Chemical defense over decadal scales: ontogenetic allocation trajectories and consequences for fitness in a foundation tree species

1. Expression of herbivore defense traits can change dramatically during the course of plant development. Little is known, however, about the degree of genetic or sexual variation in these ontogenetic defense trajectories or whether the trajectories themselves are adaptive, especially in long‐lived species.

2. We used a 13‐year dataset of chemical defense traits, growth, and survivorship from a common garden of trembling aspen (Populus tremuloides) genotypes to document long‐term defense trajectories and their relationship to tree fitness during juvenile and early mature stages.

3. Overall, concentrations of the two principal classes of aspen defense compounds (salicinoid phenolic glycosides [SPGs] and condensed tannins [CTs]) decreased to differing degrees in foliage of juvenile trees and then remained relatively constant in maturity. Initial values, juvenile rates of change, and average mature values all exhibited significant genetic variation for both SPGs and CTs.

4. Relati...