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

Summary 1  The effects of resource competition and herbivory on a target species, Triglochin 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 soil fertility over two growing seasons. Three parts of the marsh were selected to represent different successional stages; Triglochin reached local dominance at intermediate biomass of salt‐marsh vegetation. At each stage, three 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 . The impact of grazing was strongest at the stage with the lowest biomass, while both herbivory and competition had a significant impact at the stage with the highest biomass. When plants were protected from direct herbivory, competition operated at all three successional stages. 4  Grazing reduced light competition when vegetation biomass was low or intermediate, but at high biomass there was competition for light even when grazing occurred. Herbivore exclusion increased the effects of plant competition. Except at low biomass, the negative impact of plant competition on Triglochin 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 largely by grazing. Intense grazing in the younger marsh and increasing competition for light in the older marsh will restrict the distribution to sites with intermediate biomass.