Comparison of tree growth, wood density and anatomical ...

Comparison of tree growth, wood density and anatomical
properties between coppiced trees and parent crop of six

Eucalyptus genotypes

A Zbonak, T Bush and V Grzeskowiak

Forestry and Forest Products Research Centre, CSIR, P.O. Box 17001, 4013 Congella, RSA
E-mail: [email protected]

ABSTRACT

Coppicing is a very common practice in South African forestry. It allows a plantation grower to have a second
timber rotation without replanting and thus reduces reestablishment costs. While the large variation in wood
properties among different Eucalyptus resources is well documented, little information is available on the quality
of wood of the coppice shoots and whether it is different from that of the original planted trees.

In this paper, tree growth, wood density and anatomical characteristics of coppice and original parent wood were
measured using rapid screening tools. Six different Eucalyptus genotypes aged seven years were harvested from
a research trial in Zululand, South Africa in 1997. Cut stumps were allowed to coppice and managed to produce
one to two coppice sprouts. In 2005, the coppiced stems from the same original trees were sampled at the age of
eight years. In total, fifty four trees were selected for both types of wood. The wood of parents and coppiced trees
were similar in terms of vessels characteristics and fibre wall thickness. However, the parent trees had
significantly smaller fibres with smaller lumen. Consequently, the wood density of the parent trees was higher
than that of the coppiced trees. A serious drought which occurred during the growth of the parent trees was
associated with marked changes in certain wood properties. This made it difficult to assess the real effect of
coppicing on wood quality.

The ability to predict the quality of coppiced trees is important for the future strategy of growers. If productivity
yield and wood quality of coppiced trees are comparable to those of parent trees, then coppicing would be a
preferable option to replanting since it can greatly reduce reestablishment and management costs.

Key words: coppiced wood, wood density, anatomical properties, Eucalyptus

INTRODUCTION
Eucalypts are an important supply of fibre for the South African pulp and paper industry. The
genus is largely propagated by seeds and cuttings but it also has an ability to resume growth
from harvested tree stumps or the base of a damaged stem. The shoots produced from the
cambium layer beneath the bark are known as “coppice”. These grow using the same root
system that had developed for planted trees. Coppicing is a very common practice in South
Africa forestry. It allows the growers to be able to have a second timber rotation without
replanting, provided the first rotation was established well and there are enough stumps that
have regrown.

While the large variation in the fibre and pulp properties among different Eucalyptus
resources in South Africa is well documented (Malan 1988, Malan 1993, Clarke et al., 1999,
Zbonak 2006), little is known about quality of wood of the coppice shoots and whether it is
different from that of the original planted trees. If volume yield and wood quality are
comparable to those of the parent trees, then coppicing would be preferable option since it
can greatly reduce establishment and management costs (Whittock et al., 2004). Sharma et
al. (2005) investigated wood quality between non-coppiced and coppiced wood of
E. tereticornis in India. They concluded that both types of wood are comparable and can be
utilized for the same purposes. Schonau (1991) found that wood density of coppiced trees of
E. grandis is lower to their parent trees. Sesbou and Nepveu (1991) confirmed, the first
cutting decreases basic density for E. camaldulensis, however pulp yield and fibre length
were higher. In the past, Forestry and Forest Product Research Centre (FFP) of the CSIR
(the Council for Scientific and Industrial Research) conducted research to study wood and

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pulp properties of coppiced E. grandis and compared it with seedling material (Price and
Turner, 2000). No clear results emerged since the coppiced material was not compared to its
original parent trees.

Sappi research trial Mavuya, which consisted of six planted genotypes was sampled in 1997
and allowed to coppice. Since 54 sampled trees had been previously analysed in detail, this
provided an excellent opportunity to study the effect of coppicing on wood and pulp quality
(Male et al., 1998).

In this paper, the differences in tree growth, wood structure and wood density between
coppiced and original parent trees of 6 Eucalyptus genotypes are presented. Detailed pith-to-
bark variation of fibre and vessel properties was evaluated using rapid screening tools. The
advantage of those tools is that the assessment of average wood characteristics for each
tree as well as the within tree variation can be determined.

MATERIAL AND METHODS
Parent trees
Six Eucalyptus genotypes were selected from Sappi Mavuya CS1-2 trial in Zululand (Table 1).
Owing to confidential nature of the project, the real names of clones within a single species
have not been revealed in this paper. They have been identified instead as clone A and
clone B. In August 1997, nine trees from each genotype across three replications were
sampled at age 7.3 years, totalling fifty four trees for wood and pulp quality evaluation.
Detailed information about genotype selection and results were reported by Male et al.
(1998). Each sampled tree was identified by a unique number within a compartment so later
identification of coppice was possible.

Coppiced trees
Parent trees were cut 10 cm above the ground and allowed to coppice. According to Little
and Gardener (2003), the large number of coppice shoots should be reduced to the original
stocking in two operations. First thinning should leave two or three shoots per stump when
the dominant height is 4 m at around 10 months old, and the second to the original stocking
at dominant height of 8 m. The current stand was thinned when coppice material was two
years old. No further thinning was done and trees were not fertilized.

In November 2005, the trial was assessed. The unique labeling system allowed location of
the stumps of the parent trees which had been felled in 1997. From the previously fifty four
original sampled trees, forty nine trees produced coppice shoots. Four trees of E. grandis
clone and one tree of E. GxC clone B did not have any shoots at the time of sampling. It is
not clear whether these trees had failed to coppice or whether the emerged shoots from
stumps, if any, died later during their growth. When sampling, the actual age of coppiced
trees was slightly older (around 8.5 years) when compared to their parent sampled trees (7.3
years). Generally, a single stem was taken from a stump. In cases where two or three stems
were growing from a single stump, the additional stem was only considered for sampling if
minimum DBH was more than 10 cm. In total, sixty seven stems were sampled where
eighteen trees provided two stems per stump. Table 1 summarises the number of single and
double stems, and stems which did not coppice in the trees samples. A total height, height to
stem diameter of 7 cm and diameter at breast height DBH (over-bark) were measured
directly upon felling. These values were compared with values of parent trees to determine
differences in tree growth.

Wood density
The basic density of a wedge of coppiced trees from breast height (BH) was evaluated using
a the same gravimetric method used previously for the parent trees.

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In addition, the wood density was also analysed by gamma-ray densitometry. Radial strips of
uniform thickness were obtained using a twin-blade saw from the ‘North’ position in each
disk. Wood density was measured on strips conditioned under constant humidity and
temperature (65% RH, 240C) at 0.5 mm intervals, in a radial-longitudinal face from pith to
bark using a gamma-ray densitometer. This density measurement is commonly referred to as
air-dry density.

Table 1 Studied genotypes and number of felled coppiced trees (related to original 9 trees per
genotype).

Stems E. GxC E. GxU E. grandis
Clone A Clone B Clone A Clone B Clone seedling
Not coppiced
Single stem 01 00 40
Double stems 98 28 22
Total sampled 00 71 37
98 16 10 8 16

Anatomical properties
Pith-to-bark strips of thickness 2.5 mm were extracted from BH disks and were sectioned in
the transverse plane using a sledge microtome. The 25 µm sections were examined using a
Leica fluorescent microscope. An image analysis system (Leica QWin) was used to
determine fibre properties at 2 mm intervals from the pith and vessel characteristics every
0.5 mm. Anatomical characteristics measured included: vessel diameter, vessel frequency
(the number of vessels per unit area), vessel percentage (the percentage area occupied by
vessels), fibre diameter, fibre lumen diameter and cell wall thickness.

Data analysis
Five original parent trees which did not produce coppiced stems were eliminated from further
analysis. In the instances where double stems per single stump were taken, the property
values were averaged to give a single value per tree.

Weighted mean values were calculated for each wood property for each tree from pith to
bark profile. To obtain a weighted mean value, the wood property measured at each position
from pith to bark was multiplied by the area measured; these values were added together
and divided by the sum of the squares of all the distances from the pith. Paired t-test was
used to compare the properties of identical individuals between parent and coppiced trees.

The wood density and anatomical characteristics were analysed at detailed intervals from
pith to bark. Since trees in this study did not produce classical growth rings, it was not
possible to determine the mean value for a particular age of tree growth. Instead, the relative
distance approach was used. Each strip was divided into 10 consistent parts over the radius
and the mean value of the property for each fraction calculated. Using this method the
different lengths of wood strips were standardised.

RESULTS AND DISCUSSION
The average values of various tree growth and wood physical properties of both parent and
coppiced wood of 6 genotypes are given in Table 2 along with the comparison based on
paired t-test. It was found that the difference in wood properties between parent and

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coppiced trees was clone specific. In particular for E. GxC clone B and E. grandis seedling,
wood samples from parent and coppiced trees were statistically similar. For another clone
(E. GxU clone B), almost all investigated properties of wood were significantly different. For
all genotypes, the most significant differences between parent and coppiced trees were
found for wood density, fibre and lumen diameter.

Coppiced trees were taller when compared to their original trees. However, paired t-test
revealed that significant differences were observed for both E. GxU clones and E. grandis
clone (Table 2). This difference can be attributed to the fact that coppice trees were older
when sampled (8.3 years) whereas seedling trees were 7.5 years old. A larger standard
deviation in height was observed for coppiced trees. This might be due to a fact that some
original stems produced two coppiced shoots of different height.

Trees of coppiced E. GxU clone B had significantly higher DBH compared to the parent trees
(Table 2). The coppiced trees were 1 year older when sampled compared to their parent
ones. However, a difference of 33 mm cannot be explained by age only, since the annual
increment for E. GxU clone at age of eight years rarely exceeds this value. Seven from nine
trees of E. GxU clone A and E. grandis seedling consisted of two stems per a stump. This did
not seem to have an effect on total height and DBH when compared to the values of their
original trees.

Concerning basic wood density, the coppiced trees produced wood with lower density
compared to their original trees. Only 20% of all coppiced samples had higher density than
their identical parent crop samples. The differences were statistically significant for both E. GxU
clones, E. GxC clone A and the E. grandis clone (Table 2).

When compared at a disc level, coppiced trees produced wood with significantly larger fibre
and lumen diameter (Table 2). Significant differences in fibre diameter were detected across
all genotypes, except for E. GxC clone B. In the case of lumen diameter, with the exception
of E. GxC clone B and E. grandis seedling, all genotypes of coppiced wood, showed
significant differences when compared to their original trees. Results indicate that coppiced
samples had thinner fibres with better ability to collapse. However, as revealed by the paired
t-test there were not significant differences between non-coppiced and coppiced wood (Table 2).

Vessel diameter, vessel percentage and vessel frequency showed very similar trends
between genotypes for both types of wood (Table 2). Significant differences between parent
and coppiced wood in vessel diameter and vessel frequency were found only for E. GxU
clone B, coppiced samples having fewer vessels with larger diameter.

Table 2 Mean values of wood properties with their standard deviation. Comparison between
parent and identical coppiced trees for 6 genotypes is based on paired t-test.

Features Type E. GxC clone A E. GxC clone B

Total tree parent Mean STD t-test Mean STD t-test
height (m) coppiced
20.38 0.78 NS 22.17 0.88 NS
DBH (cm) parent 20.69 2.05 22.01 3.01
coppiced
Wood basic 131.9 14.6 NS 152.4 15.9 NS
density (kg m-3) parent 128.2 26.4 148.5 33.3
Fibre diameter coppiced
563 19.8 ** 545 31.6
parent 532 24.6 NS

515 34.8

11.9 0.31 * 12.4 0.41 NS

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(µm) coppiced 12.5 0.37 12.6 0.22 NS
Fibre lumen parent 5.7 0.29 6.5 0.63 NS
diameter (µm) 6.6 0.36 ** 0.59 NS
coppiced 3.11 0.27 7.2 0.31 NS
Fibre wall parent 2.95 0.24 2.98 0.23 NS
thickness (µm) 105.5 4.7 NS 7.5
coppiced 105.1 8.4 2.69 7.8
Vessel diameter parent 14.1 0.69 97.7 1.54
(µm) 13.5 1.45 NS 2.45
coppiced 13.3 0.55 93.2 1.08
Vessel frequency parent 12.8 1.31 15.7 0.59
NS
Vessel occupancy coppiced 15.9
(%) parent 12.3
NS
coppiced 11.7

Features Type E. GxU clone A E. GxU clone B

Mean STD t-test Mean STD t-test

Total tree parent 18.8 0.41 *** 18.54 1.38 ***
height (m) coppiced 20.29 0.47 22.29 1.48

DBH (cm) parent 149.4 11.6 NS 164.1 24.2 *
coppiced 151.3 18.4 200.6 42.3
Wood basic
density (kg m-3) parent 444 7.8 *** 439 12.2 *
coppiced 421 9.7 402 18.7
Fibre diameter
(µm) parent 13.7 0.34 ** 13.3 0.32 **
coppiced 14.7 0.48 14.1 0.59
Fibre lumen
diameter (µm) parent 8.6 0.36 * 8.2 0.49
coppiced 9.3 0.65 **
Fibre wall
thickness (µm) parent 9.2 0.85
coppiced
Vessel diameter 2.58 0.21 NS 2.55 0.22 NS
(µm) parent 2.74 0.35 2.45 0.17
coppiced
Vessel frequency 108.9 7.6 NS 105.7 9.7 *
parent 109.2 10.1 117.5 8.3
Vessel occupancy coppiced
(%) 13.1 0.74 NS 12.1 0.74 *
parent 13.3 1.16 10.6 1.05
coppiced
13.5 1.11 NS 11.5 1.28 *
13.8 0.95 12.6 0.82

Features Type E. grandis clone A E. grandis seedling

Total tree parent Mean STD t-test Mean STD t-test
height (m) coppiced
18.66 2.67 * 20.02 1.41 NS
DBH (cm) parent 20.66 2.76 20.83 1.78
coppiced
Wood basic 143.4 22.2 NS 180.3 26.8 NS
density (kg m-3) parent 136.3 22.6 179.8 38.2
coppiced
Fibre diameter 450 18.5 * 449 46.7
(µm) parent 398 21.1 NS
coppiced
434 35.9

13.4 0.48 ** 13.2 0.75 **
14.7 0.47 13.8 0.65

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Fibre lumen parent 8.2 0.66 8.3 0.78 NS
diameter (µm) coppiced 9.8 0.41 *** 0.83 NS
2.62 0.11 0.25 NS
Fibre wall parent 2.45 0.13 8.6 0.28 NS
thickness (µm) coppiced 101.7 5.8 2.47 12.6 NS
102.6 10.2 NS 18.8
Vessel diameter parent 13.6 1.60 2.63 1.21
(µm) coppiced 13.5 2.16 106.4 2.71
12.1 0.59 NS 2.11
Vessel frequency parent 12.4 0.91 110.4 1.62
coppiced 12.4
Vessel occupancy NS
(%) parent 12.7
coppiced 11.7
NS
12.6

NS – non-significant; * - significant at p=0.05; ** - significant at p=0.01; *** - significant at p<0.001

In the case of wood density (Figure 1), a gradual increase from the pith (relative distance
0.1) to a relative distance of 0.7 from the pith was observed for all genotypes of parent crops.
Both E. GxU clones and both E. grandis showed very similar trends of variation. Wood
density reached its maximum at a relative distance of 0.7 from the pith, whereupon a
sudden decrease was observed. There was also a marked decrease in density for E. GxC
clone B from 0.7 to 0.9 relative distance from pith. For coppiced trees, wood density for both
E. GxU clones and both E. grandis remained relatively constant from the pith upto a relative
distance of 0.8 from the pith whereas both E. GxC clones showed a moderate increase in
density. For all genotypes, there was a strong increase in wood density from a relative
distance of 0.8 from the pith upto the bark position. Juvenile wood in coppiced trees had
lower density than juvenile wood in parent trees.

GxU A GxU B E.gra clone GxC A GxC B E.gra seed
1997 1997 2006
0.85 1990
0.8
Wood density (gcm -3)
0.75
0.7

0.65
0.6

0.55
0.5

0.45
0.4

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Paresenetdlicngrop Cz _ocpopppiiccee
Relative distance from pith

Figure 1 Mean wood density for six genotypes over the period of 1992 to 1997 for original
parent trees, and from 1997 to 2006 for coppiced trees.

With the exception of an E. grandis clone, the variation of fibre diameter (FD) across the
radius was consistent for all genotypes of parent trees (Figure 2A). FD decreased slightly
from the pith to a relative distance of 0.5-0.6, where it reached a minimum value. FD
increased from a relative distance of 0.7 from the pith upto the bark position. The FD of the
E. grandis clone remained constant from the pith upto a distance of 0.6 whereupon it
increased upto the bark position. In the case of coppiced trees, both E. GxC clones showed

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no variation in FD across the radius. For the other four genotypes, FD increased slightly to a
relative distance of 0.4 from the pith, with a further sharp increase, reaching maximum values
at relative distance of 0.8-0.9.

Fibre wall thickness (FWT) displayed a very similar radial trend of variation as wood density
for both types of trees (Figure 2B). However, the variation between genotypes was smaller
than that observed for wood density. For parent trees, a drop in FWT is evident at a relative
distance of 0.9 from the pith. This was not the case for E. GxC clone A, where FWT
continued increasing towards to the bark position. For coppiced trees, the radial trend was
consistent for all genotypes. FWT increased slightly from the pith to a relative distance of 0.8
whereupon a rapid increase occurred towards to the bark position.

GxU A GxU B E.gra clone GxC A GxC B E.gra seed

16 1990 1997 1997 2006

15

Fiber diameter (um) 14

13

12

11

10

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

A Paresneetdclinrgop Copz_pcoicppeice

Relative distance from pith

GxU A GxU B E.gra clone GxC A GxC B E.gra seed
2006
4.5 1997 1997

Fiber wall thickness (um) 1990

4

3.5

3

2.5

2

1.5

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

B Parseenetdlcinrgop Cozp_pcoipcpeice

Relative distance from pith

Figure 2 Mean fibre properties (A – fibre diameter; B – fibre wall thickness) for six genotypes
over the period of 1992 to 1997 for original parent trees, and from 1997 to 2006 for
coppiced trees.

The detailed pith-to-bark variation in vessel diameter (VD) for parent and coppiced trees of
six genotypes is shown in Figure 3. During the growth of parent trees, there was an evident
drop in VD at a relative distance of 0.6-0.7 for all genotypes. Coppiced trees displayed larger
between genotype variations in VD than non-coppiced trees. VD increased continuously from
the pith, levelling off at a relative distance of 0.7 from the pith. A sudden drop in this
characteristic is evident in outer zones.

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The radial pattern of vessel frequency differed between parent and coppiced trees. While the
frequency decreased gradually from pith to bark for coppiced trees, parent trees showed a
slight increase in vessel count at a relative distance of 0.6 from the pith (results not shown).

Several authors (February et al., 1995, Searson et al., 2004) pointed out that water
availability is one of the most important environmental sources of variation of the fibre and
vessel elements. An inspection of rainfall values during the phase of parent growth (1990-
1997) revealed that from the period of 1992 to 1994 (Figure 4) the serious droughts
occurred. Mean annual precipitation dropped to 500 mm, which was very unusual for the
Zululand area. This could have caused water stress to the growing trees. Since it was not
possible to differentiate the actual growth rings, the dry period could correspond to the
distance scale of 0.5 to 0.7 on x- axis. The higher density, smaller fibres with smaller lumen
diameter, drop in vessel diameter and higher vessel frequency within this period may be a
response to the limitations in the water supply.

GxU A GxU B E.gra clone GxC A GxC B E.gra seed

140 1997 1997 2006

Vessel diameter (um) 1990

120

100

80

60 1
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Pares enetd licn grop Coppzciocp e

Relative distance from pith

Figure 3 Mean vessel diameter for six genotypes over the period of 1992 to 1997 for original
parent trees, and from 1997 to 2006 for coppiced trees.

Rainfall (mm)1600
1400
19901200
19911000
1992
1993800
1994600
1995400
1996200
1997
19980
1999
2000
2001
2002
2003
2004
2005

Growth period

Figure 4 Average annual rainfall values during the period of growth of parent trees (1990-1997)
and coppiced trees (1998-2005). Readings are taken from a weather station situated
in Kwambonambi, about 10 kms from the Mavuya research trial.

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CONCLUSIONS
Comparison in tree growth, wood structure and wood density between coppiced and their
parent trees of 6 Eucalyptus genotypes were studied. It was found that the difference in
wood properties between parent and coppiced trees was clone specific. For some clones,
wood properties of parent and coppiced trees were statistically similar (E. GxC clone B,
E. grandis seedling). For another clone (E. GxU clone A) almost all investigated growth and
wood properties were significantly different.

Overall, similar tree height and diameter at breast height were observed between coppiced
and parent trees, with the exception of coppiced E. GxC clone A which had significantly taller
and thicker trees.

When averaging data at the disc level, comparable vessel characteristics were found for
wood of parent and coppiced trees. However, the parent trees had significantly smaller fibres
with smaller lumen diameter. Fibre wall thickness did not differ significantly between parent
and coppiced wood. Fibre properties also had a major influence on wood density, making the
wood from coppiced trees significantly lighter.

The pith-to-bark profile of density and anatomical properties showed the juvenile/mature
variation for both wood in parent and coppiced trees. This suggests that wood formation in
coppice was similar to wood formation in parent trees. The radial changes in certain wood
properties were associated with serious drought which occurred during growth of the parent
trees. This could overshadow the real effect of coppicing on tree growth and wood quality.

The ability to predict the quality of coppiced trees is important for the future strategy of
growers. If productivity yield and wood quality of coppiced trees are comparable to those of
parent trees, then coppicing would be the preferable option to replanting since it can greatly
reduce reestablishment and management costs.

ACKNOWLEDGEMENTS

The work for this article was conducted in the wood anatomy laboratory of the Forestry and
Forest Products Research Centre of the CSIR. This study was funded by Mondi-Sappi-CSIR
Eucalyptus Cooperative.

REFERENCES

February, E.C., Stock, W.D., Bond, W.J., and Le Roux, D.J. 1995: Relationships between water
availability and selected vessel characteristics in Eucalyptus grandis and two hybrids.
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on growth, wood and pulp properties of nine eucalypt species at two sites. Tappi J. 82
(7), pp 89-99.

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