2.4 Analysis of the temporal variability of soil CO 2 efflux
3.3.1 Statistical data description
Monthly means of ecosystem fluxes and chamber data
Monthly means (including 95 % confidence intervals) of ecosystem fluxes (NEE, GPP, REco and soil CO2 flux) between January 2007 and July 2012 are presented in Fig. 3.8.
Monthly means that were derived of more than one third low-quality gap-filled values, are marked with a cross. On the right hand side of the plot, the average yearly cycle is presented. For a table of all monthly statistical properties, please refer to appendices E.1 and E.3.
Ecosystem respiration and soil CO2 flux follow a very similar cycle, with maxima in
July and August and minima in January and February. The maximum values for soil CO2 fluxes are in the range 7-10 µmol m−2s−1, as compared to 10-12 µmol m−2s−1
of total ecosystem respiration. Lowest monthly soil CO2 flux rates in winter are
about 1.0 µmol m−2s−1, whereas winter ecosystem respiration is lower (in the range of
0.5-1.0 µmol m−2s−1). Hence, the seasonal variability of ecosystem respiration appears
to be higher than the variability of soil CO2 flux. Gross primary production mainly
follows the yearly cycle of irradiation and has its maximum earlier in the year than respiration values (in June and July), and its minima in winter between November and February. The seasonal cycle of respiration is delayed by approximately 1-2 months in time compared to gross primary production, due to the delayed warming of the soil. The monthly means of soil CO2 flux as well as the environmental data are presented
Figure 3.8. Monthly means (including 95% confidence interval of mean) of NEE, GPP, REcoand soil
CO2 flux between January 2007 and July 2012. Since the direction of GPP is opposite to
the other fluxes and for reasons of better visibility, it is displayed as negative flux.
and PAR show a yearly cycle with maxima during summer and minima during winter months. Monthly means of soil temperature are within the range of -0.8 and 17.0◦C
with maxima in July/August and minima between December and March. Monthly mean values of PAR range between 0 and 80 µmol m−2
s−1 with maxima in June and minima in winter between November and January. Soil water content does not have a clear yearly cycle, as it is linked to precipitation events. However, it appears to be inversely proportional to temperature and PAR, with relatively low water content values during summer, when evaporation from the soil is highest (Davidson et al., 1998).
Monthly means of partitioned soil CO2 fluxes, i.e. of soil respiration and soil CO2
uptake, can be found in appendix E.2.
Information on monthly means of ecosystem fluxes and similar environmental factors could not be found in other studies, therefore no comparison with the literature was possible.
Annual means of ecosystem fluxes and chamber data
For two different years before and after the thinning, annual means were calculated and are presented in Table 3.7 (after conversion to g C m−2yr−1). In 2007/2008, i.e. before
Figure 3.9. Monthly means of soil CO2 flux, soil temperature, soil water content and PAR (with
95% confidence intervals of the mean) for the two time periods.
the thinning, soil CO2 efflux has a mean of 1.43 ± 1.07 kg C m−2yr−1. After the
thinning, in 2010/2011, it has a mean of 1.57 ± 1.16 kg C m−2yr−1. That means, after
the thinning, the CO2 efflux from the soil is larger. Ecosystem fluxes, i.e. NEE, GPP and
REco, are all lower after the thinning. NEE went down from 0.36 ± 2.33 kg C m −2
yr−1
to 0.32 ± 2.01 kg C m−2
yr−1
The conclusions that can be drawn from the comparison of the two years are limited and longer time series would be required to make reliable statements. However, a part of the changes might be related to the thinning and a possible explanation is given in the following.
The effect of thinning is a combined result of a decrease in root respiration, an increase in soil organic matter and changes in soil temperature and water.
Due to a lower tree density as consequence of the thinning, it can be expected that more solar radiation reaches the ground and warms the soil (Jandl et al., 2007). This is aligned with the chamber measurements: In the second year, soil temperature and PAR were indeed higher in the first year. Due to this, both photosynthesis of ground vegetation and microbial decomposition of carbon in the soil are expected to increase, as it was shown by the measurements. At the same time, above-ground respiration, root respiration and photosynthesis of trees could be expected to decrease due to the reduced tree density (Tang et al., 2005b). As root respiration presumably decreased,
but soil respiration increased, it can be assumed that there was a shift towards a higher fraction of heterotrophic respiration in the year after the thinning. This can be explained by the warming of the soil and possibly by the decomposition of litter and dead roots that were left after the thinning.
The measured increase in soil respiration should in theory be visible in the measurements of total ecosystem respiration, since soil respiration contributes a high fraction of total respiration. One possible explanation would be that above-ground respiration was significantly lower in the second year, causing total respiration to decrease. However, a decrease in above-ground respiration that outweighs the increase in soil respiration is unlikely to be caused by the thinning alone. Further investigations of the impacts of the thinning would require longer complete time series and additional meteorological data, such as air temperature and precipitation data.
Studies on the effect of thinning on soil CO2 fluxes and ecosystem fluxes in forests are
scarce and show very different results. Saunders et al. (2012) did not find any effect of thinning on NEE and GPP in Sitka spruce forest, however, the non-existence of an effect was explained by different climatic conditions before and after the thinning that compensated for potential changes in ecosystem fluxes. Peng et al. (2008) reviewed the impact of different forest management techniques on the carbon balance of forests and described varying results for the effect of thinning on soil CO2 fluxes: Ohashi et al.
(1999) found higher soil CO2 fluxes in the thinned part than in the unthinned part
of a Japanese cedar forest in the 3rd and 4th year after the thinning, Ma et al. (2004)
did not find any differences and Tang et al. (2005b) even observed a decrease of soil respiration by 15 % after the thinning.
Annual mean soil respiration had a value of 1.53 ± 1.15 kg C m−2yr−1 before the thin-
ning and 1.78 ± 1.26 kg C m−2yr−1 after the thinning. When compared to literature
data, the measured soil respiration values appear very high. For example, in their review, Raich and Schlesinger (1992) found a mean soil respiration rate for boreal forests and woodlands of 0.32 kg C m−2yr−1. In the summary of soil respiration rates
per biome presented by Raich and Tufekciogul (2000), only Brazilian forests have annual respiration rates of equal magnitude. Other studies, which were carried out in boreal forests, found mean annual soil respiration rates of 1.02 kg C m−2yr−1 (Khomik et al.,
2006), 0.85 and 0.90 kg C m−2yr−1 (Pumpanen et al., 2003). However, previous studies
Table 3.7. Comparison of annual mean values before and after the thinning. Half-hourly values of fluxes [µmol CO2m−2yr−1] were averaged over two periods: a) 01.06.2007 – 31.05.2008
and b) 01.08.2010 – 31.07.2011 and then converted to kg C m−2yr−1. The quality coefficient
QC is the mean of all quality coefficients “f_qcOK” of all values (QC 1 = highest quality, QC = 0 lowest quality). All data with a QC below 0.67, i.e. with a contribution of at least
one third of critically gap-filled data, should be used with caution.
2007/2008 2010/2011
Variable Mean STD CV QC Mean STD CV QC NEE [kg C m−2yr−1] 0.36 2.33 646.9% 1.00 0.32 2.01 631.9% 1.00
GPP [kg C m−2yr−1] 1.73 2.85 164.4% - 1.35 2.46 182.5% -
REco [kg C m−2yr−1] 2.09 1.51 72.1% - 1.66 1.38 82.9% -
FSoil[kg C m−2yr−1] 1.43 1.07 75.4% 1.00 1.57 1.16 74.2% 0.71
RSoil [kg C m−2yr−1] 1.53 1.15 75.0% - 1.78a) 1.26 70.9% -
Soil CO2 uptake [kg C m−2yr−1] 0.10 0.33 325.8% - 0.25a) 0.64 255.1% -
PAR [µmol m−2s−1] 11.54 36.74 318.5% 0.99 24.21 77.62 320.6% 0.97
SWC [m3/m3] 0.31 0.10 31.2% - 0.20 0.09 46.8% -
TSoil [°C] 6.68 4.88 73.1% 0.99 7.01 6.00 85.7% 0.97
STD - standard deviation, CV - coefficient of variation, QC - quality coefficient
a)For Jan and Feb 2011, no values for RSoiland soil CO2 uptake were available; the annual mean was
calculated without these months. Therefore, the difference between RSoil and CO2-uptake does not
equal annual soil CO2efflux. The values for RSoil and CO2 uptake are expected to be lower, if the
two months were included.
Lindroth (2000) estimated a mean annual respiration rate of 1.23 kg C m−2yr−1 at the
Norunda site for the year 1996. Widén (2002) measured CO2 fluxes at the forest floor
in Norunda and obtained mean annual respiration values of 1.25 and 1.36 kg C m−2
yr−1
in two subsequent years (measured at the south west stand, similar to the current chamber locations). Still, the measured values presented in this study are higher. The data quality of the first year is very good, however, the mean quality coefficient of the second year is comparatively low (0.71), i.e. ~30% of the data that was used for calculating the annual mean were gap-filled values with a low-quality flag. This is due to the gap-filling in winter 2010/2011. The very high value of the second year may be a result of the gap-filling process and should be used with caution. A more comprehensive analysis of the effect of the gap-filling procedure would be needed. It should also be noted that the annual carbon balance of the forest site (a loss of 0.32-0.36 kg C m−2yr−1) is higher than those found in previous studies. Annual mean
a mean of 52 g C m−2yr−1 (Lagergren et al., 2008). For the time period from 2007 to
2011, annual mean NEE ranges between 303 and 522 g C m−2
yr−1
, with a total mean of 396 g C m−2
yr−1
. These differences are possibly attributable to different correction methods and should be further investigated.