Intraspecific competition reduces plant size and quality and damage severity increases defense responses in the herbaceous perennial, Asclepias syriaca

Competition among plants within populations affects plant size, nutrient status and allocation to defenses. Herbivory places additional stress on plant allocation of resources. When resources are limited due to intraspecific competition, induced defenses may reduce costs of defense responses and trade-offs between allocation to growth or defense. We hypothesized that increased intraspecific competition would result in a decrease in plant size and leaf tissue nutrient quality, and that both intraspecific competition and leaf damage severity would affect inducibility of leaf defensive traits. We tested these hypotheses in common milkweed (Asclepias syriaca) using greenhouse experiments that manipulated plant density and damage severity treatments. We measured a suite of leaf traits generally related with herbivore performance, including size; nitrogen, carbon, lignin, and fiber concentrations; and latex production. Increased density decreased plant size and leaf nutrient quality, but increased lignin levels. Damage severity increased leaf lignin levels and latex production. There were no density-damage severity interactions. We additionally addressed the question of whether plants respond differently to simulated or natural herbivory and hypothesized that insect herbivores and mechanical plant tissue removal would similarly affect induced defensive responses. Leaf fiber and lignin increased in response to damage, but the response was greater on plants subjected to simulated, compared with caterpillar herbivory. Other plant traits responded similarly to either damage type. Our findings suggest that intraspecific competition has the potential to generate feedbacks among plants and herbivores as plants respond to herbivory.