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

Wetland-adapted trees are known to transport soil-produced methane (CH<inf>4</inf>), an important greenhouse gas to the atmosphere, yet seasonal variations and controls on the magnitude of tree-mediated CH<inf>4</inf> emissions remain unknown for mature forests. We examined the spatial and temporal variability in stem CH<inf>4</inf> 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<inf>4</inf> emissions from hollows and hummocks also were measured, thus enabling an estimate of contributions from each pathway to total ecosystem flux. Stem CH<inf>4</inf> 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<inf>4</inf> from their stems regardless of whether they were situated in hollows or hummocks. Soil temperature and pore-water CH<inf>4</inf> concentrations best explained annual variability in stem emissions, while wood-specific density and pore-water CH<inf>4</inf> concentrations best accounted for between-species variations in stem CH<inf>4</inf> emission. Our study demonstrates that tree-mediated CH<inf>4</inf> emissions contribute up to 27% of seasonal ecosystem CH<inf>4</inf> flux in temperate forested wetland, with the largest relative contributions occurring in spring and winter. Tree-mediated CH<inf>4</inf> 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<inf>4</inf> inventories and process-based models.