WOOD AND BARK FIBER CHARACTERISTICS OF Acacia melanoxylon AND COMPARISON TO Eucalyptus globules

Wood and bark fi bers of Acacia melanoxylon were characterized and compared to Eucalyptus globulus which is a major quality source of pulp fi bers. In 20 trees from four sites, fi ber length and wall thickness were measured at 5, 35 and 65% of total tree height and at 10, 30, 50, 70 and 90% of the distance from pith. Maceration were prepared in a 1:1 glacial acetic acid:hydrogen peroxide solution. Wood and bark fi ber length varied between 0.90 0.96 mm and 1.33 1.59 mm respectively. The cell wall thickness varied between 3.45 3.89 μm in wood and 5.01 5.40 μm in bark. Wood and bark fi ber length decreased from the bottom to the top of the tree and cell wall thickness had no specifi c pattern for axial variation. Fiber length and wall thickness increased from the pith to the bark, but the wall thickness increased slightly with some fl uctuations. In Acacia melanoxylon signifi cant site differences were found in relation to bark fi ber length and to wood wall thickness. The fi bers of Acacia melanoxylon were similar to those of Eucalyptus globulus but the wood fi bers were thinner and the bark fi bers thicker. The radial variation was similar in both species. In wood of Eucalyptus globulus, fi ber wall thickness increases from the base to the middle of tree height and decreases to the top; in the bark decreases from the base to the top. In Eucalyptus globulus fi bers bark are higher in the top.


INTRODUCTION
Acacia melanoxylon R. Br. and Eucalyptus globulus L. are tree species native to Australia that occur naturally across a wide range of forest ecosystems.They were introduced in Portugal as ornamentals in the late XIXth century and their expansion occurred through national forestation programs.
In Europe, Acacia melanoxylon is considered as invasive, characterized by vigorous tree or root sprouts and with seed germination stimulated by fi re.Eucalyptus globulus, as most eucalypt species, also releases the seed and is capable of vegetative regeneration after fi re.In Portugal, Acacia melanoxylon is found associated with pines along the Atlantic Coast and in pure or mixed stands with other hardwoods and pines in the inner part of the country, especially in fi re risk areas.
Acacia species were primarily cultivated for bark, which was used in the tanning industry (ILVESSALO-PFAFFLI, 1995) but they are important for several Tavares, F. et al.
purposes including building and furniture, fuelwood, perfume, traditional medicine, pulp and paper and in general have a good biomass potential (HARRIS, 2004).They are used also for wood-based composite and panel products (PEDIEU et al., 2009).The wood of Acacia melanoxylon is highly regarded in its natural region as a quality timber for carpentry and cabinet making (JENNINGS et al., 2003) and with potential to develop into a quality leader of short-fi ber pulp (GIL et al., 1999).
Very little has been published on the wood and bark anatomy of Acacia melanoxylon and on its variability (GHOUSE;IQBAL, 1977;IQBAL;GHOUSE, 1983;QUINTANAR ISAIAS et al., 2005;SAHRI et al., 1993).In this context, the biometric characteristics of fi bers are especially important given their relevance to properties and end-uses.
The aim of the this study is to characterize the wood and bark structure of Acacia melanoxylon in relation to fi ber biometry (length and wall thickness) and their variation within and between trees growing in different site conditions in Portugal, in a context of raw-material quality evaluation for pulping.Therefore the results are compared with the fi ber characteristics of Eucalyptus globulus, which is a major source for production of quality short fi ber pulp.

MATERIAL AND METHODS
The study was made on 20 trees (fi ve trees in each site), with a 40 cm DBH class sampled in four stands located in northern and central Portugal: Caminha, Ponte de Lima, Ovar, Viseu.The location and characterization of the sites are summarised in Table 1.
The trees were bucked and stem discs with 5 cm thickness were taken at different height levels.The axial variation of fiber length and cell wall thickness was studied in stem discs taken at 5%, 35% and 65% of total tree height, both in wood and bark.The radial variation of wood fi ber length and cell wall thickness in each height level was measured in fi ve radial positions at 10, 30, 50, 70 and 90% of the distance from pith.
Fiber length and cell wall thickness were measured in bark and wood samples macerated in a 1:1 glacial acetic acid:hydrogen peroxide solution.Two slides and 20 fi bers per slide were measured for each sampling point.Preliminary testing showed that with this sampling intensity the error was below 5% for a 95% confi dence level.The fi bers were measured with a Leitz ASM 68K semi-automatic image analyser.
Mean values were calculated for each height level as the arithmetic mean of all the measured radial positions.Mean values for the tree were calculated as the arithmetic mean of the three height levels studied in each tree.
Statistical analysis was made using the Sigma Stat Scientifi c Statistical software SigmaStat from Jandel Corporation.Analysis of variance (ANOVA) was applied to compare if the differences between wood and bark fi ber length and wall thickness from different sites are statistically signifi cant different at a 0.05 confi dence level (QUILHÓ et al., 2000).

RESULTS AND DISCUSSION
Figure 1 shows the general aspect of the dissociated wood fi bers of Acacia melanoxylon (Figure 1a and 1b) and of Eucalyptus globulus (Figure 1c).The wood fi bers in these two species are fusiform with apical extensions at both ends.In some of the trees the wood fi bers were bifurcated at one end (Figure 1b).
Bark fi bers of Acacia melanoxylon are similar to bark fi bers of Eucalyptus globulus as shown in Figure 2a and 2b respectively.They are fusiform with apical extensions at both ends.Bifurcated fi bers at one end can also be found in Acacia melanoxylon.The fi bers of Acacia melanoxylon and Eucalyptus globulus are morphologically identical in bark and wood, but longer in the bark of both species.

Fiber length
The fi ber length of Acacia melanoxylon wood and bark ranged between 0.90-0.96mm and 1.33-1.59mm, respectively (Table 2).The results are close to those described for other Acacia species, namely Acacia mangium: mean value of 1.08 mm with a range of 0.96 mm to 1.20 mm (LIM; GAN, 2000;LOGAN;BALODIS, 1982), 0.80 mm to 1.3 mm (WU; WANG, 1988) and 0.9 mm to 1.3 mm (RICHTER; DALLWITZ, 2000).Kim et al. (2008)  The average bark fi ber length determined in Acacia melanoxylon fits into the range of 0.833 -1.100 mm reported for this species by Ghouse and Iqbal (1977) and
The average bark fi ber length in Acacia melanoxylon is similar to the values determined in Eucalyptus globulus bark of 1.28 mm (QUILHÓ, 1998) and of 1.02 mm and 1.04 mm (JORGE et al., 2000).
In Acacia melanoxylon, fi ber length was in average 35% higher in bark than in wood.The same was also reported in Acacia cochliacantha (>30% higher) by Quintanar Isaias et al. (2005) and in various genus and species i.e. in Eucalyptus (20% higher) by Hillis (1972) and Jorge et al. (2000).However in Acacia nilotica Iqbal and Ghouse (1983) observed similar wood and bark fi ber length.
The wood fi ber length of Acacia melanoxylon was similar in the four sites and no statistical signifi cant differences were found between sites.For bark fiber length, signifi cant site differences (P <0.005) were found between Ovar and the other three sites.For Eucalyptus globulus, Jorge et al. (2000) and Quilhó et al. (2000) reported that wood and bark fi ber length was affected by site, while Jorge (1994) and Onofre (1999) found no statistical signifi cant differences between sites in relation to wood fi ber length.

Radial variation of wood fi ber length
Figure 3 shows the radial variation of wood fi ber length in Acacia melanoxylon with increasing distance from the pith.Fiber length increased from an average 0.75 mm near the pith, to 0.98 mm at mid position and to 1.06 mm at the periphery (90% of the distance from pith).Average fi ber length increased more rapidly between pith and middle positions than between mid and periphery positions (by 0.23 mm and 0.08 mm respectively).At the periphery, fi ber length was longer than 1.00 mm in all sites.Radial variation of fi ber length showed the minimal mean value near the pith in Caminha (0.69 mm), while the maximal mean value (1.09 mm) was found near the bark in Viseu (Figure 3).
Fiber dimensions increased in general from pith to bark.Sahri et al. (1993), in 4-and 8-year-old trees of Acacia mangium registered the same trend of radial variation as well as Honjo et al. (2005) for ages of 2 to 14 years where the fi ber length varied between 0.9 mm and 1.3 mm near the bark and between 0.4 mm and 0.6 mm near the pith.The same pattern of fi ber length variation was reported for Acacia bilimeki (1.032 mm to 1.077 mm) and it was different in Acacia cochiacantha with 0.780 mm near the pith and 0.727 mm close to the bark (QUINTANAR ISAIAS et al., 2005).

Axial variation of wood and bark fi ber length
Figures 4 and 5 show the axial variation of wood and bark fi ber length in Acacia melanoxylon, respectively.The axial variation of fi ber length in wood was characterised by a slight decrease towards the top of the tree (Figure 4), from an average of 0.97 mm and 0.94 mm to 0.91 mm at 5%, 35% and 65% of total tree height respectively.The maximal mean value was determined at 5% of total tree height in Ovar (1.01 mm) and the minimum occurred in Caminha (0.89 mm) at 65% height level.
Very little has been published on axial variation patterns of fi ber length in Acacia melanoxylon.Sahri et al. (1993) found the same axial variation in Acacia mangium with a fi ber length decrease with height (1.026 mm to 0.849 mm).Eucalyptus globulus shows the same axial variation of wood fi ber length.
The axial variation of bark fi ber length observed in Acacia melanoxylon was characterised by a decrease towards the top of the tree (Figure 5) from an average of 1.51 mm and 1.42 mm to 1.34 mm at 5%, 35% and 65% of total tree height respectively.The maximal mean value was determined at 5% of total tree height level in Ovar (1.78 mm) and the minimum in Viseu (1.26 mm) at 65% height level.This is a pattern also described by Iqbal and Ghouse (1983) for Acacia nilotica with a variation of 1.12 mm in the base, 1.16 mm in the middle and 0.986 mm in the top of the tree.
The axial variation of Acacia melanoxylon wood and bark fi bers showed a similar pattern, suggesting a similar cell differentiation mechanism in phloem and xylem.In Eucalyptus globulus, on the contrary, an inverse trend of fi ber length variation was reported for wood and bark, with longer phloem fi bers occurring at the top of the stem (JORGE et al., 2000;QUILHÓ et al., 2000).

Radial variation of wood fi ber wall thickness
In average, fiber wall thickness in Acacia melanoxylon increased from pith to bark, with some fl uctuations (Figure 6), from an average 3.55 μm near the pith, to 3.74 μm at the middle position and to 3.80 μm at the periphery (90% of the distance from pith).The minimal mean value was near the pith in Ovar (3.32 μm), and the maximum (3.94 μm) in Viseu.Average fi ber wall thickness increased more rapidly between pith and middle positions than between middle and periphery positions (0.19 μm and 0.06 μm respectively).Figura 5 -Variação axial do comprimento das fi bras da casca da Acacia melanoxylon nos quatro locais.

Tavares, F. et al.
Figure 6 -Radial variation of wood fi ber wall thickness of Acacia melanoxylon in four sites.
Figura 6 -Variação radial da espessura de parede das fi bras da madeira da Acacia melanoxylon nos quatro locais.Sahri et al. (1993) in 4 and 8-year-old samples of Acacia mangium observed that the fi ber wall is relatively thin and varies little in thickness, between 3.1 μm and 3.5 μm for 4-year old trees and 4.1 μm to 4.5 μm for 8 year old trees and Salang e Fujii (2000) reported a slight increase from the pith to the sapwood.Quintanar Isaias et al. (2005) reported in Acacia bilimekii the value of 5 μm in the centre and in the periphery and in Acacia cochliacantha an increase from 5 to 7 μm from pith to bark.
For Eucalyptus globulus, Jorge (1994) registered also slight increases in wall thickness from pith to bark from 1.8 μm to 2.5 μm and Onofre (1999) found no radial pattern of wood wall thickness.

Axial variation of wood and bark fi ber wall thickness
Figures 7 and 8 show the axial variation of wood and bark fi ber wall thickness of Acacia melanoxylon in four sites, respectively.
In wood fi ber wall thickness had no specifi c pattern for axial variation (Figure 7).Fiber wall thickness was higher in Viseu and Caminha than in Ponte de Lima and Ovar.
The maximal mean value of fi ber cell wall was determined at 5% of total tree height level in Viseu (3.97 μm) and the minimum in Ovar (3.44 μm) at the same height level.
For Eucalyptus globulus, the fi ber wall thickness increases from the base to 55 % tree height and decreases to the top from 1.97 μm, to 2.1 μm and 2.07 μm, respectively (JORGE, 1994).
In bark, fi ber wall thickness had also no specifi c pattern for axial variation at 5%, 35% and 65% of total tree height (Figure 8).Bark fi ber wall thickness values were lower in Caminha (Figure 8) and the maximal mean value of fi ber cell wall was determined at 35% of total tree height in Ovar (5.52 μm) and the minimum in Ponte de Lima (3.44 μm) at 65% height level.

CONCLUSIONS
In the wood of Acacia melanoxylon, fi ber length and wall thickness increased radially from pith to bark.Axially the wood fi ber length decreased from bottom to the top of the tree and wall thickness had no specifi c variation pattern.
In the bark of Acacia melanoxylon, fi ber length decreased from bottom to the top of the tree and bark wall thickness had no specifi c variation pattern.
Signifi cant site differences were found only in relation to wood fi ber cell wall thickness and bark fi ber length.
Fibers of Acacia melanoxylon were similar to those of Eucalyptus globulus in relation to length in wood and bark but wood fi bers were thinner and bark fi bers thicker.
in six natural hybrid clones of Acacia mangium and Acacia auriculiformes observed values from 0.86 mm to 0.93 mm.The mean wood fi ber length of Acacia melanoxylon obtained in this study is similar to values reported for Eucalyptus globulus of 1.0 mm (CARVALHO, 1962), 0.87 mm to 1.04 mm (JORGE et al., 2000), 0.923 mm (ONA et al., 2001), 0.941 mm to 1.192 mm (IGARTÚA et al., 2002) and 0.888 mm to 0.958 mm (MIRANDA; PEREIRA, 2002).

Figure 1 -
Figure 1 -Dissociated wood samples: (a) Acacia melanoxylon: libriform fi bers and one vessel element; (b) Acacia melanoxylon: libriform fi bers with one bifurcated fi ber at one end and (c) Eucalyptus globulus: libriform fi bers and one vessel element.

Figure 4 -
Figure 4 -Axial variation of wood fi ber length of Acacia melanoxylon in four sites.

Figure 3 -
Figure 3 -Radial variation of fi ber length of Acacia melanoxylon in four sites.

Figure 5 -
Figure 5 -Axial variation of bark fiber length of Acacia melanoxylon in four sites.

Figure 7 -
Figure 7 -Axial variation of wood fi ber wall thickness of Acacia melanoxylon in four sites.

Figure 8 -
Figure 8 -Axial variation of bark fi ber wall thickness of Acacia melanoxylon in four sites.

Table 1 -
Location and characterization of sampling sites.

Table 2 -
Wood and bark fi ber length (mm) for Acacia melanoxylon in four sites.Mean and standard deviation of fi ve trees per site.Tabela 2 -Comprimento das fi bras da madeira e da casca (mm) da Acacia melanoxylon nos quatro locais.Média e desvio padrão de 5 árvores por local.

Table 3 -
Wood and bark fi ber wall thickness (μm) for Acacia melanoxylon in four sites.Mean and standard deviation of fi ve trees per site.