Impacts of salinity and simulated herbivory on survival and reproduction of the threatened Gulf of St. Lawrence Aster, Symphyotrichum laurentianum

Halophytic plants may experience joint, and potentially interacting, effects of salinity and herbivory on their survival and reproduction. We investigated the impacts of salinity and (simulated) herbivory on fitness in the Gulf of St. Lawrence Aster (Symphyotrichum laurentianum (Fernald) Nesom; Asteraceae). In natural populations, this species ex- periences varying soil salinity and spill-over herbivory from insects associated with neighbouring seaside goldenrod (Sol- idago sempervirens L.). We subjected S. laurentianum individuals to three levels of simulated herbivory (0%, 20%, and 40% leaf-area removal) and three levels of salinity (0, 10, and 20 gL -1 ) in three runs of a growth-chamber experiment. The effects of salinity and herbivory were always additive. Salinity consistently and strongly reduced survivorship, by as much as 80% at 20 gL -1 salt, but significantly affected the seed set of survivors in only one run. Herbivory reduced sur- vival significantly in one run (by 42% at 40% leaf-area removal) and reduced seed set of survivors (by 36% at 40% leaf- area removal) in another. While both stresses can have important effects on S. laurentianum fitness, the effects of salinity were more consistently strong than were the effects of herbivory, for both survival and seed set.