Density Distribution in Wood of European Birch (Betula Pendula Roth.)

Article Density Distribution in Wood of European Birch (Betula pendula Roth.)

Ewa Dobrowolska, Paulina Wroniszewska and Agnieszka Jankowska *

Institute of Wood Sciences and Furniture, Warsaw University of Life Sciences – SGGW, Nowoursynowska 159, 02-787 Warsaw, Poland; [email protected] (E.D.); [email protected] (P.W.) * Correspondence: [email protected]; Tel.: +48-22-5938634

Received: 26 February 2020; Accepted: 26 March 2020; Published: 15 April 2020

Abstract: The aim of the presented research is to perform a comprehensive analysis of wood density variability on the longitudinal and transverse cross-section of log trees at the age of 70 to 72 years of Betula pendula Roth. and the creation of density distribution maps. Furthermore, the determination proportion of juvenile and mature wood was done. Wood density was determined with a non-destructive method using an isotopic densimeter. It was found that the wood location, both in cross-section and longitudinal section of the trunk, had a statistically significant effect on the average density of birch wood. The average density of whole logs was significantly higher than the average density at the breast height. On the cross-section, the distribution of average densities determined at the breast height, as well as on 1/4 of the log height, properly depicted the distribution of average densities on the cross-section determined for the whole logs. The geographical direction (north–south) did not have a statistically significant effect on the distribution of average densities on the cross-section of the tested birch logs.

Keywords: density distribution; European birch; Betula pendula Roth.

1. Introduction Most of the basic physical, mechanical, and technological properties of wood depend on its density. Synthetically, it expresses many features related to the wood anatomy as well as physical and chemical structures [1]. The density is influenced by a number of factors such as wood species, wood moisture content, annual growth rings and proportion of late-wood, age of the tree and its height, location within the trunk’s longitudinal and longitudinal cross-section, as well as habitat conditions [2–7]. From the analysis of numerous studies on the wood density of various species, the relationship between the width of annual growth and density was not found in the case of deciduous wood species [2,3,5,7–9]. Additionally, the independence from the cross-sectional area of the trunk was confirmed [5]. It is also believed that the wood density of these species increases from trunk pith to the bark [3]. According to Krzysik [2] and Niemz [9], the density minimum of deciduous trees is associated with juvenile wood. The influence of the proportion of juvenile wood, mature wood, and wood overridden on the distribution of wood tissue density on the cross-section was also found. Juvenile wood is a part of trunk created by cambium cells in the first years of the life of the tree and constitutes the central part in the form of a cylinder, consisting of several or even several dozen of annual growth rings [10]. The width of this zone is influenced by many factors such as species, genetic conditions, tree growth rate, type of habitat, planting, distance and age of the cambium, geographical location, breeding conditions, as well as the place of the tree in the stand [6,10–14]. According to Hakkilla [11], the proportion of juvenile wood should be considered during the analysis of wood density. The identification of the juvenile wood zone mainly consists of the characterization of structural elements of wood tissue.

Forests 2020, 11, 445; doi:10.3390/f11040445 www.mdpi.com/journal/forests Forests 2020, 11, 445 2 of 14

In the ring-porous wood species, vessels in juvenile wood are distributed over the entire width of the annual rings as in case diffuse-porous wood species. Another feature that occurs in juvenile wood is the type of perforation in the vessels. In juvenile wood vessels with a simple perforation, ladder perforation was noticed [10]. Mature wood is created by cambium cells when they reach their maximum dimensions. This part of the wooden tissue is characterized by relatively constant cell dimensions and properties [15]. Compared with mature wood, juvenile wood usually stands out by the smaller length of anatomical elements and the proportion of late-wood, which is associated with lower density and worse mechanical properties [6,14,16–18]. Depending on the class and wood species, the strength of juvenile wood constitutes from 50% to 70% of the strength and hardness of mature wood [19]. Wood density is one of the most important technological parameters defining its industrial use. The popular species list includes birch wood (Betula) from the birch family (Betulaceae)[20], occurring in almost all of Europe, with the exception of Spain, Greece, and the southern part of Italy, as well as in parts of Asia Minor, Caucasus, and western Siberia. Birch is a pioneer species, rapidly growing and plays an important role in the early stages of primary and secondary succession, as well as in preparing areas for settlement by more demanding species [21]. Moreover, birch belongs to the basic species in plantations of fast-growing forest trees [22]. That wood species is used in industrial applications for the production of plywood, veneers, and furniture, and is a valuable raw material for the pulp, paper, and board industry [3,23]. Due to the significant economic importance of birch wood, many studies on the distribution of density in birch wood have been found in the literature, particularly depending on its age and diameter [7,24,25]. However, so far, extensive research regarding birch wood density has been conducted on relatively young wood dominated by juvenile wood zone. Only a study on wood from mature Finnish trees has been found [26]. The aim of this research is to perform a comprehensive analysis of wood density variability on the longitudinal and transverse cross-section of log trees at the age of about 70 years of European birch (Betula pendula Roth.) and the creation of density distribution maps. Moreover, the determination proportion of juvenile and mature wood was made, as well as determination of the size and range of knots in both zones. Wood density was determined with a non-destructive method using an isotopic densimeter.

2. Materials and Methods For research, five birch (Betula pendula Roth.) logs were used, originating from north-eastern Poland. The trees were 70 to 72 years old, from the first bonitation and from the first biosocial class, according to Kraft, the so-called towering trees. The height of the trees was about 22 m, with a crown index of 0.3 and diameter at breast high approximately 42 cm (Table1). Trees were divided into the logs, which have been sawn with respect to the geographical directions (north, south). Directly to the research, core timber were selected in the total number of 69. After drying and planing, the dimensions of the samples were width approximately 70 mm, length approximately 1.5 m, and the moisture content was 12%.

Table 1. Characteristics of experimental trees of silver birch (Betula pendula Roth.).

Log Number Feature I II III IV V Age * (years) 71 72 72 70 70 Diameter at breast high in bark (cm) 42 42 42 42 42 High (m) 22.8 21.5 22.0 21.5 21.5 Crown index ** 0.3 0.3 0.3 0.3 0.3 * Based on the number of growth rings on the cross-section of the lowest part; ** the ratio of the length of the crown to the height of the tree. Forests 2020, 11, 445 3 of 14

The density of wood was determined using the MGD-05 isotope density meter (resolution Forests 2020, 11, x FOR PEER REVIEW 3 of 14 0.1 kg/m3) by using energy-transmission gamma 59.5 keV (Amerycium 241Am). The diameter of used radiationradiation collimator collimator was was 5 mm. 5 mm. The The distance distance ofof thethe radiation source source from from the the detector detector was was 150 mm. 150 mm. The timeThe oftime a single of a single measurement measurement was was 30 s. 30 The s. The result result of aof single a single measurement measurement was was subject subject to to an an error 10 kgerror/m3 ±[ 2710]. kg/m The3 diagram[27]. The diagram of the isotope of the isotope X-ray densityx-ray dens meterity meter MGD-05 MGD-05 is given is given in in Figure Figure1. 1. ±

Figure 1. Diagram of the isotope X-ray density meter MGD-05, where Z—source of radiation Figure 1. Diagram of the isotope x-ray density meter MGD-05, where Z—source of radiation (241Am), (241Am), SS—scintillation probe, A—ampliﬁer of pulses, E1–discrimination threshold of automatic SS—scintillation probe, A—amplifier of pulses, E1–discrimination threshold of automatic gain gain control, E2—measurement path discrimination threshold, PL1—pulse counter of the measuring control, E2—measurement path discrimination threshold, PL1—pulse counter of the measuring path, path,PL2—impulse PL2—impulse counter counter of automatic of automatic gain adjustme gain adjustment,nt, uP—microprocessor, uP—microprocessor, DAC—digital DAC—digital to analog to analogconverter, converter, ZWN—high ZWN—high voltage voltage power power supply, supply, ZNN—low ZNN—low voltage voltagepower supply, power RS—serial supply, RS—serial port, port, USB—USBUSB – USB interface, interface, AK AK—accumulator, – accumulator, PAK—charging PAK—charging rectifier. rectiﬁer.

ExaminationExamination of the of density the density distribution distribution of the of cross-section the cross-section of logs of waslogs carriedwas carried out in out two in directions two (in thedirections north and (in south the north geographical and south directions)geographical along directions) a measuring along a measuring lines (parallel lines to(parallel the longitudinal to the axis oflongitudinal the timber) axis spaced of the in distancetimber) spaced 20 mm. in Thedistance density 20 mm. distribution The density on the distribution longitudinal on the section was examinedlongitudinal along section measuring was examined lines along spaced measuring in distance lines 150 spaced mm, in taking distance into 150 account mm, taking defects into such as knots,account cracks, defects false such heartwood, as knots, cracks, stoppers. false hear Thetwood, density stoppers. maps were The density prepared maps using were the prepared MATLAB using the MATLAB computer program. computer program. Determination of the proportion of juvenile and mature wood was performed by the Determinationmeasurements of annual the proportion growth rings of juvenile width and and wood mature fibers wood length. was The performed measurements by the were measurements made of annualon samples growth taken rings along width the northern and wood radius, fibers using length. samples The obtained measurements from the breast were height. made on samples taken alongThe the width northern of the radius,annual increm usingents samples was marked obtained using from the the BIOTRONIC breast height. electronic gravimetric Theincrement. width ofMeasurements the annual incrementswere made with was an marked accuracy using of 0.01 the mm BIOTRONIC on strips whose electronic surfaces gravimetric were increment.previously Measurements leveled with a were microtome made knife. with anFrom accuracy annual increments of 0.01 mm of on3, 6, strips 9, 12, 15, whose 20, and surfaces beyond, were previouslyfrom every leveled fifth with to the a microtome perimeter, macerated knife. From wood annual samples increments were prepared of 3, 6, to 9, assess 12, 15, the 20, length and beyond, of from everywood fifthfibers. to The the measurements perimeter, macerated were performed wood samplesusing a MICROSCAN were prepared Imager to assess 512 computer the length image of wood analyzer with an accuracy of 0.1 μm. fibers. The measurements were performed using a MICROSCAN Imager 512 computer image analyzer The obtained results (average values from the obtained measurements of the properties of the with an accuracy of 0.1 µm. examined logs) were subjected to statistical analyzes including statistical measures [28,29], one-factor Theand obtainedtwo-factorresults analysis (average of variance values and Tukey’s from the test, obtained as well as measurements two-tailed Student’s of the t-test properties and two- of the examinedsegment logs) regression. were subjected The STATISTICA to statistical PL analyzes computer including program statisticaland the Microsoft measures Office [28 ,29Excel], one-factor 2010 and two-factorprogram were analysis used directly of variance for the statistical and Tukey’s processing test, of as results. well as two-tailed Student’s t-test and two-segment regression. The STATISTICA PL computer program and the Microsoft Office Excel 2010 program3. Results were usedand Discussion directly for the statistical processing of results.

3. Results3.1. Determining and Discussion the Proportion of Juvenile and Mature Wood The characteristics of the annual growth width were made at the breast height. The results are 3.1. Determiningpresented in the Table Proportion 2. The average of Juvenile annual and increase Mature width Wood was 2.6 mm with the coefficient of variation Theequal characteristics to 47%. Table 3 of presents the annual the results growth of the width length were of fiber made measurements. at the breast The height. average The length results of are presented in Table2. The average annual increase width was 2.6 mm with the coe ﬃcient of variation

Forests 2020, 11, 445 4 of 14 equal to 47%. Table3 presents the results of the length of fiber measurements. The average length of Forests 2020, 11, x FOR PEER REVIEW 4 of 14 fibers was 1279 µm (coefficient of variation was 20%). The average length of fibers was within the rangefibers of was numerical 1279 μm values (coefficient contained of variation in the literature was 20%). [29– The31]. average length of fibers was within the range of numerical values contained in the literature [29–31]. Table 2. Characteristics of annual growth (at the breast height) of birch debts.

Table 2. Characteristics ofFeature annual growth (at the breast Value height) of birch debts. FeatureAverage width 2.6 mm Value AverageCoe widthfficient of variation 47% 2.6 mm Coefficient of variationMinimal width 0.4 mm 47% Minimal widthMaximal width 6.5 mm 0.4 mm MaximalAverage width number of annual rings 68 6.5 mm Average number of annual rings 68 Table 3. Characteristics of fiber length. Table 3. Characteristics of fiber length. Feature Value FeatureAverage length 1279 µm Value AverageCoe lengthfficient of variation 20% 1279 μm Coefficient of variationMinimal length 440 µm 20% Minimal lengthMaximal length 2032 µm 440 μm MaximalNumber length of measurements 1380 2032 μm Number of measurements 1 380

DeterminingDetermining thethe proportionproportion ofof juvenilejuvenile andand maturemature woodwood waswas carriedcarried outout basedbased onon thethe measurementmeasurement ofof thethe length length of of wood wood fibers. fibers. TheThe obtained obtained results results were were analyzed analyzed using using two-segment two-segment regressionregression [ 32[32,33],,33], in in which which the the adjustment adjustment of of the the function function in in two two segments segments is is optimal optimal and and the the overall overall estimationestimation error error is is as as small small as as possible. possible. On On thisthis basis,basis,a a juvenilejuvenilewood wood zone zone was was detected, detected, comprising comprising thethe firstfirst 25 25 annual annual growth growth rings. rings. TheThe remainingremaining partpart was was mature mature wood wood (Figure (Figure2). 2). According According to to BonhamBonham and and Barnett Barnett [34 [34],], the the juvenile juvenile wood wood zone zone in Betula in Betula pendula pendulaRoth. woodRoth. coverswood covers the first the 15 annualfirst 15 growthannual rings.growth Di ffrings.erences Differences between the between results justifythe results the taking justify surveys the taking on wood surveys from on diff erentwood types from ofdifferent habitats types or di ffoferent habitats regions. or different regions.

Figure 2. Juvenile and mature wood in the examined birch wood. Figure 2. Juvenile and mature wood in the examined birch wood. The obtained results were not in accordance with numerical values known from the Polish The obtained results were not in accordance with numerical values known from the Polish literature [35], where the size of the juvenile wood zone in birch trees was estimated as 14 annual literature [35], where the size of the juvenile wood zone in birch trees was estimated as 14 annual increments, forming a cylinder with a diameter of 12 to 13 cm, covering the area from 30% to 60% of the increments, forming a cylinder with a diameter of 12 to 13 cm, covering the area from 30% to 60% of cross-section. In the tested birch trees, juvenile wood at the breast height was in the formed a cylinder the cross-section. In the tested birch trees, juvenile wood at the breast height was in the formed a with a diameter of about 16 cm. The proportion of the juvenile wood zone covers approximately 15% cylinder with a diameter of about 16 cm. The proportion of the juvenile wood zone covers of the cross-section. approximately 15% of the cross-section. In the zone of juvenile wood, fiber length increased from about 800 (in core) to 1300 mm (an increase of over 60%), and, in the mature wood zone, the increase was about 17%.

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In the zone of juvenile wood, ﬁber length increased from about 800 (in core) to 1300 mm (an increase of over 60%), and, in the mature wood zone, the increase was about 17%.

3.2. Wood Density Distribution Analysis Results of average density at 12% moisture content are given in Table4. The average density of tested silver birch wood was 623 kg/m3.

Table 4. Results of tests on average density of tested silver birch (Betula pendula Roth.) at 12% moisture content.

Density Average density (kg/m3) 623 Standard deviation (kg/m3) 58 Variability coeﬃcient (%) 9.3 Average density at breast high (kg/m3) 608 Average density at 1/4 high of the log (kg/m3) 630 Minimal value of density (kg/m3) 318 Maximal value of density (kg/m3) 895 Diﬀerence between maximal and minimal value of density (kg/m3) 577 Number of measurements 8320

According to Hakkila [11,36], conventional density in case of pine, spruce, and birch is close to density at 1/4 high of the log. But according to other research [37–39], wood density and other parameters determined from wood at breast height represent average values for all logs. Due to discrepancy of information in this area, a two-sided t-test was done (Table5). The results of statistical analysis conﬁrmed that there was no diﬀerence between average density of all logs and average density at 1/4 high of the log.

Table 5. Two-sided t-test of the diﬀerence between average densities (at signiﬁcance level p = 0.05) for density distribution in birch wood.

Empirical Number of Degrees Critical Statistics Parameter Statistics of Freedom t t 0.05/2; v v Average density of all logs/Average 2.413 1.960 8401 density at breast height Average density of all logs/Average 0.860 1.993 73 density at 1/4 high of the log

3.3. Density Distribution of Silver Birch Wood on the Cross Section The density distribution of all logs of silver birch wood on the cross-section is given in Figure3. The results of a two-factor analysis of variance (Table6) showed the following relations:

geographical direction (north–south) did not have a statistically significant effect on the distribution • of average densities across the entire birch log; distance from the pith had a statistically significant effect on the distribution of average densities • across the entire birch log; there was no significant interaction of geographical direction and distance from the pith. • Miler [40] also showed in his research the insignificance of the influence of the geographical direction on the density of birch wood. Forests 2020, 11, 445 6 of 14 Forests 2020, 11, x FOR PEER REVIEW 6 of 14

FigureFigure 3.3. DensityDensity distributiondistribution ofof allall logslogs ofof silversilver birchbirch woodwood onon thethe cross-section.cross-section. Table 6. Two-factor analysis of variance (at the significance level p = 0.05) for the distribution of average densitiesTable 6. acrossTwo-factor the entire analysis birch of log. variance (at the significance level p = 0.05) for the distribution of average densities across the entire birch log. Sum of Number of The Mean Empirical Test Source of Variation Squared DegreesNumber of Square of Statistics Probability Deviations Freedomof TheDeviations Mean EmpiricalF pTest Sum of Squared Source of Variation Degrees Square of Statistics Probability Intercept 2,039,755,000Deviations 1 2,039,755,000 784,716 0.000 Geographical direction 299of 1 Deviations 299 0.115F 0.735p Distance from the core 3,709,976Freedom 7 529,997 203.896 0.000 Direction Distance 22,208 7 3173 1.221 0.287 Intercept· 2039755000 1 2039755000 784 716 0.000 Error 19,786,270 7612 2599 - - Geographical 299 1 299 0.115 0.735 direction DistanceMultiple from comparisons the were made using medium Tukey’s test in order to verify the levels between 3709976 7 529997 203.896 0.000 which thecore factor “a distance from the pith” had a significant difference (Table7). Five homogeneous groupsDirection were⋅Distance obtained. The highest22208 average density 7 of 653 kg 3173/m3 was obtained 1.221 for measuring 0.287 lines 10 and 12 cmError away from the pith,19786270 while the lowest7612 average density2599 of 591 kg/m3- was obtained -for the measuringMultiple line comparisons 2 cm away from were the made pith. using The relationship medium Tukey’s between test wood in order density to and verify tree the was levels also confirmedbetween which during the studies factor on “a silver distance birch growingfrom the at pith” the lower had altitudea significant of the difference Czech Republic (Table region 7). Five [7] andhomogeneous the Lativian groups plantation were on obtained. post-agricultural The highest lands average [24]. density of 653 kg/m3 was obtained for measuring lines 10 and 12 cm away from the pith, while the lowest average density of 591 kg/m3 was Table 7. Tukey test of the factor “distance from the core” for the distribution of average densities over obtainedthe cross-section for the measuring of the entire line birch 2 cm log. away from the pith. The relationship between wood density and tree was also confirmed during studies on silver birch growing at the lower altitude of the Czech Average Test Probability RepublicDistance region from [7] and the Lativian plantation on post-agricultural lands [24]. Density p theFor Core comparison, (cm) the distribution of average densities across the cross-section at breast height (kg/m3) and ¼ of the height of the birch log2 was examined 4 (Figures 6 4 8 and 5). 10 At breast 12 height 14 and at ¼ 16 of the height of 2the birch logs 591in cross-section, the 0.000 average 0.000 density 0.000 of wood 0.000 increased 0.000 along 0.000 the radius 0.000 of the tree from4 the core to 613a distance 0.000 of about 8 cm from 0.000 the core, 0.000 and then 0.000 there 0.000 was very 0.000 little variation 0.000 in density.6 630 0.000 0.000 0.000 0.000 0.000 0.000 0.004 8 643 0.000 0.000 0.000 0.000 0.004 1.000 1.000 10 653 0.000 0.000 0.000 0.000 1.000 0.007 0.289 12 653 0.000 0.000 0.000 0.004 1.000 0.026 0.411 14 643 0.000 0.000 0.000 1.000 0.007 0.026 1.000 16 644 0.000 0.000 0.004 1.000 0.289 0.411 1.000 Homogeneous group number 1 2 3 4 5 5 4 4. 5

For comparison, the distribution of average densities across the cross-section at breast height and 1/4 of the height of the birch log was examined (Figures4 and5). At breast height and at 1/4 of the

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Table 7. Tukey test of the factor “distance from the core” for the distribution of average densities over Table 7. Tukey test of the factor “distance from the core” for the distribution of average densities over the cross-section of the entire birch log. the cross-section of the entire birch log. Average Test Probability Distance from Average Test Probability Distance from Density p the Core (cm) Density p the Core (cm) (kg/m3) 2 4 6 8 10 12 14 16 (kg/m3) 2 4 6 8 10 12 14 16 2 591 0.000 0.000 0.000 0.000 0.000 0.000 0.000 2 591 0.000 0.000 0.000 0.000 0.000 0.000 0.000 4 613 0.000 0.000 0.000 0.000 0.000 0.000 0.000 4 613 0.000 0.000 0.000 0.000 0.000 0.000 0.000 6 630 0.000 0.000 0.000 0.000 0.000 0.000 0.004 6 630 0.000 0.000 0.000 0.000 0.000 0.000 0.004 8 643 0.000 0.000 0.000 0.000 0.004 1.000 1.000 Forests 20208 , 11, 445 643 0.000 0.000 0.000 0.000 0.004 1.000 1.0007 of 14 10 653 0.000 0.000 0.000 0.000 1.000 0.007 0.289 10 653 0.000 0.000 0.000 0.000 1.000 0.007 0.289 12 653 0.000 0.000 0.000 0.004 1.000 0.026 0.411 12 653 0.000 0.000 0.000 0.004 1.000 0.026 0.411 14 643 0.000 0.000 0.000 1.000 0.007 0.026 1.000 height of14 the birch logs 643 in cross-section, 0.000 0.000 the average0.000 density 1.000 of wood0.007 increased 0.026 along the radius 1.000 of the 16 644 0.000 0.000 0.004 1.000 0.289 0.411 1.000 tree from16 the core to 644 a distance 0.000 of about0.000 8 cm from0.004 the core,1.000 and then0.289 there 0.411 was very1.000 little variation Homogeneous group in density.Homogeneous group 1 2 3 4 5 5 4 4. 5 number 1 2 3 4 5 5 4 4. 5 number

Figure 4.4. Distribution of average densities on the cross-sectioncross-section ofof birchbirch loglog atat breastbreast height.height. Figure 4. Distribution of average densities on the cross-section of birch log at breast height.

Two-sidedTwo-sided tests tests of of significance significance of correlation of correlation coeffi cientscoefficients showed showed that, in boththat, cases, in both the correlationcases, the was statisticallyTwo-sided significanttests of significance (Table8). The of distributioncorrelation coefficients of average densitiesshowed acrossthat, in the both cross-section cases, the correlationcorrelation waswas statisticallystatistically significantsignificant (Table(Table 8).8). TheThe distributiondistribution ofof averageaverage densitiesdensities acrossacross thethe determinedcross-section both determined at breast height both andat breast1/4 of theheight log heightand ¼ properly of the depictedlog height the properly distribution depicted of average the densitiescross-section across determined the cross-section both at determined breast height for the and entire ¼ birchof the log. log height properly depicted the distributiondistribution ofof averageaverage densitiesdensities acrossacross thethe croscross-sections-section determineddetermined forfor thethe entireentire birchbirch log.log.

FigureFigure 5.5.Distribution Distribution ofof averageaverage densitiesdensities onon thethe cross-sectioncross-section of of a a birch birch log log at at 1 ¼/4 of the log height. Figure 5. Distribution of average densities on the cross-section of a birch log at ¼ of the log height.

Forests 2020, 11, x FOR PEER REVIEW 8 of 14 Forests 2020, 11, 445 8 of 14 Table 8. Two-sided tests of the significance of correlation coefficients between average densities across the entire birch log and average densities across the cross-section at breast height and ¼ of the Table 8. Two-sided tests of the significance of correlation coefficients between average densities across height of the birch log (at the significance level p = 0.05). the entire birch log and average densities across the cross-section at breast height and 1/4 of the height of the birch log (at the significance level p = 0.05). Number of Correlation Critical Values Degrees of Area Type CorrelationCoefficient Number of Degrees CriticalR Values Area Type Coefficient 0.05/2; v; k+1 of FreedomFreedom R R R 0.05/2; v; k+1 v v Distribution atat breastbreast height height 0.9770.977 0.6660.666 7 7 Distribution at at 1 ¼/4 of the heightheight ofof the the log log 0.9590.959 0.6660.666 7 7 The radial density increase (average for two geographical directions) in the juvenile wood zone for theThe entire radial birch density log increasewas 11.9% (average (Table 9). for two geographical directions) in the juvenile wood zone for the entire birch log was 11.9% (Table9). Table 9. Radial density increase (in the juvenile wood zone) on the cross-section of the birch log. Table 9. Radial density increase (in the juvenile wood zone) on the cross-section of the birch log. Area Type Radial Density Increase (%) AreaAll logs Type Radial Density 11.9 Increase (%) At breastAll logs height 17.711.9 At ¼ ofAt the breast height height of the log 15.917.7 At 1/4 of the height of the log 15.9 3.4. Density Distribution on the Longitudinal Section of a Birch Log 3.4. DensityFigures Distribution 6 and 7 show on thethe Longitudinal distribution Section of average of a Birch densities Log on the longitudinal section of a birch log. Figures6 and7 show the distribution of average densities on the longitudinal section of a birch log.

Figure 6. Distribution of average densities on the longitudinal section of a birch log. Figure 6. Distribution of average densities on the longitudinal section of a birch log.

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Figure 7. Distribution of average densities on the longitudinal section of a birch log. Figure 7. Distribution of average densities on the longitudinal section of a birch log. Based on the results of one-way analysis of variance (Table 10), it was found that the location of Based on the results of one-way analysis of variance (Table 8), it was found that the location of the wood at the trunk height had a statistically significant effect on the average density of birch wood. the wood at the trunk height had a statistically significant effect on the average density of birch wood. That relation was also observed in silver birch logs from Lativia [24], even though the analyzed wood That relation was also observed in silver birch logs from Lativia [24], even though the analyzed wood consisted mainly of juvenile wood what is opposite to material used for presented research. consisted mainly of juvenile wood what is opposite to material used for presented research. Table 10. One-way analysis of variance (at significance level p = 0.05) for distribution of average Table 8. One-way analysis of variance (at significance level p = 0.05) for distribution of average densities on the longitudinal section of birch logs. densities on the longitudinal section of birch logs. Empirical Statistics Critical Statistics Number of Degrees of Freedom Empirical Statistics Critical Statistics Number of Degrees of Freedom F F0.05; u; v u, v F F0.05; u; v u, v 5.602 1.202 147. 8172 5.602 1.202 147. 8172

CorrelationCorrelation coecoefficientsfficients werewere determineddetermined andand two-sidedtwo-sided teststests ofof significancesignificance ofof correlationcorrelation coecoefficientsfficients werewere verified,verified, checkingchecking thethe significancesignificance ofof thethe statisticalstatistical correlationcorrelation betweenbetween thethe averageaverage densitydensity of birch wood and and its its location location at at the the height height of of the the trunk. trunk. The The calculations calculations were were carried carried out out for fortwo two areas: areas: from from the thebutt butt to the to thebase base of the of thecrown crown and andfor the for area the area between between the base the base of the of crown the crown and andthe top the of top the of tree the (Table tree (Table 9). On 11 the). longitudinal On the longitudinal section of section the birch of thelog, birch the average log, the density average decreased density decreasedas it moved as itfrom moved the fromtree butt the treeto the butt root to the of rootthe ofcrown the crown(a statistically (a statistically significant significant correlation correlation was wasobtained), obtained), while while in the in thecrown crown area area the the average average density density of of wood wood increased increased as as it it moved moved to to the top ofof thethe treetree (a(a statisticallystatistically significantsignificant correlationcorrelation waswas obtained).obtained).

TableTable 11.9. Two-sidedTwo-sided tests tests of of the the significance significance of correlati correlationon coefficients coefficients between the average densitydensity ofof birchbirch woodwood andand itsits locationlocation atat thethe heightheight (at(at thethe significancesignificance levellevelp p= = 0.05).0.05). Number of Correlation Number of Degrees Correlation Critical Values Area Type Coefficient Critical Values of FreedomDegrees of Area Type Coefficient R0.05/2; v; k+1 R R0.05/2; v; k+1 vFreedom R From the butt to the base of the crown 0.291 0.198 97 v − Between the base of the crown and the top From the butt to the base of the crown 0.551−0.291 0.2790.198 48 97 Between the baseof theof the tree crown and the top of 0.551 0.279 48 the tree The trunk height was determined (based on the analysis of the accuracy of function matching, by maximizing the value of the correlation coefficient), to which the average density of the tested birch wood decreased as it moved from the butt to the crown root. It amounted to 4.65 m, which was 0.2 times the height of the log. Correlation coefficients were calculated for two areas: below and above 0.2 log height (Table 12). Two-sided tests of significance of correlation coefficients showed that in both areas, the correlation was statistically significant.

Forests 2020, 11, x FOR PEER REVIEW 10 of 14

The trunk height was determined (based on the analysis of the accuracy of function matching, by maximizing the value of the correlation coefficient), to which the average density of the tested birch wood decreased as it moved from the butt to the crown root. It amounted to 4.65 m, which was 0.2 times the height of the log. Correlation coefficients were calculated for two areas: below and above 0.2 log height (Table 10). Two-sided tests of significance of correlation coefficients showed that in both areas, the correlation was statistically significant. Forests 2020, 11, 445 10 of 14 Table 10. Two-sided tests of significance of correlation coefficients between the average density of birch wood and its location at trunk height below and above 0.2 log height (at significance level p = Table 12. Two-sided tests of significance of correlation coefficients between the average density of birch 0.05). wood and its location at trunk height below and above 0.2 log height (at significance level p = 0.05). Number of Correlation Correlation CriticalNumber of Degrees of Critical Values Degrees of Area Type Area Type Coefficient Coefficient R0.05/2;Freedom R Values R 0.05/2; v; k+1 v Freedom R v; k+1 v Below 0.2 log height 0.649 0.355 29 Below 0.2 log height− −0.649 0.355 29 Above 0.2 log height 0.306 0.182 116 Above 0.2 log height 0.306 0.182 116

FigureFigure8 8shows shows the the distribution distribution of of average average densities densities on on the the longitudinal longitudinal section section of of a a birch birch log log belowbelow andand aboveabove 0.20.2 loglog height.height.

FigureFigure 8.8. DistributionDistribution ofof averageaverage densitiesdensities onon thethe longitudinallongitudinal sectionsection ofof aa birchbirch loglog belowbelow andand aboveabove 0.20.2 loglog height.height. The highest density on the longitudinal section was characterized by the butt part of the birch log, The highest density on the longitudinal section was characterized by the butt part of the birch confirming the literature data. The average densities on the longitudinal section of the birch log varied log, confirming the literature data. The average densities on the longitudinal section of the birch log in the range 590–670 kg/m3. varied in the range 590–670 kg/m3. 3.5. Density Map 3.4. Density Map The literature related to the research on density distribution in experimental tree trunks made by The literature related to the research on density distribution in experimental tree trunks made destructive methods (mainly stereometric) characterizes the technical possibilities of rational industrial by destructive methods (mainly stereometric) characterizes the technical possibilities of rational use of wood. The application of the isotope densimeter to measure the density using the X-ray industrial use of wood. The application of the isotope densimeter to measure the density using the x- (non-destructive) method allowed for very accurate determination of the density distribution on the ray (non-destructive) method allowed for very accurate determination of the density distribution on longitudinal and cross-section of the birch logs tested. The density distribution (at 12% moisture the longitudinal and cross-section of the birch logs tested. The density distribution (at 12% moisture content) averaged for all tested logs in the form of a map is presented in Figure9. content) averaged for all tested logs in the form of a map is presented in Figure 9. On the map illustrating the density distribution, wood with the highest density not caused by On the map illustrating the density distribution, wood with the highest density not caused by knots (700–750 kg/m3) appeared in the butt part (dark yellow). The correlation between the wood knots (700–750 kg/m3) appeared in the butt part (dark yellow). The correlation between the wood density and the maturity of cells was observed. It was noticed that between the measuring lines density and the maturity of cells was observed. It was noticed that between the measuring lines spaced 4 cm from the pitch, there was a strip of wood with a density of 550 to 599.9 kg/m3 (light blue). spaced 4 cm from the pitch, there was a strip of wood with a density of 550 to 599.9 kg/m3 (light blue). The density of wood behind this area was from 600 to 649.9 kg/m3 (green). This part around the pitch The density of wood behind this area was from 600 to 649.9 kg/m3 (green). This part around the pitch with a radius of about 8 cm formed a zone of juvenile wood with a lower density visible in the form of blue and green colors. Behind the measuring lines spaced 8 cm from the pith (mature wood), there was wood with a density in the range of 650 to 700 kg/m3 (light yellow stripes). Around 11 m in height, near the peripheral parts, a reduced density area appeared (dark blue and navy blue). This phenomenon could be explained by the occurrence of wedges and cracks. At two heights: 18.0 and 21.6 m, wood density was found to be 750 to 899.9 kg/m3 (orange and red). The increase in density in these areas was caused by a significant number of high-density knots. The occurrence of defects, mainly in the Forests 2020, 11, x FOR PEER REVIEW 11 of 14 with a radius of about 8 cm formed a zone of juvenile wood with a lower density visible in the form of blue and green colors. Behind the measuring lines spaced 8 cm from the pith (mature wood), there was wood with a density in the range of 650 to 700 kg/m3 (light yellow stripes). Around 11 m in height, near the peripheral parts, a reduced density area appeared (dark blue and navy blue). This Forestsphenomenon2020, 11, 445 could be explained by the occurrence of wedges and cracks. At two heights: 18.011 ofand 14 21.6 m, wood density was found to be 750 to 899.9 kg/m3 (orange and red). The increase in density in these areas was caused by a significant number of high-density knots. The occurrence of defects, formmainly of knots,in the form had aof statistically knots, had significanta statistically effect significant on the average effect on density the average of the density large-dimensioned of the large- 3 birchdimensioned wood tested. birch The wood average tested. density The of average wood with density defects of waswood higher with by defects approximately was higher 5 kg/ mby comparedapproximately to the 5 density kg/m3 compared of wood without to the density defects. of The wood slight without difference defects. isdue The toslight the significantdifference shareis due ofto large-sizedthe significant wood share in the of large- knotlesssized zone. wood in the knotless zone.

FigureFigure 9.9.Map Map ofof densitydensity distributiondistribution inin thethe studiedstudied woodwood ofof silversilver birch.birch.

3 InIn birchbirch woodwood withwith anan averageaverage densitydensity ofof 623623 kg kg/m/m 3 (Table(Table 1311),), overover 22%22% ofof averageaverage densitydensity 3 measurementmeasurement results results ranged ranged from from 500 500 to to 599.9 599.9 kg kg/m/m 3,, and and over over 75% 75% ofof averageaverage densitydensity measurementmeasurement 3 resultsresults ranged ranged from from 600 600 to to 699.9 699.9 kg kg/m/m .3.

Table 13. Proportion of density measurement results in density ranges.

Feature Average density (kg/m3) 623 Density ranges (kg/m3) Proportion (%) 0–99.9 0.0 100–199.9 0.0 The proportion of average results of density 200–299.9 0.0 measurements in density ranges 300–399.9 0.3 400–499.9 0.7 500–599.9 22.3 600–699.9 75.4 700–799.9 1.3 Forests 2020, 11, x FOR PEER REVIEW 12 of 14

Table 11. Proportion of density measurement results in density ranges.

Feature Average density (kg/m3) 623 Density ranges (kg/m3) Proportion (%) 0–99.9 0.0 100–199.9 0.0 200–299.9 0.0 The proportion of average results of density 300–399.9 0.3 measurements in density ranges 400–499.9 0.7 500–599.9 22.3 Forests 2020, 11, 445 600–699.9 75.4 12 of 14 700–799.9 1.3

4. Conclusions 4. Conclusions Based on the analysis of density variabilityBased on of the the analysis tested birch of density wood variability (Betula pendula of the Roth.) tested at birch the wood (Betula pendula Roth.) at the age of 70 to 72 years, the followingage conclusions of 70 to 72 were years, made: the following conclusions were made: 1. The place where wood was tested,1. The both place in cros wheres-section wood and was longitudinal tested, both section in cross-section of the trunk, and longitudinal section of the trunk, had a statistically significant effect on thehad average a statistically density significantof birch wood. effect on the average density of birch wood. 2. The average density of whole 2.logs wasThe statistica average densitylly significantly of whole higher logs was than statistically the average significantly density higher than the average density at breast height, while the average densityat breast at ¼ height height, did while not the differ average statistically density significantly at 1/4 height from did not differ statistically significantly the average density of the whole log. from the average density of the whole log. 3. On the cross-section, the distribution3. On of the averag cross-section,e densities the determined distribution at ofbreast average height, densities as well determined as at breast height, as well as on ¼ of the log height, properly depictedon 1the/4 of distri the logbution height, of average properly densities depicted on the the distribution cross-section of average densities on the cross-section determined for the whole logs. The distributiondetermined of for average the whole densities logs. The in the distribution cross-section of average can be densities in the cross-section can be described by a second degree polynomialdescribed in the byjuvenile a second wood degree area polynomial and two straight in the juvenilelines (in wood the area and two straight lines (in the mature wood areas); in the juvenile woodmature area, wood the areas);average in wood the juvenile density wood increased area, the significantly average wood density increased significantly along the radius of the tree; in the maturealong wood the areas, radius there of the was tree; a slight in the density mature fluctuation. wood areas, there was a slight density fluctuation. 4. On the longitudinal section of the tested birch wood, the average density decreased going from 4. On the longitudinal section of the tested birch wood, the average density decreased going from the tree butt to 20% of the log height. In the crown area, the average wood density increased as it the tree butt to 20% of the log height. In the crown area, the average wood density increased as it moved to the top of the tree. In both areas, a statistically significant correlation was obtained between moved to the top of the tree. In both areas, a statistically significant correlation was obtained the average density of wood and its location at the height of the trunk. between the average density of wood and its location at the height of the trunk. 5. The geographical direction (north–south) along which the density was determined did not have 5. The geographical direction (north–south) along which the density was determined did not have a a statistically significant effect on the distribution of average densities on the cross-section of the statistically significant effect on the distribution of average densities on the cross-section of the tested birch logs. tested birch logs. Author Contributions: Conceptualization, E.D. and P.W.; methodology, E.D. and P.W.; investigation, P.W.; resources, E.D. and P.W.; data curation,Author E.D. Contributions:and P.W.; writing—originalConceptualization, draft preparation, E.D. and P.W.; E.D.; methodology, writing— E.D. and P.W.; investigation, P.W.; review and editing, A.J.; visualization, resources,P.W. and E.D.; E.D. supervision, and P.W.; data E.D curation, and A.J. E.D. All authors and P.W.; have writing—original read and agreed draft preparation, E.D.; writing—review to the published version of the manuscript.and editing, A.J.; visualization, P.W. and E.D.; supervision, E.D and A.J. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Funding: This research received no external funding.

Acknowledgments: Publication of theAcknowledgments: article has been financedPublication by the of Polish the article National has beenAgency ﬁnanced for Academic by the Polish National Agency for Academic Exchange under the Foreign PromotionExchange Programme. under the Foreign Promotion Programme.

Conflicts of Interest: The authors declareConﬂicts no conflict of Interest: of interest.The authors declare no conﬂict of interest.

References References

1. Niemz, P.; Sonderegger, W. Holzphysik.1. Niemz, Physik P.; des Sonderegger, Holzes und der W. Holzwerkstoffe,Holzphysik. Physik 1st ed.; des HolzesFachbuchverlag und der Holzwerksto ﬀe, 1st ed.; Fachbuchverlag Leipzig im Carl Hanser Verlag: München,Leipzig Germany, im Carl 2017; Hanser pp. Verlag:138–160. München, Germany, 2017; pp. 138–160. 2. Krzysik, F. Nauka o drewnie, 1st ed.; PWN: Warsaw, Poland, 1957; pp. 615–623.

3. Kubiak, M.; Laurow, Z. Surowiec Drzewny; Fundacja Rozwój SGGW: Warsaw, Poland, 1994. 4. Group work under Suwała, M. Poradnik U˙zytkowanialasu dla Le´sników Praktyków (Forest Use Guide for Forestry Practitioners); Oficyna Edytorska Wydawnictwo Swiat:´ Warsaw, Poland, 2000. 5. Kokociński,W. Drewno: Pomiary Wła´sciwo´sciFizycznych i Mechanicznych, 1st ed.; Wydawnictwo–Drukarnia Prodruk: Poznań,Poland, 2004. 6. Bowyer, J.L.; Shmulsky, R.; Haygreen, J.G. Forest Products and Wood Science, an Introduction, 5th ed.; Blackwell Publishing: Ames, IA, USA, 2007. 7. Giagli, K.; Vavrˇcík, H.; Fajstavr, M.; Cernˇ ý, J.; Novosadová, K.; Martiník, A. Stand factors affecting the wood density of naturally regenerated young silver birch growing at the lower altitude of the Czech Republic region. Wood Res. 2019, 64, 1011–1022. Forests 2020, 11, 445 13 of 14

8. Helińska-Raczkowska,L.; Fabisiak, E. Zale˙zno´sćmi˛edzydługo´sci˛aelementów anatomicznych i g˛esto´sci˛a drewna brzozy (Betula pendula Roth.) (Relationship between the length of anatomical elements and the density of birch wood (Betula pendula Roth.). Work Wood Technol. Comm. 1995, 14, 43–48. 9. Niemz, P. Physik des Holzes und der Holzwerkstoffe; DRW-Verlag Weinbrenner GmbH &. Co.: Esslingen, Germany, 1993. 10. Fabsiak, E. Zmienno´sćpodstawowych elementów anatomicznych i g˛esto´scidrewna wybranych gatunków drzew (Variability of basic anatomical elements and wood density of selected tree species). Rocz. Akad. Rol. Pozn. Rozpr. Nauk. 2005, 369, 5–9. 11. Hakkila, P. Investigations on the basic density of Finnish pine, spruce and birch wood. Commun. Inst. For. Fenn. 1966, 61, 1–98. 12. Kocoń,J. Struktura i ultrastruktura drewna młodocianego i dojrzałego jodły pospolitej (Structure and ultrastructure of juvenile and mature silver fir). Folia For. Pol. Ser. A 1991, 31, 139–150. 13. Hejnowicz, Z. Anatomia i Histogeneza Ro´slinNaczyniowych (Anatomy and Histogenesis of Vascular Plants); PWN: Warsaw, Poland, 1973; Volume 2. 14. Niedzielska, B.; W ˛asik,R. Badania zmienno´scicech drewna, jako podstawa do racjonalnego kształtowania jego jako´sci. Stan i perspektywy badańz zakresu u˙zytkowanialasu (Research on variability of wood features as a basis for rational shaping of its quality. The state and perspectives of research on forest use). In Proceedings of the III Forest Conference, S˛ekocinLas, Poland, 30–31 March 2000; pp. 256–267. 15. Helińska-Raczkowska,L. Leksykon Nauki o Drewnie (The Lexicon of Wood Science), 1st ed.; Agricultural University of Poznań:Poznań,Poland, 1999; pp. 29–30. 16. Fukazawa, K. Juvenile wood of hardwoods judged by density variation. IAWA Bull. 1984, 5, 65–73. [CrossRef] 17. Bao, F.C.; Jiang, Z.H.; Jiang, X.M.; Lu, X.X.; Luo, X.Q.; Zhang, S.Y. Differences in wood properties between juvenile wood and mature wood in 10 species grown in China. Wood Sci. Technol. 2001, 35, 363–375. [CrossRef] 18. Gryc, V.; Vavrˇcik,H.; Horn, K. Density of juvenile and mature wood of selected coniferous species. J. For. Sci. 2011, 57, 123–130. [CrossRef] 19. Kretschmann, D.E. Properties of Juvenile Wood. Techline, Properties and Use of Wood, Composites, and Fiber Products; United States Department of Agriculture, Forest Service, Forest Products Laboratory: Washington, DC, USA, 1998. 20. Sobczak, R. Drzewa Naszych Lasów (Trees of Our Forests); Oficyna Edytorska Wydawnictwo Swiat:´ Warsaw, Poland, 1996. 21. Bernadzki, E.; Kowalski, M. Brzoza na Gruntach Porolnych (Birch on Former Agricultural Lands). Sylwan 1983, 127, 33–42. 22. Zał˛eski,A. Plantacje Le´snychDrzew Szybko Rosn ˛acych(Plantations of Fast-Growing Forest Trees); PWRIL: Warsaw, Poland, 1987. 23. Godet, J.D. Atlas Drewna. Poradnik Le´snika(Wood ATLAS. Forester’s Guide); Multico Oficyna Wydawnicza: Warsaw, Poland, 2008. 24. Liepinš, K.; Rieksts-Riekstinš, J. Stemwood density of juvenile silver birch trees (Betula pendula Roth.) from plantations on former farmlands. Balt. For. 2013, 19, 179–186. 25. Viherä-Aarnio, A.; Velling, P. Growth, wood density and bark thickness of silver birch originating from the Baltic countries and Finland in two Finnish provenance trials. Silva Fenn. 2017, 51, 18. [CrossRef] 26. Herujarvi, H. Variation of basic density and brinell hardness within mature Finnish Betula pendula and B. pubescens stems. Wood Fiber Sci. 2004, 36, 216–227. 27. Krzosek, S. Badanie G˛esto´sci,jako Kryterium Wytrzymało´sciowejJako´sciIglastej Tarcicy Konstrukcyjnej (Density Testing as a Criterion for Strength Quality Coniferous Structural Timber). Ph.D. Thesis, Warsaw University of Life Sciences, Warsaw, Poland, 1998. 28. Elandt, R. Statystyka Matematyczna w Zastosowaniu do Do´swiadczalnictwaRolniczego (Mathematical Statistics in Application to Agricultural Experimentation); PWN: Warsaw, Poland, 1964. 29. Wagenführ, R.; Scheiber, C.H.R. Holzatlas. Mit 890 zum Teil Mehrfarbigen Bilden; VEB Fachbuchverlag: Leipzig, Germany, 1985. 30. Kokociński,W. Anatomia Drewna (Anatomy of Wood); Drukarnia Prodruk: Poznań,Poland, 2005. 31. Galewski, W.; Korzeniowski, A. Atlas Najwa˙zniejszych Gatunków Drewna (Atlas of the Most Important wood Species); PWRIL: Warsaw, Poland, 1958. Forests 2020, 11, 445 14 of 14

32. Abdel-Gadir, A.Y.; Krahmer, R.L. Estimating the age of demarcation of juvenile and mature wood in Douglas-fir. Wood Fiber Sci. 1993, 25, 242–249. 33. Stanisz, A. Przyst˛epnykurs Statystyki z Zastosowaniem STATISTICA PL na Przykładach z Medycyny. Tom 2. Modele Liniowe i Nieliniowe (An Affordable Statistics Course Using STATISTICA PL on Examples from Medicine. Volume 2. Linear and Nonlinear Models); StatSoft Polska Sp. z o.o.: Kraków, Poland, 2007. 34. Bonham, V.A.;Barnett, J.R. Fibre length and microfibril angle in silver birch (Betula pendula Roth). Holzforschung 2001, 55, 159–162. [CrossRef] 35. Helińska-Raczkowska,L.; Fabisiak, E. Długo´sćmłodocianego okresu przyrostu na grubo´sćdrzew brzozy (Betula pendula Roth.) (Length of adolescent growth in birch trees (Betula pendula Roth.)). Sylwan 1995, 139, 77–84. 36. Hakkila, P. Wood density survey and dry weight tables for pine, spruce and birch stems in Finland. Commun. Inst. For. Fenn. 1979, 96, 1–59. 37. Pazdrowski, W. Warto´sćtechniczna drewna sosny zwyczajnej (Pinus Sylvestris L.) w zale˙zno´sciod jako´sci pni drzew w drzewostanach r˛ebnych(Technical value of Scots pine (Pinus Sylvestris L.) wood depending on the quality of tree trunks in forest stands). Rocz. Akad. Rol. Pozn. Rozpr. Nauk. 1988, 170. 38. Velling, P. Wood density in two Betula pendula Roth progeny trials. Folia For. 1979, 416, 1–24. 39. Niedzielska, B. Zmienno´sćg˛esto´scioraz podstawowych cech makroskopowej struktury drewna jodły (Abies alba Mill.) w granicach jej naturalnego wyst˛epowaniaw Polsce (Variability of density and basic features of the macroscopic structure of fir wood (Abies alba Mill.) Within its natural occurrence in Poland). Zesz. Nauk. Akad. Rol. Kołł ˛atajaKrakowie 1995, 198, 1–75. 40. Miler, Z. Badania nad własno´sciami drewna brzozy brodawkowatej i omszonej (Research on the properties of birch wood). Folia For. Pol. Ser. B 1966, 7, 135–159.

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