Controlling Factors of Seasonal Variation of Stem Methane Emissions From Alnus japonicain a Riparian Wetland of a Temperate Forest

Tree‐mediated methane (CH4) emissions represent an uncertain pathway of the total CH4flux released from forested ecosystems. We measured the stem CH4fluxes from Alnus japonicain a riparian wetland of a temperate forest for a year from 2017 to 2018, using automated chambers. The stem CH4fluxes varied seasonally, showing summertime maxima and wintertime minima. Medians of stem CH4fluxes were 12.9, 0.5, 0.4, and 13.4 nmol m−2s−1during autumn, winter, spring, and summer, respectively. Dissolved CH4concentration in groundwater beneath the trees was the major controlling factor of the stem CH4fluxes. Temporal variations in soil environmental conditions (temperature and groundwater level) controlled dissolved CH4concentrations in groundwater, and accordingly, stem CH4fluxes. In addition, stem CH4emissions were observed when the sap flux diminished during the defoliation period. These results indicate that CH4in the rhizosphere is mainly transported in the gaseous form through intercellular space in trees. Intense rainfall occasionally caused a transient increase in the stem CH4emissions, which was attributed to the temporal changes in the groundwater flow paths by rainfall affecting the belowground CH4concentrations. Our findings highlight the need for continuous stem CH4flux measurements for improved understanding of the controlling factors and future response of CH4dynamics in forests, with regards to climate change. Methane (CH4) is an important greenhouse gas. Aerated forest soils are assumed to be an atmospheric CH4sink. However, the results of recent experimental and field studies suggest that tree‐mediated CH4emissions have the potential to offset the soil CH4sink and may convert the forest from a net sink to a net source. Tree‐mediated CH4emissions represent an uncertain pathway of the total CH4flux from forested ecosystems. In this study, we conducted continuous field measurements for a year, on the stem CH4fluxes from Alnus japonicastanding in a riparian wetland. We found that the seasonal variation in the stem CH4fluxes is controlled by seasonal variations of the dissolved CH4concentrations in the groundwater. Temporal changes in soil environmental conditions (temperature and groundwater level) controlled dissolved CH4concentrations in groundwater. It was also found that intense rainfall causes a transient increase in the stem CH4emissions. At the study site, changes in the groundwater flow paths by rainfall affect the dissolved CH4concentrations in the rhizosphere and the stem CH4flux. Understanding these mechanisms will contribute to the development of models used to estimate the CH4emissions from forest ecosystems and predict their response to environmental changes. The stem CH4fluxes from Alnus japonica seasonally vary and increase during and after intense rainfallDissolved CH4concentration in groundwater is a major factor controlling stem CH4flux as shown by path analysisRhizospheric CH4is mainly transported in the gaseous form through intercellular space in trees and partly by the transpiration stream The stem CH4fluxes from Alnus japonica seasonally vary and increase during and after intense rainfall Dissolved CH4concentration in groundwater is a major factor controlling stem CH4flux as shown by path analysis Rhizospheric CH4is mainly transported in the gaseous form through intercellular space in trees and partly by the transpiration stream