A test of top-down control on plant production and nutrient quality in low-salinity tidal marshes

Abstract

Herbivory has been shown to play a strong role in controlling primary productivity in tidal salt marshes, but little work has been conducted in low salinity marshes. We measured aboveground plant biomass and nutrient response to insect exclusion along a salinity gradient from tidal freshwater to oligohaline marshes. We expected higher biomass in plants protected from herbivory and more so in the higher salinity marshes where tissue quality was anticipated to improve. Among three marshes along the salinity gradient, aboveground biomass within experimental plots did not vary across control and insect exclosure treatments. Overall tissue quality did not increase with increasing salinity, thus we did not find increasing grazing pressure along that gradient. Tissue N-content in Zizania aquatica (a low-salt tolerant, C3, annual grass) was significantly higher (p < 0.05) in plants within the insect exclosures, and it is the only species of those tested to demonstrate this effect. The lower tissue N-content in controls of this species could be either a response to grazing pressure that disrupts the ability of the plant to develop amino acids or a loss of N through guttation. We found that increasing levels of salt may have little effect on biomass in these marshes, but nutrient dynamics may shift as species like Zizania adjust the pool of tissue N-content.