Past responses to climate change: reconstruction of vegetation histories in three brackish marshes

Long-term persistence of coastal wetlands depends on maintenance of surface elevations relative to sea-level rise also in the presence of other natural and anthropogenic stressors. To predict coastal marsh response to future sea-level rise, it is important to understand how marshes responded to past sea-level rise. The objective of this study was to reconstruct vegetation histories from sediment cores collected in Big Branch NWR, Louisiana, USA, Pearl River WMA, Louisiana, USA, and Grand Bay NERR, Mississippi, USA. A dichotomous key was created using characteristics of modern plant species common in coastal marshes of the northern Gulf of Mexico, which was used to identify vegetative fragments preserved in cores. Cores were radiocarbon dated and also examined for soil characteristics (bulk density, organic content). Vegetation and soil characteristic data were processed using SiZer to determine significant shifts or changes of vegetative community and soil characteristics over time. Several significant shifts in the vegetative communities were detected at all sites (Big Branch: six shifts; Pearl River: four shifts; Grand Bay: four shifts) in addition to multiple significant shifts in bulk density (Big Branch: five shifts; Pearl River: three shifts; Grand Bay: two shifts). Results indicate that these brackish marshes have maintained surface elevations relative to sea-level through both physical and biological processes. Extended periods of organic matter accumulation (demonstrated by presence of preserved vegetation) were interrupted by periods/pulses of sediment deposition (demonstrated by increases in bulk density and decreases in organic matter content). The approach developed in this project will aid in identifying and interpreting past changes in coastal wetlands in response to sea-level rise and other global change factors. (Published By University of Alabama Libraries