PS 49-260 - Methane emissions from least-disturbed, restored, and disturbed tidal wetlands in the Pacific Northwest, USA

Abstract: Blue carbon ecosystems such as tidal marshes and swamps have significant capacity to sequester and store organic carbon over long time scales, but these wetlands are sometimes sources of greenhouse gasses including methane, which might offset some of their climate cooling potential. We measured methane fluxes nine times over a year in 34 sites in five estuaries in the Pacific Northwest, USA to assess the effects of wetland type and environmental drivers in least disturbed, restored, and former tidal wetlands. Methane fluxes ranged from emission to uptake and varied among wetland classes, with the highest emissions observed in slow-draining, non-tidal freshwater pastures and restored tidal marshes. We used a Boosted Regression Tree (BRT) model as part of our analysis which showed that soil temperature, groundwater level and pH, wetland surface elevation (a proxy for surface inundation), and salinity had the greatest relative predictive power for methane fluxes. Modeling using BRTs could be a useful prediction tool for methane fluxes in coastal tidal wetlands of the Pacific NW if provided with adequate training using major environmental drivers. Furthermore, such data can be used in restoration planning to select optimal salinity and elevation conditions to minimize offsets to blue carbon sequestration due to methane.