The roles of restoration, elevation, and tide in the salt marsh carbon flux game

Abstract:

Predicting and understanding wetland radiative forcing is currently limited by incomplete and temporally coarse wetland carbon accounting, as well as a deficient understanding of how tidal exchange, elevation, and salinity gradients influence the transformation, import, and export of carbon compounds in wetlands. Improving such knowledge and predictions is needed to guide management and policy decisions at local, regional, and national levels. Here we present CO₂ and CH₄ fluxes of restoring and reference brackish salt marsh eddy covariance flux tower sites (US-Nrf and US-Nrs), located at the Billy Frank Jr. Nisqually National Wildlife Refuge in Puget Sound. We further analyze how wetland elevation, tidal variation, and salinity influence fluxes from each site. From 2017 to 2019 one reference wetland was consistently a strong carbon sink, with high CO2 uptake and low CH4 release. The other reference site displayed high interannual variability, oscillating between source and sink. The restoring site was a weaker carbon sink; however, annual CO2 uptake increased over the course of this study. Further, evidence from these sites suggests that interactions between tide and wetland elevation strongly influence wetland carbon fluxes. These findings may indicate key parameters of interest for wetland carbon balance and radiative forcing modelling, and also transform our approach of, and expectations for, wetland restoration.