Effects of resource competition and herbivory on plant performance along anatural productivity gradient

1 The effects of resource competition and herbivory on a target species, Tr iglochin maritima, were studied along a productivity gradient of vegetation biomass in a temperate salt marsh. 2 Transplants were used to measure the impact of grazing, competition and s oil fertility over two growing seasons. Three parts of the marsh were selec ted to represent different successional stages; Triglochin reached local do minance at intermediate biomass of salt-marsh vegetation. At each stage, th ree competition treatments (full plant competition, root competition only, and no competition) and three grazing treatments (full grazing, no grazing on Triglochin, and no grazing on Triglochin or neighbours) were applied to both seedlings and mature plants. 3 Competition and herbivory reduced biomass and flowering of Triglochin. Th e impact of grazing was strongest at the stage with the lowest biomass, whi le both herbivory and competition had a significant impact at the stage wit h the highest biomass. When plants were protected from direct herbivory, co mpetition operated at all three successional stages. 4 Grazing reduced light competition when vegetation biomass was low or inte rmediate, but at high biomass there was competition for light even when gra zing occurred. Herbivore exclusion increased the effects of plant competiti on. Except at low biomass, the negative impact of plant competition on Trig lochin performance was greater than the positive effect of not being grazed . 5 Grazing played a minor role in seedling survival and establishment which were largely controlled by competitive and facilitative effects. 6 Once established, the persistence of Triglochin will be determined largel y by grazing. Intense grazing in the younger marsh and increasing competiti on for light in the older marsh will restrict the distribution to sites wit h intermediate biomass.