Tidal marsh tolerance for sea-level rise in response to elevated CO2 and nitrogen pollution

Background/Question/Methods

Previous work has shown that elevated CO₂ and nitrogen addition direct affect plant productivity and the mechanisms that allow tidal marshes to maintain a constant elevation relative to sea level. Furthermore, these two factors can drive changes in plant community composition that may other consequences for marsh viability. For instance, N addition has been shown to favor species that show less tolerance for sea level rise. Here we manipulated CO₂ concentration (two levels) and N availability (two levels) and relative sea level using in-situ mesocosms anchored at varying heights (6 levels), factorially in a tidal marsh dominated by a sedge, Schoenoplectus americanus, and a grass , Spartina patens, to determine if elevated CO₂ and nitrogen alter the survivorship and growth of these plants in coastal ecosystems facing rising seas.

Results/Conclusions  

After two growing seasons, we found that nitrogen addition allowed greater plant growth in higher sea-level treatments (160% stimulation in the +10 cm treatment), and the magnitude of the nitrogen effects were larger in elevated CO₂ (470% stimulation). S. americanus showed much greater tolerance for inundation than S. patens and drove the most important effects in the treatments that simulated sea-level rise. Though S. patens responds strongly to N addition at normal sea levels it does not show any enhanced ability to tolerate sea level. S. americanus responded strongly to N only in the higher sea-level treatments that excluded S. patens.  Interestingly, addition of N, which has been suggested to accelerate marsh loss, may afford some marsh plants, such as S. americanus, an enhanced ability to tolerate stands of high sea level. However, if chronic N pollution reduces the abundance of propagules of S. americanus or other flood-tolerant species on a landscape scale, then it could render tidal marshes more susceptible to marsh collapse.