Anatomical property variation in Acacia auriculiformis growing in Bangladesh

Abstract

Acacia auriculiformis A. Cunn. (ex Benth) is an extensively planted fast-growing species in Bangladesh. Understanding the anatomical property variation has significant importance in tree improvement and wood utilization. Radial variations of anatomical properties in 11-year-old trees were examined in this study. Vessel diameter increased gradually up to about 40–45% distance from pith and then levelled-off to bark. Fibre diameter gradually decreased toward bark, whereas fibre wall thickness showed a reverse pattern. In contrast to fibre proportion, vessel and ray proportions gradually increased from pith to bark. Cell wall proportion was nearly stable up to ∼70% distance from pith and then increased gradually to bark. Results suggest that air-dried density or compressive strength variation is mainly attributed by the fibre morphology, i.e. fibre diameter and fibre wall thickness (at 1% level). Significant variations among the trees in vessel and fibre diameters (at 1% level), and ray and axial parenchyma proportions (at 5% level) suggest the possibility of tree selection for breeding.

Keywords

Air dried density Cell proportion Compressive strength Vessel and fibre morphology

Introduction

Beyond the natural range in Australia, Papua New Guinea and Indonesia, Acacia auriculiformis A. Cunn. (ex Benth) has been planted in China, India and Southeast Asia, and some African countries (Pinyopusarerk et al. 1991). The wood of this species is widely used for charcoal, fuelwood, pulp and construction (Pinyopusarerk et al. 1991; Ishiguri et al. 2004). Owing to its fast-growing nature and good site adaptability, it has been considered a priority species in the short-rotation plantations in Bangladesh, such as social forestry and agroforestry projects (Islam et al. 1999). It is also the most preferred species by rural people, mainly for furniture over other fast-growing species, such as Acacia mangium Willd. and Eucalyptus camaldulensis Dehnh. (Chowdhury et al. 2005; Kabir and Webb 2005), and is thus considered as one of the major sources of raw material in the forest industry of Bangladesh.

As with other important species (Zobel and van Buijtenen 1989), tree breeding programmes in Acacia auriculiformis A. Cunn. (ex Benth) have been focused on increased stem growth through tree selection based on the growth characteristics (Hai et al. 2008). However, considering importance of wood and fibre quality in the forest industry, wood quality needs to be an integral part in the tree breeding programme (Zobel and Jett 1995). For efficient integration of wood quality traits in the tree breeding programme, basic information on wood quality of this species is therefore essential. Because of constant age and environmental conditions, the largest proportion of the total variation of wood properties among trees in the same stand can be explained by genetic differences, and thus, it is possible to improve wood quality by selecting trees with the desired trait(s) through tree breeding (Zobel and Jett 1995). In previous reports, it was demonstrated that wood density, cell length, stiffness and strength variations among the trees offer opportunities for tree selection and to improve wood quality in Acacia auriculiformis A. Cunn. (ex Benth) (Chowdhury et al. 2009a, 2012a; Hai et al. 2010). Therefore, understanding wood anatomical properties variation in this species is imperative for tree breeding programmes as well as wood utilization.

In contrast with research related to a few anatomical properties (Verghese et al. 1999; Ishiguri et al. 2004; Kojima et al. 2009; Chowdhury et al. 2009a), detailed anatomical variability along the radius has not yet been determined in this species. In this regard, the objective of this study was to investigate the possibility of plus tree selection on the basis of anatomical properties variations in Acacia auriculiformis A. Cunn. (ex Benth) grown in Bangladesh. In the present study, radial variations of anatomical properties were analysed from pith to bark. In addition, variability of anatomical traits among the trees was evaluated for plus tree selection.

Materials and methods

Sample collection and preparation

Samples were collected from an 11-year-old Acacia auriculiformis A. Cunn. (ex Benth) plantation at Cox's Bazar Forest Division, Bangladesh (Fig. 1; Chowdhury et al. 2009a, 2012a). 10 years is the usual rotation period but harvesting had been delayed in this case. The plantation was raised by seedlings with 2×2 m spacing and the area is characterised by a monsoonal climate. Seven trees were randomly harvested for sample collection. After harvesting the trees, 10 cm thick discs were collected at diameter at breast height (dbh). The mean tree diameter was 24·1±3·5 cm, excluding the bark.

Figure 1

Location of the sampling site (arrow indicated open circle): map adapted from Banglapedia (2013)

Although polished disks revealed distinct growth rings, their numbers did not correspond with plantation age at dbh (Chowdhury et al. 2009a). Owing to the diameter variation in sample trees, the radial variations of all anatomical properties were analysed in relation to relative distance from pith, rather than actual distance. Data of relative distance from pith was calculated by the method described in a previous report (Chowdhury et al. 2009b). For the anatomical analysis, small blocks (2×2 cm) were prepared from thin radial strips at 2·5 cm intervals from pith to bark.

Anatomical analysis

Cross sections (15 μm in thickness) were cut using a sliding microtome (Yamato, ROM-380) from each block, and sections were stained with safranin, and thereafter dehydrated in alcohol series. Dehydrated sections were dipped in xylene, and then mounted on the glass slide with a cover slip. Microscopic images of the cross sections were taken using a microscope (Olympus, BX-51) connected with a digital camera (Olympus, DP11). Vessel and fibre morphological parameters were measured on the printed micrographs by a digital caliper (Mitsutoyo, CD-15CP) in each radial position. A total of 50 measurements were carried out for each case in each radial position (Ishiguri et al. 2009).

For cell proportion, five images were used in each radial position, and measurements were carried out by the point counting method (Denne and Hale 1999; Ishiguri et al. 2009; Chowdhury et al. 2012b). Eighty grid points were drawn at 80 μm intervals per image using software (Adobe Photoshop, ver. 7.0.1). A total of 400 grid points were counted in each radial position. Thereafter, the cell types were categorised as vessel wall and lumen, fibre wall and lumen, ray wall and lumen, and parenchyma wall and lumen. Subsequently, the counted cells were calculated into proportion for each axial category.

Compressive strength (CS) test

The CS was measured according to Japanese Industrial Standards (JIS 1994) on samples from a previous study (Chowdhury et al. 2009a), and samples for the current study were also prepared at 2·5 cm intervals from pith. The specimen size was 23 mm (R)×23 mm (T)×50 mm (L). The compressive test was performed by using universal testing machine (Shimadzu, DCS-5000), and the load speed was adjusted at 1 mm min⁻¹.

Statistical analysis

The relationships between anatomical properties and air-dried density, as well as CS parallel to grain were analysed by Pearson's correlation analysis. Variation among the trees for each anatomical trait was analysed by one-way ANOVA test.

Results and discussion

Radial variation of anatomical properties

Figure 2 shows the transverse sections of three sample positions (tree no. 2) from pith to bark, as an example. Vessel diameter (an average of tangential and radial vessel diameters) showed a distinctive radial pattern which gradually increased up to about 40–45% distance from pith and then it was almost stable to bark (Fig. 3). In addition, vessel proportion increased from pith to bark (Table 1). Bosman et al. (1994) also reported that the vessel proportion increases from pith to bark in Shorea leprosula Miq. and Shorea parvifolia Dyer, where an increase of vessel proportion is related to an increase of vessel size with constant vessel frequency. It can be assumed that increase of the vessel proportion would have a minute effect on the density or strength properties due to its inverse relationship with density (Savidge 2003). In the previous report (Chowdhury et al. 2009a), we reported that basic density gradually increased to ∼80 mm from pith and then was nearly constant to bark. Thus, low density near the pith side might not be related to size or vessel proportion in Acacia auriculiformis A. Cunn. (ex Benth). In the present study, the average fibre diameter (an average of tangential and radial fibre diameters) decreased from the pith to bark (Fig. 4), while conversely, the fibre wall thickness increased gradually in the same radial direction (Fig. 5). Hence, basic density might be related to the cell wall thickness of wood fibres. On the other hand, the cell wall proportion showed a constant value from the pith to ∼70% radial distance and then it increased gradually to bark (Fig. 6). Therefore, it can be assumed that due to having an inverse relationship between the fibre diameter and wall thickness, the cell wall proportion tends to increase near the bark.

Figure 2

Transverse sections of three radial positions of a tree (tree no. 2): scale bar = 200 μm

Figure 3

Radial variation of average vessel diameter in relation to relative distance from pith to bark: each line indicates an individual tree

Figure 4

Radial variation of average fibre diameter in relation to relative distance from pith to bark: each line indicates an individual tree

Figure 5

Radial variation of fibre wall thickness in relation to relative distance from pith to bark: each line indicates an individual tree

Figure 6

Radial variation of cell wall proportion in relation to relative distance from pith to bark: each line indicates an individual tree

Table 1

Variation of cell proportion in relation to radial distance from pith*

	Cell proportion/%
	Vessel		Fibre		Ray		Axial parenchyma
Radial distance from pith/%	Mean	SD	Mean	SD	Mean	SD	Mean	SD
20	10	2	65	2	9	1	16	1
40	12	2	63	2	9	1	16	1
60	13	4	62	3	8	3	17	1
80	14	3	59	4	11	2	17	1
100	14	2	59	3	12	3	16	2

*SD, standard deviation; number of sample trees = 7.

In contrast to the vessel proportion, the fibre proportion gradually decreased from pith to bark, whereas the ray proportion gradually increased toward the bark (Table 1). Such a lower fibre proportion near the bark is expected to decrease the density or mechanical strength as stem increases in diameter, but possibly the need for additional strength might be compensated by increasing other cell types, for example, ray proportion. It is also reported that an increase of one cell type must necessarily be associated with a decrease of at least another cell type (Taylor and Wooten 1973). The axial parenchyma proportion was almost constant along the radius in the present study.

Table 2 shows the mean vessel and fibre morphology, and cell proportion of the present study. Verghese et al. (1999) reported that the mean vessel and fibre diameters, and fibre wall thickness were 166·6, 9·0 and 3·6 μm respectively, in 15-year-old Acacia auriculiformis A. Cunn. (ex Benth) grown at Maharastra in India. The range of the mean vessel and fibre diameters of the present study were similar to Verghese et al. (1999), while the fibre wall thickness was lower than that of the previous study. In addition, Ishiguri et al. (2004) also showed that the mean vessel, fibre and axial parenchyma proportions were approximately 16·8, 65·0 and 12·0% respectively, in 11-year-old Acacia auriculiformis A. Cunn. (ex Benth) grown in Malaysia. The mean vessel and fibre proportions in the aforementioned study were within the ranges of the present study, but the axial parenchyma proportion was slightly lower.

Table 2

Mean value of anatomical property and results of ANOVA tests among the trees†

Anatomical property		Min	Mean	Max	SD	Significance
Vessel morphology	Tangential vessel diameter/μm	75·4	114·5	169·8	19·1	**
	Radial vessel diameter/μm	90·8	133·1	185·0	19·3	**
	Average vessel diameter/μm	83·8	123·9	175·1	18·7	**
Fibre morphology	Tangential vessel fibre diameter/μm	8·1	10·9	12·9	1·2	ns
	Radial vessel fibre diameter/μm	10·3	14·4	18·1	1·9	**
	Average vessel fibre diameter/μm	9·4	12·6	16·1	1·5	*
	Fibre wall thickness/μm	1·6	2·0	2·4	0·2	ns
Cell proportion	Vessel/%	7	13	20	2	**
	Fibre/%	54	63	69	4	ns
	Ray/%	4	10	19	2	*
	Axial parenchyma/%	15	17	20	1	*
	Cell wall/%	38	44	52	4	ns

†SD, standard deviation; **significance at 1% level; *significance at 5% level; ns, not significant; number of sample trees = 7.

Variation among trees

Table 2 shows the anatomical traits variation among the trees. The implication is that significant variations in different anatomical properties among the trees offer some opportunities for plus tree selection to improve wood quality of this species. Because this variation among the trees might be attributed to genetic differences of individual trees, since trees within the stand were of the same age, spacing, and likely to have the same management treatments throughout their life (Chowdhury et al. 2009a). The vessel and fibre diameters varied significantly among the trees (Table 2) and thus, it could be possible to select trees with small vessel and fibre diameters. On the other hand, the fibre wall thickness did not vary significantly. Among the cell proportions, the vessel, ray and axial parenchyma proportions varied significantly among the trees; while conversely, the fibre and cell wall proportions did not vary among the trees (Table 2). However, in the same time pulp yield is expected to decline, as it described for other hardwood species, for example, Eucalyptus spp. (Ona et al. 2001). Hence, significant variation among the trees explored is another possibility for tree selection to reduce the ray proportion.

Correlation with density and CS

Fibre diameter showed a strong negative correlation with air-dried density or CS, whereas the fibre wall thickness showed a positive correlation (Table 3). The negative effect of fibre diameter is offset by increase of the fibre wall thickness. Denne and Hale (1999) showed that lower mean density is associated with thinner fibre wall thickness and wider vessel lumen in Nothofagus nervosa (Rauli). Ishiguri et al. (2009) also reported that the highest correlation coefficient (r = 0·87, 1% level) was obtained between cell wall thickness of wood fibre and basic density in another tropical species (Paraserianthes falcataria (L.) Nielsen) grown in Indonesia. Moreover, it is known that wood density variation is mainly related to cell diameter and thickness (Panshin and de Zeeuw 1980).

Table 3

Correlation coefficients between anatomical properties with air-dried density and CS†

Anatomical properties		Air-dried density		CS
Anatomical properties		r	Significance	r	Significance
Vessel morphology	Average vessel diameter	−0·216	ns	0·183	ns
Vessel morphology	Vessel frequency	−0·002	ns	−0·247	ns
Fibre morphology	Average vessel fibre diameter	−0·712	**	−0·515	**
Fibre morphology	Fibre wall thickness	0·671	**	0·579	**
Cell proportion	Vessel	−0·027	ns	−0·097	ns
	Fibre	−0·345	*	−0·465	**
	Ray	0·458	**	0·437	**
	Axial parenchyma	−0·267	ns	−0·095	ns
	Cell wall	0·358	**	0·373	**

†CS, compressive strength; r, correlation coefficient. **significance at 1% level; *significance at 5% level; ns, not significant, number of specimens = 48.

Among the cell proportions, the fibre proportion inversely correlated with air-dried density or CS, while the ray and cell wall proportions showed significant positive correlations with them (Table 3). Such a negative correlation in fibre proportion decreases air-dried density and CS, but the negative effect of fibre proportion is possibly counteracted by the positive influences of ray proportion and cell wall proportions. Our result similarly agreed with Ishiguri et al. (2009), where they noted a negative correlation in fibre volume (r = −0·72, 1% level), and positive correlation in cell wall proportion (r = 0·72, 1% level) in Paraserianthes falcataria (L.) Nielsen). On the other hand, non-significant negative correlations were found in the vessel morphology, and vessel and axial parenchyma proportions with density as well as CS (Table 3). This might be due to low variation of vessel and axial parenchyma along the radius (Table 1).

Conclusion

The radial variations of anatomical properties in 11-year old Acacia auriculiformis A. Cunn. (ex Benth) trees were investigated from pith to bark. The anatomical properties varied in different magnitudes along the radius due to the influence of radial growth, except axial parenchyma proportion. The air-dried density or CS variation is mainly due to the fibre morphological attributes, i.e. fibre diameter and fibre wall thickness. Considering the importance of wood for pulping, the results (significant variation among the trees in vessel and fibre diameters, ray and axial parenchyma proportions) suggest that this information could be used as additional criteria in plus tree selection to improve wood quality.

Footnotes

Acknowledgements

We thank the Bangladesh Forest Department to give us permission for the fieldworks. We also thank Mr Mihir Kumar Doe, Divisional Forest Officer, Bangladesh Forest Department and the field staff for their assistance in the fieldworks.

References

Banglapedia . 2013. http://www.banglapedia.org/httpdocs/HT/C_0364.htm (accessed 12/1/2013).

Bosman

MTM

, de Kort

, van Genderen

, Bass

. 1994. Radial variation in wood properties of naturally and plantation grown light red meranti (Shorea, Dipterocarpaceae). IAWA J. 15: 111–120.

Chowdhury

, Ishiguri

, Iizuka

, Hiraiwa

, Matsumoto

, Takashima

, Yokota

, Yoshizawa

. 2009a. Wood property variation in Acacia auriculiformis growing in Bangladesh. Wood Fibre Sci. 41: 359–365.

Chowdhury

, Ishiguri

, Iizuka

, Takashima

, Matsumoto

, Hiraiwa

, Ishido

, Sanpe

, Yokota

, Yoshizawa

. 2009b. Radial variations of wood properties in Casuarina equisetifolia growing in Bangladesh. J. Wood Sci. 55: 139–143.

Chowdhury

, Ishiguri

, Hiraiwa

, Takashima

, Iizuka

, Yokota

, Yoshizawa

. 2012a. Radial variation of bending property in plantation grown Acacia auriculiformis in Bangladesh. For. Sci. Tech. 8: 1–4.

Chowdhury

, Ishiguri

, Iizuka

, Ishido

, Takashima

, Matsumoto

, Hiraiwa

, Yokota

, Yoshizawa

. 2012b. Variation in anatomical properties and correlations with wood density and compressive strength in Casuarina equisetifolia growing in Bangladesh. Aust. For. 75: 95–99.

Chowdhury

, Shams

, Alam

. 2005. Effects of age and height variation on physical properties of mangium (Acacia mangium Wild) wood. Aust. For. 68: 17–19.

Denne

, Hale

. 1999. Cell wall and lumen proportions in relation to wood density of Nothofagus nervosa. IAWA J. 20: 23–36.

Hai

, Hannrup

, Harwood

, Jansson

, Ban

. 2010. Wood stiffness and strength as selection traits for swan timber in Acacia auriculiformis. Can. J. For. Res. 40: 322–329.

10.

Hai

, Jansson

, Harwood

, Hannrup

, Thinh

. 2008. Genetic variation in growth, stem straightness and branch thickness in clonal trials of Acacia auriculiformis at three contrasting sites in Vietnam. For. Ecol. Manage. 255: 156–167.

11.

Ishiguri

, Hiraiwa

, Iizuka

, Yokota

, Priadi

, Sumiasri

, Yoshizawa

. 2009. Radial variation of anatomical characteristics in Paraserianthes falcataria planted in Indonesia. IAWA J. 30: 343–352.

12.

Ishiguri

, Yokota

, Yoshizawa

, Ona

. 2004. Radial variation of cell morphology in three Acacia species. In: Ona, T. (edited). Improvement of forest resources for recyclable forest products. Tokyo: Springer-Verlag, 74–76.

13.

Islam

, Kamaluddin

, Bhuiyan

, Badruddin

. 1999. Comparative performance of exotic and indigenous forest species for tropical semievergreen degraded forest land reforestation in Chittagong, Bangladesh. Land Degrad. Dev. 10: 241–249.

14.

JIS . 1994. Methods of test for woods. JIS Z2101-1994. Japan Standards Association, Tokyo, Japan.

15.

Kabir

, Webb

. 2005. Productivity and suitability analysis of social forestry woodlot species in Dhaka Forest Division, Bangladesh. For. Ecol. Manage. 212: 243–252.

16.

Kojima

, Yamamoto

, Yoshida

, Ojio

, Okumura

. 2009. Maturation property of fast-growing hardwood plantation species: a view of fibre length. For. Ecol. Manage. 257: 15–22.

17.

Ona

, Sonoda

, Ito

, Shibata

, Tamai

, Kojima

, Ohshima

, Yokota

, Yoshizawa

. 2001. Investigation of relationships between cell and pulp properties in Eucalyptus by examination of within-tree property variations. Wood Sci. Tech. 35: 229–243.

18.

Panshin

, de Zeeuw

. 1980. Textbook of wood technology. New York: McGraw-Hill.

19.

Pinyopusarerk

, Williams

, Boland

. 1991. Geographic variation in seedling morphology of Acacia auriculiformis A. Cunn. (ex Benth). Aust. J. Bot. 39: 247–260.

20.

Savidge

. 2003. Tree growth and wood quality. In: Barnett, J. R., and Jeronimidis, G. (edited). Wood quality and its biological basis. Oxford: Blackwell Publishing, 1–29.

21.

Taylor

, Wooten

. 1973. Wood property variation of Mississippi Delta hardwoods. Wood Fibre Sci. 5: 2–13.

22.

Verghese

, Nicodemus

, Subramanian

. 1999. Growth and wood traits of plantation-grown Acacia mangium, A. auriculiformis and A. crassicarpa from Thane, Maharashtra. Indian For. 125: 923–928.

23.

Zobel

, Jett

. 1995. Genetics of wood production. Berlin: Springer-Verlag.

24.

Zobel

, van Buijtenen

. 1989. Wood variation: its causes and control. Berlin: Springer-Verlag.