Morphological responses to competition modulated by abiotic factors in two monoculture-forming wetland plants

We investigated the relative roles of competition and environmental stressors in two wetland plants important for coastal restoration in the northern Gulf of Mexico. Phragmites australis and Schoenoplectus deltarum are ubiquitous in tidal freshwater marshes along the northern Gulf of Mexico coast. Schoenoplectus deltarum and Phragmites australis were grown in both monoculture and competition, and subjected to two levels each of nitrogen, salinity, and soil organic matter in a factorial greenhouse experiment. Productivity in both species was consistently suppressed under competition; the extent of this depended on the abiotic treatments. In monoculture, Schoenoplectus productivity responded negatively to low organic matter and reproduction responded negatively to brackish water; Phragmites productivity responded negatively to increased nitrogen. Schoenoplectus root-to-shoot ratio responded positively to brackish water when grown in soil with high organic matter, and Phragmites root-to-shoot ratio responded negatively to increased nitrogen. The sediment diversion operation strategy that has been recommended by experts familiar with Louisiana coastal wetland restoration may result in marshes characterized by increased nitrogen, periodic brackish water conditions, and increased soil organic matter. Such a strategy may therefore promote species coexistence and increased root production, which are respectively associated with increased ecosystem services and soil stability. The differential responses of the two species to the same conditions illustrates the importance of considering individual species to a restoration project.