Tidal Freshwater Forested Wetlands (TFFWs) have a myriad of ecological and economic functions in the American Southeast. Sea level rise is leading to encroachment on these ecosystems by more halophytic communities. Vegetation community changes driven by this encroachment necessarily influence carbon (C) stocks, and the resultant C sequestration potential of these ecosystems. In order to monitor these changes, standing stocks of C were measured using a nested plot design within riverine forests along two hydrologic gradients on the Winyah Bay in South Carolina and the Savannah River in Georgia. Sites along the hydrologic gradient include nontidal, head-of-tide, upper tidal fresh (0.1 psu), low salinity TFFW (0.6-3.9 psu), and degraded TFFW that are now predominantly oligohaline marsh (> 4 psu). Stand-level measurements of live tree C stocks were collected along with monthly measures of litterfall, baldcypress growth, and root-zone salinity. Annual measurements of standing stocks were made using metric DBH tapes, while monthly baldcypress growth was assessed using dendrometer bands. Both of these measures were used in species-specific allometric equations This study aims to address the change in tree standing stocks and growth (i.e., C sequestration) along a hydrologic gradient spanning degraded TFFW to future TFFW using a long-term dataset.
Results/Conclusions
Preliminary findings show that, on average, these riverine forests contained 138.44 Mg of C/ha in live trees in 2019. Nontidal sites held the most C, averaging 177.93 MgC/ha. The most saline site along the Savannah River had a complete die-off of trees, thus containing no C for the purposes of this preliminary analysis. The most saline site on the Winyah Bay transect held the lowest levels of measurable C, only 22.98 MgC/ha. Differences in stand-level C stocks were analyzed using Kruskal-Wallis nonparametric tests. Analysis was grouped by river (e.g., Winyah or Savannah), and then by site position (e.g., head of tide, nontidal, etc.). Live C stocks were found to be different between most of the extreme ends of the salinity gradient, while differences in the middle range of our salinity gradient were confounded and not significantly distinct. This evidence suggests that C sequestration will be changing as TFFWs retreat up-stream in the Savannah and Winyah river systems due to saltwater encroachment. Analysis of C growth rates as well as stocks in coarse woody debris and shrubs is ongoing, as is analysis of litterfall and monthly growth rates of baldcypress trees.