Responses of coastal wetlands to sea-level rise and elevated CO2: Biological contributions to soil elevation

Background/Question/Methods The persistence of coastal wetlands depends on the maintenance of soil elevations relative to sea-level.  Physical processes of erosion, sedimentation, and subsidence influence the capacity of coastal wetlands to adjust to rising seas.  However, it is also important to consider biological responses, such as plant production or decomposition, that contribute to changes in surface elevations by altering organic matter contributions to soil volume.  Thus, environmental factors that influence production-decomposition dynamics, such as elevated CO₂, changes in flooding or salinity associated with sea-level rise, or nutrient availability, may affect the capacity of coastal wetlands to maintain elevations relative to sea-level.  In a series of greenhouse experiments with a brackish marsh community, we manipulated atmospheric CO₂ concentrations along with either flooding and salinity, or nutrient availability and vegetation composition of C₃ and C₄ plants to test hypotheses regarding the potential contributions of plants to soil elevations in a changing climate.  Our experimental design permitted examination of multiple factor interactions and identification of important mechanisms controlling elevation change in coastal wetlands. 
Results/Conclusions Our results indicated that plant production drove patterns of surface elevation change, and that elevated CO₂ ameliorated the negative effects of salinity on the C₃ species, thereby enhancing its contribution to elevation.  In a second experiment, we found that vegetated mesocosms exposed to elevated CO₂ for 16 months gained elevation, with the CO₂ effect greatest for C₃ plants growing alone.  In comparison, unvegetated mesocosms showed no change in soil elevation, positive or negative, illustrating not only that plants play a role in soil expansion, but also that decomposition of soil organic matter did not contribute significantly to elevation loss in this case.  Although nutrients affected plant productivity, especially for the C₄ species, nutrient additions did not affect vertical soil movement in this plant community.  The results of these studies suggest that elevated CO₂, sea-level rise, and other global drivers may interact to determine elevation dynamics in coastal marshes, which underscores the importance of considering multiple factor interactions in future management strategies.