Table 1. Locations, areas, and main characteristics of the growing stock in eight sample plots at the beginning of the pruning experiment in spring 2004 and after six growing seasons in winter 2009–2010.
Stand, location and coordinates (WGS84) Plot Plot area, m3 Number of stems, ha–1 Crown base, m Dominant
height, m 		Mean dbh,
mm 		Volume,
m3 ha–1
2004 2004 2004    2010 2004    2010 2004    2010
A Padasjoki
   (61°26´N, 25°06´E) 		1 680 1838 3.2 11.5 16.6 94 124 65 143
2 990 1444 2.0 12.2 15.5 96 130 47 116
B Kuhmoinen
   (61°36´N, 25°18´E) 		3 770 1481 1.3 10.4 15.7 79 117 31 102
4 1076 1571 1.7 9.6 15.2 73 109 25 83
5 828 1546 2.0 11.4 17.5 88 121 44 120
C Parkano
   (62°02´N, 23°02´E) 		6 600 1650 3.2 12.1 17.1 102 138 66 158
7 600 1650 3.3 11.9 16.9 102 139 66 161
D Parkano
   (61°59´N, 22°56´E) 		8 600 2033 3.1 11.4 16.2 93 126 63 167
Mean 768 1652 2.5 11.3 16.4 91 126 51 131
Table 2. Numbers (N), mean diameters at breast height (dbh) over bark in spring 2004 and winter 2009–2010, and annual dbh increments of felled study trees during the experiment for different pruning treatments (standard deviations presented in parentheses and ranges between minimum and maximum values).
dbh 2004, mm dbh 2010, mm dbh increment, mm a–1
Pruning
method 		Pruning
date 		N Mean (Sd) Range
(min-max) 		Mean (Sd) Range
(min-max) 		Mean (Sd) Range
(min-max)
Secateurs 19.3.2004 8 97 (13) 79–117 134 (11) 119–149 6.2 (1.2) 4.4–8.0
Saw 19.3.2004 8 95 (16) 73–123 131 (18) 100–155 5.9 (1.0) 4.5–7.5
Secateurs 29.4.2004 8 88 (16) 64–113 124 (15) 102–143 5.9 (0.8) 4.8–7.2
Secateurs 3.6.2004 8 93 (10) 75–103 133 (11) 118–152 6.7 (1.7) 4.3–9.2
Secateurs 23.6.2004 8 96 (9) 80–103 131 (13) 113–145 5.9 (1.1) 4.9–7.6
Secateurs 15.7.2004 8 91 (14) 76–117 124 (15) 105–149 5.4 (1.3) 3.4–7.8
Saw 15.7.2004 8 92 (12) 75–106 130 (20) 101–159 6.3 (2.1) 2.9–9.8
Secateurs 5.8.2004 8 95 (14) 68–119 135 (21) 104–176 6.6 (1.4) 5.1–9.5
Secateurs 26.8.2004 8 96 (22) 64–136 139 (31) 96–191 7.1 (1.9) 4.5–9.3
Saw 26.8.2004 8 98 (20) 69–118 134 (22) 97–161 5.9 (0.9) 4.7–7.2
Secateurs 16.9.2004 8 95 (16) 79–119 136 (21) 115–166 6.8 (1.2) 5.3–9.0
Stick autumn 2004 8 87 (13) 70–104 122 (17) 101–149 5.8 (1.5) 2.8–7.4
Stick spring 2005 8 90 (15) 70–115 127 (14) 106–146 6.1 (1.2) 3.8–7.3
Control not pruned 8 96 (15) 76–123 131 (14) 114–153 5.9 (0.6) 5.0–6.8
ALL DATA   112 94 (14) 64–136 131 (18) 96–191 6.2 (1.3) 2.8–9.8
Table 3. Main characteristics of study branches. Numbers (N) and mean vertical diameters (measured in 2004 over bark) are presented for living, dead, and all branches in total and separately for different pruning treatments (standard deviations in parentheses and ranges between minimum and maximum values). Cumulative radial increments (over bark) of felled study trees at the height of each branch during five to six growing seasons were also determined for all branches. View in new window/tab.
1

Fig. 1. Three scanned images of study knots: Knot 1A: a 12-mm-thick knot occluded without discolouration (occlusion class 1). The occlusion gap was closed and the occlusion rate was set to 100%. Knot 1B: occlusion started from the up and down sides of a 16-mm-thick knot (class 3). Because the occlusion gap (G) was 7.7 mm long, the occlusion percentage was 52%. Knot 1C: vertical diameter of the knot was 13 mm and the occlusion gap was not measured because no occlusion was observed (class 6). The occlusion rate was set to 0%. A small discolouration in the stemwood (upward only↑) was also detected.

Table 4. Observation methods used in this study to identify external and internal occlusion classes as well as occlusion percentages for knots in alternative pruning treatments and four differently formulated models to analyse the knot
occlusion.
Dependent variable of occlusion External occlusion class Internal occlusion class Occlusion percentage of a knot
Origin From stem surface From split knots From split knots
Observation method External evaluation, 6 classes Internal evaluation, 8 classes Exact measurement (0.1 mm), occlusion % of a knot
Model formulation Not modelled Ordinal regression models
with mixed-effects (Eq. 2–4) 		Linear mixed models (Eq. 5)
Modelling:
1) Effect of pruning date
Data:
Treatments: 		MODEL 1
1000 knots
Secateurs 8 dates
2) Effect of pruning method and season
Data:
Treatments: 		MODEL 2
1001 knots 1)
Secateurs 3 dates
Saw 3 dates
Stick 2 dates Control 		MODELS 3 and 4
1334 knots
Secateurs 8 dates
Saw 3 dates
1) includes only equal dates for saw and secateurs pruning.
Table 5. Distribution of silver birch knots into occlusion classes according to the pruning method (sc = secateurs, sw = saw, stick = stick pruning) and date. Classification is made internally from the cross-cut surface of longitudinally split knots five to six growing seasons after pruning.
Number of knots according to different pruning treatments (i.e., various methods and dates)
Sc Sw Sc Sc Sc Sc Sw Sc Sc Sw Sc Stick Stick Control Total
Occlusion class 19.3. 19.3. 29.4. 3.6. 23.6. 15.7. 15.7. 5.8. 26.8. 26.8. 16.9. autumn spring N %
1 – Occluded without discolouration 46 34 27 28 16 6 18 13 10 19 18 10 5 3 253 15
2 - Occluded with some discolouration 67 59 53 65 30 26 57 27 21 23 28 27 27 3 513 31
3 - Partly occluded from both the up- and downside of the knot 21 22 27 23 26 30 27 24 22 26 19 10 4 1 282 17
4 - Partly occluded from either the up- or downside of the knot 10 1 17 14 37 28 10 47 21 20 35 8 11 2 261 16
5 - Occlusion just started, a visible pro-trusion outside the knot 2 3 4 0 12 20 6 9 13 17 19 4 6 0 115 7
6 - Occlusion not started 0 0 1 1 7 9 7 5 12 9 4 19 32 3 109 7
7 - Unpruned dead branch 0 0 0 0 0 0 0 0 0 0 0 0 0 92 92 6
8 - Unpruned living branch 0 0 0 0 0 0 0 0 0 0 0 0 0 12 12 1
All knots 146 119 129 131 128 119 125 125 99 114 123 78 85 116 1637 100
All knots of living branches 43 39 69 78 55 54 68 67 50 37 44 - - 36 640 39
All knots of dead branches 103 80 60 53 73 65 57 58 49 77 79 78 85 80 997 61
Table 6. Parameter estimates and standard errors (S.E) of the ordinal regression model with mixed effects (Model 1) predicting the probability of an internally evaluated occlusion class (categories 1–6 in Table 5) for a branch pruned by secateurs at eight alternative dates (N = 1000).
MODEL 1        
Threshold coefficients Estimate S.E. z value
Occlusion classes 1|2 –1.3515 0.721 –1.874
Occlusion classes 2|3 0.6887 0.722 0.955
Occlusion classes 3|4 1.9124 0.723 2.644
Occlusion classes 4|5 3.7859 0.730 5.183
Occlusion classes 5|6 5.2673 0.744 7.078
Parameter Estimate S.E. z value Sig.
Horizontal diameter of a branch, mm 0.1689 0.019 8.830 <0.001
Cumulative net radial increment over bark in 5–6 years, mm –0.1644 0.029 –5.597 <0.001
Treatment (ref. secateurs 19.3.)
- secateurs 29.4. 0.9643 0.442 2.181 0.029
- secateurs 3.6. 1.0562 0.443 2.387 0.017
- secateurs 23.6. 2.1187 0.447 4.744 <0.001
- secateurs 15.7. 2.2457 0.468 4.796 <0.001
- secateurs 5.8. 2.0378 0.452 4.508 <0.001
- secateurs 26.8. 2.5737 0.457 5.629 <0.001
- secateurs 16.9. 1.8757 0.450 4.171 <0.001
Vitality of a branch (ref. living branch)
- dead branch 0.4315 0.140 3.081 0.002
Random part Variance SD
Stand effect (δstand2) (N = 4) 0.1276 0.3572
Sample tree effect (δtree2) (N = 64) 0.5421 0.7363
2

Fig. 2. Effect of the pruning date on the occlusion of 15-mm thick secateurs-pruned living branches with the cumulative 18-mm net radial growth (over bark at the branch height) according to Model 1. Percentage bars illustrate the probabilities of these branches pruned at eight alternative dates to fall into different occlusion classes. The same letter (a, b, c or d) in the bars indicates an insignificant difference (p > 0.05) in the occlusion between the pruning dates.

3

Fig. 3. Effects of the horizontal diameter of a branch (mm) and the cumulative net radial increment over bark (mm) of a tree (at the height of the branch) on the estimated probabilities of occlusion classes in the case of living branches pruned with secateurs on 15 July, according to Model 1. The percentage characterizes the probability of a branch to fall into a certain occlusion class.

Table 7. Parameter estimates and standard errors (S.E.) of the ordinal regression model (Model 2) with mixed-effects predicting the internally evaluated occlusion class (categories 1–8 in Table 5) for a branch pruned by saw, secateurs, or stick or unpruned (control) (N = 1001).
MODEL 2        
Threshold coefficients Estimate S.E. z value
Occlusion classes 1|2 –1.8186 0.835 –2.179
Occlusion classes 2|3 0.3571 0.835 0.428
Occlusion classes 3|4 1.5259 0.837 1.823
Occlusion classes 4|5 2.5602 0.838 3.054
Occlusion classes 5|6 3.4792 0.842 4.134
Occlusion classes 6|7 6.3755 0.907 7.032
Occlusion classes 7|8 12.0251 1.115 10.782
Parameter Estimate S.E. z value Sig.
Horizontal diameter of a branch, mm 0.1158 0.018 6.279 <0.001
Cumulative net radial increment over bark in 5–6 years, mm –0.1381 0.035 –3.935 <0.001
Treatment (ref. secateurs 19.3.)
- saw 19.3. –0.0129 0.577 –0.022 0.982
- secateurs 15.7. 1.9503 0.602 3.242 0.001
- saw 15.7. 0.8946 0.580 1.543 0.123
- secateurs 26.8. 2.1518 0.584 3.684 <0.001
- saw 26.8. 1.3180 0.599 2.200 0.028
- stick in autumn 2004 2.1664 0.617 3.513 <0.001
- stick in spring 2005 2.7407 0.620 4.418 <0.001
- no pruning (control) 10.1923 0.786 12.968 <0.001
Random part Variance SD
Stand effect (δstand2) (N = 4) 0.1813 0.426
Sample tree effect (δtree2) (N = 72) 1.0877 1.043
4

Fig. 4. Effects of the pruning method and timing on the occlusion of 15-mm thick branches with the cumulative 18-mm net radial growth (over bark at the branch height) according to Model 2. The percentage bars illustrate the probabilities of these branches to fall into different occlusion classes. The same letter (a, b, c, d or e) in the bars indicates an insignificant difference in (p > 0.05) the occlusion between the treatments.

Table 8. Numbers (N) and mean occlusion percentages with standard deviations (SD) of studied branches pruned by secateurs (Sc) and saw (Sw) at different dates during the growing season in 2004. Occlusion percentages for branches were evaluated five to six years after pruning.
      Pruning method and date
Sc Sw Sc Sc Sc Sc Sw Sc Sc Sw Sc ALL
19.3. 19.3. 29.4. 3.6. 23.6. 15.7. 15.7. 5.8. 26.8. 26.8. 16.9.
Living branches                        
  N 42 37 68 78 54 51 68 67 47 36 44 592
  Mean percentage 95.0 90.3 85.3 86.8 72.3 61.4 79.6 50.2 48.0 60.4 61.1 72.4
  SD 15.3 23.2 23.3 24.2 32.4 30.7 32.1 31.3 38.6 39.7 34.5 33.5
Dead branches
  N 101 79 57 52 70 64 57 58 48 77 79 742
  Mean percentage 88.8 88.8 74.9 87.3 46.5 40.3 69.8 58.0 50.9 53.5 49.4 65.3
  SD 23.4 24.7 33.0 23.1 40.1 40.3 40.7 39.7 42.8 40.8 43.0 40.0
All branches
  N 143 116 125 130 124 115 125 125 95 113 123 1334
  Mean percentage 90.6 89.3 80.6 87.0 57.7 49.7 75.1 53.8 49.4 55.7 53.6 68.4
  SD 21.4 24.1 28.5 23.7 39.0 37.7 36.4 35.5 40.6 40.4 40.4 37.4
Table 9. Parameter estimates and standard errors (S.E.) for Models 3 and 4 to estimate the arcsine transformation of the occlusion rate (0–1) for a saw or secateurs pruned branch (N = 1334). AIC = Akaike’s information criterion.
MODEL 3 MODEL 4
Parameter Estimate S.E. Sig. Estimate S.E. Sig.
Fixed part
Intercept 0.30398 0.30378 0.3188 1.09016 0.20446 <0.001
Vitality of a branch (ref. dead)
- living 0.08389 0.02800 0.0028 0.08189 0.02800 0.004
Pruning method (ref. saw)
- secateurs –0.25935 0.05614 <0.001 –0.27642 0.05785 <0.001
Pruning in spring (ref. no)
- yes 0.31139 0.06044 <0.001 0.38542 0.05958 <0.001
Pruning in autumn (ref. no)
- yes –0.10047 0.06293 0.1144 –0.14228 0.06505 0.032
Ln(vertical diameter of a branch, mm) –0.53603 0.03541 <0.001 –0.53061 0.03538 <0.001
Ln(cumulative net radial increment over bark, mm) 0.72287 0.10672 <0.001
Ln(dbh increment over bark, mm a–1) 0.69200 0.10401 <0.001
Height of a branch, dm 0.00707 0.00132 <0.001 0.00967 0.00127 <0.001
Random part
Random error (δe2) 0.15580 0.00626 0.15516 0.00623
Stand effect (δstand2) (N = 4) 0.00596 0.00767 0.00668 0.00810
Sample tree effect (δtree2) (N = 88) 0.04245 0.00859 0.04519 0.00895
AIC 1491 1490
5

Fig. 5. Occlusion percentages (% of branch diameter) estimated by Model 3 for living and dead branches pruned by secateurs in spring according to the vertical diameter of a branch and the cumulative radial increment of a stem at the branch height of 25 dm (during six growing seasons after pruning, good occlusion rate).

6

Fig. 6. Occlusion percentages (% of branch diameter) estimated by Model 3 for living branches pruned by saw or secateurs in summer according to the vertical diameter of a branch and the cumulative radial increment of a stem at the branch height of 25 dm (during five and a half growing seasons after pruning, moderate occlusion rate).

Table 10. Proportions of discoloured knot samples and mean spreading distances of stemwood discolouration (with standard deviations, SD) for different pruning treatments. The results are presented for living, dead, and all branches five to six growing seasons after pruning based on measurements from the images of split knots (N = 1585). Some samples were rejected due to large colour defects in the stemwood caused by unknown defects.
Pruning method Pruning
date 		Discoloured, % of samples Mean spreading distance (SD), mm Reject samples
Living Dead All Living Dead All
Secateurs 19.3. 31.7 48.5 43.6 40.6 (28.2) 35.2 (20.8) 36.3 (22.4) 6
Saw 19.3. 44.4 45.2 45.0 30.5 (23.0) 26.1 (23.4) 27.6 (23.1) 10
Secateurs 29.4. 14.7 32.2 22.8 41.1 (28.9) 22.5 (19.4) 28.9 (24.3) 2
Secateurs 3.6. 15.4 44.2 26.9 15.0 (9.2) 13.5 (9.4) 14.0 (9.2) 1
Secateurs 23.6. 14.5 47.8 32.8 16.3 (16.4) 32.8 (23.9) 29.5 (23.3) 6
Secateurs 15.7. 16.7 44.6 31.9 18.8 (11.8) 15.2 (14.3) 16.1 (13.6) 0
Saw 15.7. 22.1 48.2 33.9 15.5 (13.3) 17.8 (13.5) 17.0 (13.3) 1
Secateurs 5.8. 20.9 51.7 35.2 19.1 (20.8) 26.0 (16.4) 23.8 (18.0) 0
Secateurs 26.8. 10.0 40.0 24.2 22.8 (29.6) 19.8 (13.4) 20.5 (17.3) 4
Saw 26.8. 35.3 40.8 39.0 22.7 (21.6) 24.6 (23.4) 24.1 (22.6) 9
Secateurs 16.9. 37.2 72.5 58.9 15.4 (11.9) 25.1 (20.0) 22.8 (18.8) 11
Stick 1) autumn 2004 - 36.8 - - 19.7 (17.0) - 2
Stick 1) spring 2005 - 27.1 - - 16.9 (14.1) - 0
Control not pruned 19.4 22.5 21.6 32.8 (13.3) 21.1 (17.0) 24.4 (16.7) 0
ALL   21.7 42.6 34.3 23.8 (21.2) 23.8 (18.1) 23.8 (19.9) 52
1) Living branches were not removed in stick pruning.