Trees can communicate distress through airborne chemical signals

In the 1980s, South African researchers discovered that acacia trees being browsed by antelope began producing toxic tannins in their leaves within minutes — and that downwind trees also increased tannin production before being touched. The trees were emitting ethylene gas as a chemical warning. Subsequent research found that trees communicate insect attacks through volatile organic compounds and underground fungal networks, prompting neighbouring trees to produce defensive chemicals preemptively. This was widely dismissed when first proposed; it is now an established area of plant science called 'plant bioacoustics and signalling'.

Source: Karban, Plant Sensing and Communication (2015)