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Fig. 1. Map of Finland showing the location of the research site where the sample plots were established on the drained peatland forest (ETRS-TM35FIN 7368023,425587). On the sample plot (radius: 9 m), the red dot in the middle of the strip road is the centre of the plot and the excavation by which the peat type, the degree of peat decomposition and the mineral soil type were identified. The water level was measured weekly. The green dots are the measurement points for the thickness of the peat layer; the thickness measurement was also performed from the centre of the plot.

Table 1. Total precipitation and mean air temperature for June to August in 2017 and 2019 and long-term averages (2000–2019). The data for Hirvas have been calculated using the kriging interpolation method.
Month, Annual June July August Annual
Precipitation (mm)
   Hirvas 2017 56 81 54 490
   Hirvas 2019 86 14 97 572
   Rovaniemi 2000–2019 64 (19–125) 79 (15–152) 70 (7–178) 639 (422–874)
Temperature (°C)
   Hirvas 2017 11.8 15.6 12.8 1.7
   Hirvas 2019 13.7 14.8 13.4 1.6
   Rovaniemi 2000–2019 12.5 16.1 13.4 1.8
Table 2. Thickness of peat layer, volume of stand, depth of ditch and degree of decomposition of peat according to the type of mineral soil under the peat layer. The soil types are saSi = clay silt, siHkMr = silt sandy moraine, HkMr = sand moraine, srHkMr = gravel sand moraine, Hk = sand.
Soil N Peat layer, cm Volume of stand, m3 ha–1 Depth of ditch, cm Degree of decomposition of peat
Mean Min Max Mean Min Max Mean Min Max Mean Min Max
saSi 5 49 41 57 102 45 174 28 23 34 4.0 2 7
siHkMr 12 42 23 70 110 40 181 36 20 44 4.7(10) 3 7
HkMr 25 40 27 70 108 11 181 40 19 55 2.9(22) 1 5
srHkMr 4 38 21 53 85 23 133 42 13 52 2.0 2 5
Hk 7 38 26 61 92 55 158 42 18 56 3.0 2 5
Table 3. The average groundwater levels (in cm) on measurement days according to soil types in 2017 and 2019. Precipitation is the sum of the precipitation in the week preceding the measurement. The soil types are saSi = clay silt, siHkMr = silt sandy moraine, HkMr = sand moraine, srHkMr = gravel sand moraine, Hk = sand.
Year 2017
Soil Group N 28. June 6. July 12. July 20. July 27. July 3. Aug 10. Aug 16. Aug
saSi Silty 5 14 14 18 13 20 20 26 28
siHkMr Silty 12 15 16 25 16 26 28 36 39
HkMr Sandy 25 18 19 36 20 36 38 44 46
srHkMr Sandy 4 16 16 27 18 27 27 36 34
Hk Sandy 7 20 23 36 24 36 38 43 41
Rain, mm 29 26 5 34 5 22 10 12
Year 2019
Soil Group N 19. June 26. June 3. July 10. July 17. July
saSi Silty 5 18 14 18 21 27
siHkMr Silty 11 17 14 19 24 34
HkMr Sandy 25 28 19 30 37 47
srHkMr Sandy 3 22 16 22 26 38
Hk Sandy 7 28 23 31 36 44
Rain, (mm) 19 37 11 6 4
Rain = Previous week’s rainfall
N = Number of plots
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Fig. 2. Soil sample analyses and classification. First, a sensory evaluation (RT) was made in the field for all 53 plots. Second, for control purposes, 28 samples from the important groups of silt moraine and fine sand moraine were further analysed in the laboratory. This was done to ensure that the demarcation of samples between Silty and Sandy groups was correct in further analyses. Three soil samples were redirected over the critical demarcation line (grain size: 0.063 mm). Finally, soil samples were placed into revised soil types (GEO), which were further divided into Silty and Sandy groups using the Mann–Whitney U-test.

Table 4. Tested explanatory variables for the linear mixed effects model. The parameters of distributions (continuous variables) or the frequencies (numbers of observations in the longitudinal data) and the proportions of the categories (categorical variables) are presented in the table.
Variable Mean Median Minimum Maximum
Continuous variables:
   Groundwater level, cm (response) 27.35 24.00 8.00 65.00
   Volume of stock, m3 ha–1 108.20 111.60 10.50 181.00
   Measurement, nr. 3.93 4.00 1.00 8.00
   Rainfall (during week), mm 16.89 12.00 3.70 37.20
   Temperature (during day), °C 14.04 13.90 10.20 16.90
   Peat decomposition, scale 1–8 3.34 3.00 1.00 7.00
   Altitude, m a.s.l. 83.02 82.83 80.40 86.64
   Ditch depth, cm 35.82 35.00 13.00 56.00
   Depth of peat layer, cm 41.44 39.00 21.00 70.00
Categorical variables:
   Peat type wooden-sphagnum peat: 85% (515), wooden-carex peat 15% (91)
   Mineral soil type Silty: 31% (190), Sandy: 69% (416)
Table 5. Parameter estimates and tests of a general linear mixed effects model (Gaussian) for the groundwater level. Std. err. denotes the standard error of the estimates, df denotes the degrees of freedom, t-values are the test values for the parameter estimates, and p is the statistical significance level. R2 for the marginal model was 68.4% and that for the conditional model was 81.4%.
Variable Coefficient Std. err. df t-value p
Fixed effects:
   Intercept 13.791 4.905 511.000 2.812 0.005
   Peat type, carex-sphagnum peat, ref. wooden-carex peat –0.155 0.064 83.000 –2.412 0.018
   Volume of timber stock, m3 ha–1 0.002 0.001 83.000 2.973 0.004
   Rainfall (during the period week), mm –0.014 0.001 511.000 –26.370 0.000
   Measurement, nr 0.083 0.005 511.000 16.044 0.000
   Mineral soil type, sandy, ref. silty 1.110 0.233 83.000 4.761 0.000
   Depth of peat layer, cm –0.367 0.134 83.000 –2.748 0.007
   Ditch depth, cm 0.003 0.002 83.000 1.928 0.057
   Altitude, m.o.s.l. –0.133 0.060 83.000 –2.220 0.029
   Mineral soil type*Depth of peat layer –0.025 0.006 83.000 –4.105 0.000
   Depth of peat layer*Altitude 0.004 0.002 83.000 2.713 0.008
Random effects (variances) and phi (AR1 correlation structure), 95% confidence limits in the parenthesis:
   Random year effect 1.509e-2 (0.079e-2–0.289)
   Random sample point effect 1.354e-2 (0.593e-2–3.092e-2)
   Residual 4.092e-2 (3.206e-2–5.223e-2)
   Phi 0.511 (0.377–0.623)
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Fig. 3. The predictions of the continuous explanatory variables and their interactions for the ground water level model by predictors: volume of tree stock (a), rainfall (b), number of measurements during a growing season (c), ditch depth (d) and the interaction of soil type and ditch depth (e), altitude and thickness of peat layer (f). The predicted groundwater level values for the fifth (categorical) predictor variable peat type were: wooden-sphagnum type peat 21.7 cm and wooden-carex peat 25.3 cm. The other explanatory variables were fixed at their mean values (continuous variables) or at their average levels (for categorical variables, see Table 4).

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Fig. 4. Scatter plots for the predicted values vs. residuals (a) and observed vs. predicted values (b) of the model in the log-transformed scale. The values were computed using the fixed part of the model (marginal model).