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.