Tidal marsh plant responses to elevated CO₂, nitrogen fertilization, and sea level rise

Abstract

Elevated CO2 and nitrogen (N) addition directly affect plant productivity and the mechanisms that allow tidal marshes to maintain a constant elevation relative to sea level, but it remains unknown how these global change drivers modify marsh plant response to sea level rise. Here we manipulated factorial combinations of CO2 concentration (two levels), N availability (two levels) and relative sea level (six levels) using in situ mesocosms containing a tidal marsh community composed of a sedge, Schoenoplectus americanus, and a grass, Spartina patens. Our objective is to determine, if elevated CO2 and N alter the growth and persistence of these plants in coastal ecosystems facing rising sea levels. After two growing seasons, we found that N addition enhanced plant growth particularly at sea levels where plants were most stressed by flooding (114% stimulation in the + 10 cm treatment), and N effects were generally larger in combination with elevated CO2 (288% stimulation). N fertilization shifted the optimal productivity of S. patens to a higher sea level, but did not confer S. patens an enhanced ability to tolerate sea level rise. 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 the widespread sedge, S. americanus, the enhanced ability to tolerate inundation. However, if chronic N pollution reduces the availability of propagules of S. americanus or other flood-tolerant species on the landscape scale, this shift in species dominance could render tidal marshes more susceptible to marsh collapse.

Spatial response of coastal marshes to increased atmospheric CO 2

“…Salinity, flooding, water stress, and eutrophication can all potentially influence the effects of CO 2 fertilization on marsh plants (3,(12)(13)(14)(15)(16). Overall, the available evidence is coherent in pointing to a significant CO 2 fertilization effect on organic accretion and plant production even in the presence of other simultaneous environmental changes (2,3,17,18).…”

The impact of sea-level rise on organic matter decay rates in Chesapeake Bay brackish tidal marshes

“…At each site, we constructed platforms ("marsh organs") containing mesocosms at different elevations to simulate the response of plant growth to different flooding regimes (Kirwan and Guntenspergen, 2012;Langley et al, 2013) (Fig. 1b).…”

“…We did not measure salinity, but acknowledge that it has an important influence on decay (Craft, 2007;Weston et al, 2011;Sutton-Grier and Megonigal, 2011). We have previously reported the effect of elevation on plant growth from these experiments (Kirwan and Guntenspergen, 2012;Langley et al, 2013).…”

“…Recent work suggests that rates of productivity are maximized at some optimum flooding frequency or rate of sea-level rise (Morris et al, 2002;Kirwan and Guntenspergen, 2012;Voss et al, 2013;Langley et al, 2013; though also see Marani et al, 2004;, where moderate increases in flooding lead to flushing of salts, delivery of nutrients, and enhanced productivity. At higher rates of sea-level rise, and for longer durations of inundation, the accumulation of sulfides under anaerobic conditions limits rates of productivity (e.g.…”

Global change effects on decomposition processes in tidal wetlands: implications from a global survey using standardized litter

“…This finding and its implication may occur counterintuitive with respect to the often sharp 360 redox gradients along tidal wetland zonations and with flooding (Davy et al, 2011;Langley et al, 2013), and the mechanism by which S is decreased in the more flooded zones is unknown. Because we did not observe consistent salinity effects on S and k in our data (Figs.…”