Seawater intrusion mediates species transition in low salinity, tidal marsh vegetation

In a transitioning tidal freshwater marsh (TFM) experiencing relative sea-level rise, the native vegetation community has been introduced to both an abiotic stressor (salt) and a new plant competitor (Spartina alterniflora). Within a narrow salinity range (0-3), we examined the interplay between salt stress and plant interactions by comparing growth and nutrient status of Spartina alterniflora grown alone and in the presence of one of three TFM species (Leersia oryzoides, Peltandra virginica, and Phragmites australis). We conducted a mesocosm study pairing Spartina alterniflora with a TFM plant or a conspecific and subjecting the pairs to sub-lethal levels of salinity (0, 1.5, and 3). Measured plant traits included net photosynthesis (A(net)), aboveground and belowground biomass and tissue carbon (C), nitrogen (N), phosphorus (P), and their corresponding ratios. Relative to Spartina alterniflora grown alone, the presence of either TFM grass (Leersia oryzoides and Phragmites australis): (1) altered Spartina alterniflora A(net), (2) suppressed Spartina alterniflora biomass accumulation aboveground and belowground in all salinity treatment levels, and (3) altered aboveground and belowground tissue nutrient content. Regardless of neighbor, Spartina alterniflora's growth and nutrient status was greatest at the lowest salinity and remained largely unchanged in the presence of Peltandra virginica. These results suggest that Spartina alterniflora can successfully expand into TFM communities. Once established, salt stress eventually may hasten the transition to a Spartina alterniflora-dominated marsh community. The success of Spartina alterniflora may allow these marshes to maintain vegetation and increase their likelihood of maintaining elevation with rising sea level. (C) 2015 Elsevier B.V. All rights reserved.