Physical architecture, resource acquisition/allocation, and exposure to herbivores change during the course of plant development. Because resistance and tolerance, the two principal forms of plant defense, are differentially affected by these factors, the relative costs and benefits of individual defense traits likely exhibit ontogenetic patterns. Here, we synthesize data from numerous empirical and observational studies to address ontogenetic patterns of resistance and tolerance in trembling aspen (Populus tremuloides).

Results/Conclusions

Results from our studies have shown that levels of chemical defenses (phenolic glycosides) in aspen are particularly high and genetically variable in very young trees. Both average levels of phenolic glycosides, and genetic variation thereof, decline with tree maturation. Accordingly, chemical defenses confer resistance to herbivore attack in young aspen trees, but are ineffective in deterring wholesale, episodic defoliation in mature trees. Aspen also exhibits genetic variation in tolerance to defoliation. Tolerance is positively correlated with relative biomass allocation to stem, as opposed to root, tissues. These patterns suggest that chemical defenses may be targeting mammalian herbivores in particular, and that as trees mature, defense strategies shift toward tolerance as an adaptation to episodic insect outbreaks. A synthetic model of tree defense against herbivory, incorporating ontogenetic trajectories and genetic variation in resistance and tolerance, will be presented.