VARIABILITY of FIBRE LENGTH in WOOD and BARK in EUCALYPTUS GLOBULUS by Fátima Jorge1, Teresa Quilhó2 & Helena Pereira1

IAWA Journal, Vol. 21 (1), 2000: 41–48

VARIABILITY OF FIBRE LENGTH IN WOOD AND BARK IN EUCALYPTUS GLOBULUS by Fátima Jorge1, Teresa Quilhó2 & Helena Pereira1

SUMMARY Ten trees of Eucalyptus globulus Labill. from three different sites within Portugal were felled at 12–15 years, to study fibre length variation in bark and wood. The fibres ofE. globulus were morphologically similar in bark and wood, but generally longer in the bark (on average, 0.97 mm in wood and 1.11 mm in bark). The axial variation was small and opposite in wood and bark; fibre lengths decreased in the wood and increased in the bark from the base to the top. Fibre length in wood increased significantly from pith to bark at all height levels. The meas- urement of fibre length at 1.3 m height level was representative for the tree average for both wood and bark. Key words: Eucalyptus globulus, wood variability, fibre length, bark, wood anatomy.

INTRODUCTION

Some eucalypt species have become important sources for wood fibre and are planted worldwide for the pulpwood industry. The first species to be used in large-scale plantation forestry for pulp production was Eucalyptus globulus Labill., which combines fast growth with excellent wood quality for pulping. In Europe, E. globulus is grown in Portugal and Spain, on approximately 1 million ha, feeding a 1.5 thousand tons per year pulp industry. Fibre length is one of the quality parameters for pulpwood, and it has been exten- sively studied in relation to tree age and within-tree position (e.g., Hudson et al. 1995; Sandercock et al. 1995). Fibre dimensions are determined by the dimensions of the cambial fusiform cells from which they are derived and by processes that occur during cell differentiation (Ridoutt & Sands 1993, 1994). The length variation in phloem fibres also has been considered as a combined effect of intrusive growth and of the changes in fusiform cells associated with the aging of the cambium (Ghouse & Siddiqui 1976). In E. globulus fibres represent 64–68% of the wood (Dadswell 1972), and most studies have focused on fibre length variability (e.g., Bamber 1985; Jorge 1994). In

1) Centro de Estudos Florestais, Instituto Superior de Agronomia, Universidade Técnica de Lisboa, 1399 Lisboa Codex, Portugal. 2) Centro de Estudos de Tecnologia Florestal, Instituto de Investigação Científica Tropical, 1399 Lisboa Codex, Portugal.

Downloaded from Brill.com10/05/2021 03:41:35PM via free access 42 IAWA Journal, Vol. 21 (1), 2000 Jorge, Quilhó & Pereira — Fibre length in Eucalyptus globulus 43 the bark, which has been recently studied in detail (Quilhó et al. 1999; 2000, this issue), fibres represent 35% of all cell types, and their proportion and length were shown to be affected by site and age. In this study, we analysed the wood and bark fibres in E. globulus trees and as- sessed the within- and between-tree variability in order to gain insight in the range of variation of this important industrial quality parameter in relation to tree age and site.

MATERIAL AND METHODS

Eucalyptus globulus Labill. trees were harvested from commercial pulpwood plantations, at the end of the first rotation in three different sites in Portugal: Castelo Branco (39° 49' N; 7° 29' W; altitude 380 m; rainfall 825 mm and mean temperature 16°C), Águeda (40° 31' N; 8° 19' W; altitude 500–550 m; rainfall 1229 mm and mean temperature 14°C) and Nisa (39° 30' N; 7° 46' W; altitude 300 m; rainfall 908 mm and mean temperature 15°C ). The plantations were established using a commercial seed source at 3 × 3 m spacing. Harvest age was 12 years in Águeda, 13 years in Nisa and 15 years in Castelo Branco. In each site, two 1000 m2 plots were established and 5 trees per plot were ran- domly selected for sampling. Tree characteristics are given in Table 1. Stem sectional discs were taken from each tree at different levels of total tree height (5%, 15%, 35%, 55% and 75%) and an additional disc was taken at breast height. The radial variation was studied by sampling in each wood disc at 5 fixed percent- ages of the radius (10%, 30%, 50%, 70% and 90%). Fibre length was measured from wood and bark samples macerated in a 1 : 1 gla- cial acetic acid : hydrogen peroxide solution, and stained with astra blue. Two slides and 20 fibres per slide were measured for each sampling position. Preliminary testing showed that with this sampling intensity the error was below 5% for a 95% confidence level. The fibres were measured with a Leitz ASM 68K semi-automatic image ana- lyser.

Table 1. Characteristics of the sampled Eucalyptus globulus trees. Mean and standard deviation of 5 trees per plot in three sites.

Tree age Total height (m) DBH (cm) Castelo Branco Plot 1 15 21.7 ± 1.0 16.4 ± 1.6 Plot 2 15 18.0 ± 1.1 17.0 ± 1.6 Águeda Plot 1 12 19.7 ± 1.9 15.9 ± 0.8 Plot 2 12 15.4 ± 2.0 15.4 ± 0.9 Nisa Plot 1 13 21.8 ± 1.7 15.9 ± 0.6 Plot 2 13 21.9 ± 1.8 16.5 ± 0.8

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At each tree height level, the mean wood fibre length was calculated as an area weighted average using the measurements at the different radial positions, as:

where Fh is the mean wood fibre length at each height level, fi is the fibre length measured at each sampling position along the radius (i = 1 to 5, corresponding to 10%, 30%, 50%, 70% and 90% of total radius) and Aj is the area of a circle with increasing radius (j = 1 to 5, corresponding to 20%, 40%, 60%, 80% and 100% of total radius). The mean bark and wood fibre length for each tree was determined using a volume weighted average of the values at each height level. The tree bark volume was calculated as a difference between total tree volume and wood volume, by sections corresponding to the different height levels of sampling as a total of 5 conical sections; the top portion above 95% height (with a diameter under 3 cm) was disregarded. The tree average of bark and wood fibre length was calculated as a volume weighted mean following:

where P is the tree mean parameter, vi is the bark/wood volume of each section and pi the parameter measured at each height level. A comparison of the results obtained at a fixed height of 1.3 m (breast height) with the tree average was also made.

RESULTS AND DISCUSSION

The fibres of E. globulus are fusiform with apical extensions at both ends, and have similar morphology in the bark and wood. Fibre lengths for wood and bark are summarised in Table 2. On average, wood fibres were 1.0 mm long, agreeing with Carvalho (1962), Wilkes (1988) and Ridoutt and Sands (1993). Bark fibres were longer, especially in site Castelo Branco where bark fibres were 31% longer than wood fibres. This agrees with Pereira and Araújo (1990) for very young E. globulus trees and with Hillis (1972), who observed bark fibres approximately 20% longer than wood fibres in young Eucalyptus trees. Para- meswaran and Liese (1974) had the same results in tropical trees, and Hakkila (1989) for some hardwood species (e.g., Acer rubrum, Alnus rugosa, Betula populifolia, Popu- lus tremuloides). These observations could result from a retarded production of bark cells from the cambial initials, coupled with production of a large number of xylem elements, so that phloem fibres have more time to elongate.

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Height (%) Fig. 1. Axial variation of wood fibre length inEucalyptus globulus in Águeda, Nisa and Castelo Branco. Mean of 10 trees per site, standard deviation as bar. Fibre length (mm)

Height (%) Fig. 2. Axial variation of bark fibre length inEucalyptus globulus in Águeda, Nisa and Castelo Branco. Mean of 10 trees per site, standard deviation as bar.

An analysis of variance of wood and bark fibre length for the three sites showed significant differences (P < 0.001) only between Castelo Branco and Águeda/Nisa. In the first, wood fibres were 14% shorter than in the second, despite the trees being 2–3 years older. Within a site there were no plot differences and the between-tree variability of fibre length was small, with coefficients of variance of the mean below 10%. The axial variation of wood fibre length was characterised by a slight decrease up the tree (Fig. 1). This agrees with the pattern described by Wilkes (1988) for eucalypts where fibre length increased more often to a point well up the bole and then declined at higher levels. This pattern was also observed by Carvalho (1962) and Jorge (1994) in E. globulus, Bisset and Dadswell (1949) in E. regnans, Sardinha and Hughes (1978–1979) in E. saligna and Bhat et al. (1990) in E. grandis. The decrease of wood fibre length towards the top was also described for E. globulus by Ridoutt and Sands (1993), who found a positive relationship between wood fibre length decrease with height in the tree and a concomitant decrease in the length of fusiform initials in the vascular cambium. We found that the axial variation was small in all the trees, as did Hans and Burley (1972) in E. camaldulensis, E. citriodora, E. pilularis, E. saligna, E. tereticornis and Taylor (1973) in E. grandis.

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a Fibre length (mm)

b Fibre length (mm)

c Radial position (%) Fig. 3. Radial variation of wood fibre length in Eucalyptus globulus at the different tree height levels, 5% (a), 15% (b), and 75% (c), in Águeda, Nisa and Castelo Branco. Mean of 10 trees per site, standard deviation as bar.

In the bark, fibre length varied little axially but was generally somewhat longer in the top (Fig. 2). However, this trend of variation was neither gradual nor constant in the samples studied. Nicholls and Phillips (1970) found no apparent pattern of length variation with position, height or age of the bark in E. viminalis. Also for the bark, Ghouse and Siddiqui (1976) and Iqbal and Ghouse (1983) described various trends of fibre length variation for different species. Parameswaran and Liese (1974) noticed a slight increase of phloem cell length in the crown and explained it by a possible increase of the cambial initials from the base to the top.

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Fibre length at 1.3 m height (mm) Fig. 4. Comparison of fibre length in wood and bark, as measured at 1.3 m height level and as tree average.

In this study an inverse trend of axial variation for bark and wood fibre length was observed in the same trees (Fig. 1 & 2), suggesting a distinct rate and duration of the phases of differentiation in the phloem and in the xylem cells. In relation to radial variation in length, wood fibres showed a significant increase in length from the pith to the periphery. This variation occurred at all height levels and at all sites (Fig. 3). For instance, in Águeda at the 15% height level, fibre length

Table 2. Wood and bark fibre length (mm) forEucalyptus globulus in the three sites. Mean and standard deviation of 10 trees per site. Site values followed by the same letter are not significantly different.

Site Águeda Nisa Castelo Branco Tree mean fibre length (mm) Wood 1.04 ± 0.06 b 0.99 ± 0.04 b 0.87 ± 0.05 a Bark 1.04 ± 0.09 b 1.02 ± 0.03 b 1.28 ± 0.08 a

DBH fibre length (mm) Wood 1.04 ± 0.07 b 0.99 ± 0.04 b 0.87 ± 0.06 a Bark 0.99 ± 0.09 b 1.01 ± 0.07 b 1.27 ± 0.09 a

DBH / Tree mean fibre length Wood 1.00 0.99 1.01 Bark 0.95 0.99 0.99

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was at 10%, 30%, 50%, 70% and 90% of the radius, respectively, 0.70 mm, 0.86 mm, 1.00 mm, 1.10 mm, and 1.20 mm. The same magnitude of radial variation was found at the 75% height level, with 0.70 mm, 0.84 mm, 0.94 mm, 1.05 mm and 1.13 mm, respectively. Carvalho (1962), Tomazello Filho (1987) and Bhat et. al. (1990) also found the same trend of radial variation in other species of Eucalyptus. The increase of fibre length could be explained on the basis of the increase in length of cambial initials with increasing cambial age. Usually, wood quality of trees is evaluated using samples taken from a certain height level, often 1.30 m. To assess how well sampling at this height estimates tree mean fibre length, these two values were compared for each tree (Fig. 4, Table 2). Both results were identical, not surprisingly, considering the small axial variation of fibre length, with the difference being under 5% for all trees.

CONCLUSION

The bark and wood fibres of Eucalyptus globulus were morphologically similar, but generally longer in the bark. The axial variation in fibre length was small and opposite in wood and bark: a decrease in wood and an increase in the bark from base to top. A significant radial variation in wood fibre length occurred at all height levels, with longer fibres at the periphery. A sampling at 1.30 m high was representative for assessing the mean fibre length of wood and bark in E. globulus at 1–15 years of age.

ACKNOWLEDGEMENTS

We thank Cristiana Alves and Lídia Silva for their technical assistance.

REFERENCES

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