Trade-offs between induced plant defenses and competitive growth are regarded as being universal. This seems particularly true for often-studied early succession annuals, where exposure to competition often suppresses defense expression. However, whether such trade-offs are universal across plant life histories remains unclear, especially considering recent work demonstrating that the trade-off can be artificially uncoupled. We test the hypothesis that Solidago altissima, a perennial herbaceous plant, naturally uncouples this trade-off by adjusting its investment in chemical defenses when exposed to competitive cues, allowing for persistence in high-competition environments despite herbivore pressure. Using a factorial glasshouse experiment, we manipulated competition cues (far-red light and conspecific neighbors) and insect herbivory to assess impacts on growth, resistance, and secondary metabolite production. S. altissima maintained or even enhanced herbivore-induced resistance in the presence of competition cues. Bioassays revealed reduced herbivore performance on previously damaged plants, particularly when they were exposed to neighbors. Metabolomic profiling showed herbivory-induced production of several secondary metabolite classes. Most notably, we found competition-enhanced production of hydroxycinnamic acids, dominated by 3-O-(E)-feruloylquinic acid, associated with resistance. Our findings challenge the generality of the growth-defense trade-off and highlight the importance of ecological context and life-history strategy in shaping plastic responses.
Mueller et al. (Sat,) studied this question.