ABSTRACT
Trees and other woody plants differ from herbs in many aspects of their anatomy, physiology and ecology. Yet they are known to release the same major classes of volatile compounds as herbs. This chapter summarizes the major factors influencing volatile emission from trees and explores their biological functions. Herbivore attack consistently increases volatile emission in trees, as in most herbs, though not all classes of volatiles are equally affected. Chewing herbivores trigger greater emission from tree foliage than piercing and sucking herbivores, and herbivores as a group induce greater emission than pathogens in terms of blend complexity and rate of release. Among abiotic factors, increases in light, temperature and water supply tend to increase emission though most of the data are for terpenoid volatiles, especially isoprene. Tree volatiles are known to serve as direct defenses against herbivores and pathogens in some instances and as attractants for herbivore enemies. A role in intra-plant signaling is potentially very important in trees given their large size, multiple ramets and poor long-range vascular connections, but not enough information is yet available to evaluate the significance of volatile signaling in tree canopies. Terpenoid volatiles have long been assumed to help trees cope with high temperature and oxidative stresses, but their mode of action is still unknown.
