Temporal and spatial variations in CH4 concentrations in a Japanese warm-temperate mixed forest

Abstract

To clarify the characteristics of the variation in methane (CH4) concentrations in a warm-temperate mixed forest in Japan, we measured vertical profiles and daily and seasonal variations in CH4 concentrations, and soil and foliage (Quercus serrata) CH4 fluxes. CH4 concentrations increased during rainy periods, and decreased in August. Almost constant CH4 uptake was observed at the forest floor throughout the year, but no CH4 flux was observed from the foliage. In the profile measurements, the CH4 concentration was slightly higher at canopy height when friction velocity (u*) was lower than 0.3 m s-1. These phenomena were observed on the same day at micrometeorological towers in ridge and valley positions. Variations in CH4 concentrations increased in summer, around ground level, and at canopy height. These results suggest that certain large scale factors govern variations in CH4 concentration around ground level and at canopy height. To understand the overall role of the forest in CH4 circulation, more continuous measurements as close as possible to sinks and sources will be needed.