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Fig. 1. a) Location of the FAHM facility, b) layout of the experimental area (empty circles are control plots, filled circles are humidification plots, empty circles with dashed outline are open-top plots not included in this study) and c) layout of the humidification plot (black triangles stand for hybrid aspens and empty triangles for silver birches).

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Fig. 2. Average growth and wood characteristics of the analysed silver birch (Bir) and hybrid aspen (Asp) sample trees (n = 91). Error bars show standard error, asterisks show the significance of treatment effect (p < 0.05*, p < 0.01**, p < 0.001***, ns: not significant) based on ANOVA model (Eq. 1), letters on the columns denote differences between tree species separately for control (a, b) and humidified plots (x, y).

Table 1. Mean growth and wood characteristics of the analysed silver birch and hybrid aspen trees (n = 91) in control (C) and humidified (H) plots, factor effects based on ANOVA model (Eq. 1), superscript letters (a, b) denote significant differences between tree species within treatment according to t-test.
Characteristic Tree species Mean ± st. error Factor effects
Treatment Plot(Treatment)
C plots H plots F p F p
Height, m Birch 5.74 ± 0.19a 5.10 ± 0.18a 6.6 0.015 2.5 0.056
Aspen 5.62 ± 0.20a 5.09 ± 0.18a 4.3 0.044 2.3 0.077
DBH, mm Birch 38.6 ± 2.1a 33.0 ± 2.0a 3.6 0.066 0.9 0.483
Aspen 35.8 ± 2.1a 29.9 ± 1.6a 4.7 0.036 0.8 0.508
Cellulose, % Birch 57.6 ± 0.35a 56.1 ± 0.34a 13.2 <0.001 5.6 <0.001
Aspen 60.8 ± 0.15b 61.8 ± 0.32b 12.9 <0.001 6.9 <0.001
Lignin, % Birch 10.5 ± 0.14a 10.8 ± 0.17a 2.2 0.142 0.9 0.480
Aspen 9.7 ± 0.15b 10.1 ± 0.21b 3.0 0.089 2.7 0.045
Hemicellulose, % Birch 25.9 ± 0.21a 25.4 ± 0.33a 2.1 0.154 2.4 0.065
Aspen 21.5 ± 0.16b 19.9 ± 0.19b 42.3 <0.001 1.8 0.150
[N], % Birch 0.25 ± 0.006a 0.28 ± 0.007a 20.9 <0.001 2.6 0.052
Aspen 0.16 ± 0.004b 0.19 ± 0.006b 17.0 <0.001 5.2 0.002
[P], % Birch 0.045 ± 0.001a 0.049 ± 0.002a 5.5 0.024 3.0 0.030
Aspen 0.035 ± 0.001b 0.042 ± 0.001b 32.1 0.000 3.7 0.012
[K], % Birch 0.090 ± 0.003a 0.078 ± 0.002a 10.4 0.003 1.5 0.215
Aspen 0.091 ± 0.003a 0.085 ± 0.005a 1.2 0.290 0.6 0.668
P/N Birch 0.19 ± 0.004a 0.17 ± 0.005a 3.3 0.078 1.5 0.227
Aspen 0.21 ± 0.005b 0.22 ± 0.003b 8.6 0.006 6.0 <0.001
K/N Birch 0.37 ± 0.015a 0.28 ± 0.009a 30.4 <0.001 1.8 0.144
Aspen 0.57 ± 0.021b 0.44 ± 0.017b 23.6 <0.001 3.6 0.014
Ash, % Birch 0.41 ± 0.010a 0.41 ± 0.009a 0.1 0.749 2.0 0.118
Aspen 0.53 ± 0.008b 0.56 ± 0.018b 2.4 0.133 1.9 0.126
Calorific value,
kJ g–1 		Birch 19.62 ± 0.016a 19.60 ± 0.018a 1.0 0.309 1.0 0.275
Aspen 19.30 ± 0.026b 19.29 ± 0.024b 0.0 0.747 12.0 <0.001
Density, g cm–3 Birch 0.51 ± 0.005a 0.53 ± 0.009a 2.0 0.165 0.4 0.779
Aspen 0.41 ± 0.007b 0.41 ± 0.007b 0.1 0.768 3.0 0.031
Table 2. Pairwise Pearson’s correlation coefficients among wood and tree growth characteristics in control (C) and humidified (H) plots, bold indicates significant (p < 0.05) relations.
Wood characteristic Tree species Height DBH
C H C H
Cellulose Aspen –0.41 0.45 –0.44 0.29
Birch –0.03 0.03 –0.41 –0.23
Lignin Aspen –0.61 –0.47 –0.50 –0.45
Birch 0.38 –0.14 0.41 –0.20
Hemicellulose Aspen 0.75 0.01 0.68 0.20
Birch 0.17 0.35 0.35 0.63
[N] Aspen –0.65 –0.54 –0.53 –0.44
Birch –0.35 –0.63 –0.24 –0.28
[P] Aspen –0.60 –0.58 –0.39 –0.48
Birch –0.01 –0.57 –0.29 –0.41
[K] Aspen –0.40 –0.51 –0.35 –0.45
Birch 0.21 –0.02 –0.09 0.06
Calorific value Aspen 0.12 –0.26 0.26 –0.23
Birch 0.40 –0.17 0.64 –0.26
Density Aspen –0.83 –0.41 –0.76 –0.51
Birch 0.10 0.06 –0.13 0.31
Ash content Aspen –0.44 –0.53 –0.41 –0.38
Birch 0.15 –0.41 –0.09 –0.22
Table 3. Pairwise partial correlations (controlling for the effect of DBH) among wood characteristics in control (C) and humidified (H) plots, bold indicates significant (p < 0.05) relations. View in new window/tab.