Do elevated CO2, sea-level rise, and mineral sedimentation interact to influence biological contributions to soil elevations in coastal wetlands

Background/Question/Methods

Climate change associated increases in atmospheric CO₂ are driving changes in marine storm patterns and raising sea-levels.  Low-lying coastal wetlands are becoming increasingly threatened by encroaching seas, and thus, must gain elevation at a rate comparable to sea-level rise (SLR) or risk being converted to open-water.  Flood waters from marine storms can deposit large amounts of sediment, aiding wetland elevations over time.  Other physical processes (erosion, subsidence) oppose elevation gains, but may be counter-acted by biological inputs from plant production.  Little is known of the potential for biological contributions to wetland elevation under projected climate changes scenarios. Therefore, using the Wetland Elevated CO₂ Experimental Facility at the National Wetlands Research Center in Lafayette, LA, we asked:  How will increased atmospheric CO₂, sea-level rise and storm sedimentation interact to influence biological contributions to elevation change?  Mesocosms consisting of intact marsh sods containing a mixed community of a C₃ sedge and a C₄ grass were treated to a full-factorial application of atmospheric CO₂ (386, 720 ppm), sea-level rise (+2, +4, +8 mm/yr), and sedimentation (0, +5 cm).  Over the 2 year study, stem density and elevation were measured quarterly, and senescent aboveground biomass was removed thereby isolating belowground contributions to elevation.

Results/Conclusions

While CO₂ had no direct effect on soil elevations, SLR and sediment treatments significantly affected elevation trends, resulting in patterns that persisted over the study period.  Those mesocosms receiving the +4 and +8 mm/yr SLR treatments showed significant increases in elevation while those under +2 mm/yr SLR conditions showed no change.  Elevation trends in the +4 and +8 mm/yr SLR treatments corresponded to increases in stem density of the C₃ sedge, S. americanus, while the +2 mm/yr treatments were dominated by the C₄ grass, Spartina patens.  These results suggest that species-specific responses to flooding, not CO₂ or sediment, drove changes in community composition, which influenced soil expansion through changes in belowground biomass accumulation.  Furthermore, addition of sediment led to subsequent elevation losses, whereas substantial elevation gains occurred in mesocosms that did not receive sediment. These results suggest that soil expansion following sediment input may be diminished through negative effects on belowground biomass accumulation. Ongoing work is examining belowground production and decomposition to elucidate underlying controls on elevation dynamics. Understanding how these internal processes respond to external drivers will strengthen our ability to predict marsh susceptibility to SLR and improve management strategies designed to minimize wetland loss.