Continuous measurements of methane flux in two Japanese temperate forests based on the micrometeorological and chamber methods

Abstract:

Methane (CH₄) budget in forest ecosystems have not been accurately quantified due to limited measurements and considerable spatiotemporal heterogeneity. In order to quantify CH₄ fluxes at temperate forest at various spatiotemporal scales, we have continuously measured CH₄ fluxes at two upland forests based on the micrometeorological hyperbolic relaxed eddy accumulation (HREA) and automated dynamic closed chamber methods.

The measurements have been conducted at Teshio experimental forest (TSE) since September 2013 and Yamashiro forest meteorology research site (YMS) since November 2014. Three automated chambers were installed on each site. Our system can measure CH₄ flux by the micrometeorological HREA, vertical concentration profile at four heights, and chamber measurements by a laser-based gas analyzer (FGGA-24r-EP, Los Gatos Research Inc., USA).

Seasonal variations of canopy-scale CH₄ fluxes were different in each site. CH₄ was consumed during the summer, but was emitted during the fall and winter in TSE; consequently, the site acted as a net annual CH₄ source. CH₄ was steadily consumed during the winter, but CH₄ fluxes fluctuated between absorption and emission during the spring and summer in YMS. YMS acted as a net annual CH₄ sink.

 CH₄ uptake at the canopy scale generally decreased with rising soil temperature and increased with drying condition for both sites. CH₄ flux measured by most of chambers showed the consistent sensitivity examined for the canopy scale to the environmental variables. CH₄ fluxes from a few chambers located at a wet condition were independent of variations in soil temperature and moisture at both sites. Magnitude of soil CH₄ uptake was higher than the canopy-scale CH₄ uptake. Our results showed that the canopy-scale CH₄ fluxes were totally different with the plot-scale CH₄ fluxes by chambers, suggesting the considerable spatial heterogeneity in CH₄ flux at the temperate forests.