Microstructural Properties of Common Yew and Norway Spruce Determined with Silviscan

IAWA Journal, Vol. 30 (2), 2009: 165–178

Microstructural properties of common yew and Norway spruce determined with SilviScan

Daniel Keunecke1,*, Robert Evans2 and Peter Niemz1

Summary Yew wood holds a special position within the softwoods with regard to its exceptional elasto-mechanical behaviour. Despite a relatively high density, it is highly elastic in the longitudinal direction (the modulus of elasticity is low and the stretch to break high). In the radial-tangential plane, its elastic anisotropy is clearly less pronounced compared to other softwoods such as spruce. Knowledge of the anatomical organisation of yew wood is an indispensable precondition for the correct interpretation of this conspicuous mechanical behaviour. The aim of this study, therefore, was to interpret the difference in elasto-mechanical behaviour of yew and spruce (as a reference) through their relative microstructures as measured by SilviScan, a technology based on X-ray densitometry, X-ray diffractometry and optical microscopy. This system is able to measure a variety of structural features in a wood sample. The results reveal that the elasto-mechanical response of yew is primarily due to large microfibril angles and a more homogeneous cross-sectional tissue composition (regarding tracheid dimensions and density distribution) compared to spruce. With respect to structure-property relationships, it was concluded that yew wood combines properties of normal and compression wood and therefore takes an intermediate position between them. Key words: Taxus baccata, Picea abies, SilviScan, microstructure, microfibril angle, modulus of elasticity, density, anisotropy.

Introduction

For centuries, common yew (Taxus baccata L.) has been well known for its extraordi- nary longitudinal elasticity and toughness. Among other things, its wood was used for certain weapons (longbows, lances, crossbows) requiring these properties, particularly the low modulus of elasticity (MOE) and at the same time a high elastic strain parallel to the grain. The few available literature references (Sekhar & Sharma 1959; Jakubczyk 1966; Wagenführ 2000; Märki et al. 2005) also indicate a high elasticity (with MOE between 6.2 and 12 GPa). Although there are other elastic softwood species, none of them has an air-dry density (= density at 12% wood moisture content) as high as that

1) Institute for Building Materials (Wood Physics), ETH Zurich, 8093 Zurich, Switzerland. 2) CSIRO Materials Science and Engineering, Clayton, Victoria 3168, Australia. *) Corresponding author [E-mail: [email protected]]. Associate Editor: Lloyd Donaldson

Downloaded from Brill.com09/27/2021 08:13:19PM via free access 166 IAWA Journal, Vol. 30 (2), 2009 of yew (620–720 kg m-3). Thus, yew holds an exceptional position, especially since there is usually a strong species-spanning positive interrelation between density and axial MOE (for example, see data compilation in Sell 1997). This unusual combination (high density, low MOE) prompted us to investigate yew elasticity in more detail. In previous studies by our group (Keunecke et al. 2008a, b; Keunecke & Niemz 2008), we determined the axial MOE of adult yew and (as a reference) spruce heartwood at standard climatic conditions (20 °C, 65% relative humidity (RH)) for three different hierarchical levels (solid wood; tissue with a thickness of 220 µm in the tangential direction and a width of 3.5 mm in the radial direction; tracheids). Furthermore, we calculated the three-dimensional elastic behaviour of both species at the solid wood level (Keunecke et al. 2008b). The two crucial findings of these studies were: 1) Across all studied hierarchical levels, the axial MOE of yew was clearly lower than that of the 30% less dense spruce (Table 1). In other words: The lower MOE was exhibited not only by specimens subjected to cell-cell interactions but also by individual tracheids for which pure cell wall mechanics applied. Therefore, the axial stiffness of yew is obviously controlled by a feature that is present even at the cellular level. 2) Especially in the radial-tangential (RT) plane, the anisotropy of the elastic behaviour calculated for uniaxial tensile load was clearly less pronounced for yew than for spruce. This means in detail: In the case of spruce wood, even small deviations from the principle load axes (R, T) effect a considerable increase of compliance at the same stress level. The maximum compliance (a combination of the compli- ances in the radial and tangential directions) is reached at an angle of about 45°, central between both axes. Yew behaves differently: the deformation is clearly less anisotropic; it is largest along the principle axes and only slightly decreases to a minimum near 45°.

Table 1. Mean axial stiffness of yew and spruce determined at three hierarchical levels at 20 °C and 65% relative humidity.

Species Hierarchical level moECW (GPa) moECSA (GPa) Yew Tracheids1 13.9 (36.6%) — Tissue2 15.6 (26.9%) 7.0 (23.9%) Solid wood3 16.9 (11.4%) 10.5 (13.6%)

Spruce Tracheids1 26.2 (28.3%) — Tissue2 29.4 (18.6%) 9.9 (21.5%) Solid wood3 27.2 (8.2%) 12.8 (9.2%) The data presented are mean values. Tracheid level: yew, number of specimens (n) = 18; spruce, n = 21. Tissue level: yew, n = 41; spruce, n = 40. Solid wood level: yew, n = 12; spruce, n = 10. The data for the tissue level are mean values for two specimen series per species. Figures in parentheses are coefficients of variation. MOECW = modulus of elasticity based on the cell wall area; MOECSA = modulus of elasticity based on the cross-sectional area including lumens. 1) Keunecke et al. (2008a); 2) Keunecke & Niemz (2008); 3) Keunecke et al. (2008b).

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After evaluating supplementary measurements, microscopic images and literature sources, we assume the following reasons for these two findings: Finding 1: The low axial MOE of yew can be explained by the large microfibril angles (MFA) we measured with three different techniques (wide angle X-ray diffraction (WAXD), small angle X-ray scattering (SAXS), and the ray/tracheid cross-field pit aperture method). All of these methods, however, have their weaknesses: a) the data were difficult to obtain as the preparation was very time-consuming; b) when applying WAXD and SAXS (beam aligned in the radial wood direction), the MFA of a large number of cells was averaged from part of a whole growth ring. Thus, the broad radial MFA variations were not taken into account; c) the pit aperture method is only reliable for estimating the MFA in latewood tracheids (Huang et al. 1997). Finding 2: The large differences between yew and spruce regarding their degree of anisotropic elastic behaviour in the RT plane can be explained by a smaller earlywood (EW)/latewood (LW) density gradient of yew compared to spruce. This conclusion, however, is based on only a few references (e.g., Wagenführ 2000) and on exemplary microscopic analyses. The goal of this present study, therefore, was to support and to quantify the previous results (so far standing on “shaky ground”), and to further improve our knowledge of the structure-function relationships of yew wood. We decided to run yew and spruce samples through the SilviScan microstructure analyser, a system with a high spatial resolution and able to measure much more rapidly than conventional methods. SilviScan was developed at CSIRO (Commonwealth Scientific and Industrial Research Organisa- tion) to assess wood structural features such as density, MFA, tracheid diameters and cell wall thickness by a combination of X-ray densitometry, X-ray diffractometry, and digital microscopy (Evans 1994, 1999; Evans et al. 1999, 2000).

Material and methods Material Two samples each of yew and spruce, the origins of which (natural forests) are shown in Table 2, were cut from tree disks taken from randomly selected trunks at breast height. This means a total of four samples were investigated. As we wanted to avoid examining special tissues such as compression wood, we chose tree disks without apparent eccentric increment. The yew samples were from different sites, as were the spruce samples. However, in addition, the spruce differed greatly in the width of growth rings (Table 2). The spruce sample with narrow growth rings is also termed “spruce (n)” in the following, and the sample with wide growth rings “spruce (w)”. Since the yew samples attracted our main attention, the differences between the spruce samples are not elucidated in great detail. The samples were processed to radial sections of 2 mm thickness (tangential) and 7 mm height (longitudinal); their radial length varied between 90 and 260 mm, depending on stem diameter. They were cut out of the tree disks with a twin-blade circular saw from close to the pith (the first few rings were left out) to the sapwood; the sapwood consisted of less than ten growth rings in the yew samples; the spruce samples were free

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Table 2. Origin, radial length, number of analysed growth rings, and mean growth ring width of the samples analysed with SilviScan.

Yew no. 1 yew no. 2 Spruce (n) Spruce (w)

Regional origin Zurich, Arnsberg, montane region Foothills of Switzerland Germany in Grisons the Alps, (alt. 1700 m), Switzerland Switzerland

Radial length (mm) 150 90 160 260 Analysed growth rings 138 52 116 55 Mean growth ring width (mm) 1.1 1.7 1.4 4.7 The samples were 2 mm wide in the tangential and 7 mm high in the longitudinal direction; the radial length depended on the respective pith-to-sapwood distance of the samples. Spruce (n) = spruce with narrow growth rings; spruce (w) = spruce with wide growth rings. of sapwood. As we wanted to relate our results to our findings from previous studies (which were based on heartwood), we were interested only in the heartwood properties. Finally, the topsides (transverse surface) were sanded to a very high finish to reveal the cell cross sections. The SilviScan measurements were then carried out at 20°C and 40% RH. This corresponds to an equilibrium moisture content of approximately 8% for both species.

Methods Although originally developed to rapidly assess wood properties of standing trees, SilviScan of course can also be employed for fundamental research. The system combines three non-destructive analytical technologies (X-ray densitometry, X-ray diffractometry, image analysis) to determine wood anatomical features. Figure 1 shows schematically the experimental setup and the specimen alignment for the radial scans, and also the structural features analysed in this study. A first group of results (wood density, MFA, radial and tangential tracheid diameters) was directly derived from the measurement data; the variables of a second group (MOE, tracheids wall thickness, coarseness) were calculated from the primary measurements.

Method Optical imaging X-ray densitometry X-ray diffractometry

Primary Tracheid Wood density Microfibril angle results diameters

Secondary Tracheid wall thickness Modulus of elasticity results Coarseness

Figure 1. SilviScan measurement principles, and properties determined in this study. The secondary results are not measured directly but derived from the primary results.

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With capillary optics and a two-dimensional wide-angle X-ray detector, diffraction patterns were obtained for 100 µm intervals yielding information particularly on the MFA. When the MFA in wood is estimated by X-ray diffractometry, the strong reflec- tions from the 002 planes of cellulose-I (Cave 1997; Evans 1999) are most often used. The MFA is calculated from the lengths of the intense diffraction arcs using the relationship MFA ≈ 1.28S, where S is the standard deviation of the peak profiles corrected for local dispersion (Evans 1999; Evans et al. 1999). The wood density was determined by using an X-ray area detector with pixel size of 6 µm. The X-ray absorption images were converted to density images from which the density profiles were obtained at 25µ m intervals. The sample was moved forward and simultaneously rotated so that the beam was always parallel with the growth rings. With an auto-focussing video microscope, the cross-sectional texture of the wood matrix was recorded (1.29 µm pixel size) and profiles of tracheid widths produced at 25 µm steps. Using a simple semi-empirical model (for details see Evans 2006), the dynamic axial MOE was predicted on the basis of density and the diffraction pattern (a quantity related both to the MFA and to the proportion of S2 microfibrils in the wood), and therefore again in 100 µm steps. In this approach, the composition of wood is taken into account. In so doing, neither the measurement of MFA was specifically required, nor the position or shape of the diffraction peaks. High correlations (R2 > 0.9, SE ~1GPa) were found between dynamic MOE predicted by this model and dynamic MOE observed in experimental tests (Evans 2006). However, because of the influence of creep (Divos & Takana 2005) the dynamic MOE data are generally higher than those obtained by static bending tests (Bucur 1995). To assess the influence of extractives on the MOE, two additional samples, located directly adjacent to the original ones (in the tangential direction), were extracted with acetone and compared with the unextracted samples. We used additional samples for the extraction since we needed the original unextracted samples for further investiga- tions (not described in this paper).

Results and discussion

Seven characteristics were determined for the four samples. As we were particularly interested in density, MFA and MOE, these results are shown as radial profiles (Fig. 2–4). To increase the clarity of the trends in these graphs, a 500-point moving average has been added. In addition, a statistical overview of the results for the seven measured features and properties is given in Figure 5 using box-and-whisker plots. In the following, density, MFA and MOE are used to find structure-function relationships that may explain the special compliant behaviour of yew wood parallel to the grain. Then the anisotropic elasto-mechanical behaviour of yew and spruce in the RT plane is interpreted using tracheid dimensions, wall thickness and coarseness.

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1400 Density 500-point moving average

) 1200 -3 1000 800 600 400 200 Yew No. 1 0 0 15 30 45 60 75 90 105 120 135 150 1400 Density 500-point moving average 1200 ) D ensity (kg m -3 1000 800 600 400 200 Yew No. 2 0 0 10 20 30 40 50 60 70 80 90 1400 Density 500-point moving average 1200 ) D ensity (kg m -3 1000 800 600 400 200 Spruce with narrow growth rings 0 0 20 40 60 80 100 120 140 160 1400 Density 500-point moving average 1200 ) D ensity (kg m -3 1000 800 600 400 D ensity (kg m 200 Spruce with wide growth rings 0 0 20 40 60 80 100 120 140 160 180 200 220 240 260 Radial position (mm). Zero = close to the pith Figure 2. Radial density profiles of two yew and two spruce samples measured by SilviScan using X-ray densitometry. Note that the curves are based on two random samples only from two species – and hence are indicative only.

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45 MFA 500-point moving average 40 35 30 25 A (°) 20 15 10 5 Yew No. 1 0 0 15 30 45 60 75 90 105 120 135 150 45 40 MFA 500-point moving average 35 30 25 A (°) MF 20 15 10 5 Yew No. 2 0 0 10 20 30 40 50 60 70 80 90 45 40 MFA 500-point moving average 35 30 25 A (°) MF 20 15 10 5 Spruce with narrow growth rings 0 0 20 40 60 80 100 120 140 160 45 40 MFA 500-point moving average 35 30

(°) MF 25 A 20 MF 15 10 5 Spruce with wide growth rings 0 0 20 40 60 80 100 120 140 160 180 200 220 240 260 Radial position (mm). Zero = close to the pith Figure 3. Radial MFA profiles of two yew and two spruce samples measured by SilviScan using X-ray diffractometry. Note that the curves are based on two random samples only from two species – and hence are indicative only.

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50 MOE 500-point moving average 40 A ) 30

10 Yew No. 1 0 0 15 30 45 60 75 90 105 120 135 150 50 MOE 500-point moving average 40 A ) M OE (GP 30

10 Yew No. 2 0 0 10 20 30 40 50 60 70 80 90 50 MOE 500-point moving average 40 A ) M OE (GP 30

10 Spruce with narrow growth rings 0 0 20 40 60 80 100 120 140 160 50 MOE 500-point moving average 40 A ) M OE (GP 30

20 M OE (GP 10 Spruce with wide growth rings 0 0 20 40 60 80 100 120 140 160 180 200 220 240 260 Radial position (mm). Zero = close to the pith Figure 4. Radial MOE profiles of two yew and two spruce samples estimated on the basis of density and diffraction measurements. Note that the curves are based on two random samples only from two species – and hence are indicative only.

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Density The samples were scanned at 20°C and 40% RH, so the density figures shown are slightly lower (~2.5%) than the air-dry density usually measured at 20°C and 65% RH. As expected, the average density was clearly higher for yew (654 and 625 kg m-3) than for spruce (463 and 501 kg m-3) (Fig. 2 & 5a). This is particularly due to the two-fold higher EW density range in yew compared to spruce, resulting in the remarkably high shear modulus (particularly in the RT plane) and the high radial and tangential stiffness of yew (Keunecke et al. 2007b, 2008b). In contrast, the LW density of yew was in the same range and even slightly lower than that of spruce. However, according to Lundqvist and Evans (2004), the true maximum LW density may be somewhat underestimated, as it is commonly associated with only a couple of narrow tracheids, at the edge of the measurement resolution. Note that the terms “earlywood” and “latewood” are not used in a strictly defined manner here. Many definitions exist to specify EW, LW and the transition zone between them (e.g. Mork’s index, Mork 1928). In our case, we can use the terms only as a rough description of the tissue structure at the beginning and at the end of a growth period. The radial density profiles also reveal quite narrow growth rings for yew ranging from 0.3 to 2.5 mm at a mean of 1.1 mm for yew no. 1 and 1.7 mm for yew no. 2 (Table 2), clearly less than in normal spruce wood. Furthermore, the yew samples showed high variations in ring width in contrast to the spruce samples. The general trend in yew was a slight decrease in density from close to the pith outwards while the spruce density remained at a constant level. The smaller difference between EW and LW density, and the narrow growth rings in yew, resulted in a higher radial homogeneity than for spruce. This is reflected in the lower elastic anisotropy of yew compared to spruce, especially in the RT plane (Keunecke et al. 2008b).

MFA Density and MFA are negatively correlated within the growth rings (i.e., MFA is larger in EW than in LW). The MFA of yew was significantly higher than that of spruce (Fig. 3) not only for the whole samples (26.9° and 29.3° for yew; 9.9° and 11.2° for spruce) but also for EW and LW (Fig. 3; Fig. 5b shows the maxima and minima), in general agreement with our previous findings (Keunecke et al. 2007a, 2008a; Keunecke & Niemz 2008). Our measurements based on the pit aperture method (Keunecke & Niemz 2008) indicated at least a significantly larger LW MFA for yew than for spruce. The large MFA of yew endows it with high axial compliance (and high fracture strain). This influence of MFA on stiffness has been verified in numerous studies (e.g., Cave & Walker 1994; Reiterer et al. 1999). The EW/LW contrast of the MFA (Fig. 3) was less regular for the yew samples. In particular, yew no. 1 shows a high MFA variation along the radial profile. The general trend in the samples was non-uniform. Yew no. 1 shows a high vari- ability; the mean MFA of yew no. 2 initially increases over the first few rings and then continually decreases. The MFA of spruce (n) is highest close to the pith and then decreases rapidly to a constant level. In spruce (w), the MFA is relatively constant

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a 987 (57) 915 (66) 995 (74) 1044 (54) b 29.9 (3.5) 32.5 (2.9) 11.6 (1.5) 17.1 (4.1) 654 625 463 501 26.9 29.3 9.9 11.2 507 (48) 500 (57) 283 (23) 230 (21) 22.9 (3.9) 25.1 (3.7) 7.4 (0.8) 6.8 (0.8) 1400 45 40 1200 35 ) 1000 -3 30 800 25 A (°) 600 20 15 400 10 200 5 0 0 yew (1) yew (2) spruce (n) spruce (w) Yew (1) yew (2) spruce (n) spruce (w)

c 15.1 (2.9) 11.0 (1.4) 35.8 (3.4) 39.8 (3.5) d 32.8 (2.6) 28.8 (2.7) 40.6 (2.6) 40.8 (1.9) 9.8 6.9 16.7 17.0 28.7 25.0 33.1 33.4 6.5 (1.5) 4.1 (0.7) 8.3 (1.1) 3.6 (0.8) 17.7 (1.4) 16.8 (1.2) 17.6 (1.0) 18.6 (1.7) 50 50

45 40 40

30 35 30

20 25

20 10 15

0 10 yew (1) yew (2) spruce (n) spruce (w) Yew (1) yew (2) spruce (n) spruce (w)

e 24.6 (1.8) 23.5 (1.7) 36.6 (3.4) 36.3 (2.0) f 4.3 (0.4) 3.7 (0.3) 5.1 (0.6) 7.1 (0.9) 22.5 21.5 33.7 31.0 3.1 2.7 2.7 2.9 20.5 (1.8) 19.8 (1.2) 29.7 (2.8) 25.3 (2.0) 2.4 (0.2) 2.2 (0.2) 1.7 (0.1) 1.3 (0.1) 45 9

40 7 35

30 5

Wall Wall thickness R adial (µm) diameter (µm) MF 3 angential diameter M OE (µm) (GPa) D ensity (kg m T 20

15 1 yew (1) yew (2) spruce (n) spruce (w) Yew (1) yew (2) spruce (n) spruce (w)

g 487 (62) 397 (35) 728 (109) 1134 (175) Mean of the maxima (standard deviation in brackets) 409 328 480 478 Mean of all values 337 (27) 271 (22) 321 (34) 242 (27) Mean of the minima (standard deviation in brackets) 1600

1400 Box-and-whisker values )

-1 1200 Maximum Maximum 1000 75%

800 Median 600 25% Mean of all values

C oarseness (µg m 400 Minima Minimum 200 Cross = mean 0 yew (1) yew (2) spruce (n) spruce (w)

Downloaded from Brill.com09/27/2021 08:13:19PM via free access Keunecke, Evans & Niemz — SilviScan studies of yew and Norway spruce 175 except for some local increases (near radial positions 75 and 115 mm). However, as the density is not increased in these regions, a streak of compression wood could be excluded as the cause. Although the four samples consist largely of mature wood, the high MFA in the first rings of the profiles is typical of juvenile wood.

MOE The calculated intra-annual dynamic MOE of yew varied from 4.1–6.5 GPa (mean of the EW minima) to 11.0–15.1 GPa (mean of the LW maxima) (Fig. 5c). This range is considerably narrower than for the spruce samples, in which MOE varied from 3.6–8.3 GPa (mean of the EW minima) to 35.8–39.8 GPa (mean of the LW maxima); see also Figure 4. It is primarily the large MFA in both EW and LW of yew that gives this species a high overall compliance. As mentioned above, the dynamic MOE is higher than the static MOE. Thus the mean MOE of the spruce samples (16.7 and 17.0 GPa, respectively) was clearly higher than the static MOE (~12 GPa) usually quoted in the literature (e.g., Wagenführ 2000). In contrast, the mean yew MOE values were lower than expected, even below the range of static MOE values from the literature (Sekhar & Sharma 1959; Jakubczyk 1966; Wagenführ 2000; Märki et al. 2005) and determined in our own measurements (Keunecke et al. 2007a; Keunecke & Niemz 2008). The radial MOE profiles clearly illustrate that the low axial MOE of yew, determined at different hierarchical levels, is caused by the relatively low LW MOE which in turn results from the large MFA. Albeit certain specific structural features of both species (e.g., spiral thickenings in yew, resin canals in spruce) are ignored when the MOE is predicted on the basis of density and diffraction patterns, their influence on the MOE is presumably negligible. No significant difference in the predicted MOE was found between the original and the extracted yew samples; the radial MOE profiles were almost congruent. In addition, the distribution of MOE analysed in histograms was very similar for both treatments. There may have been insufficient extractives to make a difference; thus, we could not identify a relationship between the axial stiffness and the presence and amount of extractives, even though it is assumed that extraneous constituents can soften the cell walls (Faix 2008). Transverse stiffness, however, is clearly influenced by extractives. Grabner et al. (2005) measured higher transverse MOE with increasing extractives content for larch wood since the lumens were filled and the tracheids were less com- pressible without air in the lumens.

← Figure 5. Box-and-whisker plots of the primary and secondary results (cf. Fig. 1) of the radial property profiles determined with SilviScan: – a: density. – b: MFA. – c: MOE. – d: radial tracheid diameter. – e: tangential tracheid diameter. – f: tracheid wall thickness. – g: coarseness. Spruce (n) = spruce with narrow growth rings; Spruce (w) = spruce with wide growth rings. The boxplots show the span, i.e. the maxima (upper peaks) and minima (lower peaks) of the profiles (not to be mistaken for earlywood and latewood). — For explanations of the boxplots, see the legend in the bottom right corner. Note that the values are based on two random samples only from two species – and hence are indicative only.

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Tracheid diameters, wall thickness, and coarseness With regard to structure-function relationships, the tracheid cross-sectional features can help to interpret the mechanical response of a wood species in the RT plane. We examined these features in an attempt to find an anatomical explanation for the less anisotropic elasto-mechanical behaviour of yew wood compared to that of spruce (see finding no. 2 in the Introduction). The EW tracheids, which most often account for the predominant part of the growth rings, were, in all samples, narrower close to the pith (measured in the radial direction). Their radial width increased slightly but constantly from close to the pith outwards while the radial LW width remained fairly constant over the whole sample length. The intra-ring variations were smaller for yew than for spruce indicating a more homogeneous cellular structure in yew. The mean radial width of the yew tracheids (28.7 and 25.0 µm) was about 20% lower than for spruce (33.1 and 33.4 µm). These differences, however, were exclusively caused by the smaller EW diameters of yew, while the LW diameters of both species were in a similar range (Fig. 5d shows the mean values of the EW maxima and LW minima). The tracheid tangential diameter in all samples also increased from near the pith outwards but the rate of increase was lower than for radial diameter. Within the rings, the mean tangential diameters of yew tracheids (22.5 and 21.5 µm) were about 30% lower than those of spruce (33.7 and 31.0 µm). In contrast to the radial dimensions, the tangential tracheid diameters of yew in both EW and LW were significantly smaller than for spruce (Fig. 5e shows the mean values of the EW maxima and LW minima). Tracheid wall thickness and radial diameter are correlated negatively; the wall thickness increases from EW to LW. Mean wall thickness was similar for the two species (yew: 3.1 and 2.7 µm, spruce: 2.7 and 2.9 µm; Fig. 5f), but the range from EW to LW was again smaller for yew than for spruce. In other words: the walls were thicker in yew EW than in spruce EW, but thinner in yew LW than in spruce LW (spruce (w) had thicker LW walls than spruce (n)). Note that the variation in tracheid wall thickness was much less for EW minima than for LW maxima in all samples. The true peak value of LW wall thickness may be somewhat underestimated, for the same reason as for the wood density (see above). Coarseness is related to the cross-sectional area of the cell wall and gives the tracheid mass per unit length (µg m-1). As a result of their much thicker cell walls, LW tracheids have a higher coarseness than EW tracheids, in spite of the smaller LW tracheid perimeter. Ring-average coarseness slightly increases with increasing growth ring number, mostly because of the increasing perimeter of the tracheids. This tendency was observed for all samples. The mean coarseness was clearly smaller for yew (409 and 328 µg m-1) than for spruce (480 and 478 µg m-1) owing to the lower coarseness maxima in LW of yew; the coarseness minima in EW were similar for both species (Fig. 5g). Overall, the individual tracheid dimensions and their spatial allocations confirm our assumptions: compared with spruce, the yew anatomy is much more homogeneous. It is therefore not surprising that this well-balanced structure is reflected in a less anisotropic elasto-mechanical response in the RT plane (compared to spruce); in this respect yew resembles deciduous trees such as balsa, beech, or mahogany (cf. Grimsel 1999).

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Conclusions

Analysis of key micro-structural features of yew and spruce wood using SilviScan provided valuable information with regard to their structure-property relationships. The large MFA in both EW and LW of yew results in high longitudinal compliance and breaking strain. The contrast between yew EW and LW properties (tracheid diameter, wall thickness, coarseness, and density) was smaller than for spruce. This relatively homogenised yew tissue leads to lower elasto-mechanical anisotropy in the RT plane compared to spruce. From its anatomical features and elastic behaviour, it can be concluded that yew wood combines some of the characteristics of normal wood and compression wood (compression wood has thicker EW tracheid walls and a larger MFA than normal wood), occupying an intermediate position between them, and a distinctive position among softwoods. It has to be noted that all wood characteristics were derived from largely defect-free specimens. At larger scales (e.g., structural timber), account must be taken of knots and other discontinuities. It also has to be noted that the few samples cannot be representative of the species or forests concerned since they do not reflect widely variable characteristics as a consequence of genetic and environmental influences. Although these results can only be indicative, they allow a broad and at the same time detailed understanding of the results of our previous studies. Since yew is a protected species and therefore not used for construction purposes or in wood composites, case studies like this are clearly assigned to the field of basic research. Exploring complex wooden materials that hold a special position regarding their structure-property relationships can generally contribute to a better understanding of wood mechanics. However, the results can also be of use to researchers in biomechan- ics, for example, to expand basic knowledge of (evolutionary) structural optimisation processes in response to specific mechanical demands.

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