3.3 Results
3.3.5 Results: Total ecosystem C pools
Differences in tree and ground vegetation biomass will determine the above- and below ground litter flux to the soil as well as the chemistry and quality of the litter, and will again affect the accumulation and loss of the forest floor C pool as well as the soil C pool of the forest ecosystem.
Based on the data from 2012, the different tree species seem to affect the C pool in the total ecosystem somewhat differently: The total tree biomass in the spruce stand at Nordmoen differed significantly from the pine and birch stands (p=0.01 and p<0.0001, respectively). A similar pattern was found for the living biomass (p=0.03 and p<0.0001, respectively) and the total ecosystem C pool (p=0.01 and p=0.002, respectively; Figure 3.18), as well as for the sum of the C pool in mineral soil down to 40 cm (p=0.03 for both). As mentioned above, the difference in the mineral soil C pool may be related to differences in podsolization processes. Additionally, differences in the root distribution patterns between the tree species may also affect the mineral soil C pools, with pine and birch being characterized by deeper root systems.
The birch stand differed from the spruce and pine stand regarding the total soil C pool down to approx. 40 cm (p<0.0001 and p=0.03, respectively) and the C pool in the forest floor (p=0.01 and p=0.03, respectively). As for the standing dead wood, the C pool in the spruce stand was significantly higher than in the birch stand (p=0.03). The total litter layer sampled as part of the ground vegetation biomass harvest was, on the other hand, significantly higher in the pine stand relative to the birch and spruce stands (p=0.03, p<0.0001, respectively) (Table 3.12), which was also the situation for the ground vegetation biomass (p=0.01, p<0.0001, respectively).
At Skiptvet, the C pool in the living tree biomass was similar for the treatments T1-T4 (Table 3.14, Figure 3.18). The slightly higher tree biomass for T0 may reflect the very high number of trees, which again is linked to no management at the start of the experiment (Table 3.4, Table 3.14, n=2, no statistics). The distribution of spruce and birch in the different plots did not seem to affect the C pool in the trees, and the suggested higher total pool in the stand favoring spruce (T4) seemed to be mainly
0,0
related to a higher C pool in the forest floor and mineral soil (Figure 3.18), as also suggested by the one-sample t-test.
Figure 3.18. Total C pool in standing living tree biomass, standing dead wood, ground vegetation (above ground), forest floor and mineral soil down to approx. 40 cm soil depth with different mixtures of birch and spruce at Skiptvet SE Norway and in birch, pine and spruce stands at Nordmoen SE.Norway.
The C pool of the ground vegetation was small compared to the whole ecosystem C pool, but was 10 - 100 times larger at Nordmoen compared to at Skiptvet. When comparing the total ecosystem C pools at the two sites, C pool of the birch stands at Nordmoen and the T0 control stand at Skiptvet, which contained the most birch, were relatively similar (Figure 3.18). On the other hand, the total C pool in the spruce stands at Nordmoen and Skiptvet were at the opposite ends of the scale. This may partly be related to soil type where the soil at Skiptvet is expected to contain a higher supply of water and nutrients, as well as provide a higher physical protection against C decomposition in the mineral soil, relative to the soil at Nordmoen. The total soil C pool at Nordmoen was close to half that of Skiptvet.
Both sites tended toward a higher total soil C pool in the spruce stands relative to birch or mixed stands.
Differences in the soil as well as the biomass pools affected the C soil:vegetation ratio, as limited to the soil down to approx. 40 cm depth. The range was quite large both for the different stand types as well as when comparing the two sites (Table 3.14), however, at both sites, the C pool of the soil down to 40 cm was considerably lower than in the biomass.
0
Mineral soil approx. 40 cm soil depth
Skiptvet Nordmoen
Table 3.14 C pool in trees (standing living biomass+ dead wood), ground vegetation (above ground), soil down to approx. 40 cm mineral soil, total forest ecosystem, and the ratio between the C pool in soil and vegetation in different stand types at Nordmoen and Skiptvet, S.E. Norway.
Trees
biomass
Ground veg.
above gr.
Total soil (approx. 40 cm)
Total Ecosystem
C pool ratio Soil:vegetation Site Treatment kg C m‐2 kg C m‐2 kg C m‐2 kg C m‐2
Nordmoen Spruce 15.56 0.04 5.27 20.87 0.34
Nordmoen Pine 22.82 0.09 5.21 28.11 0.23
Nordmoen Birch 27.21 0.05 4.75 32.01 0.17
Skiptvet T0 24.25 0.0004 10.25 34.50 0.42
Skiptvet T1 20.78 0.0010 10.84 31.62 0.52
Skiptvet T2 20.23 0.0001 10.60 30.83 0.52
Skiptvet T3 20.80 0.0001 10.04 30.84 0.48
Skiptvet T4 20.82 0.0047 14.45 35.28 0.69
4 Site specific simulation studies
Holger Lange, Signe K. Borgen, Silje Skår, Lise Dalsgaard, O. Janne Kjønaas and Kjell Andreassen
4.1 Background
The ability of soil carbon models to reproduce carbon stocks from repeated measurements at the two sites Nordmoen and Skiptvet were investigated. We used two models, Yasso07 and Romul, which have rather different approaches. For Yasso07, model simulations were made with two separate objectives:
first, to produce realistic simulations using plausible initialization approaches and measured biomass inputs, and second, to explore various spin-up conditions (initial pool values and biomass inputs) that would be necessary to reproduce the measured soil C stocks and changes. For Romul, no spin-up procedure is required for initialization as measured stocks are used to define the starting condition in the model. In contrast to Yasso07, calibration is needed and two process parameters were used to calibrate the model. We also discuss the importance of litter production as the most crucial model input, and the uncertainty of estimates for ground vegetation litter.