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Fig. 1. Location of the study site in Veneselkä hill in Rovaniemi, with gap cuttings randomized into six blocks (indicated by different colors). Figure modified from Hallikainen et al. (2019).

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Fig. 2. Example of the sampling design with a gap (d = 20 m). Five circular regeneration plots in brown are shown within the gap, and two additional plots outside the gap on the northern and eastern sides represent the four control plots positioned in all four cardinal directions. The orange dots represent trees, and the dots with a green outline represent the dominant trees from which pendulous lichen abundance were assessed. The dots further away on the northern and eastern sides represent the sample trees within the forest in all four cardinal directions (~15 m from the outer border of the control plots). Figure modified from Hallikainen et al. 2019.

Table 1. Descriptive statistics of the covariates and the responses tested in the models 1–3. In addition to these, the categorical variables gap diameter and direction of a circular sample plot were tested in the models. Pendulous lichen category comes from the pendulous lichen assessed from five nearest dominant trees in each cardinal direction within each gap (following the methods described by Kumpula et al. (2006). Abbreviations used: Qu. = quartile, d1.3 = diameter at breast height (1.3 m).
Min. 1.Qu. Median Mean 3.Qu. Max.
At sample plot level
Number of seedlings with pendulous lichens on circular sample plot (response) 0.00 0.00 0.00 0.86 1.00 11.00
Number of all seedlings on plot (only plots recruited by seedlings) 1.00 9.00 17.00 21.40 29.75 102.00
Number of Scots pine seedlings on plot (only plots recruited by seedlings) 0.00 8.00 14.50 17.21 24.00 65.00
Height of Scots pine seedlings on plot, cm 2.00 12.00 20.00 26.80 36.00 139.00
Age of Scots pine seedlings on plot, years 2.00 5.00 6.00 6.05 7.00 14.00
Height of all seedlings on plot, cm 2.00 11.00 17.83 22.70 30.00 91.00
Height of longest Scots pine seedling, cm 7.00 78.00 108.00 121.60 150.00 440.00
Age of longest Scots pine seedling, years 4.00 10.00 15.00 19.86 26.00 79.00
At gap level, gap edge parameters
Basal area, m2 ha–1 0.57 13.79 17.68 18.13 22.46 35.83
Number of tree stems ha–1 50.00 400.00 625.00 644.10 850.00 1450.00
Total volume of tree trunks, m3 ha–1 3.31 112.04 144.51 149.15 185.54 327.68
Mean diameter (d1.3) of trees, cm 8.89 14.11 17.17 17.90 20.19 31.73
Mean height of trees, m 9.38 12.38 13.45 13.84 15.08 19.60
Mean of pendulous lichen category (categories 0–3, 10 trees classified) 0.70 1.10 1.30 1.29 1.40 2.00
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Fig. 3. Example of a Scots pine seedling with pendulous lichen (and some bark) attached.

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Fig. 4. Model 1 predictions and 95% confidence intervals for the number of pendulous lichen-colonized seedlings in the function of the following explanatory variables: number of Scots pine seedlings on a circular sample plot (a), Scots pine seedling height (b), pendulous lichen category at the forest edge (c), the interaction of gap size and number of seedlings at given values of 5, 15 and 30 seedlings in a sample plot (d), and interaction of gap size and direction (e). Pendulous lichen category comes from the pendulous lichen assessed from five nearest dominant trees in each cardinal direction within each gap (following the methods described by Kumpula et al. (2006).

Table 2. Generalized linear mixed model to explain the number of pendulous lichen-colonized seedlings using a negative binomial (Model 1) (NB2-parametrization) distribution assumption. VIF denotes the variance inflation factor values for the main effects of the predictors. The marginal model R2 = 52.3%.
Variable Estimate Std.error z-/chi-value p VIF
Fixed effects
(Intercept) –1.904 0.655 –2.909 0.004
Gap diameter, m (20 m being ref. category) 12.961 0.002 1.23
   40 m –0.936 0.514 –1.820 0.069
   80 m –2.430 0.695 –3.494 <0.001
Direction (Center being ref. category) 2.668 0.615 1.45
   North 0.246 0.392 0.627 0.531
   East 0.447 0.374 1.196 0.232
   South 0.479 0.383 1.251 0.211
   West 0.045 0.396 0.114 0.909
No of seedlings on circular sample plot (5 m2) 0.015 0.009 1.717 0.086 1.07
Height of seedlings, cm 0.016 0.005 3.532 <0.001 1.36
Mean of pendulous lichen category (categories 0–3) 0.794 0.393 2.022 0.043 1.07
Gap diameter * No of seedlings on plot 8.592 0.014
   40 m 0.029 0.015 1.877 0.061
   80 m 0.043 0.016 2.744 0.006
Gap diameter * Direction 18.448 0.018
   40 m * North –0.276 0.615 –0.449 0.654
   80 m * North 0.959 0.754 1.271 0.204
   40 m * East –1.173 0.674 –1.741 0.082
   80 m * East 0.172 0.781 0.221 0.825
   40 m * South –0.666 0.632 –1.053 0.292
   80 m * South –1.244 1.015 –1.225 0.220
   40 m * West 0.532 0.583 0.913 0.361
   80 m * West 1.701 0.730 2.331 0.020
Random effects Variance
Block 0.085
Gap nested within Block <0.001
Theta (NB2 parameter) 2.720
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Fig. 5. Model 2 predictions and 95% confidence intervals for pendulous lichen colonization on the tallest seedlings: effect of gap diameter (m) (a), direction (b), height (cm) of the tallest seedling (c), and pendulous lichen category (mean of 10 trees) (d). Pendulous lichen category comes from the pendulous lichen assessed from five nearest dominant trees in each cardinal direction within each gap (following the methods described by Kumpula et al. 2006).

Table 3. Generalized linear mixed-effects based on a binomial (Bernoulli) distribution for the presence of pendulous lichens on the tallest seedling (Model 2). VIF denotes the variance inflation factor values for the main effects of the predictors. The marginal model R2 = 33.6%. The area under the ROC-curve = 0.745 (marginal model).
Variable Estimate Std.error z-/chi-value p VIF
Fixed effects
Intercept –3.563 1.034 –3.447 0.001
Gap diameter, m (20 m being ref. category) 7.033 0.030 1.09
   40 m –1.014 0.423 –2.399 0.016
   80 m –0.934 0.423 –2.209 0.027
Direction (Center being ref. category) 8.931 0.063 1.06
   North 1.190 0.457 2.605 0.009
   East 0.543 0.453 1.198 0.231
   South 0.504 0.451 1.117 0.264
   West 1.104 0.467 2.366 0.018
Mean of pendulous lichen category (categories 0–3) 1.186 0.700 1.693 0.090 1.03
Height of the longest seedling, cm 0.016 0.003 4.791 < 0.001 1.11
Random effects Variance
Block <0.001
Gap nested within Block 0.428
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Fig. 6. Model 2 (alternative model) predictions and 95% confidence intervals for pendulous lichen colonization on the tallest seedlings when seedling height was replaced with seedling age.

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Fig. 7. Model 3 predictions and 95% confidence intervals for the abundance of pendulous lichens, as indicated by the pendulous lichen category, in the forest edge and interior.