Net primary productivity of tidal freshwater forested wetlands along a salinity gradient in South Carolina

Background/Question/Methods

Tidal freshwater forested wetlands occupy the interface of terrestrial and marine systems, providing vital services including storm abetment and carbon storage. Due to their discrete placement, they are some of the most vulnerable wetlands to climatic impacts such as sea level rise; however research on these systems is scarce. One of the principal threats to these coastal wetlands is salinity intrusion, which can stress the system and effect their structure, growth, and function. To determine the response of tidal freshwater forested wetlands to increased salinity exposure, we quantified net primary productivity (NPP) across three sites on the Waccamaw River in South Carolina. Sites increased in porewater salinity and included a continuously freshwater (<0.1 ppt), moderately salt impacted (1.8 ppt), and heavily salt impacted (2.8). We hypothesized that NPP would decrease as salinification increased due to inhibited ability of plants to take up key nutrients and salt-induced water stress. NPP was comprised of both aboveground and belowground productivity with all data collected from December 2010- December 2011. Leaf litter mass and change in woody biomass over a year were used for aboveground NPP; while belowground NPP was determined by calculating fine root (<3.0mm diameter) productivity through the sequential coring technique.

Results/Conclusions

Data suggests NPP is greatest at the continuously freshwater site (2409 g m^-2 yr^-1), with no difference between the moderately salt impacted (1234 g m^-2 yr^-1) and heavily salt impacted sites (707 g m^-2 yr^-1). Trends in aboveground NPP show a clear decrease as salinification increases ranging from 77.88-2278 g m^-2 yr^-1. Belowground productivity is greater at the continuously freshwater site compared to the heavily salt impacted site, with some fine root dynamics apparent when variables are analyzed for each size class. Analysis of nutrients in roots shows lower N concentration in live roots and lower C:N ratio in dead roots at the heavily salt impacted site. Preliminary results suggest a decrease in NPP with an increase in salinification, thus sea level rise is an imminent threat to tidal freshwater forested wetlands. Stress created by increased salinification is apparent in aboveground and belowground NPP resulting in decreased capacity for tidal freshwater forested wetlands to perform vital environmental services.