Variations in anatomical characteristics and predicted paper quality of three Eucalyptus species planted in Indonesia

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ORIGINAL

Variations in anatomical characteristics and predicted paper quality of three Eucalyptus species planted in Indonesia

Agung Prasetyo^1,2 · Haruna Aiso‑Sanada¹ · Futoshi Ishiguri¹ · Imam Wahyudi³ · I. Putu G. Wijaya⁴ · Jyunichi Ohshima¹ · Shinso Yokota¹

Received: 4 March 2019

Abstract

The aim of this study was to clarify the anatomical characteristics and predicted paper quality of Eucalyptus urophylla, E. grandis, and E. pellita planted in North Sumatra, Indonesia. In addition, potential hybridization among the three species was explored. The anatomical characteristics of the fiber (length, diameter, and wall thickness), vessels (element length, diameter, and frequency), the proportions of cell types, and the derived wood properties related to pulp and paper quality were examined for these three 9-year-old Eucalyptus species. The obtained results showed that vessel element length as well as fiber and ray parenchyma proportions significantly differed among the three species. Higher fiber and lower ray parenchyma proportions were observed in E. grandis, indicating that this species would be the best mother tree in a hybridization program for pulp and paper utilization, with the other two species complementary for refining the paper quality through this program. In terms of derived wood properties, the three species were considered to be promising raw material for the pulp and paper industry compared to other species that are broadly used as raw material for high-grade pulp and paper-based products.

* Futoshi Ishiguri

[email protected]

1 School of Agriculture, Utsunomiya University, Utsunomiya 321-8505, Japan

2 United Graduate School of Agricultural Science, Tokyo University of Agriculture and Technology, Fuchu, Tokyo 183-8509, Japan

3 Faculty of Forestry, Bogor Agricultural University, Bogor 16680, Indonesia

4 Toba Pulp Lestari, Tbk, Medan 20231, Indonesia

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Introduction

Indonesia is one of the countries responding to the global demand for wood by expanding plantation areas using fast-growing tree species such as Eucalyptus spp.

(Sedjo 1999; Obidzinski and Dermawan 2012). In Indonesia, Eucalyptus urophylla, E. grandis, and E. pellita are frequently grown because they are fast-growing and highly adaptable to higher elevations (Brawner et al. 2010). However, available information is still limited on the anatomical characteristics of these species in Indo- nesia although wood from these species has already been used for commercial pulp and paper production.

In Eucalyptus species, cell morphologies are significantly related to basic density (BD) and the derived wood properties, which are closely related to pulp properties (Ona et al. 2001; Rao et al. 2002). Ona et al. (2001) found that derived wood properties such as the Runkel ratio (RR), the slenderness ratio (SR), the flexibility ratio, Luce’s shape factor (LSF), and the solid factor (SF) were significantly correlated with pulp yield, sheet density, burst factor, and other pulp and paper characteristics.

Therefore, studying the anatomical characteristics and derived wood properties of these three Eucalyptus species planted in Indonesia is important for improving pulp and paper quality.

In a previous paper (Prasetyo et al. 2017), the growth characteristics (stem diameter and tree height) and wood properties [BD, microfibril angle (MFA), shrinkage, compressive strength (CS), modulus of elasticity (MOE), and modulus of rupture (MOR)] were evaluated for these three Eucalyptus species. It was concluded that E. pellita was superior in terms of both growth characteristics and wood properties, and E. grandis and E. urophylla were specifically superior in growth characteristics and wood properties, respectively.

The hybridization among these three species was predicted to refine the wood properties while maintaining the fast-growing characteristics and improved adaption to various environmental conditions, such as plantation altitude and rainfall (Prase- tyo et al. 2017).

In the present study, the anatomical characteristics [wood fiber length (WFL), vessel element length (VEL), vessel diameter (VD), fiber diameter (FD), fiber wall thickness (FWT), vessel frequency (VF), and cell proportions] were examined for E. urophylla, E. grandis, and E. pellita. The derived wood properties related to pulp and paper quality were also calculated from the obtained anatomical characteristics.

Furthermore, the relationships among the anatomical characteristics and wood properties obtained in a previous report (Prasetyo et al. 2017) were investigated. Finally, potential hybridization among the three species is discussed.

Materials and methods Materials

Nine disks (3 cm in thickness) at the DBH position (1.3 m above the ground) were collected from 9-year-old E. urophylla, E. grandis, and E. pellita trees from the Aek

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Nauli plantation estate, Sumatra, Indonesia (2° 46′ N and 98° 51′ E, ca. 1250 m).

The samples used in the present study were the same ones used to determine the wood properties in a previous study (Prasetyo et al. 2017). The mean values of the stem diameter were 13.1, 14.0, and 16.8 cm, and those for tree height were 12.1, 17.1, and 16.7 m in E. urophylla, E. grandis, and E. pellita, respectively. Rainfall ranges from 2800 to 3300 mm year⁻¹, with a total of 185–220 days year⁻¹ of rain at the plantation estate. From 2012 to 2015, November had the highest average rainfall (ca. 470 mm), and July had the lowest average rainfall (ca. 100 mm). The tempera- ture varied between 18 and 29 °C. The trees were planted at 3.0 × 1.5 m spacing, and fertilizer (nitrogen, triple super phosphate, and rock phosphate) was only applied when the trees were planted.

Anatomical characteristics

The following anatomical characteristics were measured at 1-cm intervals from pith to bark: WFL, VEL, VD, FD, FWT, and VF. A total of 54 small blocks were prepared from the discs (3 species × 3 collected trees × 6 radial positions).

For WFL and VEL, the strips collected at 1 cm intervals from pith to bark were macerated in Schulze’s solution from 100 mL of 35% nitric acid (Kanto Chemical, Japan) and 6 g of potassium chlorate (Kanto Chemical, Japan) (Ishiguri et al. 2007).

These macerated samples were washed with distilled water and then mounted on glass slides. The images of the fiber and vessel elements were magnified with a pro- file projector (V-12, Nikon, Japan), and then the length of 50 fibers and 30 vessel elements was measured using a digital caliper (CD-30C, Mitutoyo, Japan).

The small blocks were softened in a 25% (v/v) glycerin aqueous solution (Kanto Chemical, Japan) for 2–4 h (depending on the specimens) to obtain transverse sections. The transverse sections (20 μm in thickness) were prepared using a sliding microtome (REM 710, Yamato Koki, Japan). The sections were stained with 1%

safranin for 10 min and then dehydrated with a graded ethanol series. These dehydrated sections were dipped into xylene (Kanto Chemical, Japan) and then mounted onto the slide glass using biolite (Ohkenshoji, Japan). The anatomical characteristics were observed with a light microscope (BX51, Olympus, Japan), and photomicrographs were captured with a digital camera (PEN E-P3, Olympus, Japan) attached to the microscope. Using image analysis software (ImageJ, National Institute of Health, USA), the diameters (radial and tangential) were measured for 30 fibers, vessels, and FWT.

For determining cell proportions, the point-counting method (Denne and Hale 1999; Ishiguri et al. 2009) was used. Using the same photomicrographs for cell morphology determination, gridlines (80 μm intervals with 80 gridlines) were drawn for each photomicrograph with ImageJ. Five images were used at each radial position.

In total, 400 grids were made on five images in each radial position. The proportions of vessel wall/lumen (VP), fiber wall/lumen (FP), ray wall/lumen (RPP), and axial parenchyma wall/lumen (APP) were determined from the ratio of the counted number of each type to the total number of grids; cell wall proportion (CWP) was

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calculated as a ratio of the total wall proportions from the vessel, fiber, ray, and axial parenchyma measurements to the total number of grids.

Derived wood properties

The following derived wood properties related to pulp and paper quality were determined: RR (Runkel 1949), SR (Malan and Gerischer 1987), coefficient of rigidity (CR; Tamolang and Wangaard 1961), flexibility coefficient (FC; Malan and Ger- ischer 1987), LSF (Luce 1970), and SF (Barefoot et al. 1964). These properties were calculated from the obtained anatomical characteristics; the formulas are as follows:

where FLD is a fiber lumen diameter. RR, SR, CR, FC, and LSF have no unit, whereas the unit of SF is µm³.

Data analysis

Significant differences in anatomical characteristics, cell proportions, and derived wood properties among the three Eucalyptus species were analyzed using one-way analysis of variance (ANOVA) and the post hoc Tukey’s HSD test at a 5% level.

Pearson’s correlation analysis was conducted between the anatomical characteristics and wood property data. The data of wood properties, such as BD, CS, MOE, and MOR were listed in a previous study (Prasetyo et al. 2017). These data were determined by small-clear specimens prepared from almost the same sampling height position for the sample used in this study. R software version 3.3.1 (R Core Team 2016) was used for statistical analysis.

RR= (FWT×2) FLD ,

SR= WFL FD ,

CR= FWT FD ,

FC= FLD FD ,

LSF=

(FD²−FLD²) (FD²+FLD²),

SF=(

FD²−FLD²)

×WFL,

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Results and discussion

Values of anatomical characteristics and cell proportions

The statistical values for the anatomical characteristics and cell proportions of the three Eucalyptus species are shown in Table 1. The mean values of WFL and VEL were 1.01 and 0.38 mm, 0.99 and 0.33 mm, and 0.93 and 0.32 mm for E. urophylla, E. grandis, and E. pellita, respectively. The VD, FD, FWT, and VF in E. urophylla were 141 μm, 13.9 μm, 1.6 μm, and 12 vessels mm⁻², respectively. In E. grandis, the mean values of VD, FD, FWT, and VF were 135 μm, 14.3 μm, 1.6 μm, and 12 vessels mm⁻², respectively, and in E. pellita, these values were 153 μm, 14.1 μm, 1.8 μm, and 11 vessels mm⁻², respectively. Those results on anatomical characteristics, except for VD, showed the typical characteristics of Eucalyptus spp. reported by others researchers. For example, the WFL and VEL in 3–12-year-old Ugandan- grown E. grandis were reported to be 0.82–1.08 mm and 0.37–0.42 mm, respectively (Sseremba et al. 2016). They also reported that the mean VD, FD, and VF values were 93.5–123.5 μm, 11.0–11.8 μm, 11–16 vessels mm⁻², respectively (Sseremba et al. 2016). Malan (1988) reported that the WFL, VD, and VF were 0.93–1.13 mm, 110–134 μm, and 7–10 vessels mm⁻² for 8.8-year-old South African-grown E. gran- dis. In 15-year-old E. pellita, the mean values and standard deviations of WFL, VEL, VD, FD, and FWT were 0.93 ± 0.2 mm, 0.49 ± 0.12 mm, 56.1 ± 27.5 μm, 19.3 ± 3.9 μm, and 4.6 ± 1.6 μm, respectively (Poubel et al. 2011). The mean values and standard deviations of WFL, VD, FD, and FWT were 0.92 ± 0.09 mm,

Table 1 Statistical values of anatomical characteristics and cell proportions in three Eucalyptus species

Different letters after mean values indicate significant differences among the species in each property (Tukey’s HSD at a 5% level)

WFL wood fiber length, VEL vessel element length, VD vessel diameter, FD fiber diameter, FWT fiber wall thickness, VF vessel frequency, VP, FP, RPP, APP, CWP referred to vessel, fiber, ray parenchyma, axial parenchyma, and cell wall proportions, respectively

p values were obtained from analysis of variance

Property E. urophylla (n =3) E. grandis (n = 3) E. pellita (n = 3) Sig- nificance p value

WFL (mm) 1.01 (0.03) 0.99 (0.01) 0.93 (0.05) 0.061

VEL (mm) 0.38^a (0.02) 0.33^ab (0.01) 0.32^b (0.02) 0.022

VD (μm) 141 (6) 135 (18) 153 (10) 0.288

FD (μm) 13.9 (1.1) 14.3 (0.9) 14.1 (0.5) 0.879

FWT (μm) 1.6 (0.2) 1.6 (0.1) 1.8 (0.3) 0.531

VF (vessels mm⁻²) 12 (1) 12 (1) 11 (2) 0.487

VP (%) 23.3 (2.2) 19.8 (3.5) 20.5 (1.5) 0.283

FP (%) 38.1^ab (1.6) 44.2^a (5.3) 32.6^b (3.2) 0.024

RPP (%) 14.8^ab (1.3) 12.6^a (3.6) 19.5^b (1.6) 0.030

APP (%) 23.8 (0.7) 23.5 (3.6) 27.4 (1.9) 0.163

CWP (%) 40.6 (3.0) 40.5 (3.3) 39.5 (2.3) 0.879

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157.0 ± 27.0 μm, 14.3 ± 1.1 μm, and 1.99 ± 0.42 μm for 14-year-old E. globulus (Ona et al. 2001). Pirralho et al. (2014) reported that the VF at 50 months of age for E. viminalis, E. camaldulensis, and E. globulus was 9, 11, and 18 vessels mm⁻², respectively. In the present study, the mean values of VD in the three species were relatively larger compared to those of the other reports on E. grandis and E. pellita (Malan 1988; Poubel et al. 2011; Sseremba et al. 2016), while these values were the same as those reported for the 14-year-old E. globulus tree (Ona et al. 2001).

As shown in Table 1 for the cell proportions, the VP, FP, RPP, APP, and CWP in E. urophylla were 23.3, 38.1, 14.8, 23.8, and 40.6%, respectively. In E. gran- dis, these values were 19.8, 44.2, 12.6, 23.5, and 40.5%, respectively. In E. pel- lita, these values were 20.5, 32.6, 19.5, 27.4, and 39.5%, respectively. Wu et al.

(2006) reported that the mean values of cell proportions in 11-year-old E. uro- phylla were 12.5–17.7 (VP), 59.3–65.6 (FP), 14.2–18.0 (RPP), 3.6–4.6 (APP), and 68.4–74.6% (CWP). In addition, in the 11-year-old E. grandis, those values were 10.5–17.2, 62.2–67.3, 13.1–17.2, 4.4–5.0, and 65.5–71.5%, respectively (Wu et al.

2006). Malan (1988) reported that in 8.8-year-old E. grandis, VP, FP, and RPP were 10.3–13.9, 69.1–76.5, and 12.3–17.4%, respectively. In other Eucalyptus species, the mean values and standard deviations of VP, FP, RPP, APP, and CWP in 14-year-old E. camaldulensis were 15.9 ± 3.7, 49.4 ± 6.5, 20.6 ± 0.5, 14.1 ± 3.5, and 27.8 ± 3.8%, respectively, and those in 14-year-old E. globulus were 11.4 ± 3.1, 67.1 ± 4.1, 15.4 ± 1.9, 6.2 ± 2.2, and 34.4 ± 6.0%, respectively (Ona et al. 2001). In the present study, the FP was relatively similar to that reported in E. camaldulensis, but it was much less compared to that reported for E. urophylla, E. grandis, and E. globulus (Malan 1988; Ona et al. 2001; Wu et al. 2006). However, relatively higher VP and APP as well as the large VD values were observed (Table 1), which influence the FP in the three Eucalyptus species.

Radial profiles of the anatomical characteristics and cell proportions

Figure 1 shows the radial profiles of the anatomical characteristics in the three Eucalyptus species. In the three species, WFL and VEL gradually increase from pith to bark. For VD, the three species show an increasing pattern from pith to bark. In addition, the mean values of FD were almost constant, and FWT slightly increased from pith to bark in all species. The VF in the three species sharply decreased from pith to bark. Sharma et al. (2005) reported the slightly increasing radial profiles from pith to bark for anatomical characteristics in 10–12-year-old E. tereticornis, such as WFL, FD, FWT, and VD, except for VEL (constant values from pith to bark). The same results of radial profiles from pith to bark for WFL, FD, and FWT were also observed in 15–20-year-old E. grandis grown in South Africa (Taylor 1973). Poubel et al. (2011) also reported that in 15-year-old E.

pellita, the mean values of WFL, FD, FWT, and VD were higher for the sapwood than for the heartwood although these values did not significantly differ between these two positions. In 14-year-old E. camaldulensis and E. globulus planted in Western Australia, increasing VD and FWT values and decreasing VF values from pith to bark were also observed (Ohshima et al. 2003, 2004). Based on

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the comparison, the obtained results in this study were almost the same as those of other reports (Taylor 1973; Ohshima et al. 2003, 2004; Sharma et al. 2005;

Poubel et al. 2011), with the exception that the VEL slightly increased from pith to bark in the three species (Fig. 1).

As shown in Fig. 2, for the cell proportions, the VP increased from the pith up to 4 cm and then became almost constant toward the bark in the three species. The mean FP values varied among the three species. In E. urophylla, the FP firstly decreased from the pith to 3 cm, sharply increased to 4 cm, and then was constant toward the bark. The FP was constant from pith to bark in E. grandis, and this value gradually increased from pith to bark in E. pellita. The radial profiles of both RPP and APP gradually decreased from pith to bark in all species.

Meanwhile, the CWP showed about 40% in all three species. In 23-year-old E.

grandis, the cell proportions did not significantly differ among juvenile, transition, and mature wood zones (Palermo et al. 2015). Wu et al. (2006) reported that the VP and CWP increased, while RPP and APP decreased from the pith toward the bark in both E. urophylla and E. grandis. On the other hand, FP showed para- bolic radial variations from the pith toward the bark in E. urophylla, while these values decreased in E. grandis (Wu et al. 2006). In the present study, similar

0.0 0.3 0.6 0.9 1.2

WFL (mm)

E. urophylla E. grandis E. pellita

0.0 0.2 0.4 0.6 0.8

VEL (mm)

0 50 100 150 200

VD (μm)

0 5 10 15 20

FD (μm)

0 1 2 3 4

0 2 4 6 8

FWT (μm)

Distance from pith (cm) 0 5 10 15 20

0 2 4 6 8

VF (no. mm-2)

Distance from pith (cm)

Fig. 1 Radial profiles of anatomical characteristics in three Eucalyptus species. WFL, wood fiber length;

VEL, vessel element length; VD, vessel diameter; FD, fiber diameter; FWT, fiber wall thickness; VF, vessel frequency. Number of trees is three trees for each species

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patterns in radial variations to those reported for the VP, RPP, and APP in E. uro- phylla and E. grandis were observed (Wu et al. 2006). On the other hand, radial variations in the FP and RPP varied among the species (Fig. 2).

Derived wood properties

Referring to Table 2, the mean values for the derived wood properties RR, SR, CR, FC, LSF, and SF in the three species were 0.28–0.34 (RR), 66.1–73.5 (SR), 0.11–0.13 (CR), 0.75–0.78 (FC), 0.24–0.28 (LSF), and 78.6–80.3 × 10³ μm³ (SF).

No significant differences in the measured derived wood properties were observed among the three species. In addition, the radial profiles from pith to bark for the RR, SR, LSF, and SF were increasing (Fig. 3). On the other hand, CR and FC were constant and slightly decreased from pith to bark in the three species, respectively.

Rao et al. (2002) reported that the RR and LSF in some 4.5-year-old E. tereti- cornis clones were 0.65–0.90 and 0.46–0.56, respectively. The mean values and standard deviations of the RR, FC, SR, and LSF in 50-month-old E. camaldulen- sis were 0.79 ± 0.21, 0.56 ± 0.06, 39.4 ± 7.7, and 0.51 ± 0.08, respectively, and these

0 2 4 6 8

APP (%)

Distance from pith (cm) 0

10 20 30 40 50

RPP (%)

0 10 20 30 40 50

VP (%) FP (%)

0 10 20 30 40 50

0 2 4 6 8

CWP (%)

Fig. 2 Radial profiles of cell proportions in the three Eucalyptus species. VP, FP, RPP, APP, and CWP referred to vessel, fiber, ray parenchyma, axial parenchyma, and cell wall proportions, respectively. Num- ber of trees is three trees for each species

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values were 1.75 ± 0.58, 0.37 ± 0.07, 47.1 ± 8.8, and 0.74 ± 0.08, respectively, in 50-month-old E. globulus (Pirralho et al. 2014). The CR values of the commercial pulpwood species (7-year-old Acacia mangium, A. auriculiformis, and their

Table 2 Derived wood properties in three Eucalyptus species

SD standard deviation, RR Runkel ratio, SR slenderness ratio, CR coefficient of rigidity, FC flexibility coefficient, LSF Luce’s shape factor, SF solid factor. RR, SR, CR, FC, and LSF have no unit

p values among species were obtained from analysis of variance. Number of trees is three trees for each species

Property E. urophylla E. grandis E. pellita Significance

Mean SD Mean SD Mean SD p value

RR 0.30 0.07 0.28 0.03 0.34 0.08 0.611

SR 73.5 6.9 69.6 4.8 66.1 6.2 0.383

CR 0.12 0.02 0.11 0.01 0.13 0.02 0.582

FC 0.77 0.04 0.78 0.02 0.75 0.04 0.614

LSF 0.26 0.05 0.24 0.02 0.28 0.05 0.624

SF (× 10³ μm³) 80.1 9.0 78.6 5.6 80.3 9.8 0.961

0.0 0.1 0.2 0.3 0.4 0.5

RR

0 20 40 60 80 100

SR

0.0 0.1 0.2 0.3 0.4 0.5

CR

0.0 0.2 0.4 0.6 0.8 1.0

FC

0.0 0.1 0.2 0.3 0.4 0.5

0 2 4 6 8

LSF

0 30 60 90 120 150

0 2 4 6 8

SF×103(μm3)

Fig. 3 Radial profiles of derived wood properties in the three Eucalyptus species. RR, Runkel ratio; SR, slenderness ratio; CR, coefficient of rigidity; FC, flexibility coefficient; LSF, Luce’s shape factor; SF, solid factor. Number of trees is three trees for each species. All properties except for SF have no unit

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hybrid) were 0.13, 0.17, and 0.13, respectively (Yahya et al. 2010). The SF values of 14-year-old E. camaldulensis and E. globulus were 45.8 and 91.2 × 10³ μm³, respectively (Ona et al. 2001). Ona et al. (2001) also found that the RR and LSF were positively correlated to pulp yield and unbleached brightness, but they were nega- tively correlated with sheet density, burst factor, breaking length, folding endurance, and kappa number in 14-year-old E. camaldulensis. The results of the derived wood properties in the present study showed that the three species have lower RR and LSF values and higher SR and FC values, with similarity in the CR and SF compared to those reported by other researchers (Table 2, Ona et al. 2001; Rao et al. 2002;

Yahya et al. 2010; Pirralho et al. 2014). In addition, the increasing RR, SR, LSF, and SF values from pith to bark were reported in 14-year-old E. camaldulensis and E. globulus (Ohshima et al. 2005a). The RR and LSF were noticeably influenced by wall thickness/fiber wall material near the bark in E. tereticornis, E. camaldulensis, and E. globulus (Rao et al. 2002; Ohshima et al. 2005a). On the other hand, the FC decreased from pith to bark for E. camaldulensis, while this value had no clear trend in E. globulus (Ohshima et al. 2005b). Similarly, the FC decreased from pith to bark in E. grandis (Malan and Gerischer 1987). In the present study, except for the FC and CR, the radial profiles of the derived wood properties showed similar patterns to those of other reports (Fig. 3, Malan and Gerischer 1987; Ohshima et al. 2005a, b).

The constant values of CR from pith to bark are believed to be influenced by greater wall thickness toward the bark in the three Eucalyptus species (Figs. 1, 3). Thus, the increasing trend of FWT with the increase in the FD toward the bark seems to result in constant CR values from pith to bark (Figs. 1, 3). Furthermore, those fiber characteristics (i.e., larger fibers with narrower lumens) appear to result in slightly decreasing FC values and sharply increasing SF values from pith to bark in the three Eucalyptus species.

Correlations between anatomical characteristics and wood properties

It has been reported that physical and mechanical properties are influenced by the complexity of the wood structures, especially the size and wall thickness of cells (Sharma et al. 2005; Ishiguri et al. 2009; Poubel et al. 2011; Chowdhury et al.

2013). Poubel et al. (2011) reported that the correlation coefficients (r) between BD and WFL, BD and FD, and BD and FWT were 0.36, 0.40, and 0.34, respectively, in 15-year-old E. pellita (Poubel et al. 2011). Sharma et al. (2005) reported that air-dry density was significantly correlated with WFL (r = − 0.40), FD (r = − 0.43), and VD (r = − 0.43) in 10-year-old E. tereticornis. As shown in Table 3, in the three species, the BD and mechanical properties such as CS, MOE, and MOR were significantly positively correlated with cell length, such as WFL and VEL. Especially for E. pel- lita, these relationships were the strongest (r = 0.64–0.93) among the three species.

In E. urophylla, the VF and CWP influenced the physical and mechanical properties, whereas both FWT and FP positively affected BD and CS. In E. grandis, VD, FD, FWT, and VF were significantly correlated with the three measured mechanical properties although only a weak correlation coefficient was observed between FWT and BD (r = 0.39). Based on the obtained results, it is believed that, with some

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exceptions, cell lengths and some cell morphologies or proportions influence the physical and mechanical properties of the three species.

Table 3 Correlation analysis between anatomical characteristics and wood properties

Data for wood properties were obtained from a previous paper (Prasetyo et al. 2017); one small-clear specimen of E. urophylla was not used due to decay (for other abbreviations, see Table 1)

BD basic density, CS compressive strength parallel to grain, MOE modulus of elasticity, MOR modulus of rupture

*,** Refer to significant at 5 and 1% level, respectively

Species Property BD CS MOE MOR

E. urophylla n = 17 WFL 0.61* 0.70** 0.42 0.58*

VEL 0.33 0.51* 0.32 0.42

VD 0.30 0.28 0.00 0.08

FD − 0.28 − 0.07 − 0.15 − 0.12

FWT 0.48* 0.60* 0.11 0.22

VF − 0.64** − 0.58* − 0.44 − 0.53*

VP − 0.16 − 0.21 − 0.23 − 0.19

FP 0.51* 0.51* 0.41 0.45

RPP − 0.26 − 0.43 − 0.11 − 0.17

APP − 0.52* − 0.34 − 0.43 − 0.47

CWP − 0.59* 0.51* 0.50* 0.54*

E. grandis (n = 18) WFL 0.60** 0.62** 0.76** 0.78**

VEL 0.50* 0.60** 0.74** 0.80**

VD 0.16 0.45 0.49* 0.73**

FD 0.10 0.42 0.47* 0.58*

FWT 0.39 0.70** 0.45 0.40

VF − 0.27 − 0.59* − 0.55* − 0.76**

VP − 0.22 0.06 0.22 0.45

FP 0.02 0.11 − 0.12 − 0.21

RPP 0.31 − 0.05 − 0.08 − 0.20

APP − 0.13 − 0.17 0.03 0.03

CWP 0.25 − 0.06 0.03 − 0.26

E. pellita (n = 18) WFL 0.85** 0.71** 0.90** 0.88**

VEL 0.77** 0.64** 0.93** 0.90**

VD 0.70** 0.56* 0.59* 0.55*

FD − 0.21 − 0.21 − 0.24 − 0.27

FWT 0.33 0.35 0.23 0.16

VF − 0.31 − 0.15 − 0.18 − 0.18

VP 0.57* 0.51* 0.41 0.39

FP 0.29 0.40 0.45 0.45

RPP − 0.71* − 0.59* − 0.62** − 0.62**

APP − 0.34 − 0.52* − 0.47 − 0.47

CWP 0.31 0.21 0.31 0.29

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Potential hybridization among the three species

It is well known that an interspecific hybridization program in Eucalyptus could achieve improvements in wood quality and adaptability (Malan 1993; Potts and Dungey 2004; Quilhó et al. 2006; Carrillo et al. 2017). Zobel and Jett (1995) pointed out that the anatomical characteristics, especially cell length, have high heritability (0.6–0.7). In contrast, the heritability for BD in E. grandis was only 0.4–0.5 as reported by Malan (1988). However, the performance of offspring from the hybridization only produced intermediate characteristics to those parents (Zobel and Jett 1995). E. pellita breeding population is a major part of the tree breeding program in the Indonesian paper industry, including other advanced-generation breeding populations from E. grandis and E. urophylla at various stages of domestication, focused on increasing pulp yield and survivability in different plantation areas (elevation) (Brawner et al. 2010). In the present study, significant differences among the three species were observed in VEL, FP, and RPP (Table 1), indicating that these anatomical characteristics are inherited. Therefore, they can be used as key parameters in tree improvement programs (Table 1). Among these anatomical characteristics, FP is one of the key parameters commonly subjected to improvements in paper quality (Zobel and van Buijtenen 1989; Zobel and Jett 1995). On the other hand, Ona et al. (2001) reported that decreasing RPP and APP are breeding objectives for increasing pulp yield in E. camaldulensis and E. globulus. Considering that E.

grandis has desirable FP and RPP (Table 1, see Tukey’s HSD results), E. grandis can be used as a mother tree and the other two species can be complementary in a hybridization program. Gwaze et al. (2000) reported similar results on the domi- nance of E. grandis over E. urophylla and E. pellita when hybridized. In Zimba- bwe, crossing between E. grandis and E. camaldulensis or E. tereticornis showed that E. grandis was likely to be better parents for growth compared to the other two species under high altitude and wet hybrid trial conditions (Madhibha et al. 2013).

The current study was also confirmed by a previous report (Prasetyo et al. 2018) on two interspecific hybrids G × U and G × P (E. grandis as a mother tree), having significant hybridization effects on growth characteristics and wood properties. From the results obtained on the anatomical characteristics (Table 1), the predicted hybrid species (i.e., E. grandis × E. pellita and E. grandis × E. urophylla) would have desirable FP and RPP derived from E. grandis. Furthermore, the narrower VD and the low VP of E. grandis would also give benefits to these two hybrids. Some researchers have reported that VD and VF adversely affect wood quality (Zobel and van Bui- jtenen 1989; Savidge 2003; Kien et al. 2009). Because the relatively large VD was observed in the three species, especially in E. pellita, producing narrower VD and lesser VF would be crucial key improvements for both paper and solid wood-based products.

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Conclusion

The present study has clarified the anatomical characteristics, correlations with wood properties, and derived wood properties of three Eucalyptus species. Among the anatomical characteristics, the following characteristics significantly differed among the three species: VEL, FP, and RPP. Among the three species, E. gran- dis had higher FP and lesser RPP, which indicates that E. grandis can be used as a mother tree when the three species are subjected to hybridization programs to produce wood for better quality paper. The predicted hybrids (i.e., E. grandis × E. pel- lita and E. grandis × E. urophylla) would have desirable FP and RPP derived from E. grandis. Furthermore, the narrower VD and the lesser VP of E. grandis would also give benefits to these two hybrids. A correlation analysis between anatomical characteristics and wood properties showed that with some exceptions, mostly cell lengths and some configurations of cell morphologies and proportions influenced the physical and mechanical properties of the three species. In terms of the derived wood properties, the three Eucalyptus species in the present study showed better fiber characteristics compared to other species that are broadly used as raw material for high-grade pulp and paper-based products. Therefore, it is believed that the three species in the present study can produce wood that is promising for pulp and paper quality.

Acknowledgements The authors acknowledge Toba Pulp Lestari, Tbk., for permission to collect wood samples, Mr. P. A. Clegg for his guidance and suggestions, and all company staff that assisted us during the sampling activity.

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