Atmospheric Environment: X (Dec 2021)
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.
