Methane exchange in a boreal forest estimated by gradient method

Figures & data

Fig. 1 Digital surface model (DSM) of the Norunda site. The circles denote 500 m (orange) and 800 m (yellow) distance from the tower (red dot).

Fig. 2 Comparison of net CO₂ exchange measured by EC at 33 m and net CO₂ exchange calculated from gradients between 31.7 and 58.8 m by combined EC and gradient method (ECG) in July 2010. For the ECG method, only values with good accuracy, i.e., total errors <20 µmol m⁻² h⁻¹, are shown (see text for details).

Fig. 3 Example of (a) daytime and (b) nighttime CH₄ concentration measurements. Each level (31.7, 58.8 and 100.6 m) was measured for 5 min=300 s starting at level 1.

Fig. 4 Occurrence of CH₄ concentration differences for different height intervals and for day (global radiation >50 Wm⁻²) and night (global radiation <50 Wm⁻²). Data between the 2nd and 98th percentiles are shown.

Fig. 5 CH₄ concentration measured at 100.6 m height from June 2010 to December 2011. Error bars are given as plus/minus SD of monthly mean concentrations.

Fig. 6 Windrose for Norunda site at 33 m height above ground for the entire measurement period.

Fig. 7 Average CH₄ exchange at the tower for wind directions binned in 30 degree intervals.

Table 1. Monthly mean values of CH₄ exchange for main wind directions, 180°–240°, derived using the modified Bowen ratio (BR) method and the combined eddy covariance and gradient method (ECG) for gradients between 31.7 and 58.5 m and between 31.7 m and 100.6 m

	BR1 CH4 exchange (µmol m^−2 h^−1)	BR2 CH4 exchange (µmol m^−2 h^−1)	ECG1 CH4 exchange (µmol m^−2 h^−1)	ECG2 CH4 exchange (µmol m^−2 h^−1)	Chambers CH4 exchange (µmol m^−2 h^−1)
Jun 2010	7.5	8.7	4.0	8.0	–
Jul 2010	1.95	5.6	1.2*	3.8	−11.7
Aug 2010	−1.7	2.1	−2.7	2.4	−9.8
Sep2010	3.9	5.9	2.5	5.9	−9.0
Oct 2010	2.7	5.3	4.1	5.5	–
Nov2010	0.7*	2.8	−0.7*	3.0	–
Dec2010	−1.6	1.7*	−1.9	1.1*	–
Mar 2011	0.3*	3.5	1.0	2.2	–
Apr 2011	3.1	3.9	5.9	6.6	–
May 2011	−0.3*	1.7	−1.4*	1.2	–
Jun 2011	0.2*	5.2	1.1*	3.9	–
Jul 2011	−1.2*	1.3*	−1.2*	0.1*	–
Aug 2011	−4.0	1.1*	−3.7	1.8	–
Sept2011	−2.7	1.1*	−1.6*	0.4*	–
Oct 2011	0.3*	0.9*	0.1*	0.2*	–
Nov 2011	−1.1	−0.1*	−1.4	−1.4	–
Dec 2011	−1.4	0.4*	−3.1	−0.4*	–

*Not significantly different from zero.

Fig. 8 Half-hourly footprint estimates cumulated for the entire measurement period for (a) gradients between 31.7 and 58.5 m and (b) between 31.7 and 100.6 m. Contour lines are plotted for each 10% of the cumulated footprint. The tower location is depicted as a red dot. The background maps are tree height from LiDAR measurements, for illustration. Coordinates (x and y axes) are in UTM.

Fig. 9 Weekly mean values of CH₄ exchange, water table depths, air temperature and soil temperature and also the weekly amount of precipitation.

Table 2. Coefficients calculated by multiple linear regression analyses of daily mean values of CH₄ exchange derived at two levels and for two different methods (the modified Bowen ratio (BR) method and combined eddy covariance and gradient method (ECG)); and the explanatory variables air temperature, soil temperature, soil moisture, water table depth, CO₂ exchange and GPP. The analysis was made on standardised data to adjust for the disparity in variable sample sizes. The adjusted R ² value displays how much of the variance in CH₄ exchange can be explained

	ECG1		ECG2		BR1		BR2
	Jun–Sep, N=97, R ^2=0.03	Total, N=200, R ^2=0.07	Jun–Sep, N=126, R ^2=0.05	Total, N=281, R ^2=0.07	Jun–Sep, N=114 R ^2=0.1	Total, N=265, R ^2=0.18	Jun–Sep, N=136 R ^2=0.14	Total, N=300, R ^2=0.23
Air temperature	−0.09	0.035	−0.0005	0.05	−0.06	−0.05	0.04	−0.1
Soil temperature	−0.1	−0.004	−0.03	0.09	−0.07	−0.09	−0.01	−0.36*
Soil moisture	0.05	0.1	0.09	0.14	0.009	0.01	0.03	0.09
Water table depth	0.24*	0.30*	0.23*	0.25*	0.39*	0.32*	0.27*	0.19*
CO2 flux	−0.1	0.0006	0.23*	0.23*	0.1	0.36*	0.30*	0.64*
GPP	−0.07	−0.06	−0.03	0.03	0.04	0.1	0.12	0.37*

*The variable significantly contributes to the variance in CH₄.

Fig. 10 Diel pattern of CH₄ exchange binned hourly for the entire measurement period.

Fig. 11 Diel patterns of soil CH₄ exchange measured by individual soil chambers, 7 July–31 September 2010.

Table 3. Pearson correlations between hourly binned CH₄ exchange for BR1 and hourly binned air temperature, soil temperature, GPP and VPD

	Air temperature	Soil temperature	GPP	VPD
Mar–Apr	−0.50*	0.54*	0.85**	−0.68**
May–Jun	−0.54*	−0.15	0.49*	−0.46*
Jul–Aug	−0.77**	0.19	0.80**	−0.58*
Sep–Oct	−0.79**	−0.19	0.68**	−0.68**
Nov–Dec	−0.26	−0.12	0.24	−0.17
Total	0.26	0.56*	0.88**	−0.52*

*Significant at p<0.05, *significant at p<0.001.