Rate and Process of Mangrove Forest Expansion on Carbon Relations in Coastal Louisiana

Science Center Objects

Field observations over recent decades have confirmed mangrove expansion landward in tropical zones and poleward in temperate saltmarsh settings around the northern Gulf of Mexico.

Elevation mapping, plant cover and biomass sampling

Elevation mapping, plant cover and biomass sampling

The Science Issue and Relevance: Field observations over recent decades have confirmed mangrove expansion landward in tropical zones and poleward in temperate saltmarsh settings around the northern Gulf of Mexico. A USGS aerial survey over coastal Louisiana by seaplane in 2009 accounted for the spread and distribution of recolonized black mangrove, Avicennia germinans, over a large state-wide extent from complete dieback by a killing freeze in 1989. An understanding of the rate and process of recolonization is lacking, which lead to using radioisotopes and mensuration techniques to investigate vegetation and carbon relations of mangrove/marsh succession. Federal and private land managers in Louisiana require information on the benefit of planting mangrove in broken marsh habitats as a means to curb wetland loss and stabilize soils. It is expected that future climate change may bring warmer, freeze-free, conditions that naturally favor continued expansion and encroachment of tropical tree species such as mangrove along the northern Gulf Coast. The knowledge to guide species selection and success of mangrove planting is not yet certain or fully investigated to make management recommendations without further study and research.

Methodology for Addressing the Issue: Study sites were established near Port 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 backmarsh 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. Sites with tall mangrove support significantly higher above ground (AG) biomass than Spartina marsh and lower grade mixed marsh/mangrove zone. AG biomass was positively correlated with soil surface elevation and negatively with frequency of tidal flood inundation. Tall mangrove zones also recorded a twofold increase in soil accretion and carbon burial rates compared with saltmarsh soils based on 137Cs dating method. These findings support a positive feedback of mangrove ingrowth on carbon production and burial allowing higher accretion rates and elevation gains when favored by warmer climate periods lacking episodic freeze events. 

Study sites were established near Port Fourchon, Louisiana

Study sites were established near Port Fourchon, Louisiana

Future Steps: The project will produce an understanding of the interaction of tidal range and flood frequency effects on mangrove expansion and encroachment into temperate saltmarsh dominated wetlands at northern latitudes under warming climate lacking episodic freeze events. This new understanding can be applied in explicit landscape simulation models to improve predictive modeling of future ecosystem consequences under climate change and sea-level rise.