BIOLOGICAL REACTIONS OF THICK-BRANCHED SCOTS PINES (PINUS SYLVESTRIS L) TO PRUNING IN RELATION TO THE SEASON OF THE YEAR OF THE OPERATION

Dieter Franciszek Giefing¹, Marta Złota², Paweł Stypik², Paulina Wykpisz^2
1 Department of Forest Utilization, University of Life Sciences in Poznań, Poland
² Department of Forest Utilization, August Cieszkowski Agricultural University of Poznan, Poland

ABSTRACT

The aim of the study was to assess the impact of the season of the year in which the pruning of large-branched pine trees was carried out on their biological response to this operation. The experimental material derived from a plantation which was established in 1984 on which pine trees at 4 x 4 m spacings were planted. In 1994, during tending cutting, the spacing arrangement was loosened to 8 x 8 m. The trees were pruned during three different seasons of the year: in spring and summer of 1995 and in winter 1996. Investigations were conducted on the material collected 7-8 years after the pruning of pine trees. The total of 354 knots derived from 20 trees was examined.

The thickness of knots without bark in the pruned trees ranged from 3 to 70 millimetres. All the examined knots were healthy. Knots pruned in spring occluded the fastest (90% of all knots occluded within 7 years), while those pruned in winter – slowest (after 7 years only 53% of knots became occluded).

Key words: thick-branched pine trees, pruning, pine plantation cultivation, knots, season of the year when pruning was done.

INTRODUCTION

The most important defect which makes it impossible to qualify timber raw material into higher quality category during the process of its evaluation and classification is knots. “According to the investigations conducted by numerous researchers, about 70% of large-sized pine raw material, 82-91% of spruce raw material, 92% of Douglas fir raw material and 50-80% of Caucasian fir raw material cannot be classified into higher classes due to defects in knottiness. The frequency of the occurrence of other defects is considerably smaller and concerns maximum 4.5% of classified pine trees” [4].

We can, therefore, presume that at least two thirds of the large-sized timber raw material cannot be classified into higher quality classes due to defects in knottiness. An operation which reduces radically the occurrence of this defect is pruning. This treatment allows to forestall the process of natural self-cleaning of trees from branches, eliminates the zone of bad knot, limits the occurrence of the zone of the falling knot and makes it possible to obtain a wide zone of knotless timber.

The pruning guidelines for pine trees recommend pruning trees from branches whose thickness does not exceed 2 cm. In addition, for a long time an opinion prevailed concerning the pruning of pine and spruce trees that it is unacceptable [4] to remove live branches from then as this increases the probability of infection. However, experiments carried out so far on the biological responses of large-branched pines to their pruning failed to show any pathological changes in the wood neighboring with the occluded and occluding knots [5], despite the fact that during the pruning operation live branches with the diameter reaching 8 cm (measurement with bark) were removed. There is no information in the available literature on the subject about experiments aiming at the assessment of the season of the year during which the pruning is performed on the biological response of pruned trees. The undertaken experiments aim at filling in this gap and assessing the response of thick-branched pine trees to pruning in relation to the season of the year when the operation is carried out. These investigations will make it possible to recognise better problems associated with the removal from pines of thick branches and allow elaborating recommendations concerning possible pruning of wolf-trees as well as possibilities of the plantation cultivation of the pine. In addition, such studies will allow verification of the research results published so far on the pruning of thick-branched pine trees.

LOCATION, SCOPE AND METHODOLOGICAL ASSUMPTIONS OF THE RESEARCH PROJECT

The experimental material was obtained form the plantation cultivation of Scots pine (Pinus sylvestris L.) situated in the Zdrojowa Góra Forest District in the Piła Regional Directorate of State Forests (RDSF). The plantation cultivation was established in 1984. The initial tree spacing was 4 x 4 m but, in 1994, it was loosened to 8 x 8m. Pine trees were pruned during three different seasons of the year: in spring (25^th April 1995), summer (14^th July 1995 and winter (1^st March 1996 – maximum temperature from 8 a.m. to 3 p.m. was minus 3°C).

The total of 354 knots from 20 trees pruned in spring, summer and winter was examined. Using the band saw, the knot cylinders were cut in the laboratory across the plane determined by the cores of the knot and trunk. The health condition of the knots and the adjacent trunk timber was assessed and the length and width of the inbark, time of occlusion as well as the occlusion <footnote1> condition were measured. The results of measurements were subjected to the analysis which allowed the evaluation of the impact of the season of the year when the treatment was carried out (spring, summer, winter) on the course of the wound healing process following the branch removal.

The experimental material from the individual experimental variants was subjected to statistical analysis using the Tukey’s LSD test to check the significance of the knot thickness variability in the individual experimental variants.

RESEARCH RESULTS

The diameter (without bark) of the 354 knots subjected to investigations ranged from 3 mm to 70 mm, on average 24 mm. The thickness variability of knots in individual experimental variants was statistically non-significant (Tab. 1) allowing the authors to conclude that the thickness variability of knots in individual experimental variants had no influence on the obtained research results. It is worth stressing that knots pruned in spring occluded fastest despite the fact that their mean diameter (26 mm) was bigger than the mean diameter of knots pruned in summer (23 mm) and in winter (22 mm).

Table 1. Results of the Tukey’s LSD test (for different N) for the thickness of knots in individual experimental variants HSD (unequal N); variable max. knot diameter Approximate probabilities for tests post hoc Error: inter-group MS – 143.16, df = 351.00

Season of the year (mean diameter[mm])	Spring (26 mm)	Summer (23 mm)	Winter (22 mm)
Spring		0.051764	0.062196
Summer	0.051764		0.953767
Winter	0.062196	0.953767

The pruned trees were characterised by large increments. The mean width of annual rings was 7.46 mm. All the examined knots were healthy and no cases of rot of the knots or wood surrounding them were recorded. Therefore, it can be said that the performed pruning eliminated completely the zone of rotten knot (Fig. 1, 2). The number of occluded and non-occluded knots varied and depended on the time of the year the pruning was carried out (Fig. 3).

In the case of pine trees pruned in spring, 90% of knots became occluded after 7 years. Knots were observed to occlude slowest following winter pruning, where only 53% of knots were found occluded. It is quite possible that it was the result of frozen cambium. Most probably in winter, cambium cells die and the process of occlusion of wounds by the tissue surrounding them does not take place. In this situation, the process proceeds in the way similar to that which occurs following the removal of dead branches when the wound is occluded by the trunk wood which skins over the wound after the removal of the branch. The occlusion time of such wounds, even when only thin branches are removed, is much longer and can take up to several years [10, 16, 22]. The observed relatively poor occlusion dynamics of wounds after the removal of branches in summer results, most probably, from the poor cambium activity during this season of the year and relatively short period of time of wound occlusion in the first year after tree pruning (from the moment of pruning until the end of the vegetative season). The most favorable season for pruning which guarantees the shortest period of wound occlusion after the removal of branches is spring, when cambium is characterised by the highest activity.

Figure 1. Cross section by a completely occluded knot

Figure 2. Cross section by a non-occluded knot

Figure 3. Percentage proportion of occluded and non-occluded knots depending on the season of the year of the pruning operation

In the case of pine trees pruned in spring, 90% of knots became occluded after 7 years. Knots were observed to occlude slowest following winter pruning, where only 53% of knots were found occluded. It is quite possible that it was the result of frozen cambium. Most probably in winter, cambium cells die and the process of occlusion of wounds by the tissue surrounding them does not take place. In this situation, the process proceeds in the way similar to that which occurs following the removal of dead branches when the wound is occluded by the trunk wood which skins over the wound after the removal of the branch. The occlusion time of such wounds, even when only thin branches are removed, is much longer and can take up to several years [10, 16, 22]. The observed relatively poor occlusion dynamics of wounds after the removal of branches in summer results, most probably, from the poor cambium activity during this season of the year and relatively short period of time of wound occlusion in the first year after tree pruning (from the moment of pruning until the end of the vegetative season). The most favorable season for pruning which guarantees the shortest period of wound occlusion after the removal of branches is spring, when cambium is characterised by the highest activity.

Irrespective of the time of pruning, the process of wound occlusion was most dynamic during the first three years (Fig. 4). In the 3^rd year after pruning, 46% of knots from spring pruning, 44% of knots from summer pruning and 33% of knots from winter pruning became occluded. It should be emphasised that, in the case of winter pruning, the process of wound healing ran relatively dynamically during the first five years (Fig. 4) and later on only single knots were observed to scab over. In the case of spring and summer pruning, the occlusion process proceeded identically during the first four years. During the 5^th and consecutive years differences in the occlusion process intensified systematically. During the next two years the occlusion of consecutive, albeit rare, knots was observed but the process was distinctly slower. No knot was found to occlude in the 8^th year after pruning, irrespective of the season the operation was performed.

Figure 4. Percentage proportion of occluded knots in consecutive years after tree pruning Percentage [%] proportion of occluded knots in consecutive years.

The course of the discussed process was subjected to statistical analysis (Tab. 2). The performed two-factorial analysis of variance shows that the above-mentioned variability is statistically non-significant (calculated f is bigger than table f – f_calc.>f _tab., while the value of the true error p is smaller than 0.05).

Table 2. Analysis of variance of the number of occluded knots in consecutive years after tree pruning depending on the season the pruning was conducted

ANALYSIS OF VARIANCE
Source of variance	SS	df	MS	F	Value p	Test F
Consecutive years	2079.238	6	346.5397	13.35699	0.000108	2.996117
Season of the year	488	2	244	9.404711	0.003491	3.88529
Error	311.3333	12	25.94444

The above-presented observations are further corroborated by the regression analysis (Fig. 5). It is evident from this analysis that the dynamics of the occlusion of knots pruned in individual seasons deceases in consecutive years. The smallest number of occluded knots was recorded from the winter pruning and the equation adopts the following form: y = 19.6071429 – 2.60714286*x, hence the increase in the number of occluded knots in consecutive years will decrease by the value of 2.60. It is evident from the equations that, in the case of trees pruned in spring and summer, the dynamics of knot occlusion will also decline with age by the value of 4.5 to 4.7 (summer). However, it must be emphasised that in the case of trees pruned during these seasons, the majority of knots got occluded during the first 4-5 years anyway.

Figure 5. Regression diagram of the occlusion time of knots in individual seasons of the year spring: Zmn5: r2 = 0.4453; r = –0.6673. p = 0.1015; y = 40.7857143 – 4.5*x summer: Zmn6: r2 = 0.7621; r = –0.8730. p = 0.0103; y = 37.6428571 – 4.78571429*x winter: Zmn7: r2 = 0.6903; r = –0.8308. p = 0.0206; y = 19.6071429 – 2.60714286*x

Spring turned out to be the most favourable season of the year to perform the pruning operation. Trees pruned in spring, despite large knot diameters reaching even 70 mm (80 mm in bark), with the average diameter of 24 mm, occluded in a very short time. Majority of wounds resulting from removed branches got healed after six years.

It is also worth mentioning that, regardless of the season of pruning, no symptoms of any infection were observed. Therefore, there are no health counter indications for carrying out the pruning treatment of pine trees during any season of the year, although it must be added that the wounds got healed most rapidly when the branches were removed during spring pruning.

When trees are pruned from thick branches, inbarks often develop during the process of wound healing (Fig. 6). The proportion of knots with inbarks is considerable and may reach 45% (Fig. 7).

Figure 6. Cross section through the partially occluded knot with inbark

Figure 7. Percentage proportion of knots with inbarks and without inbarks depending on the season of the year when the pruning treatment was carried out

Differences between the frequency of the occurrence of inbarks depending on the season of the year of tree pruning are not very significant. However, the obtained research results clearly show that the smallest number of knots with inbarks occurred after summer pruning (36%). In the case of spring and winter pruning, the proportion of knots with inbarks was almost identical and amounted to 45 and 44%, respectively. Differences between the number of inbarks depending on the season of the year are not significant and, therefore, it can be presumed that the season of the year does not have a major impact on the limitation of the inbark occurrence. Furthermore, it should be stressed that the presented analysis refers only to occluded knots whose number depended on the time of pruning (Fig. 3) which is associated with a different dynamics of the knot occlusion process depending on the season of pruning. Therefore, it can be expected that, following the complete occlusion of knots, the above-presented variability may change.

DISCUSSION

For some time, both teaching materials and literature on the subject presented an approach that no live branches should be pruned from pine and spruce trees. In Poland, this opinion was presented by some authors even in 1980s and 1990s [12, 18]. In the world literature on this subject, this kind of approach was questioned already in the second half of 1930s [13, 17, 20]. Following further investigations, which were carried out during the second half of the last century [14, 10, 9, 7, 22, 11, 8], the above-mentioned opinion was completely abandoned at the beginning of 1970s. The research results obtained in this study fully corroborate this decision.

However, pruning guidelines maintain the limitation concerning the thickness of the removed branches, usually to 2 cm [1, 2]. In Bavaria, foresters are allowed to prune pine trees from branches of up to 4 cm [19]. It should be emphasised that the above-mentioned pruning instructions were elaborated on the basis of the knowledge and research results from that period and on the basis of opinions about pruning prevailing then which were often based on general knowledge and intuition of foresters and not on the experimental results of scientific research. Understandably, foresters’ opinions have always been characterised by great caution. In view of the absence of research results which would entitle them to adopt a different stand on this subject, caution – which excludes the risk of a serious mistake – appears to be fully justified.

The research results presented in this paper confirm that there are no counter indications against pruning live branches in pine trees and even without the need to restrict their thickness to 2 cm. The results of the presented experiments prove that pine is a species which tolerates pruning particularly well. Resin which covers profusely the surface of wounds after the removal of live branches protects them very well against pathogens. The performed investigations included occluded and occluding knots with diameters exceeding significantly 2 cm and no signs of infections were observed. Considerable annual increments of pine trees on the plantation cultivation resulted in a substantial acceleration of the knot occlusion process.

Wounds after the removal of live branches heal faster than wounds after the removal of dead branches. According to Zumer [22], in the case of spruce, six years after the removal of live branches, all knots were occluded. Krigul [10] maintains that wounds after the removal of dead branches heal in spruce by 30-40% and in pine – by 10% slower than wounds after the removal of live branches. In this study, following the removal of live branches, the greatest number of knots became occluded within 3-4 years (Fig. 4), irrespective of the season when the pruning was conducted (spring pruning – 46% of knots, summer – 44% of knots and winter – 33% of knots became occluded). Wounds on trees pruned in winter showed the worst occlusion dynamics but even here, 33% of them got healed after 3 years, 41% – after 4 years and up to 50% – after 5 years. During the consecutive years, the occlusion of knots was less dynamic and only 3% of knots became occluded during the period of the next 2 years. In the case of spring and summer pruning, the knot occlusion process was spread relatively regularly over the period of 7 years from the moment of pruning, whereas in the case of winter pruning – only during 5 years. In the consecutive years, the occlusion process continued very slowly.

Wolf-trees are trees which are characterised by strong branching and, by dominating over their neighbours, they restrict their growth. They attain considerable annual increments and, when young, achieve considerable dimensions similar to trees growing in open spaces and on plantation cultivations. The removal of such a tree, in some cases, may result in the creation of a large opening in the stand which may have a negative influence on the development of the adjacent trees, usually of worse biosocial position. As a rule, even after the removal of a wolf-tree, they do not have enough vital forces to return to the group of dominating or co-dominating trees. Results of the presented investigations confirm the expediency of intensive pruning of the wolf-tree. The strong reduction of its crown will check the dynamics of its growth as well as its negative impact on its neighbours. In addition, this operation will facilitate the accumulation of knotless timber in the pine wolf-tree which is characterised by exceptionally intensive growth dynamics.

A wide range of differing opinions can be found in the literature on the subject concerning the most and least favourable times of pruning. For example, Muratowa [15] recommends the end of winter and beginning of spring as the best period to carry out the pruning operation of live branches. The author claims that if the treatment is conducted in summer or winter, it exposes trees to infection with fungi. Results obtained in this study contradict this opinion. Winter pruning, also when it is carried out at the end of this season, causes considerable slowing down of the wound healing process after the removal of branches. On the other hand, spring pruning is advisable because of the greatest dynamics of the occlusion process after the removal of branches and not because of the small risk of infection. Naturally, this does not mean that winter pruning should be abandoned; despite the longer wound healing process observed in trees pruned in winter, no cases of infection among these trees were recorded. Pine is a species which tolerates pruning very well; therefore, this operation can be carried out in any season of the year.

Some pruning instruction indicate seasons of the year which are the most and the least suitable to perform this operation. A Swedish instruction [21] recommends June, July and August as the most suitable pruning time. This suggestion is in keeping with the research results obtained in this study, although May appears a more suitable period to begin pruning as wounds left on trees by August pruning healed much slower. German guidelines advise foresters not to prune trees with the KS 31 saws during the period of spring growth and autumn transition to the state of rest [6]. However, the reason for this recommendation was not the occlusion dynamics of knots but reported cases of cambium damage as a result of pressure exerted by the wheels of the device. Cambium is very sensitive during the above-mentioned periods and, consequently, numerous cases of deaths of spruce trees pruned with this device were reported. The above guidelines were compiled on the basis of the state of knowledge and practical experience of their authors. Nevertheless, the available literature on the subject fails to provide experimental data on the biological response of pine trees to pruning depending on the season of the year of this operation.

CONCLUSIONS

1. The removal of live large-sized branches from pine trees does not lead to timber infection irrespective of the timing of the treatment.

2. The wound healing process is fastest after the removal of branches following spring pruning (90% of knots became occluded after 8 years).

3. Spring pruning is the most advantageous from the point of view of the dynamics of wound healing.

4. Knots occluded as a result of live branch pruning are healthy and do not show any signs of infection. This allows the complete elimination of the zone of unsound knot.

5. The pruning of trees from large-sized branches leads to the development of inbarks. The smallest numbers of knots with inbarks were found in the case of summer pruning. Similar amounts of inbarks were recorded during winter and spring pruning. The results of the performed experiments failed to provide sufficient evidence to indicate correlations between the timing of pruning and the number of inbarks. Further experiments are required after the occlusion of all knots which would verify the obtained results.

6. Large-branched pine trees growing on the plantation cultivation are characterised by high wound-healing dynamics. Majority of wounds resulting from the removal of branches healed after 7 years despite the fact that their diameters were considerable, sometimes reaching up to 70 mm.

REFERENCES

1. Entwurf (project) 1993. Merkblatt – Wertästung – [guide – pruning], Materiały Szkoły Lesnej w Münchenhof poz.: 6442 [in German].

2. Giefing D. F., Cybulko T., Padrowski W., Ceitel J. 1993. Podkrzesywanie sosny – instrukcja [pruning of Scots pine – guide]. Dyrekcja Generalna Lasów Państwowych, Warszawa [in Polish].

3. Giefing D.F. 1994. Czy warto podkrzesywać drzewa w lesie? [is it worth to prune trees in the forest?]. Las Polski, 7:7 [in Polish].

4. Giefing D.F. 1999. Podkrzesywanie drzew w lesie [tree pruning in the forest]. Wyd. AR Poznań [in Polish].

5. Giefing D.F., Jonasz K., Wesoły W. 2004. The response of thick- branched pine trees to pruning. Electronic Journal of Polish Agricultural Universities, Forestry, Volume 7, Issue 2.

6. Harlfinger T. 1981. Jungbestandspflege und Ästung [tending and pruning in young stands]. Forst- und Holzwirt, 36. Helf 12. [in German].

7. Henman D. W. 1963. Pruning conifers for the production of quality timber. For. Comm. Bull. 35.

8. KniggeW., Olischläger K. 1970. Möglichkeiten der Grünästung der Fichte [pruning opportunities of live branches of spruce]. Holz- Zentralbl. 103: 1497-1500 [in German].

9. Kramer H. 1962. Kronenaufbau und Kronenentwicklung Gleichalter Fichtenbestande [development and structure of spruce crowns in evenage stands]. Allg. Forst- Jagdztg 133, 11: 249- 256 [in German].

10. Krigul T. 1961. Manni – ja Kuusepuistute laasimine [processing of timber]. Tartu. [in Estonian].

11. Lücke H. 1968. Grünästung der Kiefer [pruning of pine live branches ]. Forst- u. Holzwirt 20: 421- 423 [in German].

12. Mała Encyklopedia Lesna [small frorest encyclopedia] 1980. PWN, Warszawa [in Polish].

13. Mayer-Wegelin H. 1936. Ästung [pruning]. Hannover [in Polish].

14. Meyer H. 1959. Der Einfluss der Kronenkürzungen an Fichten der II Altersklasse auf deren Hohen- und Starkenwachstum [impact of shortening spruce crown in 2^nd age class on hight and diameter increment]. Arch. Forstw. 8, 9: 819- 849 [in German].

15. Murat E. 2002. Szczegółowa hodowla lasu [profund silviculture]. Wyd. Swiat, Warszawa [in Polish].

16. Nägeli W.1952. Aufästungversuche in gleichaltrigen Nadelholzbestanden schweizerischen Mittellandes [reasearch on homogenius coniferous stands in Central Swizerland]. Schweiz. Anst. Forstl. Versuchswes. Mitt. 28: 271-354 [in German].

17. Paterson A. 1938. The occlusion of pruning wounds in Norway spruce [Picea excelsa]. Ann. Bot. 2: 681-698.

18. Poradnik lesniczego [forester’s guide] 1991. Stowarzyszenie Inżynierów i Techników Lesnictwa i Drzewnictwa Wyd. Swiat. Warszawa [in Polish].

19. Richtlinien 1980. Richtlinien für die Wertastung im Bayerischen Staatswald [principles of pruning in the Bavarian State Forest]. Wyd. Bawarskie Ministerstwo Wyżywienia, Rolnictwa i Lesnictwa [in German].

20. Romell L. G. 1940. Kvistningsstundier d tall och gran. Medd. fran Statens Skogsforsoksant [branch studies of pine and spruce; message from Forest Research Institute]. 32, 5:143-194 [in Swedish].

21. STAMKVISTING AV.TALL 1994. [guide for pine pruning]. Wyd. Skogsvardsstyrelsen [in Swedish].

22. Zumer M. 1966. Ästungsversuche an Föhre, Fichte, Birke, Aspe, Esche und Eiche [research on pruning of pine, spruce, birch, aspen, ash and oak]. Med. d. Nor. Skogforsoksves. 77 [in German].

FOOTNOTE

<footnote1>
The occlusion time was determined on the basis of the number of annual increments from the moment of the pruning of trees to the moment of knot occlusion

 

Accepted for print: 10.10.2006

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Dieter Franciszek Giefing
Department of Forest Utilization,
University of Life Sciences in Poznań, Poland
Wojska Polskiego 71A, 60-625 Poznań, Poland
Phone: +48 61 848 77 54
email: giefing@au.poznan.pl

Marta Złota
Department of Forest Utilization,
August Cieszkowski Agricultural University of Poznan, Poland
Wojska Polskiego 71A, 60-625 Poznan, Poland

Paweł Stypik
Department of Forest Utilization,
August Cieszkowski Agricultural University of Poznan, Poland
Wojska Polskiego 71A, 60-625 Poznan, Poland

Paulina Wykpisz
Department of Forest Utilization,
August Cieszkowski Agricultural University of Poznan, Poland
Wojska Polskiego 71A, 60-625 Poznan, Poland

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