Cut into the roots of a Douglas fir and you find threads of fungus wrapped around them, extending outward into the soil for meters. Those threads connect neighboring trees. Carbon moves along them. Chemical signals move too. The question of how do trees communicate has a real answer, but it is narrower and messier than the version that circulates online.
First, What “Communicate” Would Even Mean
A tree has no brain, no nerves, and no way to intend anything. When biologists use the word, they mean something specific: a signal produced by one organism changes the behavior of another, and both the production and the response are shaped by natural selection.
That definition is loose enough to include a lot. A wounded leaf releases a gas that makes a neighboring leaf ramp up its defenses. A root tip floods with sugar that a fungal partner carries elsewhere. None of it requires a mind, and all of it is measurable.
The Underground Network
Most plants on Earth form mycorrhizae — a partnership between roots and fungi. The fungus gets sugar; the tree gets water and phosphorus that the fungal threads reach far more efficiently than roots can. In forests, one fungus can colonize many trees at once, linking them into a shared network.
So how do trees communicate underground? Through those fungal threads, mostly. In a now-famous 1997 experiment, Suzanne Simard traced carbon isotopes and found sugar moving from paper birch to shaded Douglas fir seedlings through the shared network. Later work suggested that older “hub” or mother trees are more heavily connected than young ones, and that seedlings can receive carbon from neighbors.
What Actually Moves Through the Network
- Carbon — sugars from a tree with surplus, moving toward shaded or defoliated neighbors
- Water and nutrients — phosphorus and nitrogen shuttled between plants and fungi
- Defense signals — jasmonic acid and similar compounds associated with pest attack
- Pathogens — the same connections that carry help can carry disease
That last point is the one popular accounts usually skip. A network that lets resources flow is also a network that lets infection flow, which is part of why some researchers think the “sharing” framing is too generous.
The Pushback, and Why It Is Fair
In 2023, a review of the field concluded that the strongest claims had outrun the evidence. The authors found a pattern of positive citation bias: dramatic findings from small greenhouse studies were cited as established fact, and later papers cited the reviews rather than the original data.
| Claim | Evidence status |
|---|---|
| Fungal networks physically link trees | Well established |
| Carbon can move between connected trees | Demonstrated in several systems |
| Trees preferentially feed their own kin | Weak and inconsistent |
| Trees “warn” each other and mount shared defense | Possible, not demonstrated in forests |
| Forests are a cooperative economy | Interpretation, not measurement |
None of this means the network is imaginary. It means the network is real and the stories built on top of it are partly speculation. Fungi are not charities; they are trading partners with their own interests, and a tree may be paying for services rather than donating to a cause.
Airborne Signals Are Better Documented
The cleaner evidence for plant signaling comes through the air, not the soil. When sagebrush is damaged by insects, it releases volatile organic compounds, and nearby plants exposed to those volatiles produce more defensive chemicals. Lima bean plants do something similar. Acacias browsed by antelope release ethylene, and neighboring acacias begin loading tannins into their leaves within minutes.
The mechanism is straightforward chemistry: a damaged plant leaks volatile compounds, and neighbors with receptors for those compounds respond. It is not a message in any intentional sense, but it is information transfer, and it changes outcomes.
Why the Argument Matters
Forest management is downstream of this question. If old trees are hubs that support regeneration, then cutting them has costs beyond the timber. If the network is mostly a market in which everyone acts in their own interest, the case for retention forestry is still strong but the reasoning changes.
Mycorrhizal inoculants sold to gardeners run into the same problem. Adding fungal spores to soil can help in degraded or sterile ground, but in healthy forest soil the resident community usually outcompetes whatever you add. The honest summary is that the underground economy is real, influential, and considerably less sentimental than the version on social media.
Keep Exploring
- What is pollination? — the other partnership plants cannot live without
- How plants turn sunlight into food — where the sugar being traded comes from
- What is soil? — the medium all of this happens in
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