Induced Defense: Herbivory on Juvenile vs. Adult Growth Stages of Trees
Plants contain “plant secondary metabolites” (PSMs) that confer resistance against herbivores and pathogens. The largest classes of PSMs are phenolics, terpenoids, and alkaloids. We distinguish constitutive and induced chemical defenses. The first occur in an entire plant or some of its parts as normal constituents, the latter are formed in response to herbivory or some other trauma to the plant. In some cases mere clipping induces defenses, in others additional factors, such as those in saliva of a herbivore are essential. Some northern trees such as green alder or paper birch respond to browsing by mammals such as snowshoe hares by growing adventitious shoots that are richer in PSMs than older twigs (Bryant 1981). These new shoots represent the juvenile-type growth form, while the older twigs on the tree or shrub are of the adult-type growth form. The fresh shoots with induced PSMs are avoided by hares for 1–3 years. In our area, quaking aspen, Populus tremuloides, that is growing back after cutting by beavers is more heavily defended during its first few years of regrowth (Basey et al. 1990).