The relative importance of plant-plant competition, plant-insect herbivory, and plant-fungal mutualism in two forbs with alternative defenses

We sought to investigate the relative importance of three major types of interactions: competition, herbivory, and mutualism in a field experiment using bergamot (Monarda fistulosa) and ox-eye (Heliopsis helianthoides). These plants were chosen because Monarda produces qualitative secondary compounds for defense, whereas Heliopsis relies on quantitative secondary compounds. The experiment employed a 3 x 2 factorial design examining above-ground competition (with/without competitors), herbivory (with/without pesticide) and mycorrhizal symbionts (with/without fungicide) within a larger grassland restoration experiment. Herbivore damage was assessed bimonthly as percent of leaves damaged. After 90 d, above-ground plant biomass was harvested and weighted, and leaf chlorophyll was measured. Herbivore damage did not differ between species and was reduced 3X and 1.5X in pesticide and fungicide treatments, respectively. The sole factor affecting above-ground plant biomass was competition, with competitor-free plants having 90% more biomass. By contrast, herbivores caused a 2-fold reduction in leaf chlorophyll whereas competition had no significant effect. The next step will be to assess how plant functional traits (e.g., leaf surface area) and herbivore assemblage structure respond to the factors of competition, herbivory, and mutualism. To date our results suggest that competition and herbivory may each be important for determining plant fitness, but their effects may manifest themselves in different ways.