Temporal variation in herbivore defense has been documented between and within different plant ontogenetic stages, but continuous studies of plant defense trajectories in long-lived species are lacking. To investigate such trajectories, we gathered data on defense characteristics over fourteen years from a population of aspen trees (Populus tremuloides) initially established with 2-year-old saplings. The trees represent twelve replicated genotypes and were grown in a common garden in Wisconsin, USA. Aspen is a highly genetically diverse foundation tree species and has a well-studied defense syndrome comprised of two main classes of compounds: condensed tannins (CTs) and phenolic glycosides (PGs). We collected leaves for chemical analysis in mid-summer each year and measured tree size after each growing season from 2002-2015.
Temporal trajectories of plant defense varied between chemical constituents and among tree genotypes. At the population level (averaged across genotypes) condensed tannin (CT) concentrations remained fairly constant as trees aged. There was significant genetic variation, however, in both average CT concentrations and in developmental trajectories (genotype-average slopes ranged from -3.4% to +3.7% change in dry weight per year). Concentrations of phenolic glycosides (PGs), on the other hand, exhibited exponential decay for all genotypes, with variation in both the starting concentrations and the rate of decline (-6.3% to -12.4% change per year). Our results demonstrate that different plant defenses exhibit different developmental trajectories over multi-decadal time scales, and that such changes are genetically variable. These trajectories will be discussed in the context of growth-defense tradeoffs and sexual dimorphism.