Article Physical and Mechanical Properties of Fast Growing Polyploid Hybrids (A. auriculiformis × A. mangium) from Vietnam

Dang Duc Viet 1,2, Te Ma 1, Tetsuya Inagaki 1, Nguyen Tu Kim 2, Nghiem Quynh Chi 2 and Satoru Tsuchikawa 1,* 1 Graduate School of Bioagricultural Sciences, Nagoya University, Furo-Cho, Chikusa, Nagoya 464-8601, Japan; [email protected] (D.D.V.); [email protected] (T.M.); [email protected] (T.I.) 2 Vietnamese Academy of Forest Sciences, 46 Duc Thang, Bac Tu Liem, Hanoi 100000, Vietnam; [email protected] (N.T.K.); [email protected] (N.Q.C.) * Correspondence: [email protected]; Tel.: +81-52-789-4155

 Received: 4 June 2020; Accepted: 26 June 2020; Published: 29 June 2020 

Abstract: Acacia are globally important resources in the industry, but particularly in Southeast Asian countries. In the present study, we compared the physical and mechanical properties of polyploid Acacia (3x and 4x) clones with those of diploid (2x) clones grown in Vietnam. We randomly selected 29 aged 3.8 years from different taxa for investigation. BV10 and BV16 clones represented the diploid controls; X101 and X102 were the triploid clones; and AA-4x, AM-4x, and AH-4x represented neo-tetraploid families of Acacia auriculiformis, Acacia mangium, and their hybrid clones. The following metrics were measured in each : stem height levels, basic density, air-dry equilibrium moisture content, modulus of rupture (MOR), modulus of elasticity (MOE), compression strength, and Young’s modulus. We found that the equilibrium moisture content significantly differed among clones, and basic density varied from pith-to-bark and in an axial direction. In addition, the basic density of AA-4x was significantly higher than that of the control clones. Furthermore, the MOR of AM-4x was considerably lower than the control clones, whereas the MOE of X101 was significantly higher than the control values. The compression strength of AM-4x was significantly lower than that of the control clones, but AH-4x had a significantly higher Young’s modulus. Our results suggest that polyploid Acacia hybrids have the potential to be alternative species for providing wood with improved properties to the forestry sector of Vietnam. Furthermore, the significant differences among the clones indicate that opportunities exist for selection and the improvement of wood quality via selective breeding for specific properties.

Keywords: Acacia hybrid; polyploid acacia; triploid acacia; tetraploid acacia; moisture content; basic density; modulus of rupture; modulus of elasticity; ultimate stress in compression parallel to grain; Young’s modulus

1. Introduction Plants of the genus Acacia are an important resource in global furniture manufacturing and are vital to the wood industry of Southeast Asian countries [1–11]. Improvements in technology and breeding techniques have increased the interest in Acacia products [12]. An Acacia hybrid (an interspecific hybrid of Acacia mangium and Acacia auriculiformis) was first recognized in Malaysia in 1972 [10]; since then, this hybrid has rapidly become an important tree in the industrial output of many countries, including Vietnam. Many studies on this Acacia hybrid, aimed at providing information for end-use applications, have shown that it is fast growing, has a medium strength, and can be utilized in many ways. For example,

Forests 2020, 11, 717; doi:10.3390/f11070717 www.mdpi.com/journal/forests Forests 2020, 11, 717 2 of 15

Kijkar (1996) demonstrated that the Acacia hybrid can grow to 8–10 m in height and 7.5–9.0 cm in diameter at breast height (DBH) in two years [13]. Another study indicated that, at 2.5–3.0 years of growth, the Acacia hybrid had produced a stem volume that was several times higher than that of A. mangium and A. auriculiformis, while at 4.5 years, it had produced twice the stem volume of A. mangium [14]. Although the Acacia hybrid has been shown to have a lower basic density than A. auriculiformis and a similar density to A. mangium, the shrinkage, modulus of rupture, and maximum strength in compression of these plants are not significantly different [7]. In contrast to these findings, Jusoh et al. [15] reported that the basic density and strength properties of the Acacia hybrid were significantly greater than those of second-generation A. mangium. Indeed, Rokeya et al. [16] compared the characteristics of the Acacia hybrid to those of teak, i.e., it is a moderately strong wood that is suitable for making furniture and household items. Three eight-year-old clones of the Acacia hybrid (namely HD3, K47, and H4) have also been compared with teak in their suitability for end-use applications [17]. In the study of these clones, the Acacia hybrid had a larger DBH than its original parent plant at the same age; however, the suitability indices of the three clones were slightly lower than those of the parent form [17]. The Acacia hybrid has also been shown to meet the requirements for use as pulp, use in the paper industry, and for certain various uses such as tool handles, furniture, and pallets [17]. Investigations into the wood properties of the Acacia hybrid clones [17–19] showed significant differences in their growth and in some wood properties. The effects of genetics and the environment on different Acacia hybrid clones have also been investigated. Studies show that the clone type and grown site significantly affect DBH and specific gravity (which can be estimated for the tree at a height of 3 m and anticipated at a young age [18]) [19–21]. However, significant differences in height and DBH are known to exist among clones at four and five years, but not at three years [21]. In Vietnam, the variation in Acacia wood properties is affected by climatic conditions; this variation is more visible in northern than in southern Vietnam [19]. Paiman et al. [22] examined the machining and physical properties of Acacia hybrid wood when it was stressed and non-stressed: non-stressed wood had better machining characteristics than stressed wood. Several studies of the properties and anatomy of the Acacia hybrids have shown that it has great potential for fiber production. For example, Yahya et al. [23] found that the hybrid produced a higher pulp yield and had better paper strength than its pure form parents, A. mangium and A. auriculiformis, because it had thinner cell walls, a smaller proportion of ray cells, and a higher wood density than A. mangium. Jusoh et al. [15] also agreed that the Acacia hybrid could be used for pulp and paper production as its fiber length was greater than that of A. mangium. Similarly, Sharma et al. [24] found that three Acacia hybrid clones from had longer fibers than those of A. mangium and A. auriculiformis. In contrast, Nirsatmanto et al. [12] reported no significant differences in the fiber lengths of the Acacia hybrid and its parent species [12]. Haque et al. [25] provided a broader perspective: the Acacia hybrid was the best among acacia species for pulpwood production in papermaking; however, the a-cellulose content and pulp properties of the Acacia hybrid were similar to those of A. mangium and A. auriculiformis. Other studies have also shown that the Acacia hybrid has properties, such as paper pulp yield and breaking strength, that are superior to its parent species [14,25]. Recently, Acacia hybrid plantation productivity and quality have been improved by using both cutting and tissue culture breeding technologies effectively in a large-scale clonal forestry. Research into, and the development of, breeding technologies has led to innovations in forestry and created cutting-edge methodologies [26], such as the breeding of polyploid Acacia hybrids that possess three or more sets of chromosomes [27]. This method is introducing diversity into breeding populations, reducing reproduction, fertility, and producing new wood fibers [3,28]. The advantages of polyploid Acacia hybrids have been described in several studies. For example, the Kraft pulp of eight-year-old tetraploid clones has been shown to have higher bulk, porosity, and tear strength than the pulp of diploid clones [29]. In addition, tetraploid clones are known to have longer and wider fibers than diploid clones [3,29]. Finally, the basic density of diploid and triploid Acacia hybrids has been compared in two locations in Vietnam [30]. Forests 2020, 11, 717 3 of 15

Although some research on the properties of polyploid Acacia hybrids has been conducted in the studies cited above, there is currently a limited understanding of the various wood properties of the clones. Therefore, in the present study, we investigated the physical and mechanical properties of the wood from polyploid Acacia hybrid clones from Vietnam. The relationships among the wood properties and clones were also evaluated, with the aim of improving breeding programs.

2. Materials and Methods

2.1. Acacia Hybrid Polyploid Clones and Sampling The materials used in this study were collected from nine Acacia hybrid clonal trials established by the Institute for Forest Tree Improvement and Biotechnology (Vietnamese Academy of Forest Sciences), between 2014 and 2018. The trial sites are located in Dong Nai (10◦570 N, 106◦490 E) in southern Vietnam. The soil type in the site is sandy alluvium, the mean annual rainfall is 1640 mm, and the mean annual temperature is 27 ◦C. Acacia hybrid diploid (2x) clones available for commercial breeding, namely BV10 and BV16, were chosen as control clones. The best selection of commercial Acacia hybrids were transformed into tetraploid (4x) clones by in vitro colchicine treatment. Triploid (3x) clones could then be produced by pollination between 2x and 4x. Five ramets of each of the four 2x and 3x clones (i.e., BV10, BV16, X101, and X102), together with three 4x trees of tetraploid taxa (i.e., A. auriculiformis, A. mangium, and the Acacia hybrid) were felled at age 3.8 years to collect wood samples. Details of the clones are provided in Table1.

Table 1. Polyploid materials at 3.8 years old used in the study.

No. of Stem Clone/Tree DBH Height Taxa Ploidy Parental Information Ramets/Trees Volume ID (cm) (m) Sampled (m3) BV10 2x 5 13.0 16.0 0.21 Acacia hybrid Commercial diploid BV16 2xAH clones 5 12.6 15.0 0.19 X101 3x Controlled pollination 5 15.0 17.9 0.32 Acacia hybrid (CP) between diploid X102 3xAA and tetraploid AM 5 14.4 16.6 0.27

Colchicine-induced A. auriculiformis AA 4x 3 11.8 10.1 0.11 tetraploid trees of AA Colchicine-induced A. mangium AM 4x 3 17.9 13.2 0.33 tetraploid trees of AM Colchicine-induced Acacia hybrid AH 4x 3 11.6 14.7 0.16 tetraploid clones of AH DBH, diameter at breast height.

Figure1 illustrates the samples collected in this study. Logs that were 0.2–1.3 m and 1.5–3 m in length were taken from each tree stem after felling, to extract samples for measurement of mechanical properties. 5-cm disks were also taken to test physical properties at various height levels (0.2–13.5 m). Disk edges were coated with wax to prevent decay and other environmental alterations. DBH (1.3 m) was determined as the mean of two cross diameters. After felling, total height was also measured. Forests 2020, 11, 717 4 of 15 Forests 2020, 11, 717 4 of 15

FigureFigure 1. Sampling 1. Sampling methods methods for discs for anddiscs logs and and logs test and samples test samples for physical for physical and mechanical and mechanical properties. (MC, moistureproperties. content; (MC, moisture BD, basic content; density). BD, basic density)

2.2. Air-Dry2.2. Air-Dry Moisture Moisture Content Content and and Basic Basic Density Density Pith-to-barkPith-to-bark strips strips (diameter (diameter30 × 30 ×20 20 mm) mm) werewere cutcut from each each disk disk after after air air drying. drying. The The strips strips × × were thenwere divided then divided into small into small pieces pieces at a distanceat a distance of 1 cmof 1 fromcm from pith-to-bark pith-to-bark with with a razor a razor blade. blade. Each Each piece piece was weighed at the air-dry stage and then dried at 103°C ± 3°C to determine dry weight. was weighed at the air-dry stage and then dried at 103 ◦C 3 ◦C to determine dry weight. Equilibrium Equilibrium air-dry moisture content was determined± at room temperature (around 25°C) and air-dry moisture content was determined at room temperature (around 25 ◦C) and humidity (25–35% humidity (25–35% relative humidity (RH)). Basic density was measured with an electronic relative humidity (RH)). Basic density was measured with an electronic densimeter (EW-300SG, Alpha densimeter (EW-300SG, Alpha Mirage, Osaka, Japan). Samples for basic density measurements were Mirage, Osaka, Japan). Samples for basic density measurements were soaked in distilled water for at soaked in distilled water for at least 24 h to ensure full hydration. They were then immersed and least 24suspended h to ensure using full a metal hydration. shield inside They the were water then bath immersed of the densimeter and suspended to determine using wood a metal volume shield insideby the water water displacement. bath of the Samples densimeter were tothen determine oven-dried wood at 103 volume °C for around by water 48 h displacement. until they reached Samples a were thenconstant oven-dried mass; they at were 103 ◦thenC for re-weighted. around 48 Basi h untilc density they was reached calculated a constant as oven-dry mass; mass they divided were then re-weighted.by wood Basic volume. density was calculated as oven-dry mass divided by wood volume. The changeThe change in basic in basic density density with with the the diameter diameter growth growth ofof each disk disk is is described described as asfollows: follows: 1 1 3 1 3 5 2n 1 BD1 = BD1BD2 = BD1 + BD2... BDn =1 BD13+ BD2 + BD3 + ... + − BDn 4 4 BD2 n2 BDn 2 n2 n2 4 4 (where BD1, BD2, ... BDn are basic densities for disk… diameters of 1, 2, ... n cm, respectively; BD1, BD , ... BD are basic densities for samples at position 1, 2, ... n cm). 2 n 1 3 5 2 1 ⋯ 2.3. Mechanical Testing

Specimens for static bending (20 20 320 mm, radial tangential axial) and compression (where BD1, BD2, … BDn are basic densities× × for disk diameters× of 1, 2, ... n cm,× respectively; BD1, BD2, (20 20 30 mm, radial tangential axial) were cut from a 20-mm-thick board. The specimens were × … ×BDn are basic densities× for samples× at position 1, 2, … n cm). conditioned to a constant mass at 20 ◦C 2 ◦C and a RH of 65% 5%. They were then maintained ± ± in this condition until required for testing. The average moisture content of the test samples at this stagewas 12%. The modulus of rupture (MOR) and modulus of elasticity (MOE) were determined by a three-point bending test according to ISO 13061–3:2014 and ISO 13061–4:2014 standards [31,32]. Forests 2020, 11, 717 5 of 15

After these tests were completed, samples were taken from undamaged portions to determine density for estimating the relationship between bending and density. Compression parallel to the grain was performed in a 100 kN universal testing machine (AG-I, Shimazdu, Japan). The displacement was measured using strain gage (FLAB 511, TML, Japan) with a gage factor of 2.1% 1%. Compression strength and Young’s modulus were determined according to ± ISO 13061–17:2017 and ISO 130614:2014 standards [32,33].

2.4. Statistical Analysis The statistical analysis involved a completely randomized design. ANOVA was used to test for differences in the outcomes of experiments with different numbers of samples. Averages were compared using Tukey’s test. The Mann–Whitney U test was also used to test for differences between clone in terms of their physical and mechanical properties. The significance level in all tests was p < 0.05.

3. Results and Discussion

3.1. Moisture Content and Basic Density The equilibrium air-dry moisture content of seven clones is described in Figure2. As shown in Figure2, X102 had the highest EMC value of the clones, whereas AA-4x had the lowest value. As presented in Table2, the mean moisture contents of the seven clones were significantly di fferent. Moreover, the data could be divided into two groups: the 4x genotypes in one group and the other clones in a second group. Because wood is always exposed to varying climatic conditions, defining the equilibrium moisture content (EMC) is important for the effective use of wood. Mechanical properties and shrinkage changes, which are probably the most important issues in the end-use applications of solid wood, are positively correlated with the EMC of wood [34–36]. The wood properties are related to heartwood and sap wood ratio, where the heartwood absorbs less water than sapwood [37–42]. This is same tendency with changing in EMC, which is affected by heartwood to sapwood ratio [43] as well as wood density [41,42]. The relationship between basic density and EMC in Figure3 pointed out that basic density had a weakly correlation with EMC in this study. Thus, the ratio of heartwood-sapwood may explain the difference in EMC between clones. In previous research, triploid Acacia had a higher proportion of heartwood than diploid Acacia when the plants were of equal age (3.8 years) and from similar locations [30]. Thus, the EMC of clones may be higher when the ratio of heartwood increases. This observation was consistent with the findings of a previous study, which showed that the EMC for the heartwood of Pinus radiata was higher than that of its sapwood at all humidity and temperature conditions tested [43]. In further research, it will be necessary to investigate the heartwood ratio of tetraploidForests 2020 clones,, 11, 717 in order to better explain the relationship between EMC and the heartwood6 of 15 ratio.

7 6.5 6 5.9 5.5 5.4 5.1 5 5.1 4.6 4.5 4.5 4.6

Moisture content (%) content Moisture 4 3.5 BV10 BV16 X101 X102 AA-4x AM-4x AH-4x Clone FigureFigure 2. 2.Variation Variation of moisture content content (%). (%).

0.9 y = 0.0456x + 0.2215 0.8 R² = 0.0235 0.7 ) 3 0.6

0.5

0.4

0.3

Basic density (g/cm density Basic 0.2

0.1

0 4 4.2 4.4 4.6 4.8 5 5.2 5.4 EMC (%)

Figure 3. Relationship between basic density and equilibrium moisture content (EMC).

Figure 4 shows the basic densities of the seven clones. The clone AA-4x had the highest basic density, while AM-4x had the lowest. This result contrasts with those for EMC, where AA-4x had the lowest rather than highest value. This trend is consistent with previous studies, in which EMC was reported to decrease as basic density increased [34,35]. As shown in Table 2, the basic densities of AA-4x and AM-4x were significantly different from those of BV10 and BV16; however, the basic densities of X101, X102, and AH-4x did not differ from those of BV10 and BV16. The wood density of Acacia hybrids was previously reported by PV et al. [30] They showed that the basic density of diploid and triploid Acacia hybrids was 0.36–0.49 g/cm3, and their mean values for diploid and triploid hybrids in southern Vietnam are consistent with our results. They also showed that Acacia hybrids from the same clone at the same age planted in southern Vietnam had similar basic densities. In another study, Ismail and Farawahida [7] indicated that the density of 6-year-old Acacia hybrids was 0.47 g/cm3. Likewise, Acacia hybrids at 7–8 years old are reported to have 0.43–0.49 g/cm3 densities [15,17]. In other studies, the densities of Acacia hybrids at 8 and 9–12 years old was 0.61–0.69 and 0.58 g/cm3, respectively, whereas A. mangium density was 0.52 g/cm3 and A. auriculiformis densities were 0.54 g/cm3 at 5.5 years and 0.69 g/cm3 at 11 years [44–46]. Previous research showed that a direct comparison of species is difficult because the wood density of Acacia species varies greatly and depends on site conditions and tree age [46]. For our study, the most interesting observation was that the basic density of AA-4x was significantly higher than that of the 2x clone.

Forests 2020, 11, 717 6 of 15

Table 2. Wood properties and ANOVA results in different clones. Forests 2020, 11, 717 6 of 15 MC (%) BD (g/cm3) MOR (MPa) MOE (GPa) σ (N/mm2) E (GPa) 5.1 c 0.47 b 71 ab 8.7 bc 46.0 a 9.6 bc BV107 (0.2) (0.06) (17) (1.1) (6.7) (1.3) 6.5 5.1 c 0.44 cd 75 ab 8.6 bc 44.6 a 9.2 c BV16 (0.3) (0.07) (15) (0.8) (5.4) (1.5) 6 5.4 b 0.45 c 79 a 9.7 a 45.3 a 9.9 ab X101 5.9 (0.3) (0.07) (14) (1.2) (5.5) (1.3) 5.5 5.9 a 0.43 d 705.4 b 8.5 bc 44.4 a 9.3 c X102 5.1 5 (0.4) 5.1 (0.07) (12) (1.3) (5.8) (1.3) 4.5 d 0.50 a 72 ab 8.2 c 44.0 a 8.9 cd AA-4x 4.6 4.6 4.5 (0.2) (0.05) (11) (1.2) 4.5 (6.0) (1.7)

Moisture content (%) content Moisture 4.6 d 0.37 e 56 c 7.2 d 38.1 b 8.2 d AM-4x4 (0.4) (0.10) (11) (0.9) (5.6) (1.3) 3.5 4.6 d 0.42 d 71 ab 9.2 ab 46.5 a 10.3 a AH-4x (0.3)BV10(0.06) BV16 X101(11) X102(0.9) AA-4x AM-4x(3.8) AH-4x(1.0) Clone Data are show as mean with different group in letters; values in parentheses represent for standard deviation. MC, moisture content; BD, basic density; MOR, module of rupture; MOE, module of elasticity; σ, ultimate stress in compression parallel to the grain;Figure E, Young’s 2. Variation modulus of in compressionmoisture content parallelto (%). the grain.

0.9 y = 0.0456x + 0.2215 0.8 R² = 0.0235 0.7 ) 3 0.6

0.5

0.4

0.3

Basic density (g/cm density Basic 0.2

0.1

0 4 4.2 4.4 4.6 4.8 5 5.2 5.4 EMC (%) Figure 3. Relationship between basic density and equilibrium moisture content (EMC). Figure 3. Relationship between basic density and equilibrium moisture content (EMC). Figure4 shows the basic densities of the seven clones. The clone AA-4x had the highest basic density,Figure while4 shows AM-4x the had basic the densities lowest. This of resultthe seve contrastsn clones. with The those clone for EMC, AA-4x where had AA-4x the highest had the basic density,lowest while rather AM-4x than highesthad the value. lowest. This This trend result is consistent contrasts with with previous those for studies, EMC, in where which AA-4x EMC was had the lowestreported rather to than decrease highest as basic value. density This increased trend is [34 co,35nsistent]. As shown with in previous Table2, the studies, basic densities in which of AA-4x EMC was reportedand AM-4x to decrease were significantly as basic density different increased from those [34, of35]. BV10 As andshown BV16; in however, Table 2, thethe basic basic densities densities of AA-4xof X101,and X102,AM-4x and were AH-4x significantly did not diff erdifferent from those from of BV10those andof BV10 BV16. and The woodBV16; density however, of Acacia the basic densitieshybrids of wasX101, previously X102, and reported AH-4x by did PV not et al.differ [30] fr Theyom showedthose of that BV10 the and basic BV16. density The of wood diploid density and of triploid Acacia hybrids was 0.36–0.49 g/cm3, and their mean values for diploid and triploid hybrids in Acacia hybrids was previously reported by PV et al. [30] They showed that the basic density of diploid southern Vietnam are consistent with our results. They also showed that Acacia hybrids from the same and triploid Acacia hybrids was 0.36–0.49 g/cm3, and their mean values for diploid and triploid clone at the same age planted in southern Vietnam had similar basic densities. In another study, Ismail hybridsand Farawahidain southern [ 7Vietnam] indicated are that consistent the density with of 6-year-old our results.Acacia Theyhybrids also wasshowed 0.47 gthat/cm3 .Acacia Likewise, hybrids from the same clone at the same age planted in southern Vietnam had similar basic densities. In another study, Ismail and Farawahida [7] indicated that the density of 6-year-old Acacia hybrids was 0.47 g/cm3. Likewise, Acacia hybrids at 7–8 years old are reported to have 0.43–0.49 g/cm3 densities [15,17]. In other studies, the densities of Acacia hybrids at 8 and 9–12 years old was 0.61–0.69 and 0.58 g/cm3, respectively, whereas A. mangium density was 0.52 g/cm3 and A. auriculiformis densities were 0.54 g/cm3 at 5.5 years and 0.69 g/cm3 at 11 years [44–46]. Previous research showed that a direct comparison of species is difficult because the wood density of Acacia species varies greatly and depends on site conditions and tree age [46]. For our study, the most interesting observation was that the basic density of AA-4x was significantly higher than that of the 2x clone.

Forests 2020, 11, 717 7 of 15

Acacia hybrids at 7–8 years old are reported to have 0.43–0.49 g/cm3 densities [15,17]. In other studies, the densities of Acacia hybrids at 8 and 9–12 years old was 0.61–0.69 and 0.58 g/cm3, respectively, whereas A. mangium density was 0.52 g/cm3 and A. auriculiformis densities were 0.54 g/cm3 at 5.5 years and 0.69 g/cm3 at 11 years [44–46]. Previous research showed that a direct comparison of species is difficult because the wood density of Acacia species varies greatly and depends on site conditions and tree age [46]. For our study, the most interesting observation was that the basic density of AA-4x was Forests 2020significantly, 11, 717 higher than that of the 2x clone. 7 of 15

0.8

0.7 ) 3 0.6

0.5 0.50 0.47 0.45 0.44 0.43 0.42 0.4 0.37 0.3 Basic density (g/cm density Basic

0.2

0.1 BV10 BV16 X101 X102 AA-4x AM-4x AH-4x Clone Figure 4. Variation of basic density (g/cm3). Figure 4. Variation of basic density (g/cm3). The mean DBHs and tree heights are shown in Table1. As shown, X101 was the tallest tree and AM-4x had theTable largest 2. DBH.Wood In properties addition, and X101 ANOVA and AM-4x results had in different higher stem clones. volumes than those of other clones. Furthermore, X101 and X102 showed higher growth than BV10 and BV16. Nonetheless, 3 2 faster growth MC does (%) not aBDffect (g/cm wood) density, MOR because (MPa) a relationship MOE (GPa) between σ (N/mm wood density) E and(GPa) growth 5.1 c 0.47 b 71 ab 8.7 bc 46.0 a 9.6 bc rateBV10 has not been found [47,48]. This trend is applicable to Acacia hybrids, as shown in a similar study of diploid and(0.2) triploid hybrids(0.06) [ 30]. (17) (1.1) (6.7) (1.3) The radial5.1 variation c 0.44 of basic cd density at75 each ab stem height8.6 is bc shown in Figure44.6 a5. The basic9.2 densityc BV16 increased from(0.3) pith to the bark(0.07) in the seven clones,(15) which is consistent(0.8) with the(5.4) findings of Walker(1.5) [49], Kim et al. [185.4], and b Machado0.45 et c al. [50], who79 reported a that the9.7 specific a gravity45.3 of Acacia a increased9.9 ab from X101 the pith to the(0.3) outer region(0.07) near the bark.(14) Variance in this condition(1.2) may be(5.5) aff ected by(1.3) variability inside the trees and other factors, such as climate conditions, during tree growth [50]. The axial 5.9 a 0.43 d 70 b 8.5 bc 44.4 a 9.3 c variationX102 of the basic density is presented in Figure6. The basic density was highest at the stump, and it tended(0.4) to initially decline(0.07) moving up(12) the tree, before increasing(1.3) again toward(5.8) the top.(1.3) Similarly, 4.5 d 0.50 a 72 ab 8.2 c 44.0 a 8.9 cd recentAA-4x studies have indicated that density decreases above breast height to about the middle of the tree before increasing(0.2) toward the(0.05) top [18,51]. (11) (1.2) (6.0) (1.7) 4.6 d 0.37 e 56 c 7.2 d 38.1 b 8.2 d AM-4x (0.4) (0.10) (11) (0.9) (5.6) (1.3) 4.6 d 0.42 d 71 ab 9.2 ab 46.5 a 10.3 a AH-4x (0.3) (0.06) (11) (0.9) (3.8) (1.0) Data are show as mean with different group in letters; values in parentheses represent for standard deviation. MC, moisture content; BD, basic density; MOR, module of rupture; MOE, module of elasticity; σ, ultimate stress in compression parallel to the grain; E, Young’s modulus in compression parallel to the grain. The mean DBHs and tree heights are shown in Table 1. As shown, X101 was the tallest tree and AM-4x had the largest DBH. In addition, X101 and AM-4x had higher stem volumes than those of other clones. Furthermore, X101 and X102 showed higher growth than BV10 and BV16. Nonetheless, faster growth does not affect wood density, because a relationship between wood density and growth rate has not been found [47,48]. This trend is applicable to Acacia hybrids, as shown in a similar study of diploid and triploid hybrids [30]. The radial variation of basic density at each stem height is shown in Figure 5. The basic density increased from pith to the bark in the seven clones, which is consistent with the findings of Walker [49], Kim et al. [18], and Machado et al. [50], who reported that the specific gravity of Acacia increased from the pith to the outer region near the bark. Variance in this condition may be affected by variability inside the trees and other factors, such as climate conditions, during tree growth [50]. The axial variation of the basic density is presented in Figure 6. The basic density was highest at the stump, and it tended to initially decline moving up the tree, before increasing again toward the top.

Forests 2020, 11, 717 8 of 15 Forests 2020, 11, 717 8 of 15

0.6 6m height BV10 (2x) 0.5 BV16 (2x) X101 (3x) 0.4 X102 (3x) 0.6 4.5m height 13.5m height

0.5 (k)

0.4 (e) 0.3

0.2 0.6 3m height 12m height

0.5 (j)

0.4 (d) 0.3 ) 3 0.2 0.6 1.5m height 10.5m height

0.5 (i)

0.4

Basic density (g/cm (c) 0.3

0.2 0.8 1.3m height 9m height 0.7 (h) 0.6 0.5 0.4 (b) 0.3 0.2 0.8 0.3m height 7.5m height 0.7 (g) 0.6 0.5 0.4 (a) 0.3 0.2 01234567891011012345 Distance from pith to bark (cm)

FigureFigure 5. Basic 5. Basic density density in the in radial the radial direction direction at diff erentat different stem heights stem heights (bars show (bars standard show standard deviations). deviations). Forests 2020, 11, 717 9 of 15 Forests 2020, 11, 717 9 of 15

13.5

BV10 (2x) 12 BV16 (2x)

10.5 X101 (3x) X102 (3x) 9 AA (4x) AM (4x) 7.5 AH (4x)

6

4.5

Height above the ground (m) ground the above Height 3

1.5

0 0.25 0.35 0.45 0.55 0.65 Basic density (g/cm3)

Figure 6. Profile of basic density in the axial direction. Figure 6. Profile of basic density in the axial direction. 3.2. Bending Test 3.2. Bending Test Figure7 shows the results for MOR in the di fferent clones. The X101 clone had the highest mean MORFigure at 78.8 7 MPa, shows whereas the results the AM-4x for MOR had in the the lowest differe valuent clones. at 56.2 The MPa. X101 As clone shown had in Tablethe highest2, there mean was aMOR significant at 78.8 diMPa,fference whereas between the AM-4x the MOR had of the X101 lowest and value AM-4x at and56.2 thoseMPa. As of theshown control in Table clones 2, BV10there Forestsandwas 2020 BV16.a significant, 11, 717 difference between the MOR of X101 and AM-4x and those of the control10 clonesof 15 BV10 and BV16. 12The MOE of the seven clones is shown in Figure 8. As shown, X101 had the highest MOE at 9.7 GPa, whereas AM-4x had the lowest MOE at 7.2 GPa. Table 2 indicates that the difference between the MOE11 of X101 and that of the control clones was significant. Taken together, the results of the bending test suggest that the MOR and MOE of clone X101 were significantly higher than that of the control10 clones. However, the bending properties of AM-4x were considerably lower than those of the 9.7 controls. 9.2 The9 MOR and MOE values from the static bending test in the present study are lower than those 8.7 8.6 8.5 reported by Rokeya et al. [16], Jusoh et al. [15], and Sharma et al. [17] This inconsistency may be 8 8.2 explained by variations in age, genotype, or location condition in our study and the others [46,47]. 7.2 Figures7 9 and 10 show the relationships of density with MOR and MOE, respectively. Contrary to expectations, we did not find a significant correlation between bending properties and density. This findingof(GPa) elasticityModule 6 differs from previous studies, which have suggested that density is strongly correlated with bending properties [45,47]. These differences may be explained in part by the relationship between strength5 and anatomical properties. For example, Nakada et al. [52] and Zhu et al. [53] reported that BV10 BV16 X101 X102 AA4x AM4x AH4x strength may be related to the microfibril angle of wood fibers to which wood density is less sensitive Clone [52,53]. The reasoning here is that density increases while microfibril angle decreases with age; this Figure 7. impacts mechanical tests andFigure resu 7. ltsVariation Variationin weak of ofmodulecorrelations module of ofrupture rupture when (MPa). density (MPa). alone is measured [54–56]. GrainThe angle MOE has of also the been seven found clones to affect is shown the corre in Figurelation8. between As shown, density X101 and had mechanical the highest properties MOE at 9.7[57]. GPa, whereas110 AM-4x had the lowest MOE at 7.2 GPa. Table2 indicates that the di fference between 100 90

80 79 75 70 71 70 72 71 60 56 50 Module of rupture (MPa) rupture of Module 40 30 BV10 BV16 X101 X102 AA4x AM4x AH4x Clone

Figure 8. Variation of module of elasticity (GPa).

Figure 9. Relationship between module of rupture and density.

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12

11

10 9.7 9 9.2 8.7 8.6 8.5 8 8.2 7.2 7 Forests 2020, 11, 717 10 of 15

Forestsof(GPa) elasticityModule 2020, 11, 717 10 of 15 6 12 5 the MOE of X101BV10 and that of BV16 the control X101 clones was X102 significant. AA4x Taken together,AM4x the AH4x results of the bending11 test suggest that the MOR and MOE of cloneClone X101 were significantly higher than that of the control clones. However, the bending properties of AM-4x were considerably lower than those of 10 Figure 7. Variation of module of rupture (MPa). the controls. 9.7 9 9.2 110 8.7 8.6 8.5 8.2 8100 7.2 7 90

Module of(GPa) elasticityModule 80 6 79 75 70 71 70 72 71 5 60 BV10 BV16 X101 X102 AA4x AM4x AH4x Clone 56 50

Module of rupture (MPa) rupture of Module Figure 7. Variation of module of rupture (MPa). 40

30110 BV10 BV16 X101 X102 AA4x AM4x AH4x 100 Clone Figure 8. Variation of module of elasticity (GPa). 90 Figure 8. Variation of module of elasticity (GPa). The MOR80 and MOE values from the static bending79 test in the present study are lower than those reported by Rokeya et al. [16], Jusoh et75 al. [15], and Sharma et al. [17] This inconsistency may be 70 71 70 72 71 explained by variations in age, genotype, or location condition in our study and the others [46,47]. Figures9 and60 10 show the relationships of density with MOR and MOE, respectively. Contrary 56 to expectations, we did not find a significant correlation between bending properties and density. 50 This finding differs from previous studies, which have suggested that density is strongly correlated Module of rupture (MPa) rupture of Module with bending40 properties [45,47]. These differences may be explained in part by the relationship between strength and30 anatomical properties. For example, Nakada et al. [52] and Zhu et al. [53] reported that strength may be relatedBV10 to the BV16 microfibril X101 angle X102 of wood AA4xfibers to which AM4x wood AH4x density is less sensitive [52,53]. The reasoning here is that density increasesClone while microfibril angle decreases with age; this impacts mechanical tests and results in weak correlations when density alone is measured [54–56]. Grain angle has also been foundFigure to aff 8.ect Variation the correlation of module between of elasticity density (GPa). and mechanical properties [57].

Figure 9. Relationship between module of rupture and density.

Figure 9. Relationship between module of rupture and density. Figure 9. Relationship between module of rupture and density.

Forests 2020, 11, 717 11 of 15 ForestsForests 20202020,, 1111,, 717717 11 of 15

14 14 y = 9.515x + 3.7122 12 y =R² 9.515x = 0.1749 + 3.7122 12 R² = 0.1749 10 10 8 8 6 6 4 4

Module of(GPa) elasticityModule 2 Module of(GPa) elasticityModule 2 0 0.350 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 Density (g/cm3) Density (g/cm3)

Figure 10. Relationship between module of elasticity and density. Figure 10. Relationship between module of elasticity and density. Figure 10. Relationship between module of elasticity and density. 3.3. Compression Test 3.3. Compression Test 3.3. CompressionUltimate stress Test and Young’s modulus in the compression test are shown in Figures 11 and 12, respectively.Ultimate stress For bothand Young’s compression modulus properties, in the compression AH-4x had thetest highestare shown values, in Figures while 11 AM-4x and 12, had Ultimate stress and Young’s modulus in the compression test are shown in Figures 11 and 12, respectively.the lowest. For There both was compression a significant properties, difference AH in-4x the had stress the valuehighest between values, AM-4xwhile AM-4x and the had control the respectively. For both compression properties, AH-4x had the highest values, while AM-4x had the lowest.clones. There Likewise, was a thesignificant Young’s di modulusfference in of the AM-4x stress was value significantly between AM-4x lower and than the that control of the clones. diploid lowest. There was a significant difference in the stress value between AM-4x and the control clones. Likewise,group, whereas the Young’s that of modulus AH-4x was of AM-4x considerably was signif highericantly than lower the values thanof that the of control the diploid group (Tablegroup,2 ). Likewise, the Young’s modulus of AM-4x was significantly lower than that of the diploid group, whereasThis result that suggests of AH-4x that was other considerably triploid and higher the tetraploid than the values clones haveof the similar control compression group (Table strengths 2). This to whereas that of AH-4x was considerably higher than the values of the control group (Table 2). This resultthe control. suggests By that comparison, other triploid the and stress the values tetraplo observedid clones in have the presentsimilar compression study were not strengths different to fromthe result suggests that other triploid and the tetraploid clones have similar compression strengths to the control.those ofBy 8-year-old comparison,Acacia the hybridsstress values [17], butobserved lower in than the those present of Acaciastudy werehybrids not at different 6–7 years from [7,15 those]. control. By comparison, the stress values observed in the present study were not different from those of 8-year-old Acacia hybrids [17], but lower than those of Acacia hybrids at 6–7 years [7,15]. of 8-year-old Acacia hybrids [17], but lower than those of Acacia hybrids at 6–7 years [7,15]. 65 65 60 60 55 55 50 50 46.0 46.5 45 46.0 44.6 45.3 44.4 44.0 46.5 45 44.6 45.3 44.4 44.0

(MPa) 40 38.1 (MPa) 40 35 38.1 35 30 30

Ultmate stress parallel to grain stress Ultmate 25

Ultmate stress parallel to grain stress Ultmate 25 20 20 BV10 BV16 X101 X102 AA-4x AM-4x AH-4x BV10 BV16 X101 X102 AA-4x AM-4x AH-4x Clone Clone FigureFigure 11. 11. VariationVariation of ofultimate ultimate stress stress in incompression compression parallel parallel to tothe the grain grain (MPa). (MPa). Figure 11. Variation of ultimate stress in compression parallel to the grain (MPa).

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14 13 12 11 10.3 10 9.9 9.6 9.3 9 9.2 8.9 8 8.2 7

Young's modulus (GPa) modulus Young's 6 5 BV10 BV16 X101 X102 AA-4x AM-4x AH-4x Clone Figure 12. Variation of Young’s modulus in compression parallel to the grain (GPa). Figure 12. Variation of Young’s modulus in compression parallel to the grain (GPa). In the Acacia breeding program in Vietnam, there has been a focus on increasing stem volume and growthIn the rate Acacia [3,8,21 breeding,44,58]. Improving program in wood Vietnam, quality, there however, has been should a focus also beon an increasing important stem consideration volume andin newgrowth breeding rate [3,8,21,44,58]. generations. Furthermore,Improving wood developing quality, improved however, clones should of also the Acaciabe an hybridimportant with considerationincreased properties in new breeding of strength generations. would be useful,Furthermore, since the developing hybrid su improvedffers related clones problems of the inAcacia some hybridextreme with conditions increased in Vietnamproperties [44 ].of Previous strength research would suggestedbe useful, that since the the hybrid hybrid clones suffers from Vietnamrelated problemshad lower in densitiessome extreme and mechanical conditions propertiesin Vietnam than [44].A. Previous auriculiformis research, or weresuggested intermediate that the betweenhybrid clonestheir parents’from Vietnam characteristics had lower [14 densities,18]. However, and mechanical according properties to our data, than the A. triploid auriculiformis clone X101, or were had a intermediatesimilar basic between density,similar their parents’ compression characteristics test values, [14,18]. and higher However, bending according test properties to our than data, those the of triploiddiploid clone clones. X101 Thus, had this a similar clone couldbasic bedensity, considered similar as compression a choice for improvedtest values,Acacia and higherwood quality.bending test propertiesIt should bethan noted those that of some diploid variation clones. in Thus physical, this and clone mechanical could be properties considered was as unexplained a choice for by improvedthe measured Acacia factors. wood Forquality. instance, the effect of different site conditions and anatomical characteristics on theIt should clone willbe noted require that additional some variation attention in inphysical further and research. mechanical properties was unexplained by the measured factors. For instance, the effect of different site conditions and anatomical characteristics4. Conclusions on the clone will require additional attention in further research.

4. ConclusionsConsidering wood density, strength, and compression properties will be important for the improvement of the polyploid Acacia breeding program. Various clones had various advantages and disadvantagesConsidering in wood this study. density, For example,strength, AA-4x and compression had significantly properties higher woodwill be density important than thefor otherthe improvementclones, while of AM-4x the polyploid had greater Acacia stem breeding volume program. than the Various control clones.clones had In addition, various advantages X101 had higher and disadvantagesstem volume andin this bending study. properties For example, (MOR AA-4x and ha MOE)d significantly than the other higher clones, wood and density similar than wood the density other clones,to the control.while AM-4x In general, had greater triploid stem and volume tetraploid thanAcacia the controlhybrids clones. have In the addition, potential X101 to be had alternative higher stemspecies volume to supply and bendingAcacia wood properties as valuable (MOR hardwood and MOE) timber. than the Moreover, other clones, X101 couldand similar potentially wood be densityused for to selection the control. to improve In general, and increasetriploid theand production tetraploid ofAcacia high-quality hybrids Acaciahave thewood. potential to be alternative species to supply Acacia wood as valuable hardwood timber. Moreover, X101 could potentiallyAuthor Contributions: be used for D.D.V.selection wrote to theimprove first draft and of increase the manuscript, the production implemented of high-quality the experiment Acacia andanalyzed wood. the data. T.M., T.I., N.T.K., N.Q.C., and S.T. designed the experiment, and reviewed and edited the manuscript. AuthorAll authors Contributions: have read andD.D.V. agreed wrote to the the first published draft of version the manuscrip of the manuscript.t, implemented the experiment and analyzed theFunding: data. T.M.,This T.I., research N.T.K., and N.Q.C., APC wasand fundedS.T. designed by JSPS the KAKENHI experiment, Grant and Number reviewed 19H03015. and edited the manuscript. All authors have read and agreed to the published version of the manuscript. Acknowledgments: The authors gratefully acknowledge Kosei Ando, Nagoya University, for his dedicated and Funding:enthusiastic This help research in mechanical and APC measurements. was funded by The JSPS authors KAKENHI would Gr alsoant like Number to thank 19H03015. colleagues in the Vietnamese Academy of Forest Sciences who helped to collect samples for this study. Acknowledgments: The authors gratefully acknowledge Dr. Kosei Ando, Nagoya University, for his dedicated Conflicts of Interest: and enthusiastic help Thein mechanical authors declare measurements. no conflict ofTh interest.e authors would also like to thank colleagues in the Vietnamese Academy of Forest Sciences who helped to collect samples for this study. References Conflicts of Interest: The authors declare no conflict of interest. 1. Bueren, M. Van Acacia Hybrids in Vietnam; Impact Assessment Serie Report; Australian Centre for International ReferencesAgricultural Research: Canberra, , 2004.

1. Bueren, M. Van Acacia Hybrids in Vietnam; Impact Assessment Serie Report; Australian Centre for International Agricultural Research: Canberra, Australia, 2004.

Forests 2020, 11, 717 13 of 15

2. Do, T.; Thuyet, D.; Thang, N.; Trung, P.; Huyen, L.; Phuong, N.; Ha, D.; Tuan, N.; Hanh, L.; Nhung, H.; et al. Effect of Planting Density on Production of Acacia Plantations in Northeast Vietnam. Asian J. Soil Sci. Plant. Nutr. 2018, 3, 1–5. [CrossRef][PubMed] 3. Griffin, A.R.; Chi, N.Q.; Harbard, J.L.; Son, D.H.; Harwood, C.E.; Price, A.; Vuong, T.D.; Koutoulis, A.; Thinh, H.H. Breeding polyploid varieties of tropical : Progress and prospects. South. For. J. For. Sci. For. J. For. Sci. 2015, 77, 41–50. [CrossRef] 4. Griffin, A.R.; Nambiar, E.S.; Harwood, C.E.; See, L.S. Sustaining the future of Acacia plantation forestry—A synopsis. South. For. J. For. Sci. 2015, 77, v–viii. [CrossRef] 5. Hai, P.H. Genetic Improvement of Plantation-Grown Acacia Auriculiformis for Sawn Timber Production. Ph.D Thesis, Swedish University of Agricultural Sciences, Uppsala, Sweden, 2009. 6. Hung, T.T. Productivity and Sustainability of Acacia Plantaions in Vietnam. Ph.D Thesis, University of Tasmania, Tasmania, Australia, 2016. 7. Ismail, J.; Farawahida, A.Z. Physical and Mechanical Properties of Acacia Mangium x Acacia Auriculiformis Hybrid (Acacia Hybrid) and Acacia Mangium Superbulk Planted in Sarawak. In Proceedings of the Conference on Forestry & Forest Products Research 2007, Kota Samarahan, Malaysia, 27–29 November 2007; pp. 270–278. 8. Nambiar, E.S.; Harwood, C.E.; Kien, N.D. Acacia plantations in Vietnam: Research and knowledge application to secure a sustainable future. South. For. J. For. Sci. 2015, 77, 1–10. [CrossRef] 9. Nor Asmah, H.; Nor Hasnida, H.; Noraliza, A.; NZNA; NSN. In vitro propagation of Acacia hybrid through alginate-encapsulated shoots and axillary buds. Afr. J. Biotechnol. 2012, 11, 12814–12817. [CrossRef] 10. Pinso, C.; Nasit, R. The Potential Use of Acacia mangium x Acacia auriculiformis Hybrid in Sabah. In Proceedings of the Breeding Technologies for Tropical Acacias, Sabah, Malaysia, 1–4 July 1991; International Agricultural Research: Canberra, Australia, 1991; Volume 37, pp. 101–109 132. 11. Turnbull, J.W.; Midgley, S.J.; Cossalter, C. Tropical acacias planted in Asia: An overview. In Proceedings of the Recent developments in acacia planting: Proceedings of an international workshop, Hanoi, Vietnam, 27–30 October 1997; pp. 14–28. 12. Nirsatmanto, A.; Sunarti, S.; Praptoyo, H. Wood Anatomical Structures of Tropical Acacias and its Implication to Tree Breeding. Int. J. Hortic. 2017, 3, 9–16. 13. Vozzo, J.A. Tropical Tree Seed Manual; U.S. Department of Agriculture, Forest Service: Washington, DC, USA, 2002. 14. Kha, L.D. Studies on natural hybrids of Acacia mangium and A. auriculiformis in Vietnam. J. Trop. Sci. 2000, 12, 794–803. 15. Jusoh, I.; Zaharin, F.A.; Adam, N.S. Wood quality of Acacia hybrid and second-generation Acacia mangium. BioResources 2014, 9, 150–160. [CrossRef] 16. Rokeya, U.K.; Hossain, M.A.; Ali, M.R.; Paul, S.P. Physical and mechanical properties of hybrid Acacia. J. Bangladesh Acad. Sci. 2010, 34, 181–187. [CrossRef] 17. Sharma, S.K.; Shukla, S.R.; Sujatha, M. Physical and mechanical evaluation of 8-years-old acacia hybrid (a. mangium x a. auriculiformis) clones for various end uses. Indones. J. Res. 2018, 5, 95–102. [CrossRef] 18. Kim, N.T.; Ochiishi, M.; Matsumura, J.; Oda, K. Variation in wood properties of six natural acacia hybrid clones in northern Vietnam. J. Wood Sci. 2008, 54, 436–442. [CrossRef] 19. Kim, N.T.; Matsumura, J.; Oda, K. Effect of growing site on the fundamental wood properties of natural hybrid clones of Acacia in Vietnam. J. Wood Sci. 2011, 57, 87–93. [CrossRef] 20. Aimin, A.A.S.; Abdullah, M.Z.; Muhammad, N.; Ratnam, W. Early growth performance of full-sib Acacia auriculiformis x Acacia mangium F1 hybrid progenies at three different sites. In Proceedings of the 2014 Ukm Fst Postgraduate Colloquium, Selangor, Malaysia, 9–11 April 2014; AIP Publishing: Melville, NY, USA, 2014; pp. 769–771. 21. Kha, L.D.; Harwood, C.E.; Duc Kien, N.; Baltunis, B.S.; Hai, N.D.; Thinh, H.H. Growth and wood basic density of acacia hybrid clones at three locations in Vietnam. N. For. 2012, 43, 13–29. 22. Paiman, B.; Lee, S.H.; Zaidon, A. Machining properties of natural regeneration and planted Acacia Mangium X A. Auriculiformis hybrid. J. Trop. Sci. 2018, 30, 135–142. 23. Yahya, R.; Sugiyama, J.; Silsia, D.; Gril, J. Some anatomical features of an Acacia hybrid, A. mangium and A. auriculiformis grown in with regard to pulp yield and paper strength. J. Trop. Sci. 2010, 22, 343–351. Forests 2020, 11, 717 14 of 15

24. Sharma, S.K.; Shukla, S.R.; Sujatha, M. Radial variation of anatomical properties in 8-year-old clones of Acacia hybrid (A. mangium x A. auriculiformis). Afr. J. Wood Sci. 2016, 4, 238–246. 25. Haque, M.; Iqbal Aziz, M.; Sharif Hossain, M.; Quaiyyum, M.A.; Zahangir Alam, M.; Sarwar Jahan, M. Pulping of hybrid acacia planted in a social forestry program in Bangladesh. Cellul. Chem. Technol. 2019, 53, 739–745. [CrossRef] 26. Kha, L.D.; Ha, H.T. Research and development of acacia hybrids for commercial planting in Vietnam. Vietnam J. Sci. Technol. Eng. 2017, 59, 36–42. 27. Ramsey, J.; Schemske, D.W. Pathways, mechanisms, and rates of polyploid formation in flowering plants. Annu. Rev. Ecol. Syst. 1998, 29, 467–501. [CrossRef] 28. Nghiem, C.Q.; Griffin, R.A.; Harbard, J.L.; Harwood, C.E.; Le, S.; Nguyen, K.D.; Van Pham, B. Reduced fertility in triploids of Acacia auriculiformis and its hybrid with A. mangium. Euphytica 2018, 214. 29. Griffin, A.R.; Twayi, H.; Braunstein, R.; Downes, G.M.; Son, D.H.; Harwood, C.E. A comparison of fibre and pulp properties of diploid and tetraploid acacia mangium grown in Vietnam. Appita J. 2014, 67, 43–49. 30. Bon, P.; Harwood, C.; Chi, N.; Thinh, H.; Kien, N. Comparing wood density, heartwood proportion and bark thickness of diploid and triploid acacia hybrid clones in Vietnam. J. Trop. Sci. 2020, 32. 31. ISO 13061-3. Physical and Mechanical Properties of Wood—Test Methods for Small Clear Wood Specimens—Part 3: Determination of Ultimate Strength in Static Bending; ISO: Geneva, Switzerland, 2014. 32. ISO 13061-4. Physical and Mechanical Properties of Wood—Test Methods for Small Clear Wood Specimens—Part 4: Determination of Modulus of Elasticity in Static Bending; ISO: Geneva, Switzerland, 2014. 33. ISO 13061-17. Physical and Mechanical Properties of Wood—Test Methods for Small Clear Wood Specimens—Part 17: Determination of Ultimate Stress in Compression Parallel to Grain; ISO: Geneva, Switzerland, 2017. 34. Hernández, R.E. Moisture sorption properties of hardwoods as affected by their extraneous substances, wood density, and interlocked grain. Wood Fiber Sci. 2007, 39, 132–145. 35. Mottonen, V.; Herajarvi, H.; Koivunen, H.; Lindblad, J. Influence of felling season, drying method and within-tree location on the brinell hardness and equilibrium moisture content of wood from 27–35-year-old Betula pendula. Scand. J. Res. 2004, 19, 241–249. [CrossRef] 36. Peng, H.; Jiang, J.; Zhan, T.; Lu, J. Influence of Density and Equilibrium Moisture Content on the Hardness Anisotropy of Wood. Prod. J. 2016, 66, 443–452. [CrossRef] 37. Metsa-Kortelainen, S.; Antikainen, T.; Viitaniemi, P. The water absorption of sapwood and heartwood of

Scots pine and Norway spruce heat-treated at 170 ◦C, 190 ◦C, 210 ◦C and 230 ◦C. Holz Als Roh—Und Werkstoff 2006, 64, 192–197. [CrossRef] 38. Sjökvist, T.; Blom, Å. The influence of coating color, heartwood and sapwood, on moisture content and growth of microorganisms on the surface during outdoor exposure of Norway spruce boards. J. Coat. Technol. Res. 2019.[CrossRef] 39. Sjökvist, T.; Wålinder, M.E.; Blom, Å. Liquid sorption characterisation of Norway spruce heartwood and sapwood using a multicycle Wilhelmy plate method. Int. Wood Prod. J. 2018, 1–8. [CrossRef] 40. Fredriksson, M.; Lindgren, O. End grain water absorption and redistribution in slow-grown and fast-grown Norway spruce (Picea abies (L.) Karst.) heartwood and sapwood. Wood Mater. Sci. Eng. 2013, 8, 242–252. [CrossRef] 41. Sivertsen, M.S.; Vestøl, G.I. Liquid water absorption in uncoated Norway spruce (Picea abies) claddings as affected by origin and wood properties. Wood Mater. Sci. Eng. 2010, 5, 181–193. [CrossRef] 42. Sjökvist, T.; Blom, Å.; Wålinder, M.E. The influence of heartwood, sapwood and density on moisture fluctuations and crack formations of coated Norway spruce in outdoor exposure. J. Wood Sci. 2019, 65. [CrossRef] 43. Ball, R.D.; Simpson, I.G.; Pang, S. Measurement, modelling and prediction of equilibrium moisture content in Pinus radiata heartwood and sapwood. Holz Als Roh—Und Werkstoff 2001, 59, 457–462. [CrossRef] 44. Hai, P.H.; Hannrup, B.; Harwood, C.; Jansson, G.; Van Ban, D. Wood stiffness and strength as selection traits for sawn timber in Acacia auriculiformis. Can. J. Res. 2010, 40, 322–329. [CrossRef] 45. Chowdhury, M.Q.; Ishiguri, F.; Hiraiwa, T.; Takashima, Y.; Iizuka, K.; Yokota, S.; Yoshizawa, N. Radial variation of bending property in plantation grown acacia auriculiformis in Bangladesh. For. Sci. Technol. 2012, 8, 135–138. 46. Gérard, J.; Guibal, D.; Paradis, S.; Cerre, J.-C. Tropical Timber Atlas: Technological Characteristics and Uses; Editions Quae: Versailles, France, 2017; ISBN 9782759227709. Forests 2020, 11, 717 15 of 15

47. Zobel, B.J.; Buijtenen, J.P.v. Wood Variation: Its Causes and Control; Springer-Verlag: Berlin, Germany, 1989; ISBN 9788578110796. 48. Makino, K.; Ishiguri, F.; Wahyudi, I.; Takashima, Y.; Iizuka, K.; Yokota, S.; Yoshizawa, N. Wood properties of young acacia mangium trees planted in Indonesia. Prod. J. 2012, 62, 102–106. 49. Walker, J.C.F. Primary Wood Processing: Principals and Practice, 2nd ed.; Springer: Dordrecht, The Netherlands, 2006; ISBN 9781402043925. 50. Machado, J.S.; Louzada, J.L.; Santos, A.J.A.; Nunes, L.; Anjos, O.; Rodrigues, J.; Simões, R.M.S.; Pereira, H. Variation of wood density and mechanical properties of blackwood (Acacia melanoxylon R. Br.). Mater. Des. 2014, 56, 975–980. [CrossRef] 51. Lim, S.; Gan, K. Some physical properties and anatomical features of 14-year-old Acacia mangium. J. Trop. Prod. 2000, 6, 206–213. 52. Nakada, R.; Fujisawa, Y.; Hirakawa, Y. Effects of clonal selection by microfibril angle on the genetic improvement of stiffness in Cryptomeria japonica D. Don. Holzforschung 2003, 57, 553–560. [CrossRef] 53. Zhu, J.; Tadooka, N.; Takata, K.; Koizumi, A. Growth and wood quality of sugi (Cryptomeria japonica) planted in Akita prefecture (II). Juvenile/mature wood determination of aged trees. J. Wood Sci. 2005, 51, 95–101. [CrossRef] 54. Cave, I.D.; Walker, J.C.F. Stiffness of Wood in Fast-Grown Plantation Softwoods—the Influence of Microfibril Angle. Prod. J. 1994, 44, 43–48. 55. Machado, J.S.; Cruz, H.P. Within stem variation of Maritime Pine timber mechanichal properties. Holz Als Roh—Und Werkstoff 2005, 63, 154–159. [CrossRef] 56. Yang, J.L.; Evans, R. Prediction of MOE of eucalypt wood from microfibril angle and density. Holz Als Roh—Und Werkstoff 2003, 61, 449–452. [CrossRef] 57. Ross, R.J. Wood Handbook—Wood as an Engineering Material. General Technical Report FPL-GTR-190; Centennial; Department of Agriculture, USDA Forest Service, Forest Products Laboratory: Madison, WI, USA, 2010; ISBN 978-1484859704. 58. Harwood, C.E.; Hardiyanto, E.B.; Yong, W.C. Genetic improvement of tropical acacias: Achievements and challenges. South. For. J. For. Sci. 2015, 1, 11–18. [CrossRef]

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