Insights into the mechanism of diurnal variations in methane emission from the stem surfaces of Alnus japonica.

Recent studies have suggested that in certain environments, tree stems emit methane (CH 4 ). This study explored the mechanism of CH 4 emission from the stem surfaces of Alnus japonica in a riparian wetland. Stem CH 4 emission rates and sap flux were monitored year-round, and fine-root anatomy was investigated. CH 4 emission rates were estimated using a closed-chamber method. Sap flux was measured using Granier-type thermal dissipation probes. Root anatomy was studied using both optical and cryo-scanning electron microscopy. CH 4 emissions during the leafy season exhibited a diurnally changing component superimposed upon an underlying continuum in which the diurnal variation was in phase with sap flux. We propose a model in which stem CH 4 emission involves at least two processes: a sap flux-dependent component responsible for the diurnal changes, and a sap flux-independent component responsible for the background continuum. The contribution ratios of the two processes are season-dependent. The background continuum possibly resulted from the diffusive transport of gaseous CH 4 from the roots to the upper trunk. Root anatomy analysis indicated that the intercellular space of the cortex and empty xylem cells in fine roots could serve as a passageway for transport of gaseous CH 4 .