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The large scale system

As it was stated before, data gaps occur inevitably during long term ecosystem studies due to instrument malfunction or other factors (e.g. calibration periods, data storage problems, human factor, etc.). The profile method turned out to be inadequate for long term NEE calculation but useful for filling the data gaps.

Yearly net carbon dioxide exchange can be inferred from the monthly averages of NEE (Moncrieff et al., 1996), or from the daily sums of CO\( _{2}\protect \) exchange between the atmosphere and the biosphere. The latter method requires NEE information for each day of year. To complete this task, a data gap filling procedure must be developed (Falge et al., 2001) as an important step towards a defensible NEE study.

In our case, the first guess for each day is the monthly average daily NEE cycle calculated directly from the measured NEE (eddy flux at 82 m + rate of change of CO\( _{2}\protect \) storage below, i.e. \( F_{c}+F_{s} \)). All further step modifies this daily cycle where the improved estimate is available, leaving the rest of the cycle intact.

The second estimate is the modelled NEE calculated from the NEE-PPFD function during daytime (if PPFD is known), and from the NEE-\( t_{10} \) function during nighttime (if \( t_{10} \) is known).

The third estimate is the modified profile CO\( _{2}\protect \) flux described in section [*].

The fourth estimate is only applied in cases when \( F_{c} \) is available, but there is no \( F_{s} \) available. If this is the case, the monthly average daily \( F_{s} \) cycle is used to estimate the current storage term.

The fifth -- and best -- estimate is the measured NEE (=\( F_{c}+F_{s} \)). It was described in section [*] that the percentage of instationarity is calculated for each hourly period to test the quality of the measurement (Foken and Wichura, 1996). During the fifth step NEE values measured during periods with instationarity exceeding 30% are rejected.

Many authors use the empirical light-response function and the NEE-temperature function to fill data gaps (e.g. Valentini et al., 1996; Anthoni et al., 1999) at their site. Our approach is to use the semi-empirical similarity theory for this purpose, since it relies on a dynamic method compared to the empirical, statistics-based environmental forces method. As it was seen in section [*], the nighttime NEE-\( t \) function exhibits large scatter (e.g. Greco and Baldocchi, 1996; Valentini et al., 1996), which means that estimates based on the empirical NEE-\( t \) function may provide inaccurate estimates.

Having all neccessary measured data, and having a methodology to fill measurement gaps, we are capable to present the long term NEE time series for the two measuring system.

Figure: Annual and interannual variation of the daily net CO\( _{2}\protect \) exchange in 1997, 1998 and 1999 for the large scale system. Negative values indicate CO\( _{2}\protect \) uptake by the biosphere.

Figure [*] shows the temporal variation of the daily net CO\( _{2}\protect \) exchange during 1997, 1998 and 1999 as a function of time. Since direct flux measurements started at the end of April 1997, monthly average daily cycles were not available for January-April. The neccessary daily cycles are calculated as the average of the 1998 and 1999 monthly values from January to April. Further steps of the gap filling is performed as it was described earlier in this section. The neccessay empirical environmental functions are presented in section [*].

Seasonality is evident in Fig. [*]. CO\( _{2}\protect \) uptake exceeded 25 g CO\( _{2}\protect \) m\( ^{-2} \) day\( ^{-1} \) on some days during the growing season in some cases. However, there were also days when the biosphere lost carbon to the atmosphere. Wintertime NEE remains considerably larger than zero indicating active respiration even during the coldest days.

Figure: Cumulative carbon exchange for 1997, 1998 and 1999 for the large scale system.

Figure [*] shows the cumulative carbon exchange for the 1997, 1998 and 1999 based on the presented data.

The calculations show that during 1997 CO\( _{2}\protect \) NEE was -491 g CO\( _{2}\protect \) m\( ^{-2} \) year\( ^{-1} \) (-134 g C m\( ^{-2} \) year\( ^{-1} \)), during 1998, -537 g CO\( _{2}\protect \) m\( ^{-2} \) year\( ^{-1} \) (-146 g C m\( ^{-2} \) year\( ^{-1} \)) and during 1999, -337 g CO\( _{2}\protect \) m\( ^{-2} \) year\( ^{-1} \) (-92 g C m\( ^{-2} \) year\( ^{-1} \)). It means that during the 3 year period of 1997-1999 the region sequestered 372 g C m\( ^{-2} \).

The dynamics of these fluxes can be better understood by breaking them down into subcomponents. NEE is defined as the sum of gross primary production (GPP) and total ecosystem respiration (R\( _{t} \)). R\( _{t} \) is defined as the sum of autotrophic respiration (R\( _{a} \)) and heterotrophic respiration (R\( _{h} \)), but this decomposition needs the measurement of at least one of the two components. As it is not performed at our site because of the large diversity of species, we only investigate GPP and R\( _{t} \). Neither net primary production (NPP=NEE-R\( _{h} \)) nor soil carbon flux estimate is possible for the same reason. This is generally possible in sites with homogeneous vegetation and appropriate measuring devices (e.g. using the cuvette or chamber method).

GPP and R\( _{t} \) is calculated using the NEE-\( t_{10} \) relationship determined in section [*]. Daily respiration is calculated from the nighttime NEE (which is actual respiration) and the modelled daytime respiration (missing temperature data is estimated from the monthly average daily course of temperature). GPP is calculated as NEE-R\( _{t} \).

Figure: Annual variation of carbon NEE, GPP and R\( _{t} \) for the 1997-1999 period. Data are smoothed with boxcar average of 10 days for clarity. Solid line: carbon NEE: dashed line: R\( _{t} \); dash dot line: GPP. Negative value indicate carbon uptake by the ecosystem.

Figure [*] shows the annual and interannual cycle of NEE, R\( _{t} \) and GPP for 3 years between 1997 and 1999. Variability is present in the ecosystem respiration, but the interannual variability of NEE is mainly caused by differences in GPP (i.e. photosynthesys).

Maximum values of NEE are around 4 g C m\( ^{-2} \) day\( ^{-1} \) in 1997 and 1999, but reach 6 g C m\( ^{-2} \) day\( ^{-1} \) in 1998. Maximum rates of GPP are less than 9 g C m\( ^{-2} \) day\( ^{-1} \) in 1997 but almost reach 10 g C m\( ^{-2} \) day\( ^{-1} \) in 1998 and 1999. The maximum respiration occurs in 1998.

Table [*] summarizes the calculated NEE and its subcomponents for each year from 1997 to 1999.

Table: Yearly sums of carbon NEE, GPP and R\( _{t} \) for 1997, 1998 and 1999.
  NEE [g C m\( ^{-2} \) year\( ^{-1} \)] GPP [g C m\( ^{-2} \) year\( ^{-1} \)] R\( _{t} \) [g C m\( ^{-2} \) year\( ^{-1} \)]
1997 -134 -1151 1017
1998 -146 -1215 1069
1999 -92 -1155 1063

next up previous contents
Next: The local scale system Up: Net ecosystem exchange, gross Previous: Net ecosystem exchange, gross   Contents
root 2001-06-16