Assessment of Aboveground Net Primary Productivity and Carbon Pools, Detrital Biomass, Community Structure, and Species Composition Across a Floodplain Forest of the Congaree River

Floodplain forests offer services to society in the form of species diversity, wood production, sequestration of carbon, and buffering capacity critical to water quality. These services are dependent upon maintenance of hydrological connections and sedimentation dynamics of these systems that determine site specific heterogeneous microsite distributions yielding specific community structure, species composition, and productivity rates. The study area was within South Carolina’s Congaree National Park (CONG), and consisted of a micro-topographic gradient across the Congaree River floodplain differing in sedimentation rates and inundation periodicity and intensity. The research was designed to take advantage of this gradient to help bridge information gaps regarding how communities vary across the floodplain. Specific objectives included: (i) describing community composition and structure of both live and dead wood to define community types, (ii) quantifying net aboveground primary productivity, (iii) determining litter decomposition rates and litter nutrient dynamics, (iv) calculating sequestered carbon in aboveground wood, and (v) synthesizing these data to qualitatively assess the stability of, and possible shifts in, community composition. Utilizing 20 plots, we characterized four distinct forest community groups: younger mixed bottomland hardwoods at higher elevations; old-growth mixed bottomland hardwoods at both higher and intermediate elevations; and mature cypress backswamp at lowest elevations. The annual percentage of mass gain for tree wood was greater at higher and intermediate elevations compared to the backswamp. Additionally, the percentage of tree wood gained annually is greater at a given elevation for plots having the majority of tree biomass in younger trees. However, the actual magnitude of mass gain, and thus carbon sequestration, is greater when old growth trees are present. Litter turnover times were 64-80 weeks, thus litter decomposition and cycling of nutrient contents of the litter are comparatively rapid processes within CONG. For all pools of aboveground sequestered carbon, we found cumulative values of 14840, 18878, and 7460 (younger stands) grams C/m² for bottomland hardwood forest and 19076 grams C/m² for the bald cypress swamp. The species composition, wood mass and volume, and unit quantities between the dead and live wood fractions, in conjunction with decay class of dead wood used as a temporal proxy, suggests that there are ongoing shifts in both the canopy and understory in the bottomland hardwood forest stands while the cypress swamp appears to be stable. We conclude that topographic position, a surrogate for both hydrology and sedimentation, as well as stand age and size metrics result in differing productivity rates and thus carbon sequestration within forested floodplains of the southeast