Measurement of methane flux over an evergreen coniferous forest canopy using a relaxed eddy accumulation system with tuneable diode laser spectroscopy detection

Abstract

Very few studies have conducted long-term observations of methane (CH 4 ) flux over forest canopies. In this study, we continuously measured CH 4 fluxes over an evergreen coniferous (Japanese cypress) forest canopy throughout 1 year, using a micrometeorological relaxed eddy accumulation (REA) system with tuneable diode laser spectroscopy (TDLS) detection. The Japanese cypress forest, which is a common forest type in warm-temperate Asian monsoon regions with a wet summer, switched seasonally between a sink and source of CH 4 probably because of competition by methanogens and methanotrophs, which are both influenced by soil conditions (e.g., soil temperature and soil moisture). At hourly to daily timescales, the CH 4 fluxes were sensitive to rainfall, probably because CH 4 emission increased and/or absorption decreased during and after rainfall. The observed canopy-scale fluxes showed complex behaviours beyond those expected from previous plot-scale measurements and the CH 4 fluxes changed from sink to source and vice versa.

Methane exchange in a boreal forest estimated by gradient method

“…Patches of wetter soils and small ponds can be found in the main wind direction and it is possible that emissions from these wet areas switch the site from being a sink of CH 4 to being a source of CH 4 as shown by BR2 and ECG2 and for some months by BR1 and ECG1 (Table 1). Temporal shifts of soils from net sinks to net sources in wet periods were found in several studies (e.g., Hudgens and Yavitt, 1997;Moosavi and Crill, 1997;Sakabe et al, 2012). Observations further indicate that large emissions from small source areas can shift a site from a sink to a source (Fiedler et al, 2005;Sakabe et al, 2012).…”

“…Periods with lower than average CH 4 emissions at Norunda might be due to methanotrophs dominating the net flux. Diel trends with maximum CH 4 uptake around noon were found at sites where consumption dominates the CH 4 exchange (Smeets et al, 2009;Sakabe et al, 2012;Wang et al, 2013). Wang et al (2013) explained the increased daytime uptake with increased turbulence during the day that facilitated the transport of atmospheric CH 4 to the methanotrophs in the soil, while Sakabe et al (2012) found a dependency of the diel amplitude of CH 4 exchange on air temperature.…”

“…It is possible that the CH 4 uptake rate in forests, especially in moist forests, is overestimated since emissions from wet soils are not adequately accounted for (Grunwald et al, 2012). Even if CH 4 emissions are sporadic and limited to restricted areas, their occurrence frequency and source strength might turn a forest from a net sink to a net source of CH 4 (Fiedler et al, 2005;Sakabe et al, 2012). The capacity of plants with roots in CH 4 -rich water-saturated soils, even woody plants, to transport and release CH 4 (Terazawa et al, 2007;Gauci et al, 2010) adds another level of the complexity to the analyses.…”

“…CH 4 production is strongly favoured by increasing temperatures, while CH 4 oxidation is not as sensitive to temperature changes (Dunfield et al, 1993;Yvon-Durocher et al, 2014). Soils have been observed to switch from sinks to sources following precipitation (Keller and Reiners 1994;Wang F. L. and Bettany 1995;Hudgens and Yavitt 1997;Sakabe et al, 2012).…”

“…Measuring CH 4 exchange is more challenging than measuring CO 2 exchange due to low flux rates from spatially restricted areas within a forest and therefore relatively few MM studies exist to date (Nicolini 2013). CH 4 exchange between forest stands and the atmosphere has been measured by EC (Smeets et al, 2009;Shoemaker et al, 2014), relaxed EC (Sakabe et al, 2012;Ueyama et al, 2012), and by gradient methods (Simpson et al, 1997;Bowling et al, 2009;Querino et al, 2011).…”