Seawater Exposure Increases Tree Methane Emissions

Abstract:

Seawater exposure generally decreases methane (CH₄) exchange between coastal environments and the atmosphere due to competing soil redox reactions. However, CH₄ emissions from trees has recently emerged as an important, but poorly quantified process. In coastal forests, seawater flooding events expose low-lying trees to a range of salinities, potentially altering soil-tree biogeochemical linkages and gas exchange. Trees may act as conduits for CH₄ to escape soils even under redox conditions that don’t favor soil CH₄ emissions, but it remains unclear how the magnitude of stem CH₄ emissions are affected by periodic seawater flooding. A survey of coastal forest stem CH₄ concentrations found that seawater flooding was significantly correlated with increased stem CH₄ concentrations, however the effect of seawater flooding on stem CH₄ emissions was not quantified. Here we seek to understand the mechanisms driving stem CH₄ emissions to the atmosphere in coastal forests exposed to seawater flooding, which is critical for predicting future CH₄ emissions linked to sea level rise and coastal forest flooding events. To test the impact of seawater exposure on stem and soil CH₄ emissions, we studied tree stem and soil CH₄ emissions within a coastal forest floodplain located in the Pacific Northwest. We found that seawater flooding is significantly correlated to increased soil and stem CH₄ emissions. Our findings suggest that in association with seawater exposure, coastal forests experience increased soils and tree CH₄ emissions. Thus, seawater flooding events may act as a switch, causing soil CH₄ emissions to change from a sink to a source, highlighting that CH₄ emissions are likely to increase from coastal forests experiencing seawater exposure.