Insect herbivory and defoliation on Centaurea species: The roles of neighbors, allelopathy, and arbuscular mycorrhizal fungi

Herbivory is assumed to have negative effects on plants, but many species compensate for herbivory by accelerating growth and reproduction after damage. Little is known about how biotic interactions influence variation in plant responses to herbivory. The assumption that herbivory significantly reduces plant performance and competitive effects on native neighbors is central to biocontrol theory. In this context, numerous biocontrols have been introduced to control invasive plants, including Centaurea species. I examined how herbivory altered the interactions between Centaurea and Festuca species in three different systems. First, I investigated how Festuca species influenced the compensatory response of Centaurea maculosa to herbivory and whether herbivory altered the allelopathic effects of C. maculosa on Festuca. Centaurea maculosa compensated for herbivory (up to 40% leaf damage) and Festuca neighbors did not increase the negative effects of herbivory. Centaurea maculosa was allelopathic to F. idahoensis; however, herbivory on C. maculosa did not increase these allelopathic effects. In a second study I determined whether a North American genotype (the tetraploid C. stoebe ssp. micranthos) differed ecologically from a western European genotype (the diploid C. stoebe ssp. stoebe). Both subspecies responded similarly to defoliation, the presence of F. ovina, and arbuscular mycorrhizal (AM) fungi. Defoliation negatively affected C. stoebe , but F. ovina positively affected C. stoebe . AM fungi positively affected C. stoebe, but did not increase compensatory growth. In a third study I examined interactions between C. uniflora and F. paniculata in subalpine communities of the French Alps. Centaurea uniflora fully compensated for defoliation in the first year, but biomass was negatively affected after repeated defoliation for two growing seasons. Festuca paniculata neighbors had competitive effects on C. uniflora , but did not affect its compensatory response. My results suggest that herbivory has weak or no effect on Centaurea and competitors do not exacerbate the effects of herbivory. In addition, herbivory on Centaurea does not decrease its competitive effects on neighbors nor does herbivory increase allelopathic effects. In this study, AM fungi did not enhance Centaurea compensatory growth. My results suggest that biocontrols may be less effective at reducing the competitive ability of Centaurea than previously thought.