Are Trees Really Plumbed Into Each Other by Fungi?

You may have heard a beautiful story about forests. Beneath our feet, it goes, trees are linked by a vast underground web of fungal threads, sharing food, passing warnings and even caring for their young. It is sometimes called the wood wide web, and it has inspired books, films and countless articles. Part of this story is solidly true. But in 2023, three researchers who study these fungi published a careful review arguing that much of the rest has run far ahead of the evidence.

Start with what is well established. Scientific American explains that fungal filaments called hyphae intertwine with the tips of tree roots. The fungi break down minerals in the soil that the trees can then take into their roots, and in return the fungi get a steady supply of sugar from the trees. Writing in Undark, the researchers Melanie Jones, Jason Hoeksema and Justine Karst confirm that fungi grow inside and on tree roots in a partnership called a mycorrhiza, and that these partnerships are essential for the normal growth of trees.

The fungal threads can also connect the roots of different trees, forming what scientists call common mycorrhizal networks. The popular story goes much further, claiming that trees use these networks to share carbon, water and nutrients, to send chemical warnings about insect attacks, and to let older trees support nearby seedlings. According to Jones, Hoeksema and Karst, however, there is very little that scientists can say with certainty about how, and to what extent, trees actually interact through these networks.

Their review, published in the journal Nature Ecology and Evolution, looked closely at the evidence. In Undark they report reviewing 26 field studies of fungal networks in forests. On the question of whether seedlings benefit, they examined 28 experiments, and in only 18 percent of them were the positive effects of the networks strong enough to overcome the negative effects of root interactions. Scientific American adds that, according to the review, substances that do move between neighbouring trees might travel through root contact or through the soil rather than through fungi.

The idea that trees send underground warnings is on especially shaky ground. Scientific American reports that this claim rests on a single greenhouse study and has not been tested in a forest. Justine Karst told the magazine that the researchers did not want to kill anyone’s joy, curiosity or wonder about the forest, but did want to tamp down on some of the misinformation. The authors also found that fewer than half of the statements made about the original field studies in recent papers could be considered accurate.

So are trees really plumbed into each other by fungi? Fungi certainly wrap around tree roots, and the partnership matters enormously for how trees grow. Some fungal connections between trees exist too. But the grand picture of a forest-wide sharing network is, for now, an open scientific question rather than a settled fact. That makes a walk in the woods no less wonderful. There is a hidden world beneath every tree, and scientists are still working out what it is really doing.