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Fig. 1. Locations of six research stands in Finland and experimental design of the study within the stand. SPOT = spot mounding, INV = inverting.

Table 1. Main characteristics of the six research stands to study the effects of spot mounding and inverting.
Stand Regen.
year
Tree
species
Number of blocks Site typea Stump
harvesting
Soil texture TWIb ± SD Stoniness ± SD, cm Particles
<0.063 mm ± SD, %
1 2013 Spruce 4 MT No Medium-coarse 7.4 ± 0.5 11.2 ± 4.8 34.4 ± 3.4
2 2013 Spruce 3 MT No Medium-coarse 6.8 ± 1.2 12.8 ± 4.9 45.0 ± 4.7
3 2013 Spruce 4 OMT Yes Medium-coarse 6.4 ± 0.6 14.1 ± 2.6 40.9 ± 10.6
4 2013 Spruce 4 OMT Yes Medium-coarse 7.5 ± 0.6 10.1 ± 4.3 30.2 ± 6.0
5 2015 Pine 4 MT No Fine 8.9 ± 2.1 25.5 ± 2.8 57.5 ± 14.0
6 2015 Spruce 4 MT No Fine 10.0 ± 1.6 24.8 ± 4.5 47.5 ± 19.0
a Forest site type: MT = Myrtillus type forest, OMT = Oxalis-Myrtillus type forest. Soil fertility: MT < OMT (Cajander 1926).
b TWI refers to the Topographic Wetness Index (Salmivaara et al. 2017).
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Fig. 2. Schematic illustration for the treatments of the study: a) spot mounding and b) inverting (Luoranen et al. 2007). In spot mounding, humus and mineral soil are inverted and placed beside the patch onto undisturbed soil whereas in inverting, inverted mound is returned to the patch it came from.

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Fig. 3. Cumulative mortality (a) after first and third growing season and height (b) of seedlings at planting and after first, second, and third growing seasons in areas of spot mounding (SPOT) and inverting (INV). In stand two height was not measured one growing season after planting. View larger in new window/tab.

Table 2. The effects of the site preparation method (spot mounding vs. inverting) on the number of birches regenerated (seedlings ha–1), the proportion of exposed mineral soil on site preparation (%),and the height of the planted seedlings (cm) three growing seasons after regeneration (linear mixed models). Parameter estimates, standard error (SE), and p-values are presented as well as standard deviation of random effects. Statistically significant p-values (p < 0.050) for the model coefficients are in bold.
Explanatory variables Birch regeneration Exposed mineral soil The height of the seedling
Coeff. ± SE p Coeff. ± SE p Coeff. ± SE p
Intercept 4.802 ± 0.288 <0.001 –1.040 ± 0.095 <0.001 57.301 ± 1.967 <0.001
Site prep. [INVERTING] –0.310 ± 0.094 0.004 –0.161 ± 0.099 0.118 0.767 ± 1.698 0.656
Standard deviation of random effects
Stand level 0.674 0.152 3.755
Block level 0.236 0.000 0.000
Residual 0.307 0.326 5.597
Table 3. Site preparation effects (spot mounding and inverting) on the occurrence of soil surface disturbance categories (mean ± standard deviation, %).
Site preparation Mound Patch Otherwise disturbed Undisturbed Stone Stump
Whole data Spot mounding 17.6 ± 4.6 9.2 ± 5.0 20.9 ± 16.1 46.6 ± 15.5 4.9 ± 4.4 0.8 ± 0.6
Inverting 17.8 ± 4.1 5.8 ± 4.7 19.0 ± 19.1 53.0 ± 19.3 3.8 ± 4.3 0.7 ± 0.5
Stump harvesting excluded Spot mounding 17.6 ± 3.8 10.8 ± 4.4 11.8 ± 5.2 55.2 ± 11.9 3.5 ± 4.7 1.0 ± 0.6
Inverting 17.4 ± 2.8 4.8 ± 4.2 9.6 ± 5.5 64.5 ± 9.6 3.0 ± 4.5 0.7 ± 0.5
Table 4. Influence of fixed and random effects on the total variation of models of the number of birches regenerated (seedlings ha–1), the proportion of exposed mineral soil on site preparation (%), and the height of planted seedlings (cm). Full models refer to Table 2 and empty models to models estimated without fixed predictors. ICC refers to Intra Class Correlation.
Response Empty model Full model Influence of fixed effects, %
Variance estimate ICC, % Variance estimate ICC, %
Stand level
Birch regeneration 0.438 71.5 0.454 75.2 –1.8
Exposed mineral soil 0.022 16.7 0.023 17.9 –2.0
The height of the seedling 13.920 31.2 14.100 31.0 –0.64
Block level
Birch regeneration 0.039 77.9 0.056 84.4 –19.2
Exposed mineral soil 0.000 16.7 0.000 17.9 –4.2
The height of the seedling 0.000 31.2 0.000 - 15.7
All estimates of random effects converged in the calculations, but some were so small that they round to zero.
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Fig. 4. Number and standard deviation of naturally regenerated birch seedlings (103 ha–1) in areas of spot mounding (SPOT) and inverting (INV).