Trees have a symbiotic relationship with fungi, which are neither plant nor animal. Fungi cannot make their own food through photosynthesis, and their cell walls consist of chitin, a substance that is found in insects. As such, they are dependent on their “organic connections” with things that they can eat (50). Wohlleben clarifies that not all fungi are beneficial to trees; some are parasites that kill trees by living and feeding on them. Over time, underground networks of fungi called “mycelium” can become huge, occupying the soil under hundreds of acres of land (50). The author notes that this ability makes them the “largest living organisms in the world” (50). When trees want to partner with a beneficial fungus they must keep their roots hairs open so that the threads of fungi can grow into them.

Once they are connected, the fungi function as an extension of the tree roots, enabling them to reach out further and acquire more nutrients and water, and connect with neighboring fungal networks. The fungi will also help keep the tree healthy by filtering out pollutants and heavy metals and by countering harmful bacterial or fungal attacks. Fungi also benefit from this partnership. They receive sugar and carbohydrates from their host tree, sometimes taking as much as one third of the tree’s food (51).