The contribution of trees to ecosystem methane emissions in a temperate forested wetland

Wetland‐adapted trees are known to transport soil‐produced methane ( CH 4 ), an important greenhouse gas to the atmosphere, yet seasonal variations and controls on the magnitude of tree‐mediated CH 4 emissions remain unknown for mature forests. We examined the spatial and temporal variability in stem CH 4 emissions in situ and their controls in two wetland‐adapted tree species ( Alnus glutinosa and Betula pubescens ) located in a temperate forested wetland. Soil and herbaceous plant‐mediated CH 4 emissions from hollows and hummocks also were measured, thus enabling an estimate of contributions from each pathway to total ecosystem flux. Stem CH 4 emissions varied significantly between the two tree species, with Alnus glutinosa displaying minimal seasonal variations, while substantial seasonal variations were observed in Betula pubescens . Trees from each species emitted similar quantities of CH 4 from their stems regardless of whether they were situated in hollows or hummocks. Soil temperature and pore‐water CH 4 concentrations best explained annual variability in stem emissions, while wood‐specific density and pore‐water CH 4 concentrations best accounted for between‐species variations in stem CH 4 emission. Our study demonstrates that tree‐mediated CH 4 emissions contribute up to 27% of seasonal ecosystem CH 4 flux in temperate forested wetland, with the largest relative contributions occurring in spring and winter. Tree‐mediated CH 4 emissions currently are not included in trace gas budgets of forested wetland. Further work is required to quantify and integrate this transport pathway into CH 4 inventories and process‐based models.