots sugars they need to grow.

As fungal filaments spread out through forest soil, they will often, at least temporarily, physically connect the roots of two neighboring trees. The resulting system of interconnected tree roots is called a common mycorrhizal network, or CMN.

When people speak of the wood-wide web, they are generally referring to CMNs. But there’s very little that scientists can say with certainty about how, and to what extent, trees interact via CMNs. Unfortunately, that hasn’t prevented the emergence of wildly speculative claims, often with little or no experimental evidence to back them up.

One common assertion is that seedlings benefit from being connected to mature trees via CMNs. However, across the 28 experiments that directly tackled that question, the answer varied depending on the trees’ species, and on when, where, and in what type of soil the seedling is planted. In other words, there is no consensus. Allowed to form CMNs with larger trees, some seedlings seem to perform better, others worse, and still others seem to behave no differently at all. Field experiments designed to allow roots of trees and seedlings to intermingle — as they would in natural forest conditions — cast still more doubt on the seedling hypothesis: In only 18 percent of those studies were the positive effects of CMNs strong enough to overcome the negative effects of root interactions. To say that seedlings generally grow or survive better when connected to CMNs is to make a generalization that simply isn’t supported by the published research.

Other widely reported claims — that trees use CMNs to signal danger, to recognize offspring, or to share nutrients with other trees — are based on similarly thin or misinterpreted evidence. How did such a weakly sourced narrative take such a strong grip on the public imagination?