EFFECT OF THE REDUCTION OF THE ASSIMILATION APPARATUS FOLLOWING THE PRUNING TREATMENT OF SCOTS PINE (PINUS SYLVESTRIS L.) ON THE TRACHEID LENGTH OF THE EARLY AND LATE WOOD

Witold Pazdrowski¹, Jarosław Szaban^2
1 Department of Forest Utilization, University of Life Sciences in Poznań, Poland
² Department of Forest Utilisation, University of Life Sciences in Poznań, Poland

ABSTRACT

This study constituted an attempt to assess the influence of the reduction of the assimilation apparatus caused by the reduction of the length of live tree crowns following the pruning treatment of Scots pine (Pinus sylvestris L.) on the length of tracheids of early and late wood developed after the treatment. The authors analysed changes in the length of wood tracheids in years following the pruning treatment in relation to the extent of the crown reduction.

Key words: pruning, length of tracheids, early wood, late wood.

INTRODUCTION

When considering wood as construction material, it is important to take into account its microscopic structure. The shape and dimensions of tracheids, which make up over 90% of the volume of coniferous timber [5], have a significant influence on its technical quality. Their development depends on numerous outside factors [1, 8, 12]. The structure and quality of the developing wood anatomical elements determine its physico-mechanical properties [9]. The knowledge of interrelationships between the reduction of the assimilation apparatus and the quality of the developing wood tissue may have both cognitive and utilitarian significance. These relationships may find a practical expression in the recommendations for manufacturers of wood (foresters), its receivers (wood processing industry) and designers of wood constructions.

Investigations on pruning, which have been carried out so far, focused mainly on issues connected with either increment losses or the value increase of the timber raw material. The absence of research results concerning the impact of the reduction of the assimilation apparatus on the structure of the developing wood tissue encouraged the authors to conduct such experiments.

The starting point, which initiated the investigations, was the assumption that the reduction of the live tree crown during the pruning process of Scots pine may influence the wood tissue depositing in the tree in the years following the operation. It was assumed that the removal of part of the assimilation apparatus, in other words the artificial `shifting´ of the base of the live crown upwards may change or interfere with the process of the formation of tracheids of both early and late wood. From the point of view of technical properties of wood tissue and, hence, also technical properties of wood, this impact may be beneficial, neutral or harmful.

The objective of this research project was to assess the effect of the reduction of the assimilation apparatus expressed by the shortening of the live tree crown length during the pruning operation of Scots pine (Pinus sylvestris L.) on the length of tracheids of the early and late wood developed after the treatment.

METHOD AND DESCRIPTION OF THE PERFORMED EXPERIMENTS

Materials used in the investigations were collected in the Forest Experimental Station Murowana Goślina (Wielkopolska-Kujawy Province, Region lll Wielkopolska-Pomorze) from the area on which the live tree crown reduction was carried out. When classifying areas for investigations, the authors decided to select only the position with regard to which there was documentation containing reliable information concerning the degree of reduction of the live crown. Eventually, the material for investigations was collected from the experimental area established by professor K. Suchocki in the compartment 83C in the Potasze Forest Range, of the Zielonka Forest District. This experimental area was established in February 1951, in a 14-year old sapling pine stand.

The experimental area had the shape of a rectangle divided into ten plots of 20 ares each. In March 1951 trees on 8 plots were pruned, whereas two plots were left unpruned as controls. Four levels of the intensity of live tree crown reduction were applied on the pruned plots, namely: up to 1/4, 1/3, 1/2 and 2/3 of the live crown height. As two plots fell for the same level of pruning, the plots were arranged in such a way that the plots with the same pruning intensity were never next to each other.

The selected experimental area was particularly useful with regard to the main theme of this study, since there is full documentation of the performed pruning, detailed description of the performed operations, precise specification of the extent of the applied treatment and an interesting photographic documentation. When looking through the documentation, the authors came across the photograph, which shows the experimental area one year after the treatment. It was evident, both from the photograph and the accompanying description, that the pine trees, which were pruned most severely, developed dwarf shoots. This phenomenon as well as the analysis of the photographic documentation appears to imply that important changes must have taken place in the organisms of the pruned trees, which could have influenced the wood tissue developed in the trees later on. These reflections led to the development of the working hypothesis mentioned above.

Measurements of breast height diameters and heights of trees on the experimental area were carried out. Having obtained the diameter-height characteristics, the authors proceeded to calculate dimensions of samples trees using the Urich ll method [2].

The total of thirty trees was selected in the experimental area.

The trees were always selected from the centre of the given plot trying to avoid the direct influence on their growth of the neighbouring stand. All sample trees were marked off permanently with paint. Next, using the Presler´s increment borer of 12 mm diameter and 40 cm length, one increment core was taken from each tree. The increment cores were taken from the height of 1.3 m above the ground and always from the north direction in an attempt to avoid variations in the wood structure associated with the position in the tree trunk. The wood material was collected in November 1995.

After sampling, the wood material was stored for several months at the temperature of 5^oC with the aim to dry the collected wood properly and to avoid its depreciation.

Every increment core was measured with an increment meter which allowed determining the value of the linear increment of early and late wood. Measurements were performed using the following principle:

• increment No.1 - the first increment counting from the bark, i.e. the increment from 1994 (the 44^th increment after pruning)

• increment No.2 - the second increment counting from the bark... etc.

The length of tracheids was measured from each bore hole on the first increment before the pruning treatment (1950), on the increment from the year of pruning (1951) as well as on the second, third, sixth, eleventh and forty fourth increments after pruning.

Each annual ring was divided into two parts:

• early wood,

• late wood.

This wood was subjected to de-fibering in nitric acid (HNO₃) with an addition of potassium perchlorate (KClO₃). After heating in a test-tube, bundles of tracheids were obtained which were rinsed in distilled water, fixed in alcohol and dyed with neutral safranine. The last of the treatments was performed to facilitate the observation of the examined tracheids in a microscope. This material was used to make semi-permanent preparations. which were encapsulated in glycerine and then measured.

Measurements were made separately for the early and late wood in individual increments from the following years:

1950 - one year before pruning
1951 - the year when the treatment was carried out (early spring) and the first increment was deposited,
1952 - one year after the treatment - second increment after treatment,
1953 - 2 years after the treatment - third increment after treatment,
1956 - 5 years after the treatment - sixth increment after treatment,
1961 - 11 years after the treatment - twelfth increment after treatment,
1991 - 41 years after the treatment - forty second increment after treatment.

RESEARCH RESULTS

Length of tracheids from early wood

The authors analysed changes which occurred in the anatomical structure (length) of tracheids with the passage of time after the pruning treatment. Following the performed one-factorial analyses of variance, separately for each reduction variant, statistically significant differences were found in the length of tracheids in the following analysed years: 1950 (one year before the performed treatment), 1951, 1952, 1953, 1956, 1961 and 1991.

Next, using the Dunnet procedure, mean early wood tracheids dimensions in successive years after reduction treatment were compared with the mean wood tracheids dimensions from 1950, which was adopted as the control.

The performed investigations revealed, for the 1/4, 1/3, 1/2 as well as the control, lack of variations (at the level of α = 0.05) in the tracheid lengths of the early wood from 1950, 1951 and 1952. Beginning with the third year after the treatment, it was possible to see that mean lengths of tracheids were significantly greater than in 1950 (Fig. 1). Differences in the mean tracheid length from 1950 and consecutive years were compared with the least significant difference at the significance level of α = 0.05.

A very different picture emerges, in relation to the remaining variants, when the change of the mean tracheid length in the strongest variant of the live tree crown reduction is considered. Already in the first year after the reduction of the assimilation apparatus, the mean tracheid length of the early wood is significantly greater than their mean lengths from 1950. The mean length of tracheids in 1950 was 1003.9 μm, whereas in 1951 - 1307,5 μm. During the subsequent years, the mean tracheid lengths increased gradually and were significantly greater than in 1950.

The analysis of the length of tracheids of early wood in individual variants of the reduction shows that it was only when the length of tree crowns was reduced by 2/3 that we can observe a rapid increase of the length of tracheids.

Applying the Tuckey procedure to compare simultaneously mean tracheid lengths in the second and third years after the treatment and comparing differences between means with the appropriate values of the least significant differences, the authors arrived at the same conclusion, namely that mean tracheid lengths for the variant of the 2/3 reduction were significantly greater in comparison with the mean tracheid lengths of the earlywood in all the remaining reduction variants.

The examination by means of the Tuckey procedure of mean tracheid lengths of the early wood of individual variants in the consecutive years, it is possible to notice the absence of the regularity observed during the first three years after the treatment, i.e. the greater length of tracheids in the 2/3 variant. Beginning with the sixth year after the treatment, the variability of mean tracheid lengths of the early wood was different in the consecutive years and it can be assumed that the impact of pruning on the tracheid length of early wood, with the exception of the first few years, was slight. It appears that, in this period, it is the diversification of the position of tress in the stand that exerts a more significant influence on the length of tracheids (maturation of the wood tissue).

Length of tracheids from late wood

Identical procedures were applied to analyse changes taking place in the tracheid anatomical structure of late wood with the passage of time after the pruning operation. Also in this case, single factorial analyses of variance for the individual variants of crown reduction were performed. As in the case of wood tracheids of the early wood, the following years were the source of variability of the examined trait (length of tracheids): 1950 (a year before the treatment), 1951, 1952, 1953, 1956, 1961 and 1991.

Next, using the Dunnet procedure, mean late wood tracheid dimensions in the successive years after the reduction treatment were compared with the mean wood tracheid dimensions from 1950, which was adopted as the control.

For the control surface (not pruned) as well as for the 1/4 and 1/3 crown reduction variants the analysis showed that mean tracheid lengths revealed highly significant differentiations in the examined years. For the above crown redaction variants, using the Dunnet procedure, no variation (at the level of α = 0.05) in the tracheid lengths of the late wood from 1950, 1951 and 1952 was observed. Beginning with the third year after the treatment, it was possible to see that mean lengths of tracheids were significantly longer than in 1950.

The length of tracheids of the late wood in the consecutive years after the treatment for the 1/2 live crown reduction variant showed a highly significant variability. For the 1/2 variant, no significant variability was observed in the mean tracheid lengths between year 1950 and 1951, 1952 and 1953. In the same variant, the tracheid length, beginning with 1956, was significantly greater in comparison with their lengths in 1950.

A different picture, in relation to the remaining variants, emerges in the case of changes in the mean tracheid lengths of the late wood in the strongest crown reduction variant. Already in the first year after the reduction of the assimilation apparatus, the mean tracheid length was significantly greater than that of 1950 (Fig. 2). In the following years, mean tracheid lengths increased gradually and were significantly greater than in 1950.

Using the analysis of variance to compare mean tracheid lengths of the late wood between individual reduction variants separately for each year, it was possible to notice a lack of variation of the mean tracheid lengths between individual reduction variants for 1950 and highly significant differences in all the remaining years. Applying the Tuckey procedure to compare simultaneously means of tracheid lengths in the second year after treatment and comparing differences between means with appropriate values of the least significant differences, it was concluded that the mean tracheid lengths for the 2/3 reduction variant were significantly greater in comparison with the mean tracheid lengths of the latewood in all the remaining reduction variants. Analysing the mean tracheid lengths in the third year after pruning and comparing differences between means with appropriate values of the least significant differences it was concluded that mean tracheid lengths for the 2/3 reduction variant continued to be significantly grater than the mean tracheid lengths of the late wood in all the remaining reduction variants. The above relationship also occurred in the increment from 1956 (sixth increment after the treatment)

The examination, by means of the Tuckey procedure, of mean tracheid lengths of the late wood of individual variants in 1961 and 1991, reveals the absence of the regularity observed during the first three years after the treatment, i.e. the greater length of tracheids in the 2/3 variant. Beginning with the eleventh year after the treatment, the variability of the mean tracheid lengths of the late wood was different in the consecutive years and it can be assumed that the impact of pruning on the tracheid length of late wood, with the exception of the first few years, was slight. It appears that, in this period, it is the diversification of the position of trees in the stand that exerted a more significant influence on the length of tracheids (maturation of the wood tissue).

DISCUSSION

The obtained research results allowed concluding that, both in the control variant as well as in the case of the crown reductions by 1/4, 1/3 and 1/2, the mean length of wood tracheids remained at the similar level in the period from three years after the pruning operation. It was only in the case of the strongest pruning variant (shortening of the live crown by 2/3) that the length of tracheids of both early and late wood increased significantly one year after the treatment and they remained significantly longer for several years. Analysing the causes of this phenomenon, one of the most obvious hypotheses is that the strongest reduction of the assimilation apparatus disturbed life processes in the examined trees. Each living organism possesses appropriate protective mechanisms, which allow it to survive in critical situations. Undoubtedly, the reduction of the live tree crown is a stressful situation for the tree. The observed phenomenon of the evident reaction of trees in the form of changes in the length of tracheids following a strong reduction of live crowns can be explained by the concept of the hormonal regulation of the formation of wood annual rings [6]. According to this theory, there is a distinct correlation between the content of auxins transferred from the assimilation apparatus through the phloem to cambium and the formation of tracheids characteristic for early wood. High levels of auxins are associated with the intensive growth and development of shoots and leaves.

In normal conditions, the change of length of wood tracheids in successive annual increments is a natural process. Perhaps the causes of the rapid increase of the length of tracheids should be sought in the decrease of the vital capabilities of trees to such an extent that they were threatened with death resulting from their inability to compensate for the removed crown assimilation surface by the reserve substances found in wood.

Wood anatomical elements (tracheids) developed under the strong influence of the crown (leaves) are shorter and form the so called `juvenile wood [3, 4, 7, 10, 11]. Therefore, lateral branches exert an important influence on the kind of tissue which occurs in the tree trunk. In the case of pine trees whose live crowns were reduced considerably, we can observe their intensive reconstruction in the consecutive years after pruning. Trees deprived of a smaller number of whorls of live branches required a shorter time to rebuild their assimilation apparatus than trees from which a large part of the live crown was removed. Perhaps trees whose crowns were not reduced so severely managed to defend themselves using stored reserve substances. It is also possible that trees which were pruned moderately could defend themselves by increasing quantities of chlorophyll in needles. However, these questions could not be answered by the above-described investigations since too many years elapsed from the time the assimilation apparatus was reduced.

CONCLUSIONS

1. The reduction of live crown by 1/4, 1/3 and 1/2 did not affect the length of tracheids in the early and late wood.

2. A very strong crown reduction (by 2/3) caused statistically significant changes of the length of tracheids already in the first year after pruning in the case of early as well as late wood.

3. In comparison with weaker crown reductions, longer tracheids in both early and late wood following reduction by 2/3 of the live crown remained noticeable until the 6^th year after pruning.

4. The most severe reduction of the crown caused the increase of the length of tracheids and, hence, accelerated the development of mature wood. This, undoubtedly, exerted a positive influence on the technical quality of timber.

5. However, the reduction of tree crowns by 2/3 cannot be recommended in silvicultural practice because of the negative influence of this type of treatment on the condition and developmental possibilities of trees.

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