Controlling Factors of Seasonal Variation of Stem Methane Emissions From <i>Alnus japonica</i> in a Riparian Wetland of a Temperate Forest
Ayaka Sakabe, Kenshi Takahashi, Wakana Azuma, Masayuki Itoh, Makiko Tateishi, Yoshiko Kosugi
Abstract
Abstract Tree‐mediated methane (CH 4 ) emissions represent an uncertain pathway of the total CH 4 flux released from forested ecosystems. We measured the stem CH 4 fluxes from Alnus japonica in a riparian wetland of a temperate forest for a year from 2017 to 2018, using automated chambers. The stem CH 4 fluxes varied seasonally, showing summertime maxima and wintertime minima. Medians of stem CH 4 fluxes were 12.9, 0.5, 0.4, and 13.4 nmol m −2 s −1 during autumn, winter, spring, and summer, respectively. Dissolved CH 4 concentration in groundwater beneath the trees was the major controlling factor of the stem CH 4 fluxes. Temporal variations in soil environmental conditions (temperature and groundwater level) controlled dissolved CH 4 concentrations in groundwater, and accordingly, stem CH 4 fluxes. In addition, stem CH 4 emissions were observed when the sap flux diminished during the defoliation period. These results indicate that CH 4 in the rhizosphere is mainly transported in the gaseous form through intercellular space in trees. Intense rainfall occasionally caused a transient increase in the stem CH 4 emissions, which was attributed to the temporal changes in the groundwater flow paths by rainfall affecting the belowground CH 4 concentrations. Our findings highlight the need for continuous stem CH 4 flux measurements for improved understanding of the controlling factors and future response of CH 4 dynamics in forests, with regards to climate change.