Trees Can Send Warnings to Other Trees Through the Air

When a tree is under attack from insects, it doesn't just sit there and take it; it broadcasts a warning. This remarkable form of plant communication was first documented in 1983 and has since been observed in over 30 different plant species. The tree releases a specific cocktail of airborne chemicals known as volatile organic compounds (VOCs). These compounds, which we might perceive as a grassy smell, travel through the air to neighboring trees, carrying a message of the impending threat.

This airborne alert system is surprisingly sophisticated. The chemical composition of the VOCs can indicate the specific type of threat, distinguishing between different insect attackers. Trees downwind receive these signals and begin to mount a defense before the insects even arrive. This pre-emptive response can involve increasing the production of toxins in their leaves to make them less palatable or even releasing other VOCs designed to attract the natural predators of the attacking insects, essentially calling for backup.

One classic example of this is the African acacia tree. When a giraffe begins to eat its leaves, the tree releases ethylene gas. Nearby acacia trees detect this gas and respond by pumping tannins into their leaves, which can be toxic (Review) to herbivores in large quantities. This intricate chemical dialogue demonstrates a complex and dynamic community defense system within forests, where trees work together to fend off common threats. This fascinating natural phenomenon showcases the hidden and complex lives of trees.