Chemical defense over decadal scales: Ontogenetic allocation trajectories and consequences for fitness in a foundation tree species
Cope, Olivia L. | Kruger, Eric L. | Rubert‐Nason, Kennedy F. | Lindroth, Richard L.
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. 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. 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. Relationships between defense trajectory parameters and metrics of tree fitness (growth and survivorship) depended on compound type and tree sex. Females with higher‐allocation SPG trajectories (high initial juvenile concentrations, slow juvenile declines, high mature concentrations) grew more slowly relative to females with lower‐allocation trajectories. In males, higher‐allocation SPG trajectories had a lesser effect on growth but were associated with reduced mortality. Juvenile CT trajectories were not correlated with tree fitness, but average CT concentration in maturity was positively related to growth in females. These results suggest that ontogenetic defense trajectories are adaptive and subject to natural selection. Genotypic variation and ontogeny shape tree defensive chemistry, both independently and interactively. These patterns of defense expression have the potential to structure trophic interactions and the genetic composition of forests in both space and time. A free Plain Language Summary can be found within the Supporting Information of this article.Show more [+] Less [-]