Is the "wood-wide web" real or exaggerated?

Since the discovery of mycelial networks in the roots of trees back in the 1990s, there have been an awful lot of scientific citations and mainstream media mentions of it. This has given rise to all sorts of exaggerated claims that trees share food and water across these underground networks, and even communicate with each other and warn each other of danger, etc. This is partly, I think, because it's such a nice idea - often described by the cute moniker the "wood-wide web" for its similarity to the Internet in many people's minds - that people really WANT it to be true.

And it is kind of true, but maybe not in the way that many people think, or at least not to the extent that people believe, as more recent meta studies are starting to make clear. Yes, trees in a forest are linked together underground by a network of thread-like strands of fungi called hyphae, which grow out from tree roots, and are often (but not always) manifested as mushrooms above ground. The trees receive essential nutrients from the soil through these networks, and in turn the fungi receive sugars and fats from the trees through their roots.

These fungal networks, or mycorrhizas, do also, to some extent, connect and interact with each other, and can transmit some resources from one tree system another. For example, it has been shown that carbon can be transferred from the mycelial system of one tree to that of another, but it turns out that the amounts transferred are tiny, and remain almost exclusively within the mycorrhizal networks, and so are not really transferred from one tree to another. Also, such transfers are almost certainly initiated and driven by the appetites of the fungi themselves and not by the trees. This is not plant-based altruism as such; it is more the fungi just doing what they need to do to survive and thrive.

Some of the evidence being quoted is not as clear-cut as we might like. There is some evidence of the transmission of defensive information in bean plants, and some evidence of carbon and water being transferred between Japanese red pine and Scots pine tree saplings, but only in lab conditions. There are also possible alternative explanations to many of the findings, so it is hard to be definitive about causal relationships. Positive citation bias and increasing exaggeration over time, as well as the problem of using simplistic anthropomorphic language to explain these complex scientific phenomena in layman's terms, all add to the confusion.

All in all, the science is pretty woolly and inconclusive in the area, and the function of common mycorrhizal networks, however cool, may well be significantly overstated. There is certainly an awful lot to learn about soil ecology and what happens under the earth. Just don't expect the trees to be talking to each other.