Growth and photosynthesis responses of two co-occurring marsh grasses to inundation and varied nutrients

For tidal marshes of the US Northeast, the late twentieth century decline of Spartina patens(Aiton) Muhl. has been attributed to increased flooding associated with accelerated sea level rise and nitrogen over-enrichment from cultural eutrophication. The objective of this study was to examine the impacts of inundation and nutrient availability on growth, photosynthesis, and interactions of S. patens and Distichlis spicata (L.) Greene, which co-occur and are common marsh species. Plants were grown in a factorial greenhouse experiment, where flow-through seawater was used to simulate semidiurnal tides. Field surveys were additionally conducted to relate plant distributions to environmental conditions. For S. patens grown in monoculture, nutrient additions did not enhance growth for the high inundation treatment. In addition, the combination of high nutrient availability and high inundation adversely affected S. patens tiller density, photosynthetic efficiency, and leaf CO₂ uptake. For D. spicata, nutrient additions enhanced growth for both inundation treatments with respect to aboveground biomass and tiller density. For species pairings, S. patens expanded relative to D. spicata under low inundation, low nutrient availability conditions, but declined relative to D. spicata under daily inundation in combination with nutrient amendments. These findings were additionally supported by field data, which indicated that D. spicata was more common than S. patens where nutrient availability was high. These results suggest that S. patens persistence is favored by low nutrient inputs and well-drained conditions, and supports the interpretation that this species is vulnerable to loss where high nutrient loads coincide with accelerated sea level rise.