Diurnal and seasonal variations of CH 4 exchange observed in a black spruce forest

Transcription

1 Diurnal and seasonal variations of CH 4 exchange observed in a black spruce forest Hiroki Iwata (Kyoto Univ) Collaborators: Y. Harazono, H. Nagano (IARC), M. Ueyama (Osaka Pref Univ), Y. Kosugi, A. Sakabe, K. Takahashi (Kyoto Univ)

2 Background: CH 4 eddy covariance observation CH 4 eddy covariance observation is becoming a possible option. Ecosystem-scale exchange Maximum value during daytime Temperate wetland: 300 nmol m -2 s -1 (Detto et al., 2011) Arctic wetland: 35 nmol m -2 s -1 (Sachs et al., 2008; Zona et al., 2009) Forests: ±5 nmol m -2 s -1 (Smeets et al., 2009; Querino et al., 2011) How about black spruce forests?

3 Objectives and approaches Objectives - Clarify the CH 4 exchange dynamics in a black spruce forest - Quantify the ecosystem-scale CH 4 exchange Approaches - Employ the eddy covariance and chamber techniques to examine the temporal and spatial patterns of CH 4 exchange

4 Observation site - Black spruce ecosystem. Tree height ranging from 1-4 m. Typical tree age: 85 years (Ueyama et al., submitted). - Vegetation at the forest floor: dwarf birch, sedge, mosses etc. - Permafrost. The active layer depth is cm. - Microtopography with hummocks and hollows - Soil water ph: Mean monthly T: (Jan), 16.9 (Jul), Mean annual precipitation: 263mm Iwata et al. (2010, 2012)

5 Eddy covariance and chamber observations Closed-path eddy covariance LI flow rate: 10L/min CSAT3 Air inlet tube - Permapure dryer - High-frequency attenuation corrected Dynamic open chamber - two wet areas and two dry areas (Iwata et al., submitted) Permapure dryer Pump RMT-200 LGR LGR methane analyzer

6 Abstract of preliminary results The methane exchange in the black spruce ecosystem observed with the eddy covariance technique had a small but clear seasonal variation. The emission was low until mid-june, and it increased with the deepening of soil thaw in the latter half of summer. This ecosystem was a small net methane source with an emission of about 30 mmol/m2 during five months in summer. In a drier year, the emission was lower probably due to the drier soil and thus less developed anaerobic condition. Key controlling variables for methane emission was soil water content and thaw depth. Regression tree analysis showed that the methane emission was larger when the soil water content for cm was higher and thaw depth was greater. No evident dependence of methane emission on air pressure was found, which may imply that the ebullition contributed little to the exchange in this site. No clear diurnal variation of methane exchange was found. The chamber observation revealed a heterogeneous methane source/sink distribution: emission in wet areas and uptake in dry areas. The magnitude of fluxes observed with the eddy covariance technique was within the range of fluxes observed with the chambers, suggesting both techniques reasonably agree.

7 References Detto, M. et al.: Comparing laser-based open- and closed-path gas analyzers to measure methane fluxes using the eddy covariance method, Agric. For. Meteorol., 2011, 151, Iwata, H. et al.: Influence of source/sink distributions on flux-gradient relationships in the roughness sublayer over an open forest canopy under unstable conditions, Boundary-Layer Meteorol., 2010, 136, Iwata, H. et al.: The role of permafrost in water exchange of a black spruce forest in Interior Alaska, Agric. For. Meteorol., 2012, 161, Iwata, H. et al.: Cross-validation of open-path and closed-path eddy covariance techniques for observing methane fluxes, submitted to Boundary-Layer Meteorol. Querino, C. A. S. et al.: Methane flux, vertical gradient and mixing ratio measurements in a tropical forest, Atmos. Chem. Phys., 2011, 11, Sachs, T. et al.: Environmental controls on ecosystem-scale CH4 emission from polygonal tundra in the Lena River Delta, Siberia, J. Geophys. Res., 2008, 113, G00A03. Smeets, C. J. P. P. et al.: Eddy covariance methane measurements at a Ponderosa pine plantation in California, Atmos. Chem. Phys., 2009, 9, Ueyama, M. et al.: Autumn warming reduces the CO2 sink of a black spruce forest in interior Alaska based on a nine-year eddy covariance measurement, submitted to Global Change Biol. Zona, D. et al.: Methane fluxes during the initiation of a large-scale water table manipulation experiment in the Alaskan Arctic tundra, Global Biogeochem. Cycles, 2009, 23, GB2013.