Competitive Interactions Between Two Salt Marsh Halophytes Across Stress Gradients

Abstract

In tidal marshes, facilitative and competitive interactions between plant species can shift in response to variation in stress gradients (e.g., inundation and salinity) and resource gradients (e.g., nutrient availability). We measured plant competitive response using relative yield based on total biomass of salt marsh species Salicornia virginica and Triglochin concinna. We tested interactions between these species in two greenhouse experiments using an additive experimental design which included nitrogen supply, water level, and duration (4-months and 1-year) treatments. We repeated two N-supply treatments in a 1-year field experiment at two elevations. We found that competitive and facilitative processes were dependent on the type of stress, the species, and the experiment duration. We found a facilitation-competition shift along the N-availability gradient, but only for Salicornia in the 4-month experiment. We found no effect of N addition on competitive response in the 1-year experiment; relative yield values remained <1 for both Triglochin and Salicornia. Our results have implications for understanding competitive interactions across multiple stress gradients and for community structure in restored sites; for example, planting Triglochin in restored marshes could reduce aboveground biomass of Salicornia and aid in the development of a species-rich canopy.