1

Fig. 1. Locations of the study plots with all available measured data (n = 82; map projection: Gauss_Kruger; Finland_Zone_3, central meridian: 27.0).

Table 1. Descriptive statistics of the main used variables. Soil characteristics are averaged (mean of soil layers).
Variable Min Max Mean Median Standard
deviation
Altitude (m a.s.l.) 10 290 134.63 120 67.24
Average temperature sum (d.d., at 2 m, threshold 5 oC) 807 1746 1383.91 1425.5 235.91
Slope (degree) 0 26 4.89 4 5.67
Thickness of humus layer (cm) 0.35 10.4 4.5 4.2 2.22
Proportion of clay (<0.002 mm, %) 0.4 31.78 3.21 2.3 4.41
Proportion of silt (0.002–0.063 mm, %) 2.5 74.6 29.53 28.82 16.41
Proportion of sand (0.063–2 mm, %) 7.47 97.17 67.26 69.08 19.18
Proportion of gravel (>2 mm, %) 0.04 32.34 8.52 7.04 6.72
Fine fraction (<0.063 mm, %) 2.90 92.57 32.74 30.98 19.18
Bulk density (g cm–3) 0.97 1.77 1.32 1.32 0.15
Organic matter (%) 0.42 10.85 2.67 2.37 1.89
Topsoil pH (0–6 cm)* 3.60 5.30 4.12 4.00 0.31
Total porosity (=WC0, vol.%) 32.85 64.00 49.8 49.7 5.6
Water retention at –1 kPa (WC1, vol.%) 8.09 51.20 36.78 38.47 7.43
WR at –10 kPa (WC10, vol.%) 3.93 40.80 22.12 22.46 9.67
WR at –100 kPa (WC100, vol.%) 0.67 33.28 13.18 12.71 7.02
WR at –1500 kPa (WC1500, vol.%) 0.55 16.06 6.67 6.40 4.13
Air-filled porosity at –1 kPa (AFP1, vol.%) 1.68 35.81 13.26 12.76 6.2 
Air-filled porosity at –10 kPa (AFP10, vol.%) 2.57 40.93 27.86 29.29 9.27 
Age of forest stand (years) 0 248 73.16 72.00 47.19
Height of dominant trees at age of 100 years (H100) 8.68 38.08 22.17 23.16 6.42
Basal area of tree trunks (m2) 0 33.37 17.93 19.20 9.33
Volume of growing stock (m3 ha–1) 0 366.35 148.83 142.34 98.39
Proportion of Scots pine (%) 0 100 52.87 55.02 41.27
Proportion of Norway spruce (%) 0 99.84 30.37 8.43 36.37
Proportion of deciduous trees (%) 0 100 14.58 3.47 25.3
Canopy cover of trees (%) 0 96 47.74 50 24.04
Canopy cover of bushes (%) 0 88 8.48 3 15.99
Cover of seedlings or saplings (%) 0 13.78 1.27 0.33 2.28
Cover of scrubs indicating sub-xeric sites (%)1 0 71.25 11.50 7.25 13.23
Cover of bottom layer vegetation indicating xeric sites (%)2 0 42.95 5.85 1.30 9.95
Cover of mosses and scrubs indicating mesic sites (%)3 0 70.75 18.99 11.05 19.76
Cover of vascular species indicating herb rich sites (%)4 0 87.12 9.50 5.00 15.09
Cover of hay species (%) 0 52.3 5.62 1.26 10.25
Cover of grass species (%) 0 68.21 6.56 0.97 12.99
Cover of mosses (%) 0.22 154.25 59.26 63.81 34.22
Cover of lichens (%) 0 51.62 3.32 0.02 8.38
Cover of Sphagnum species (%) 0 49.37 2.11 0 6.99
Cover of Salix species (%) 0 1 0.03 0 0.14
Cover of Rhododendron tomentosum (%) 0 2 0.09 0 0.35
Cover of Polytrichum commune (%) 0 16.5 1.16 0 3.31
* pH values are calculated from H+ concentrations.
Site types (n): herb-rich (19), mesic (30), sub-xeric (27), xeric and barren heath forests (6).
Field estimated soil particle size (n): Fine <0.06 mm (18), Medium 0.06–0.6 mm (56), Coarse >0.6 mm (8).
Soil layers (n for water retention): 0–6 cm (82), 30–36 cm (82).
Vegetation groups indicating site quality, formed by Item Cluster Analysis (Revelle 2017): 1 sub-xeric scrubs: Vaccinium vitis-idaea L., Vaccinium uliginosum L., Empetrum nigrum L., Calluna vulgaris (L.) Hull; 2 xeric lichens: Cladina sp., Dicranum fuscescens Sm.; 3 mesic mosses and scrubs: Dicranum majus Sm., Hylocomium splendens (Hedw.) Schimp., Vaccinium myrtillus L.; 4 herb-rich vascular plants: Rubus saxatilis L., Pteridium aquilinum C.N. Page & R.R. Mill, Oxalis acetocella L., Maianthemum bifolium (L.) F. W. Schmidt, nutrient-demanding bushes.
Table 2. Summary statistics for the selected major response variables with the three most significant (Anova F-test) explanatory variables. Site type and soil layer are categorical variables.
Fixed models:    
Variable Multiple R2 Adjusted R2 Residual SE Explainer1 Explainer2 Explainer3
H100 (model a) 0.829 0.813 2.75 Site type Temperature sum Fine fraction
H100 (model b) 0.718 0.702 3.50 Temperature sum pH (0–6cm) WC10
(H-layer)0.5 0.498 0.479 0.39 pH (0–6cm) Xeric species* Mesic species*
(Fines)0.5 (model a)** 0.482 0.424 1.21 Site type Temperature sum Slope
Mixed models:
Variable Conditional R2 Marginal R2 Residual SD Explainer1 Explainer2 Explainer3
(Fines)0.5 (model b) 0.834 0.624 0.732 WC10 Soil layer Site type
Db (model a) 0.717 0.450 0.085 Org. Soil layer Soil layer:Org.
Db (model b) 0.665 0.473 0.094 (Org.)0.5 Slope Soil texture class
TP (=WC0) 0.694 0.403 3.26 Soil layer Org. Slope
WC1 0.597 0.477 4.49 Total porosity Fine fraction Slope
WC10 0.806 0.655 4.21 Fine fraction Db Slope
(WC100)0.5 0.796 0.478 0.46 Fine fraction Soil layer Org.
(WC1500)0.5 0.597 0.319 0.61 Db Herb-rich species* -
(AFP1)0.5 0.688 0.537 0.54 Soil layer:Db Soil layer Soil texture class:Db
AFP10 0.853 0.651 3.55 Fine fraction Db Soil layer:Db
Db = bulk density, Org. = organic matter (%), Fines = soil fine fraction,
* the cover percentages of plant species indicating site quality (see Table 1), ** the fine fraction as a mean of layers.
Table 3. Estimate statistics of fixed effects models for the H100 site index, thickness of humus layer (cm) and soil fine fraction content (%) (without hydraulic explanatory variables). Estimates for categorical variables refer to the zero reference level (i.e. site types vs. reference category of herb-rich site type).
Variable Effect Estimate SE t-value Pr(>|t|)
H100 Intercept 0.848 3.190 0.266 0.791
Mesic site type –5.960 1.128 –5.282 <0.001
Sub-xeric site type –4.409 1.279 –3.447 0.001
Xeric site type –6.962 2.073 –3.358 0.001
Temperature sum 0.017 0.002 9.786 <0.001
Fine fraction 0.071 0.026 2.738 0.008
Residual SE 2.751 Multiple R2 0.829
  Df 52 Adj. R2 0.813
(H-layer)0.5   Intercept 6.455 0.657 9.826 <0.001
Xeric species* –0.0193 0.00461 –4.189 <0.001
Mesic species* 0.00685 0.00251 2.728 0.00787
pH (0–6 cm depth) –1.0727 0.154 –6.952 <0.001
Residual SE 0.394 Multiple R2 0.498
  Df 78 Adj. R2 0.479
(Fine fraction)0.5 Intercept 8.628 1.191 7.245 <0.001
Mesic site type –1.106 0.514 –2.153 0.036
Sub-xeric site type –1.626 0.525 –3.095 0.003
Xeric site type –3.203 0.840 –3.036 <0.001
Temperature sum –0.0075 0.0009 –3.036 0.004
Slope 0.0617 0.0299 2.065 0.044
H100 0.0676 0.0340 1.985 0.052
Residual SE 1.205 Multiple R2 0.482
  Df 53 Adj. R2 0.424
* Cover percentages of plant species indicating site quality (see Table 1).
2

Fig. 2. Predicted relation of the thickness of the humus layer to topsoil pH (0–6 cm) (see the fixed model in Table 3). The shaded area denotes a 95% confidence interval.

3

Fig. 3. Predicted relations of the soil’s fine fraction content to the temperature sum and site type (see the fixed model in Table 3). The shaded area and vertical bars denote a 95% confidence interval.

Table 4. Estimate statistics of mixed effects models for the soil’s fine fraction content and bulk density as well as for water retention (WC10) and air-filled porosity (AFP10) at –10 kPa matric potential (SE for fixed and SD for random effects). Estimates for categorical variables refer to the zero reference level (i.e. site types vs. reference category of herb-rich site type and deeper soil layer vs. upper layer and field estimated soil particle size vs. reference category of fine sites).
Variable Effect Estimate SE/SD t-value Pr(>|t|)
(Fine fraction)0.5 Intercept 2.138 0.713 2.999 0.004
Mesic site type –0.890 0.330 –2.697 0.009
Sub-xeric site type –1.063 0.346 –3.074 0.003
Xeric site type –1.675 0.538 –3.112 0.003
WC10 0.170 0.026 6.480 <0.001
Layer 30–36 cm 1.748 0.607 2.878 0.006
WC10: Layer 30–36 cm –0.061 0.026 –2–354 0.022
Plot (random) 0.826 n = 81
Residual (random) 0.712   n = 103
Marginal R2 0.624
Conditional R2 0.834    
Db Intercept 1.369 0.027 49.945 <0.001
Layer 30–36 cm 0.160 0.031 5.107 <0.001
Slope angle –0.00899 0.002 –3.897 <0.001
Org. matter –0.0135 0.007 –1.948 0.054
Layer 30–36 cm: Org. –0.0468 0.012 –4.024 <0.001
Plot (random) 0.083 n = 81
Residual (random) 0.085   n = 105
Marginal R2 0.450
Conditional R2 0.717
WC10 Intercept 35.644 6.667 5.346 <0.001
Layer 30–36 cm –4.595 1.760 –2.610 0.011
Db –16.253 4.819 –3.373 0.001
Fine fraction 0.355 0.031 11.292 <0.001
Slope angle –0.335 0.121 –2.776 0.007
Org. matter –0.214 0.338 –0.631 0.530
Layer 30–36 cm: Org. 1.432 0.613 2.333 0.022
Plot (random) 3.721 n = 81
Residual (random) 4.210   n = 103
Marginal R2 0.655
Conditional R2 0.806    
AFP10 Intercept 74.955 8.655 8.660 <0.001
Layer 30–36 cm –23.310 8.429 –2.766 0.010
Medium-coarse sites 2.911 1.830 1.591 0.116
Coarse sites 5.598 2.735 2.047 0.044
Db –32.403 6.258 –5.178 <0.001
Fine fraction –0.261 0.041 –6.337 <0.001
Slope angle 0.285 0.121 2.368 0.020
Layer 30–36 cm: Bulk density 19.422 6.389 3.040 0.005
Plot (random) 4.160 n = 78
Residual (random) 3.548   n = 98
Marginal R2 0.651
Conditional R2 0.853    
4

Fig. 4. Predicted relations of the soil bulk density to the terrain slope angle and soil organic matter by soil layer (see the mixed effects model in Table 3). The shaded area denotes a 95% confidence interval.

5

Fig. 5. Predicted relation of the water content (WC10) and air-filled porosity (AFP10) at –10 kPa matric potential to the soil’s fine fraction content (clay + silt) (see the mixed models in Table 4). The shaded area denotes a 95% confidence interval.