Plant biomass and rates of carbon dioxide uptake are enhanced by successful restoration of tidal connectivity in salt marshes.

Salt marshes, due to their capability to bury soil carbon (C), are potentially important regional C sinks. Efforts to restore tidal flow to former salt marshes have increased in recent decades in New England (USA), as well as in some other parts of the world. In this study, we investigated plant biomass and carbon dioxide (CO 2) fluxes at four sites where restoration of tidal flow occurred five to ten years prior to the study. Site elevation, aboveground biomass, CO 2 flux, and porewater chemistry were measured in 2015 and 2016 in both restored marshes and adjacent marshes where tidal flow had never been restricted. We found that the elevation in restored marsh sites was 2–16 cm lower than their natural references. Restored marshes where porewater chemistry was similar to the natural reference had greater plant aboveground biomass, gross ecosystem production, ecosystem respiration, as well as net ecosystem CO 2 exchange (NEE) than the natural reference, even though they had the same plant species. We also compared respiration rates in aboveground biomass (AR) and soil (BR) during July 2016, and found that restored marshes had higher AR and BR fluxes than natural references. Our findings indicated that well-restored salt marshes can result in greater plant biomass and NEP, which has the potential to enhance rates of C sequestration at 10-yrs post restoration. Those differences were likely due to lower elevation and greater flooding frequency in the recently restored marshes than the natural marsh. The inverse relationship between elevation and productivity further suggests that, where sea-level rise rate does not surpass the threshold of plant survival, the restoration of these salt marshes may lead to enhanced organic and mineral sedimentation, extending marsh survival under increased sea level, and recouping carbon stocks that were lost during decades of tidal restriction. The photos showing the tidal channel before (A) and after restoration (B) at Bass creek, Barnstable, MA, USA. Our study indicated that successful restoration of salt marshes leads to greater rates of C sequestration for a decade, at minimum. Moreover, the negative relationship between elevation and plant productivity suggested that sea level rise may lead to enhanced sedimentation, extending marsh survival under the increased sea level, and recouping carbon stocks that were lost during tidal restriction periods. Unlabelled Image • Coastal wetlands are important regional C sinks. • The plant biomass and CO 2 fluxes was compared in restored marsh and reference. • The recently restored salt marsh had higher plant aboveground biomass and NEE than the natural marsh. • Negative feedbacks between plant productivity and sea levels rise help salt marsh survival. [ABSTRACT FROM AUTHOR]