Disentangling the effects of drought and salinity on carbon fluxes at varying temporal and spatial scales from a coastal forested wetland
Due to their landscape position, coastal forested wetlands are particularly vulnerable to altered precipitation regimes, droughts, and sea-level rise. Human activities such as canal dredging and flow regulation also alter the hydrology and salinity of coastal forested wetlands. These stressors are increasing the frequency, duration, and magnitude of saltwater intrusion into forested wetlands in the Southeast US. We used field and laboratory studies to examine the consequences of changes in hydrology and salinity for the carbon balance of a large scale restored wetland in the coastal plain of North Carolina. To understand changes in both the uptake and release of carbon, we examined the growth of dominant trees, greenhouse gas fluxes, and export of dissolved organic carbon (DOC) in response to natural and experimental variations in salinity and water level. We asked: 1) How does salinity and water level affect the growth of bald cypress trees in the field and a greenhouse experiment? 2) How are greenhouse gas emissions and DOC export affected by an experimental ecosystem-scale drawdown? 3) How are greenhouse gas emissions affected by drought and salinity in a soil column experiment? And 4) How are greenhouse gas emissions affected by salinity in an experimental field salt addition?
We found consistent decreases in C uptake by bald cypress trees in the field and in a greenhouse experiment. Salinities as low 0.5 ppt led to 20-60% decline in bald cypress growth. A rapid draining of ~10 ha of the wetland in less than a day led to rapid increase in CO2 fluxes to the atmosphere. Surprisingly we also observed a spike in CH4 fluxes after the drawdown. DOC concentrations increased initially, but declined over the two weeks of the experimental drawdown. Long-term monitoring has shown that drought-induced increases in salinity have led to 40-60% decreases in DOC export from the site. In both the soil column experiment and an experimental field salt addition, we found declines in CH4 fluxes in response to increased salinity. We also observed declines in CO2 fluxes in response to salt addition, which was contrary to what we expected based on the current literature. Overall, our results suggest that both uptake and release of carbon from this wetland are sensitive to changes in hydrology and salinity. Alterations in frequency, duration, and magnitude of droughts, storms, and saltwater intrusion events will make it difficult to forecast the carbon balance of wetlands in the coming decades.