Net ecosystem exchange

Much like Reco, NEE showed a characteristic seasonal pattern at all four sites (Fig.4.5-4.8). In summer, gross uptake of CO2 through GPP outbalanced the release through Reco, while during the colder months net fluxes were near 0 or net emissions occurred. At the Eriophorum site and S. cuspidatum site highest monthly cumulated net uptake of CO2 occurred in average in June, at the Molinia site the highest averaged uptake of CO2 was detected in July and at the S.

papillosum site in August. There was a gradient from highest net uptake at the Molinia site to lowest net uptake at the S. papillosum site during the summer months June until August.

During the remaining part of the year, the Molinia site emitted net CO2, the Eriophorum site emitted also net CO2, but less than the Molinia site. The S. cuspidatum site and the S.

papillosum site sequestrated net CO2, but the S. papillosum site sequestrated more net CO2. Monthly cumulated NEE was mostly highly significantly different between the sites, especially in winter and spring. The Eriophorum site and the S. cuspidatum site revealed comparatively often similar monthly cumulated NEE compared to the other sites, while monthly cumulated NEE at the Molinia site was very seldom similar to monthly cumulated NEE of the S. cuspidatum site and the S. papillosum site, respectively.

Maximum daily uptake and release at the examination sites are presented in table 4.7.

-10 -5 0 5 10 15 20 25

0 5 10 15

Oct-09 Nov-09 Dec-09 Jan-10 Feb-10 Mar-10 Apr-10 May-10 Jun-10 Jul-10 Aug-10 Sep-10 Oct-10 Nov-10 Dec-10 Jan-11 Feb-11 Mar-11 Apr-11 May-11 Jun-11 Jul-11 Aug-11 Sep-11 Oct-11 Nov-11 Dec-11 °C

CO2-C [g m-2d-1]

Molinia site Eriophorum site S. cuspidatum site S. papillosum site soil temperature

144 Fig. 4.5: Monthly cumulated net ecosystem exchange (NEE) of the Molinia site. Note: 2009 only from October to December. Error bars are standard errors.

Fig. 4.6: Monthly cumulated net ecosystem exchange (NEE) of the Eriophorum site. Note: 2009 only from October to December. Error bars are standard errors.

-2000 -1500 -1000 -500 0 500 1000 1500

Jan Febr March April May June July Aug Sept Oct Nov Dec CO2-C [kg ha-1]

month

2009 2010 2011

-1500 -1000 -500 0 500 1000

Jan Febr March April May June July Aug Sept Oct Nov Dec CO2-C [kg ha-1]

month

2009 2010 2011

145 Fig. 4.7: Monthly cumulated net ecosystem exchange (NEE) of the S. cuspidatum site. Note: 2009 only from October to December. Error bars are standard errors.

Fig. 4.8: Monthly cumulated net ecosystem exchange (NEE) of the S. papillosum site. Note: 2009 only from October to December. Error bars are standard errors.

-1000 -800 -600 -400 -200 0 200 400 600

Jan Febr March April May June July Aug Sept Oct Nov Dec CO2-C [kg ha-1]

month

2009 2010 2011

-700 -600 -500 -400 -300 -200 -100 0 100 200

Jan Febr March April May June July Aug Sept Oct Nov Dec CO2-C [kg ha-1]

month

2009 2010 2011

146

2

and standard error (s.e.).

^site max uptake +/- s.e. max release +/- s.e.

[g CO₂-C m^-2 d^-1] [g CO₂-C m^-2 d^-1]

Molinia site -7.2 +/- 2.2 13.1 +/- 1.0

Eriophorum site -5.3 +/- 1.5 8.9 +/- 0.9

S. cuspidatum site -3.3 +/- 0.6 2.6 +/- 0.1

S. papillosum site -3.0 +/- 0.6 1.8 +/- 0.6

The variability between the years was in general high, but in several cases monthly cumulated NEE were similar. At the Molinia site the coefficient of variation was extremely high in August and September (cv = 626 % and cv = 1,079 %, respectively), because in 2010 monthly net uptake occurred, while in 2011 CO2 was released. Contrastingly, at the S. cuspidatum site monthly balances in September were almost identical (cv = 1 %). In contrast to monthly Reco, monthly NEE were not as often significantly different between the two years 2010 and 2011, but inter annual variation of gas fluxes was still high.

The differences of the annual courses of NEE between the years 2010 and 2011 are apparent in figure 4.9, which show the cumulated daily NEE. In the Leegmoor all sites exhibited an annual course that can be divided into three units: Release in winter and spring, uptake in summer as well as release in autumn and winter. At the Molinia site the course in 2011 was shifted towards the past for one or two months, compared to 2010. CO2 uptake in the summer was similar between the two years, but during the remaining part of the year CO2 release was higher in 2011, leading to different balances. At the Eriophorum site higher emissions were also observed in winter/spring and autumn/winter in 2011 compared to 2010. In addition, during the summer months there was a much higher uptake during 2010. Consequently, the two examination years had very different gas balances. The annual courses at the S.

cuspidatum site were quite similar, except from May until July and in December. In May and June, uptake rates in 2010 were much higher, but in July 2010 the site released CO2. The S.

papillosum site sequestrated almost year round CO2 with highest uptake rates in July and August. The two years exhibited similar courses, however in 2010 uptake rates were higher.

147 Fig. 4.9: Annual courses of cumulated NEE the examination sites.

Annual course of daily GPP at the Molinia site was similar to the pattern at the Eriophorum site, while the annual course at the S. cuspidatum site was analogous to the pattern at the S.

papillosum site (Fig.4.10). However, as already established, the vegetation at the Eriophorum site developed earlier than the vegetation at the other sites. In July 2010, the GPP at the S.

cuspidatum site was contrary to the general trend, and dropped to lower values. Subsequently, it increased again. By comparing the annual course of the GPP with the course of the PAR, there is a delay visible, similar to the delay of the Reco (see above).

-200 -150 -100 -50 0 50 100

cum. NEE [g CO2-C m-2 d-1] 2010 2011

Moliniasite

-300 -250 -200 -150 -100 -50 0 50 100

150 2010

2011

Eriophorum site

-150 -100 -50 0 50

D J F M A M J J A S O N D cum. NEE [g CO2-C m-2 d-1]

month

2010 2011

S. cuspidatum site -150

-100 -50 0 50

D J F M A M J J A S O N D month

2010 2011

S. papillosum site

148 Fig. 4.10: Annual courses of daily GPP (left axis) and PAR (right axis) of the measurement sites.