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Fig. 1. Distribution of the study plots in the territory of Latvia. View larger in new window/tab.

Table 1. Description of explanatory variables tested in the model. Number of observations for categorical variables; mean ± standard deviation (SD) for continuous variables.
Abbreviation Description Classes Number of observations / mean ± SD
st10 Dominant tree species ( > 50 % of stand volume) 1 – birch 2010
2 – pine 2854
3 – spruce 1095
stand_str Composition of canopy-layer 1 – mixed 3635
2 – monodominant ( ≥ 75% of standing volume) 2324
mix_spruce Admixture of spruce in canopy-layer 0.07 ± 0.11
storey_spruce Admixture of spruce in understory 0 – no spruce 4918
1 – spruce admixture 1041
dens_group Relative stand density (calculated as ratio between basal area per hectare and the maximum stand basal area for a species and mean height) 1 – ( ≤ 0.4) 163
2 – (0.5–0.8) 3544
3 – (0.8–1.0) 2252
h10 Mean height of the dominant canopy trees (m) 21.27 ± 4.76
d10 Mean diameter of the dominant canopy trees (cm) 23.56 ± 6.73
h2d Height2/diameter ratio 19.58 ± 4.59
soil_group Soil type (based on forest type group) 1 – dry mineral soil 1353
2 – wet mineral soil 2752
3 – peat soil 201
4 – drained mineral soil 903
5 – drained peat soil 750
thin_befk Timing of thinning 1 – thinned more than 10 years ago or no information on thinning 4888
2 – thinned within last 6 to 10 years 475
3 – thinned within last 5 years 596
wind_dir Adjacent upwind zone of the forest edge BL – new forest edge (fresh clearcut/ young stand < 5 years) 190
GL – old forest edge (young stand with mean tree height < 5 m / wetland / agriculture land) 840
H – high-forest edge (forest stand with mean height > 5 m) 4929
gust_group Wind gusts region 1 – (17–19 m s–1) 2830
2 – (20–21 m s–1) 1502
3 – (22–24 m s–1) 980
4 – (25–30 m s–1) 647
Table 2. Parameter estimates of fitted model. (Df): degree of freedom, * – interaction between factors.
Explanatory variables Chi-squared Df p-value
st10 296.6 2 <0.001
gust_group 44.8 3 <0.001
stand_str 1.4 1 0.22
mix_spruce 131.9 1 <0.001
storey_spruce 0.5 1 0.46
dens_group 44.6 2 <0.001
h10 49.6 1 <0.001
h2d 7.2 1 <0.01
thin_befk 266.7 2 <0.001
wind_dir 302.6 2 <0.001
soil_group 10.7 4 0.03
st10 * gust_group 13.1 6 0.04
st10 * thin_befk 16.4 4 <0.01
Random effects (study plot): variance 2.9; standard deviation 1.72
Table 3. Fixed effects parameter estimates of fitted model. (SE): standard error, * – interactions between factors.
Explanatory variables Estimate SE t-value p-value
Intercept 1.96 0.18 10.4 <0.001
st10: pine 0.16 0.07 2.1 <0.05
st10: spruce 1.03 0.08 11.7 <0.001
gust_group: 2 1.06 0.18 5.6 <0.001
gust_group: 3 1.06 0.22 4.8 <0.001
gust_group: 4 0.73 0.26 2.7 <0.01
stand_str: monodominant 0.05 0.04 1.2 0.22
mix_spruce 0.26 0.02 11.4 <0.001
storey_spruce: 1 0.04 0.05 0.7 0.45
dens_group: 2 –0.66 0.12 –5.5 <0.001
dens_group: 3 –0.79 0.12 –6.4 <0.001
h10 0.27 0.03 7 <0.001
h2d –0.1 0.03 –2.6 <0.01
thin_befk: 2 0.53 0.13 4.1 <0.001
thin_befk: 4 0.93 0.12 7.3 <0.001
wind_dir: GL –1.4 0.12 –11.6 <0.001
wind_dir: H –1.83 0.11 –16.3 <0.001
soil_group: 2 –0.07 0.06 –1.1 0.25
soil_group: 3 0.2 0.07 2.5 <0.05
soil_group: 4 0.05 0.06 0.7 0.44
soil_group: 5 0.00008 0.07 0 0.99
st10: pine * gust_group: 2 –0.06 0.12 –0.5 0.58
st10: spruce * gust_group: 2 –0.1 0.13 –0.7 0.44
st10: pine * gust_group: 3 –0.21 0.13 –1.5 0.11
st10: spruce * gust_group: 3 0.08 0.16 0.5 0.61
st10: pine * gust_group: 4 –0.44 0.15 –2.8 <0.01
st10: spruce * gust_group: 4 –0.06 0.21 –0.3 0.75
st10: pine * Thin_befk: 2 –0.18 0.15 –1.1 0.24
st10: spruce * Thin_befk: 2 –0.57 0.21 –2.7 <0.01
st10: pine * Thin_befk: 4 0.07 0.15 0.4 0.63
st10: spruce * Thin_befk: 4 0.51 0.19 2.6 <0.01