This research seeks to explain how genetic, environmental, and ontogenetic factors shape the expression of defense traits (chemical resistance and tolerance) in plants, and how these in turn affect tree resistance to herbivore (insect and mammal) attack, herbivore susceptibility to natural enemies, and ecosystem processes such as nutrient cycling. Our major experimental system consists of aspen (Populus tremuloides) and a variety of aspen-feeding animals (ranging from gypsy moth to elk). The secondary metabolites of importance include phenolic glycosides and proanthocyanidins (condensed tannins). Current and future emphases address costs of defense (resistance and tolerance) to mammals and insects in evolutionary context. Several aspen common gardens, including a gene association mapping garden (WisAsp), provide opportunities for evaluating phenotypic plasticity and for characterizing genomic components of phenotypic traits.