Tree populations can exhibit tremendous genotypic variation in allocation to chemical defense against herbivores. Genotypes with higher defense allocation are predicted to have slower growth rates, and vice versa, because production of these chemical defenses shares both substrates and pathways with components of plant growth. However, growth-defense tradeoffs are not always observed and can be context-specific. Selection for growth vs. defense traits can vary among environments; for example, plant genotypes with high growth rates are predicted to dominate in competitive environments. These high-growth genotypes are likely to be poorly defended, and if so their dominance in high-competition environments could lead to a reduction in herbivore resistance at the population level. We used replicated, mixed-genotype populations of trembling aspen (Populus tremuloides) under different intraspecific competition intensities to test whether growth-defense tradeoffs, coupled with variation in growth environment, affect population-level trajectories of genotypic composition and herbivore resistance traits.
Results/Conclusions
Our results show that tree genotypes that invest heavily in chemical defense have slower growth rates than those with lower defense allocation, regardless of competition intensity. These slow-growing genotypes have reduced rates of survivorship in both high- and low-competition stands. However, the relationship between growth and survivorship is especially strong high-competition stands. Thus, the observed growth-defense tradeoff interacts with intraspecific competition to select against high-defense genotypes in high-competition stands. Different selective pressures result in divergent trajectories of genotypic composition in the two environments. For example, one high-defense genotype dropped to 33% survivorship in high-competition stands while maintaining 89% survivorship in low-competition stands. Preliminary results show that changes in genotypic composition (in terms of both relative growth and survivorship) shape stand functional trait composition, with high competition leading to reduced levels of herbivore resistance traits and higher herbivory at the stand level.