The Rate and Process of Mangrove Forest Expansion on Above and Below Ground Carbon Relations in Coastal Louisiana

Recent field studies have documented measureable mangrove expansion landward in tropical zones and poleward in temperate saltmarsh around the northern Gulf of Mexico. Louisiana, in particular, has been recolonized by black mangrove, Avicennia germinans, over a large spatial extent since complete dieback from freeze dating back two decades. Study sites were established near Fourchon, Louisiana to account for the rate and process of marsh/mangrove dominance on carbon burial and elevation change over the past 50 years. Transects were oriented perpendicular from waterway berm of tall and scrub mangrove cover on higher ground grading to low marsh settings of saltmarsh and mixed mangrove ingrowth. Elevation mapping, plant cover and biomass sampling, and soil core dating were conducted to evaluate carbon relations above and below ground. Results showed that sites with tall mangrove have significantly higher above ground biomass than Spartina marsh and mixed marsh/mangrove zone. AG biomass was positively correlated with soil surface elevation and negatively with tidal flood frequency. In addition, tall mangrove zones recorded a twofold increase in soil accretion and carbon burial rates compared with saltmarsh soils based on 137Cs dating. These findings support a positive feedback of mangrove ingrowth and persistence on carbon production and burial allowing higher accretion rates and elevation gains when favored by warmer climate periods lacking or following episodic freeze events.