RADIAL VARIABILITY OF THE STRENGTH QUALITY COEFFICIENT OF SCOTS PINE (PINUS SYLVESTRIS L.) WOOD IN RELATION TO THE TREE BIOSOCIAL POSITION IN THE STAND

Marcin Jakubowski¹, Arkadiusz Tomczak², Tomasz Jelonek², Witold Pazdrowski^2
1 Department of Forest Utilization, August Cieszkowski Agricultural University of Poznan, Poland
² Department of Forest Utilization, University of Life Sciences in Poznań, Poland

ABSTRACT

In their study, the authors made an attempt to determine the radial variability of the strength quality coefficient of Scots pine (Pinus sylvestris L.) wood in relation to the biosocial position of trees in the stand.

The performed investigations comprised trees representing the first three Kraft classes from the pine stands of the age classes V and VI growing in conditions of the following forest site types: fresh coniferous forest (FCF) and mixed fresh coniferous forest (MFCF).

The experiments revealed that the growth site conditions and the biosocial tree position in the stand exerted some influence on the radial variability of the strength quality coefficient of Scots pine (Pinus sylvestris L.) wood. Both the size of the tree crown as expressed by its volume and, first and foremost, the height of the position of the crown centre of gravity were important.

Key words: strength quality coefficient, radial variability, forest site type, age class, biosocial position.

INTRODUCTION

The extent to which wood is utilized as construction material depends, to a considerable degree, on the lightness of the material accompanied by its high strength properties as expressed by its strength quality coefficient, which takes into consideration not only wood strength but also its density [12, 14]. It is commonly accepted that the higher the wood strength and the lower its density, the higher becomes its technical value.

The most valuable construction timber is obtained from the material called `the primary material´. Theoretically, the primary material derives from the part of the log, which is confined by the square written into the cross-section of the log [3, 18].

The tree trunk tissue well adapted to actively counteract tree deformations resulting from its own weight as well as the action of outside forces, e.g. the wind, is characterised by a modified structure tailored to withstand greater loads in order to protect the tree against damage, for example, breaking. Growing in a dense community, the tree is characterised by greater deflections in the upper parts of the trunk. After opening up the stand in the result of, among others, the realization of tending operations (cleanings, thinnings), the places of deflections are lowered and the entire tree begins to sway, including the bottom part of the trunk [4]. Therefore, the above-mentioned factors contribute to the development of the tree tissue of increased strength and to the formation of the trunk shape that is capable to prevent the breaking of the tree at various levels of its height, especially in the plane of the maximum bending moment, namely the root collar.

The objective of the research project was to assess the radial variability of the strength quality coefficient of Scots pine, i.e. the main forest-forming tree species, which provides the majority of the construction wood, set against tree growth conditions and their biosocial positions in the stand.

RESEARCH MATERIAL AND METHODS

Investigations were carried out in the Miastko Forest District in pine stands of the age classes V and VI growing in conditions of the following forest site types: fresh coniferous forest (FCF) and mixed fresh coniferous forest (MFCF). Breast height diameters of all growing trees as well as their heights proportionally to the number in the assumed (2 cm) size gradations were measured on the established experimental plots of 0.5 ha each. Having at their disposal the diameter-height characteristics of trees and using the Urich ll dendrometric method [5], the authors calculated dimensions of mean sample trees. Next, on each of the experimental plots, trees with dimensions corresponding to the calculated model trees were identified. The entire experimental material was represented by 12 trees, of which 6 trees represented the site type of fresh coniferous forest and the other six - mixed coniferous site type. The trees derived from the three first Kraft classes, two trees from each class. The study employed the simple Kraft classification in which the biosocial position of the tree in the population is assessed. This classification assumes that the tree growth dynamics in the stand is expressed by its position and the crown construction.

The biosocial position of the tree portrays excellently its developmental potentials in the stand and is very likely to have an impact on the properties of the developing tree tissue.

In the next stage of the investigations, the identified trees were felled. After felling the mean sample trees, the length of the part of the bole from the plane of cutting to the base of the live crown as well as the length of the live crown itself were measured. These data allowed the authors to determine the height of the position of the crown centres of gravity of the model trees. In addition, these trees were also used to collect material for further investigations. Wood for physico-mechanical investigations was sampled from pith boards 25 mm wide. All samples derived from the part of the board corresponding to the area of the breast height diameter, i.e. the height of about 1.3 m on the direction north-south.

The distribution of the samples collected for investigations of the analyzed properties is presented in Figure 1.

Fig.1. Diagram of sample distribution collected for investigations on wood physical and mechanical properties (S - sample with moisture content 0%; M - sample with moisture content above the point of fibre saturation)

In the course of further research, conventional density was ascertained using, for its determination, the stereometric method [20].

The compression strength along wood fibres was determined on the Tira Test 2300 testing machine equipped in the Matest Service Company computer software. All assays were carried out with 0.01 MPa accuracy. The sample strength (designated with symbol M) was determined at the wood moisture content above the fibre saturation point [19, 21]. This strength, also referred to as `the strength of wet wood´ or `basic strength´, shows the true wood quality as construction material and depends only on primary bonds [6, 7, 8].

On the basis of the obtained wood densities and compression strengths along fibres, the strength quality coefficient (J) was calculated. This parameter, referring in the described investigations both to the density and strength, was used as the basic element for the evaluation of the wood technical value. The radial variability of this coefficient at the height of 1.3 m, i.e. at the level of the breast height diameter of trees which grew in different site conditions and represented different Kraft biological classes was subjected to detailed analysis.

The obtained empirical material was analyzed using methods of mathematical statistics employing, for this purpose, the Statistica 6 PL statistical program.

RESEARCH RESULTS

The obtained research results show that the value of the strength quality coefficient (J) changed considerably depending on site conditions and Kraft biological classes. The mean value of the coefficient in conditions of the fresh coniferous forest (FCF) was 4.78 km, whereas in the conditions of mixed fresh coniferous forest (MFCF) - 4.17 km (Fig. 2). However, the complete picture emerged only when its value in both forest site types was compared with Kraft biological classes. In the case of the first Kraft class, the mean value of the coefficient (J) on the FCF was 4.98 km, while on the MFCF - 3.82 km. This is quite a considerable difference, bearing in mind a very small variability of this trait (variability coefficient: 4.64% for the FCF and 5.75% for the MFCF). The mean values were also significantly different in the second Kraft class (4.51 km for the FCF and 4.15 km for the MFCF), but the variability coefficient was greater in this case (Tab. 1). However, the differences between mean values of the strength quality coefficient as affected by the site were the smallest in the third Kraft class. In the discussed cases, differences in mean values turned out statistically significant in the first Kraft class on the FCF. On the other hand, in the case of the MFCF, statistically significant differences were found between the first and third Kraft classes as well as between the second and the third Kraft class (Tab. 2).

Fig.2. Mean values of the strength quality coefficient (J) in relation to the forest site type and Kraft biological class

Table 1. Descriptive statistics of the strength quality coefficient (J) of the wood of pine trees (Pinus sylvestris L.) developed in different site conditions and occupying different biosocial positions in the stand

Measures of position and dispersion	1 Kraft class	1 Kraft class	2 Kraft class	2 Kraft class	3 Kraft class	3 Kraft class
	FCF	FMCF	FCF	FMCF	FCF	FMCF
	J	J	J	J	J	J
Mean [km]	4.9800	3.8219	4.5069	4.1519	4.8573	4.6797
SD [km]	0.2315	0.2193	0.5299	0.4567	0.1650	0.7318
Median [km]	4.9572	3.8693	4.4720	4.1291	4.8900	5.0283
Maximum [km]	5.3484	4.1269	5.5542	4.8501	5.0259	5.3792
Minimum [km]	4.7535	3.4883	3.8829	3.3489	4.5819	3.7123
Var.coeff. [%]	4.6495	5.7388	11.7568	10.9990	3.3970	15.6378

Table 2. Results of the LSD test for the strength quality coefficient Results are significant at the level p£ 0.05.

			Kraft biological class
			1	2	3	1	2	3
			FCF	FCF	FCF	FMCF	FMCF	FMCF
Kraft biological class	1	FCF		0.0485	0.6132	0.0000	0.0011	0.2205
	2	FCF	0.0485		0.1546	0.0055	0.1339	0.4776
	3	FCF	0.6132	0.1546		0.0001	0.0060	0.4819
	1	FMCF	0.0000	0.0055	0.0001		0.1625	0.0011
	2	FMCF	0.0011	0.1339	0.0060	0.1625		0.0352
	3	FMCF	0.2205	0.4776	0.4819	0.0011	0.0352

However, the mean values alone cannot be relied on as dependable indicators of changes occurring in the value of the coefficient (J) in different Kraft classes and different site conditions. A very interesting situation was observed when the mean values of the discussed coefficient on the radial cross-section were presented graphically (Fig. 3). The diagrams show the change in the value of the coefficient (J) in relation to the relative distance from the tree pith and the maxima deserve special attention. The maximum in the first Kraft class, in the case of the FCF, happened to occur at a very small distance from the pith (about 0.2 length of the radius), whereas in the case of the MFCF - it was closer to the circumference than the pith (about 0.6 of the relative radius length). The situation was identical in the second Kraft class; on the FCF, the maximum occurred at the distance of 0.4 of the radius counting from the pith, while on the MFCF - at the distance of 0.8 of the radius. Only in the third Kraft class, it was difficult to notice similar trends.

Fig.3. Values of the strength quality coefficient (J) of pine tree wood on the radial cross-section in conditions of the fresh coniferous forest and mixed fresh coniferous forest in the three first Kraft biological classes

At this stage of investigations, a question arose: what caused the observed course of the strength quality coefficient? The coefficient was calculated on the basis of the conventional density and the compression strength along fibres determined at the moisture content above the saturation point of cell walls. This was, therefore, the moisture content which described perfectly the conditions occurring in living trees. Apparently during their lifetime, conditions must have existed which resulted in this particular development of the tissue in tree stems. From the natural point of view, the answer must probably be sought in the developmental stages of trees and the performed tending operations, whereas from the point of view of bole mechanics - in the formation of the crown, the bole, the influence of the tree weight, the effect of such outside forces as, for example, wind. Bearing in mind the above-mentioned factors, the size of crowns of the examined trees at the moment of cutting, i.e. at this particular stage of development, was analyzed.

Considerable differences were found in tree crown volumes in individual Kraft classes. The mean crown size characterised by the volume measure was: 393 m³ in the first Kraft class, 218 m³ - in the second and 175 m³ - in the third. Furthermore, the size of the crown from individual Kraft classes was also influenced by the considered forest site types (Tab. 3). When analyzing the tree crown size alone, the authors failed to observe any close connection with the recorded changes in the coefficient (J) on the cross-section along the radius. Therefore, it was assumed that the changes could be associated with the height of the placement of the crown centre of gravity on the bole and since, in the methodology, the authors assumed the crown volume to be that of the cylinder volume [16, 17], the centre of gravity was assumed to be situated half through the length of the crown. In all of the analyzed cases, the centre of gravity was placed higher for trees which grew in conditions of the MFCF (Tab. 3).

Table 3. Mean crown size and the height of the placement of its centre of gravity in the bole of the examined mean sample trees

Forest site type	Kraft biological class	Crown volume [m3]	Height of the position of the crown centre of gravity [m]
FCF	1	473	20.02
	2	223	20.00
	3	167	19.37
FMCF	1	313	20.85
	2	214	21.25
	3	182	20.60

Within sites, differences in the height of placement of the centre of gravity in all Kraft classes were at the level of about 1 meter. Probably this value was sufficient to create an appropriately greater bending moment, which led to the development of wood tissue more resistant to loads in the circumferential part of the trunk. It can be presumed that, during their earlier stages of development, the examined trees occupied biosocial positions similar to those they had before cutting and, therefore, had similar crown proportions.

The obtained results indicate that the initiated investigations should be continued. Tree tissues on the radial cross-section are characterized by considerable heterogeneity of structure and properties. Our investigations showed that the observed heterogeneity depended, to a large extent, on the tree growth and development factors but also on the applied tending operations in the individual stand developmental stages. A better understanding of mechanisms affecting the quality of the developing wood tissue may allow us to control the process of wood production and a better utilization of the wood raw material. In addition, such experiments may also be helpful in investigations on damages of living trees, especially those caused by high winds.

DISCUSSION

The strength quality coefficient plays an important role in the selection and application of construction materials. Timber technical value depends on its strength and density [13, 14] - the higher its strength and the lower its density - the higher is the wood technical value. The analysis of values of the (J) coefficient in studies on wood quality carried out in the moisture content above the saturation point of the cell wall allow depicting its quality in natural conditions without taking into account secondary bonds, which depend on changes in wood moisture content. The choice of the discussed parameters seems appropriate with regard to the type of the correlations determined in the wood such as: the impact of the biosocial position occupied by the tree in the stand and the height of placement of the centre of the crown gravity on wood strength and density presented in the calculated coefficient (J). According to studies carried out by other researchers [15, 22], both the wood density and strength of coniferous trees on the radial cross-section increase in the direction from the pith towards their circumference. However, the majority of these investigations focus on the analysis of the means alone, which refer to larger populations. In this system, the results are perfectly acceptable and in agreement with the physiological development of the wood tissue. The reported results point to differences of the strength quality coefficient in relation to the forest site type. It is quire probable that site conditions influenced the obtained results only indirectly. It appears that it was the shape of tree crowns in combination with the biosocial position of trees in the stand that exerted the direct influence on the modification of the cell wall structure in the course of the process of the wood tissue formation. This is proved by the height of placement of the crown gravity centre, which was higher in conditions of the mixed fresh coniferous forest. This, in turn, led to greater bending moments occurring in tree boles resulting in the increased resistance of the wood tissue to the compression and tensile stresses in the circumferential part of the trunk. The described interrelationships should be understood only as suggestions of certain phenomena taking place in nature since there were infinite other factors which, to a smaller or greater extent, also exerted some influence on the obtained results. At this stage of research, there are some cognitive gaps and that is why it appears necessary to continue investigations on the interrelationships between wood microstructure and factors affecting it.

The radial variability of wood tissue properties, including the strength quality coefficient (J) on the bole cross-section should also be associated with the occurrence of initial stresses in the tree trunk [1, 2, 9, 10, 11]. The newly developed wood exhibits a tendency for shrinkage, while the existing wood resists it. Because of the state of equilibrium, the sum of tensile stresses must be equal to the sum of compression stresses. It seems that the initial stresses play a very important role in the tree trunk mechanics by increasing their resistance to gusts of winds. The value and distribution of these stresses change in the tree trunk together with its growth and development. [10, 11].

CONCLUSIONS

The performed investigations allowed the authors to draw the following conclusions:

1. The mean value of the strength quality coefficient of pine trees in all the analysed Kraft classes was higher on the less fertile site than on the more fertile one. The statistically significant difference was found in the first Kraft class on the FCF. On the other hand, on the MFCF, statistical differences were recorded between the first and third as well as between the second and third Kraft classes.

2. A distinct radial variability of the strength quality coefficient occurred on the bole cross-section (at the level of the breast height diameter). At a certain distance, in the direction from the pith to the circumference, a distinct increase of its value occurred and after reaching its maximum, its value dropped again. This regularity was independent of site conditions.

3. Site conditions were decisive regarding the location on the bole cross-section radius of the maximum value of the strength quality coefficient. On the less fertile site, the maximum occurred closer to the pith, whereas on the more fertile site, the maximal value of the coefficient was identified at the distance distinctly further away from the pith.

4. The tree biosocial position in the stand exerted an important impact on the value variability of the strength quality coefficient on the cross-section radius of the tree but the observed influence occurred with different strength and varying intensity.

5. The tree crown size expressed by its volume as well as the height of the placement of its centre of gravity were also found to exert a significant impact on the radial variability of the strength quality coefficient (J) on the tree cross-section.

6. Because of the complexity of the investigated problem and its cognitive and practical importance, it is advisable to continue this type of research in order to better understand the above regularities.

REFERENCES

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8. Grzeczyński T. 1985. Zależność wytrzymałości drewna od wartości kurczenia się jego przekroju obciążonego [Dependence of wood strength on the shrinkage value of its loaded cross-section]. Przemysł Drzewny nr 2, 24 - 26 [in Polish].

9. Hejnowicz Z. 1973. Anatomia rozwojowa drzew [Developmental anatomy of trees]. PWN Warszawa [in Polish].

10. Hejnowicz Z. 1980. Anatomia i histogeneza roślin naczyniowych [Anatomy and histogenesis of vascular plants]. PWN Warszawa [in Polish].

11. Hejnowicz Z. 2002. Anatomia i histogeneza roślin naczyniowych [Anatomy and histogenesis of vascular plants]. Wyd. Naukowe PWN Warszawa [in Polish].

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13. Kollmann F.F.P., Côte W. A. 1968 Principles of wood science and technology, vol. I Solid Wood. Springer - Verlag Berlin Heidelberg New York.

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15. Kučera B. 1994. A hypothesis relating current annual height increment to juvenile wood formation in Norway spruce. Wood and Fiber Science 26/1, 152 - 167.

16. Lemke J. 1966. Korona jako kryterium oceny dynamiki wzrostowej drzew w drzewostanie sosnowym [Crown as the evaluation criterion of tree growth dynamics in a pine stand]. Folia Forestalia Polonica, seria A, z. 12. 185 - 236.

17. Lemke J. Wodniak A. 1976. Struktura niektórych cech ilościowych korony w drzewostanach sosnowych [Structure of some quantitative crown traits In pine stands]. PTPN, Prace komisji Nauk Roln. i Komisji Nauk Leśnych, t. XIII, 75 - 83.

18. Milewski A. 1970. Materiały i wyroby z drewna [Wood materials and products]. PWE Warszawa.

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21. Polska Norma - PN - 79/D - 04102 Drewno. Oznaczanie wytrzymałości na ściskanie wzdłuż włókien [Polish Standard PN - 79/D - 04102 - Wood. Determination of compression strength along fibres].

22. Thörnqvist T. 1993. Juvenile wood in coniferous trees. Document D. 13. Uppsala.

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Marcin Jakubowski
Department of Forest Utilization,
August Cieszkowski Agricultural University of Poznan, Poland
Wojska Polskiego 71 A, 60-625 Poznan, Poland
email: nicram@au.poznan.pl

Arkadiusz Tomczak
Department of Forest Utilization,
University of Life Sciences in Poznań, Poland
Wojska Polskiego 71 A, 60-625 Poznań, Poland
Phone: (+48 61) 8487754
email: atomczak@au.poznan.pl

Tomasz Jelonek
Department of Forest Utilization,
University of Life Sciences in Poznań, Poland
Wojska Polskiego 71 A, 60-625 Poznań, Poland
Phone: (+48 61) 8487754
email: tjelonek@au.poznan.pl

Witold Pazdrowski
Department of Forest Utilization,
University of Life Sciences in Poznań, Poland
Wojska Polskiego 71 A, 60-625 Poznań, Poland
Phone: (+48 61) 8487757
email: kul@au.poznan.pl

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