Seasonal and weather-related controls on methane emissions from the stems of mature trees in a cool-temperate forested wetland

Methane (CH4) emission from tree stems (stem CH4 emission) is a previously overlooked emission pathway in forested wetlands. Despite growing evidence of its significance to both regional and global CH4 budgets, the drivers and mechanisms of stem CH4 emission in wetlands are not fully understood. To clarify the environmental drivers of stem CH4 emission, we conducted chamber-based flux measurements for mature Alnus japonica and Fraxinus mandshurica trees in a forested wetland in northern Japan over four snow-free seasons, as well as a short-term flooding experiment and an isotopic analysis of stem-emitted and soil porewater CH4. Stem CH4 fluxes varied seasonally and annually. The highest flux, exceeding 10,000 mu g CH4 m(-2) h(-1), occurred in August 2016 following a record-breaking rainfall. Soil temperature and water-table depth had significant effects on stem CH4 flux. Artificial flooding remarkably increased stem CH4 flux more than tenfold within 4 days and elevated fluxes persisted for at least 1 day after the floodwater receded. Stem-emitted CH4 was depleted in C-13 by an average of 2-3 parts per thousand relative to porewater CH4, implying isotopic fractionation of CH4 by diffusion and/or oxidation along its emission pathway. Belowground environmental conditions exhibited considerable control over stem CH4 emissions in forested wetlands; the biogeochemical and biological mechanisms of this influence should be further explored.

Automated summary

Stem methane emission from trees in forested wetlands is an overlooked pathway that plays a significant role in methane budgets. Environmental factors such as soil temperature and water-table depth influence the flux of stem methane seasonally and annually. Flooding can dramatically increase stem methane emission, and isotopic analysis shows fractionation along the emission pathway. Further research is needed to explore the mechanisms behind the control of stem methane emission by belowground environmental conditions.