The Internet connects humans all over the world. But did you know that trees also have a social life? Trees share key resources and communicate with one another through a vast underground network. Although you would expect a tree to be a solitary, lonely fellow, they make complex connections with many other trees using this secret language. Not only can trees share information using this system, but they can also share vital nutrients, supporting the health and survival of fellow trees. As biologists learn more about these complex interactions, they have begun to describe these connections between trees as the “wood wide web.”

So how do trees make these connections? The answer lies in fungi! Although we commonly think of fruiting bodies when we talk about fungi, which lie above the ground, the real connections that allow trees to communicate are found underneath the surface. Fungi have long, tubular threads called mycelia that spread out over a vast distance below the surface and on top of tree roots. Soil fungi form critical symbiotic relationships with many plants and trees; both organisms benefit from these interactions. The mycelium of the fungi spread out and collect water and nutrients, especially phosphorus and nitrogen, and bring these back to the tree. In return, the tree or other plant provides the fungi with sugars, which it generates during photosynthesis. However, trees can share more than nutrients–they also send signals to communicate with each other, warning each other about droughts, disease, and insect attacks, for example, using chemical, hormonal and slow-pulsing electrical signals.

Fungi that colonize roots and form symbiotic relationships with plants are called mycorrhizae. These mycorrhizae form networks that connect to multiple trees, enabling the exchange of nutrients between not only the fungi and the tree, but also between trees. Trees are able to cooperate through these networks and can lend a hand when other trees are struggling for nutrients. This is particularly important for young saplings in deeply shaded parts of the forest. Since they lack sunlight to photosynthesize, they rely on their parents and other older, taller trees for sugars, which are shared through their mycorrhizal networks. Trees can also detect stress signals from their neighbors through these networks and increase the flow of nutrients to these trees to support their survival.

The fact that trees share resources rather than competing makes sense evolutionary–trees live the longest and reproduce the most when they are in healthy, diverse forests. There is even evidence that “mother trees” can recognize their kin. These “hub” or “mother” trees are those that are the oldest and largest in the forest, forming the most fungal connections. Research has shown that trees can recognize the roots tips of their kin and distinguish them from the root tips of unrelated seedings. They seem to favor sending carbon to their kin over unrelated trees through their mycorrhizal networks. However, scientists are still not exactly sure how trees can recognize which trees are related to them. 

Not only is learning about the social network of trees fascinating, but it is also important for us to share this knowledge with others to increase public appreciation of trees and prevent the logging of old-growth forests. Mycorrhizal connections are dynamic and complex, and destroying them can affect the health of not only a few trees but the entire forest. Mother trees can be hundreds of years old and form deep, interconnected partnerships with hundreds of other trees. There is still so much we do not know about trees, and we have only recently begun to decode their secret underground language. Having a greater understanding of this language will help us learn how to preserve these vital networks under the threats of climate change and human destruction.