Vertical profiles of methane concentration above and within the canopy of a temperate Japanese cypress forest

Abstract

Methane (CH₄) is one of the major greenhouse gases, and therefore its source identification and quantification are quite important. So far water-unsaturated soils of upland forests are usually recognized as sinks, but recent increasing evidence supports tree stems in certain environments can be a source of CH₄, and thereby research on the dynamics of CH₄ in forest ecosystems has entered a new phase. In this study, we measured vertical distributions of CH₄ concentrations within and above a forest canopy to examine how they are influenced by local ecological processes and synoptic-to local-scale atmospheric processes. Profile measurements were conducted using a meteorological tower in a temperate Japanese cypress forest with a 30-min time resolution. Time series data of CH₄ concentrations above the canopy at our site showed occasional spikes relative to background levels, and these spikes corresponded with temporal variations in CO₂ concentrations. Backward trajectory analyses showed that most of the air masses associated with the spikes in CH₄ and CO₂ concentrations had continental origins. Seasonal-mean diurnal properties in CH₄ concentrations above and within the canopy were weaker than those for CO₂ concentrations. It was considered likely that nighttime accumulation of CH₄ in a stable surface layer and its convective diffusion into a mixed layer accounted for the diurnal properties in CH₄, while ecological processes played a major role in the diurnal properties observed in CO₂ levels. Vertical distributions in CH₄ concentration within the canopy showed a monotonic decrease from the canopy to the forest floor, with variations depending on the season. The profiles agreed with soil-atmosphere CH₄ fluxes measured using a closed-chamber method at our site, indicating that the surface soil in upland areas plays a major role for determining the within-canopy distributions of CH₄. Also, it was inferred that at least at our site, CH₄ emissions from stems and leaves, which has become a hot topic over the recent years, was not significant for cypress trees in upland area enough to influence the within-canopy distributions of CH₄ during the study period.