Maple trees release chemicals when attacked by insects. This is absorbed by leaves of other branches which then make callose to protect themselves if they are attacked.There is some evidence that plants make chemicals in the roots which it uses to communicate with neighbouring plants if it is under water stress.
Volatile organic compounds (VOCs) diffuse through the air from the plants which may cause gene switching that produce chemicals that repel insects or attract predators or parasites of the insects. Sometimes VOCs stimulate neighbouring plants to produce the VOC before they are actually attacked.