Influence of Microfibril Angle on Within-Tree Variations in the Mechanical Properties of Chinese Fir (Cunninghamia Lanceolata)

IAWA Journal, Vol. 32 (4), 2011: 431–442

INFLUENCE OF MICROFIBRIL ANGLE ON WITHIN-TREE VARIATIONS IN THE MECHANICAL PROPERTIES OF CHINESE FIR (CUNNINGHAMIA LANCEOLATA)

Yafang Yin1, Mingming Bian1,2, Kunlin Song1, Fuming Xiao3 and Jiang Xiaomei1,*

SUMMARY Radial variations in microfibril angle (MFA) and their effect on the mechanical properties of plantation-grown Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) were investigated with the aim of achiev- ing an effective utilization of the wood. Correlations between MFA and mechanical properties, including longitudinal modulus of elasticity (MOEL), static bending strength (MOR) and compression strength parallel-to-the-grain (CS), were analyzed for predicting the quality of timber. The results indicated that MFA had a greater variation in juvenile wood than in mature wood. The biggest change occurred close to the pith in Chinese fir. The outer-rings (rings 9–30 from the pith) have a relatively low MFA, together with high mechanical properties and high density, when compared with the inner-rings (rings 1–8 from the pith). The MFA had significant negative curvilinear correlations with all the mechanical properties (MOEL, MOR and CS) of Chinese fir, with the value of r2 being 0.88, 0.69 and 0.74 respectively. The correlation between the MFA and basic density (BD) was strong in certain consecutive rings (rings 5–30 from the pith), but this did not apply across the whole billet, i.e. from the pith to the bark. Key words: Cunninghamia lanceolata, MFA, mechanical properties, basic density.

INTRODUCTION

Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) is one of the most important, fast growing and major commercial plantation species that mainly occurs in 17 prov- inces of the central and southern regions of temperate China. The wood of Chinese

1) Wood Anatomy and Utilization Department, Chinese Research Institute of Wood Industry, Chinese Academy of Forestry, No.1 Dongxiaofu, Beijing, CN 100091, China. 2) State Academy of Forestry Administration, No. 8 Linxiao North Road, Beijing, CN 102600, China. 3) Jiangxi Academy of Forestry, No. 1629 Fenglin Street, Nanchang, Jiangxi Province, CN 330032, China. *) Corresponding author [E-mail: [email protected]]. Associate Editor: Lloyd Donaldson

Downloaded from Brill.com10/01/2021 02:19:26PM via free access 432 IAWA Journal, Vol. 32 (4), 2011 fir has traditionally been used for furniture making, bridge and boat building, general carpentry and timber constructions. However, plantation-grown Chinese fir has not been effectively and fully utilized, due to a high proportion of juvenile wood and large variations in wood properties. With a decrease in natural wood resources and an increase in the demand for wood materials, the wood supply from this tree species is undergoing a recent increased demand in China. As a result, there is an urgent need for much more detailed information on wood quality to ensure the production of high value wood products from this species. As is generally known, when wood properties are analyzed transversely across the radius, around the perimeter or longitudinally along the stem, the wood of Chinese fir varies in its chemical, mechanical, and anatomical properties (Bao & Jiang 1998; Savidge 2003). Therefore, in order to achieve the best utilization of this wood, it is essential that research work should be carried out on the radial and longitudinal variations in wood properties of the tree. Correlations among wood properties may be of help in effectively predicting the quality of the wood. A few studies have been conducted on the variation of MFA within or between trees (Donaldson 1992; Donaldson & Burdon 1995; Zhang et al. 2007; Yin et al. 2011). The variation of MFA between growth rings indicates a consistently decreasing trend from pith to bark at all heights. MFA between heights shows greater values at breast height of the stem and a trend of general decline within the stem with height (Zhang et al. 2007; Donaldson 2008; Yin et al. 2011). Significant variation among trees and growth rings was also indicated in Pinus radiata D. Don (Donaldson & Burdon 1995). The MFA of the S2 layer in the cell wall is known to be one of the main determi- nants affecting the mechanical wood properties in different species (Cave & Walker 1994; Walker & Butterfield 1996; Treacy et al. 2000; Deresse et al. 2003). A strong correlation between MFA and MOEL (longitudinal modulus of elasticity) was obtained, when the distribution along the stem of butt logs of Pinus radiata was studied (Xu & Walker 2004). The MFA was also significantly related to the modulus of rupture (MOR) in small clear wood samples from different species (Bendtsen & Senft 1986; Treacy et al. 2000; Deresse et al. 2003). Using clear wood samples of Pinus radiata, high correlations among MOEL, MFA and density (r = -0.78 and 0.69 respectively) were obtained, while path analysis showed that MFA was the only significant causal factor for MOE per unit of mass (Booker et al. 1998). Cown et al. (2004), in a study of the relative effects of MFA and basic density on MOEL in boards of Pinus radiata clones, found only a low contribution from MFA, when compared to other factors, such as spiral grain and knot area ratio. The correlation between MFA and CS of small clear wood has rarely been investigated (Donaldson 2008). Nevertheless, micromechanical properties parallel-to-the- grain have been obtained using microtomed sections, single fibers or the nano-indentation of cell walls. A high correlation was observed between MFA and the extensibility of wood microtomed sections (Reiterer et al. 1999), whereas single fibers with a larger MFA showed greater extensibility (Mott et al. 2002). Hardness obtained using the nano- indentation of cell wall regions displayed no strong correlation with MFA (Gindl et al. 2004).

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Previous research work has indicated that there is still uncertainty regarding the relationship between MFA and mechanical properties in different species, or at different positions in the tree, and the level of correlation between MFA and specific mechanical properties. Only a few reports on the wood properties of Chinese fir are available and, consequently, correlations between MFA and mechanical properties still need to be ascertained. The aim of this paper is to assess the radial variations in MFA and mechanical properties of plantation-grown Chinese fir, in order to achieve an effective utilization of this wood, and to investigate whether and to what degree the mechanical properties (bending modulus of elasticity (MOE), the bending modulus of rupture (MOR) and the compression strength parallel-to-grain (CS)) of this species are affected by its MFA properties.

MATERIALS AND METHODS Sample preparation Twenty vigorous sample trees were randomly collected from a 36-year-old Chinese fir plantation at the Shanxia Forestry Centre (114° 30' E, 27° 30' N), in Dagangshan in the Jiangxi Province, which is located at the Experimental Centre of Subtropical For- estry of the Chinese Academy of Forestry. The plantation was cultivated from seedlings, planted at a spacing of 4 by 6 m.

Small clear specimen 10 mm (R) × 10 mm (T)

a L/15 mm South North b L/20 mm c L/30 mm L/300 mm 8 ... 1 1 ... 8 a - MFA d L/150 mm b - CS c - BD d - MOEL and MOR

A B C Figure 1. A schematic diagram of the sampling. – MFA = microfibril angle; CS = compression strength; BD = basic density; MOE = modulus of elasticity; MOR = modulus of rupture.

A stem disk (billet) sample with a thickness of 300 mm was taken from each tree, at a height of 1.3 to 1.6 m from ground level (Fig. 1A). After marking the north and the south facing stem surfaces, a radial section 10 mm thick and including the pith, was taken from each billet in the north-south direction of the tree stem. A series of longitudinal wood strips were then sawn from the pith to the bark in each section (Fig. 1B). Each strip was subsequently cut into four blocks for measuring MFA, basic density (BD) and mechanical properties (MOE, MOR and CS) respectively (Fig. 1C). No significant variations of selected properties were observed between north and south

Downloaded from Brill.com10/01/2021 02:19:26PM via free access 434 IAWA Journal, Vol. 32 (4), 2011 sections. Therefore, it was assumed that the average value of samples that had been collected from both the north and south sections at the same growth ring from the pith would represent the average properties.

X-ray diffraction for MFA measurement Tangential sections, approximately 1 mm thick and containing latewood, were obtained from wood blocks using a stereo microscope. The method for measuring MFA using X-ray diffraction was similar to that described in an earlier paper (Yin et al. 2011). In brief, a tangential section was attached to a holder in an X-ray scattering system (Dmax-3BX). When the sample was rotated, the intensity curve was measured as a function of the rotation angle φ at a step of 0.5° and a measuring time of 180 s per point. The radiation source was a nickel-filtered radiation of 40 kV, and 30 mA. The mean MFA of the tracheid cell wall layer was then calculated, according to the measuring method of the 002 peak made in transmission, developed by Cave (1966) and Meylan (1967), using 0.6T, where T is the half-width of the peak, taken from the tangents drawn at the points of inflection (Anderssonet al. 2000).

Mechanical testing The mechanical properties, i.e. bending MOE, MOR and CS, were used for evalu- ating the quality of the wood using small clear wood samples without any defects. Small clear wood samples, with dimensions of 10 × 10 mm in cross section and 150 mm in the longitudinal direction, were used for static bending MOE and MOR tests. Small samples, 10 mm (R) × 10 mm (T) × 20 mm (L), were used for obtaining the compressive strength parallel-to-grain (CS). Before mechanical testing, all the small clear samples were conditioned until they had attained an equilibrium moisture content (EMC) of approximately 12%, after storing them in a climate room maintained at a constant temperature of 20 °C and a RH of 65%. The center point loading method was carried out over a span of 120 m to determine both the MOEL and MOR values, using a universal testing machine (AMSLER-4T, Switzerland). A deflectometer mounted on an aluminium alloy frame was suspended over the outer supports at the neutral axis of the sample to measure the mid-span deflec- tion relative to the support points. Measurements of the CS were made with the same equipment. Subsequently, a wood block, 10 mm (R) ×10 mm (T) × 30 mm (L), was used for measuring the wood basic density (BD), using a water displacement method (Chinese standard GB1933-91).

Data analysis A one-way analysis of variance (ANOVA) was used for comparative analyses. A statistical analysis of the differences among all the positions was made, using SAS programs (SAS Institute 9.0). The correlation coefficients between MFA and all the mechanical properties (MOEL, MOR and CS) of the small clear specimens were calculated to determine their relationship.

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RESULTS AND DISCUSSION

Variations in the microfibril angle The average value of all properties at breast height among twenty trees was calculated and listed according to growth ring number from the pith (Fig. 2). The MFA varied from 20.1° to 8.5°, with a coefficient of variation (COV) value of 36.6% from the pith to the sapwood. The average MFA value obtained using X-ray diffraction, decreased sharply by 9.7° from the pith to ring 6 and then displayed a constant value of approximately 8.5° towards the bark. Based on the t-test, at a 95% confidence level, significant variation was observed (p < 0.05) in the inner-rings (rings 1–8 from the pith), while no significant variation was observed (p > 0.05) for the outer-rings (rings 9–30 from the pith) (Table 1). In addition, as inferred from the standard deviation in Figure 2, the between-tree variations of MFA in rings 1–4 are slightly greater than those of rings 5–30, although no significant tree-to-tree variations were observed in MFA for all eight selected positions among all twenty trees (p > 0.05). It can therefore be concluded that variations in MFA are greater in rings 1–8 than in rings 9–30. Studies of Cryptomeria japonica (L. f.) D. Don (Yamashita et al. 2000), and Pinus massoniana Lamb. (Zhang et al. 2007) agree with the result that a larger MFA is clearly as- sociated with the juvenile wood in the first few rings near the pith, followed by a steep decline towards the bark (Donaldson 2008).

Table 1. Variance analysis of wood characteristics at different positions across the radius from the pith to the sapwood.

Rings MFA MOEL MOR CS BD (degree) (GPa) (MPa) (MPa) (kg/m3) 1 – 2 A A A A A 3–4 B A A A B 5–6 C B A A B 7–8 D BC A B AB 9–10 D CD B C C 11–19 D D B C C 20–23 D D B C C 24–30 D D B C C F value 35.32 14.86 10.64 17.15 7.31 p > F < 0.0001 < 0.0001 < 0.0001 < 0.0001 < 0.0001

Note: MFA = microfibril angle; MOEL = longitudinal modulus of elasticity; MOR = static bending strength; CS = compression stress parallel to the grain; BD = basic density. Different letters in the same column indicate that there is a significant difference between rings at p < 0.05 (Student Newman Keuls test).

Variation in wood mechanical properties As seen in Figure 2, for MOEL the average value at breast height ranged from 6.65 to 10.28 GPa with a COV value of 15.4 %, while for MOR the range was from 56.2 to 80.4 MPa with a COV value of 13.7%, and for CS from 30.3 to 39.3 MPa with a

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COV value of 11.9%. All the mechanical properties increased rapidly in the inner-rings (rings 1–8 from the pith) and then showed a tendency to flatten out towards the bark. Moreover, no significant variations were observed in the selected mechanical properties at each position among the twenty trees (p > 0.05). To better discern the changes in all mechanical properties, the within-tree variations among the different positions were compared using the t-test at a 95% confidence level (Table 1). For MOEL values, a significant difference (p < 0.05) was obtained between rings 5 and 6 and rings 7 and 8. MOR, CS and BD values all showed significant differences between rings 7 and 8 and rings 9 and 10 (p < 0.05) (Fig. 2).

Juvenile wood Mature wood ) 375 3 340 305 270 235 200 100 85 70 55 40

10 (GPa) CS (MPa) MOR (MPa) BD (kg/m L 5

MFA (°) MOE MFA 10

0 1–2 3–4 5–6 7–8 9–10 11–19 20–23 24–30 Growth rings Figure 2. Variations in the mechanical properties and microfibril angle (MFA) at breast height as a function of growth ring from pith to bark. – MFA = microfibril angle; MOEL = longitudinal modulus of elasticity; CS = compression stress parallel-to-grain; MOR = static bending strength; BD = basic density. The bars represent the standard deviation across the radial positions within the 20 trees. The dashed line indicates the transition between juvenile wood and mature wood zones at breast height.

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The variations in properties observed in the current research imply that there are two significantly different zones from the pith to the sapwood that could be utilized for different end uses: 1) the inner-rings (rings 1–8 from the pith) that have low quality and 2) the outer-rings (rings 9–30 from the pith) that have a relatively high quality. Different parameters have been used to distinguish between juvenile and mature wood including tracheid length and MFA (Watanabe et al. 1963), tracheid length and wood density (Zobel & Sprague 1998), or growth rate and tracheid length (Sudo 1986), which have been used to identify the juvenile wood. In the present study, the inner-rings (rings 1–8 from the pith), which have a greater MFA, a lower BD and lower mechanical properties, are considered to be juvenile wood (Fig. 2) in agreement with previous reports (Zobel & Sprague 1998; Burdon et al. 2004; Gapare et al. 2006). In radiata pine, Zobel and Sprague (1998) indicate that juvenile wood can occur between 7 and 13 rings from the pith. The transition from juvenile to mature wood can be influenced by site and silvicultural practice (Downeset al. 2003; Gapare et al. 2006). However, this is somewhat different from Bao and Jiang (1998), who came to the conclusion that the boundary between the juvenile wood and the mature wood can be defined as ring 16 in plantation-grown Chinese fir, taking into account tracheid length and wood basic density at breast height. High density might be desirable for pulping, but is less necessary for low-density decorative timbers, with surface-hardening technologies commercially available for some structural uses. However, it is possible to achieve a greater degree of stiffness more effectively by improving MFA. Wood quality can therefore be improved as a result of distinguishing juvenile wood from mature wood in fast-grown pine plantations (Cave & Walker 1994). All the properties that are set out in Figure 2 indicate that wood utilization can be optimized by selecting juvenile wood and mature wood for more effective utilization of plantation-grown Chinese fir.

Relationships between the microfibril angle and wood properties The relationships between MFA and wood mechanical properties are shown in Figure 3A and B. Mechanical properties, including MOEL, MOR, and CS, all increase logarithmically with the decreasing of MFA. The relationship of MFA with MOEL, MOR and CS displays significant negative curvilinear correlations, with values of r2 being 0.88, 0.69 and 0.74 respectively. This result indicates that MFA can be considered as an important factor affecting the wood mechanical properties of Chinese fir. Previous researchers have also confirmed that MFA can be used to predict wood mechanical properties in other species (Bodig & Jayne 1982; Cown et al. 1999; Bergan- der 2001). Many researchers have determined a curvilinear relationship between MFA and MOEL (Cown et al. 1999; Yamashita et al. 2000; Xu & Walker 2004). MFA is also correlated with the modulus of rupture (MOR) in plantation-grown Pinus taeda L. (Bendtsen & Senft 1986) and in the early juvenile wood of Pinus resinosa Aiton (Deresse et al. 2003). Although the correlation between MFA and CS of small clear wood has rarely been studied (Donaldson 2008), we have shown a strong correlation between MFA and compression strength parallel-to-grain (CS), when using small clear wood samples of Chinese fir.

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14 14

11 11 (GPa) (GPa)

L 8 L 8

5 5 y = -4.40 Ln (x) + 19.45, R2 = 0.84 y = 0.065x – 10.17, R2 = 0.75 2 2 7 9 11 13 15 17 19 21 260 270 280 290 300 310 320 330 MFA (°) A C BD (kg/m3) B D MOR 100 MOR 90 y = -26.01 Ln (x) + 131.20, R2 = 0.70 y = 457.05x – 65.77, R2 = 0.90 CS 85 CS 75 70 60 55 45

30 40 Strength MOE (MPa) Strength MOE (MPa) 15 y = -11.96 Ln (x) + 64.25, R2 = 0.74 25 y = 198.58x – 22.92, R2 = 0.85 0 10 7 9 11 13 15 17 19 21 260 270 280 290 300 310 320 330 MFA (°) BD (kg/m3) Figure 3. The relationships between mechanical properties, microfibril angle (MFA), and basic density (BD). – A: The longitudinal modulus of elasticity (MOEL) as a function of microfibril angle (MFA) (p < 0.001). – B: The static bending strength (MOR) and compression strength parallel-to-grain (CS), as a function of the microfibril angle (MFA) (p < 0.01). – C: The longitudinal modulus of elasticity (MOEL) as a function of the basic density (BD) (p < 0.01). – D: The static bending strength (MOR) and compression strength parallel-to-grain (CS), as a function of the basic density (BD) (p < 0.001).

2 For Chinese fir, the correlation between MFA and MOEL (r = 0.88) was greater than that between MFA and MOR (r2 = 0.69). Donaldson (2008), in summary, stated that the properties of the cell wall material (specifically the MFA) and the amount of cell wall (density) both affect the mechanical properties of wood. Consequently, in order to ascertain which of these is the key factor affecting a specific mechanical property in Chinese fir, the relationship between wood basic density (BD) and mechanical properties was also investigated in this study. As shown in Figure 3C and D, significant positive linear correlations between wood 2 BD and MOEL, MOR and CS were obtained, with values of r being 0.65, 0.88 and 0.81 respectively. This result is in accordance with that of other researchers (Cown 1999; Holmberg 2000), who stated that strong relationships or linear relationships exist between wood density and mechanical properties. For Chinese fir, regression analysis 2 showed that the correlation between MOEL and MFA (r = 0.88) was higher than with BD (r2 = 0.65), while the correlation between MOR and MFA (r2 = 0.69) was lower than with BD (r2 = 0.88). Using small clear wood samples of Pinus radiata, Booker et al. (1998) concluded that the correlation between MOEL and MFA (r = -0.78) was higher than the correlation between MOEL and BD (r = 0.69). MFA also had a stronger

Downloaded from Brill.com10/01/2021 02:19:26PM via free access Yin, Bian, Song, Xiao & Jiang — Microfibril angle in Cunninghamia 439 correlation with MOEL, when the distribution of MFA, density and MOEL along the stem of the butt logs of Pinus radiata were studied (Xu & Walker 2004). The MOEL value of plantation-grown Pinus taeda obtained from both density and MFA was compared showing that the increase in density accounted for only a small fraction of the increase in stiffness, in contrast to MFA (Bendtsen & Senft 1986). In addition, it was concluded that MFA had a higher correlation with MOE than with MOR and that wood density had a higher correlation with MOR than with MOE, when factors affecting the juvenile wood of Pinus radiata (Ivkovic et al. 2009) were investigated. In Chinese fir, MFA varies more in the juvenile wood than BD, while both MFA and BD tend to show small variations in the mature wood. Comparisons of correlations among MFA, density and MOEL have shown that MFA has a stronger effect in the butt log and in the juvenile wood because MFA tends to vary within and among trees, mainly in the juvenile wood. In comparison, density varies mostly in the mature wood, thus explaining the difference in importance between MFA and density in relation to MOE (Cown et al. 1999). The correlation between MFA and wood density is variable (Donaldson 2008). Some studies have shown a correlation between MFA and wood density while other studies show no correlation (Bergander 2001). In Chinese fir, we have investigated the relationship between MFA and BD, not only in certain consecutive growth rings (rings 5–30 from the pith) but also in the whole billet from pith to the bark (Fig. 4). The correlation between MFA and BD in some consecutive growth rings, i.e. rings 5–30 from the pith, was very strong presenting r2 of 0.97, while the correlation between MFA

330 320 y = -0.024x + 0.52

) 310 3 R2 = 0.97 300 290

BD (kg/m 280 270 260 7 9 11 13 15 17 19 21 MFA (°) Figure 4. Linear relationship between the microfibril angle (MFA) and basic density (BD) in certain consecutive rings (rings 5–30 from the pith) at breast height. (p < 0.001). and BD in the whole billet, i.e. from the pith to the bark, was not significant. Therefore, it can be concluded that MFA has a significant negative-linear relationship with BD in consecutive growth rings (rings 5–30 from the pith), although it is not correlated with BD in the whole billet. The large variation of MFA and small variation of BD in the juvenile wood region, and small variations of both MFA and BD in mature wood (Fig. 2) may account for this phenomenon.

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CONCLUSIONS In Chinese fir, MFA shows greater variation in the juvenile wood than in the mature wood, with the largest variation occurring close to the pith. At breast height, using MFA as a criterion, there is evidence of a small juvenile wood zone which in these samples averaged 8 growth rings. As an important anatomical feature, MFA displays significant negative curvilinear correlations with mechanical properties (MOEL, MOR and CS) at breast height of plantation-grown Chinese fir, with r2 values of 0.88, 0.69 and 0.74 respectively. In addition, MFA has a higher correlation with MOEL than with MOR, while BD had a higher correlation with MOR than with MOEL. Furthermore, the relationship between MFA and BD is highly significant in some consecutive rings, i.e. rings 5–30 from the pith, although it is not observed across the whole billet, i.e. from the pith to the bark. This can be interpreted by the different variations of MFA and BD in juvenile and mature wood region.

ACKNOWLEDGEMENTS

This work was supported financially by a project at the Chinese National Natural Science Founda- tion (No. 30972303) and the Chinese State Forestry Administration project (201104058). It should be noted that Mr. M. Bian contributed equal work as the first author to the article. We would like to express our gratitude for the technical help given by Professor J. Lv, H. Ren, X. Luo and Mrs. M. Xu at the Research Institute of Wood Industry, the Chinese Academy of Forestry. We grate- fully acknowledge Dr. L. Donaldson of Scion, New Zealand, Dr. L. Salmén of Innventia AB and Ms. E. Bergström from the KTH Royal Institute of Technology, Sweden, for their invaluable com- ments on the report.

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