Within-crown variability in herbivore performance and leaf traits

Functional trait variation within individual plants is predicted to have important ecological consequences. However, our understanding of the sources contributing to subindividual trait heterogeneity, and the ramifications thereof, is poor.
In a common garden, we sampled multiple genotypes of mature trembling aspen (Populus tremuloides) at different vertical crown levels and quantified the contributions of genetic, spatial and biotic (herbivory) factors to subindividual morphological and chemical leaf trait variance. Bioassays using gypsy moth (Lymantria dispar L.) caterpillars were conducted to study impacts of spatial and genotypic factors on insect performance.
Crown position was the primary source of subindividual variation in leaf morphology and explained more of the variance in morphological traits than tree genotype. Effects of spatial, genetic and biotic factors on within-crown chemistry were highly compound-specific. For many compounds, changes in within-crown concentrations differed among genotypes, revealing that genotypic variation not only contributes to phytochemical diversity among individuals of the same population, but also affects subindividual chemical trait heterogeneity. For many defense related compounds, total within-crown variance itself differed among genotypes and was in many cases a heritable trait. Bioassays revealed that effects of crown position on caterpillar performance varied among tree genotypes and caterpillar sexes.
We demonstrate that even relatively small trees can express subtle but distinct levels of within-crown leaf-trait variation. In aspen, this variation is shaped not only by spatial factors, but also by genetics and to a lesser degree by herbivory. Within-crown variation in defense chemistry might itself provide a defensive phenotype against herbivores and contribute to the biological diversity of canopy insect communities.