LATEWOOD DENSITY IN RELATION TO WOOD FIBRE DIAMETER, WALL THICKNESS, AND FIBRE AND VESSEL PERCENTAGES IN QUERCUS ROBUR L - Brill
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IAWA Journal, Vol. 18 (2), 1997: 127-138
LATEWOOD DENSITY IN RELATION TO WOOD FIBRE DIAMETER,
WALL THICKNESS, AND FIBRE AND VESSEL PERCENTAGES IN
QUERCUS ROBUR L.
by
R.V. Rao l , D.P. Aebischer 2 & M.P. Denne 2
SUMMARY
To elucidate the influence of growth rate and cambial age on latewood
density in oak (Quercus robur L.), the diameter and wall thickness of
libriform fibres, and percentage of latewood area occupied by libriform
fibres and vessels, were analysed across discs often 87-year-old oak trees
from Germany.
Fibre wall material per unit fibre area did not vary significantly with
cambial age, since increase in fibre diameter with ring number from the
pith was offset by increase in wall thickness. There were no significant
variations in fibre diameter or wall thickness with latewood width.
Changes in latewood density with cambial age or with growth rate were
primarily associated with changes in tissue percentages per unit area,
latewood density being more closely correlated with vessel than with
fibre percentages. There appeared to be a curvilinear relationship be-
tween latewood width and latewood density, density reaching a maxi-
mum when latewood width was about 2 mm.
Key words: European oak, libriform fibres , tissue proportions, vessel
percentage, fibre dimensions, wood properties, computer tomography.
INTRODUCTION
The wood of broadleaved trees includes a variety of cell types that are specialised to
accomplish a range of functions affecting the survival and efficiency of the living
plant. Whilst vessels and tracheids promote the transport of water and dissolved nutri-
ents, and living parenchyma maintains metabolic activity for a number of years, thick-
walled libriform fibres provide much of the stiffening and resilience essential to sup-
port the weight of the crown of the tree. The proportions of different cell types in the
wood, together with their cell wall and lumen dimensions, are important in determin-
ing not only the hydraulic conductivity, toughness and flexibility of the living tree, but
also the permeability, density, mechanical strength properties, and end uses of the tim-
ber after felling.
I) Wood Properties and Uses Division, Institute of Wood Science and Technology, FRL Cam-
pus, 18 Malleswaram, Bangalore, 580003, India.
2) School of Agricultural and Forest Sciences, University of Wales, Bangor, Gwynedd, LL57
2UW, United Kingdom.
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In ring-porous hardwoods such as oak (Quercus spp.), the latewood part of the
growth ring has a relatively high density, having a higher proportion of libriform fibres
and lower proportion of vessels than the earlywood zone at the beginning of each ring.
Change in ring width of oak has been shown to be more closely associated with change
in width of latewood than in that of earlywood (Gas son 1987, Lei et al. 1996), percent-
age latewood increasing alongside ring width. From that, the wood density of oak (and
hence many of its strength properties) would be expected to increase as growth rate
increases. However, as shown by Zhang & Zhong (1991), Nepveu (\993) and Zhang
et al. (1993), the relationship between density and ring width tends to be curvilinear up
to a maximum value, the curves for some trees indicating that density may decline
with further increase in ring width at very fast growth rates (Nepveu 1993). Although
Zhang et al. (1990) reported no significant effect of growth rate on the cell characteris-
tics of earlywood or latewood in Quercus liaotungensis, the curvilinear variation in
density with ring width suggests that growth rate has some influence on the anatomical
structure of the wood in addition to change in the proportion of earlywood to latewood.
From that premise, the present work started with the hypothesis that, in oak, variation
in latewood density may be associated with variation in dimensions and/or frequency
of libriform fibres, which are themselves influenced by growth rate.
In view of the importance of wood density to the physical and mechanical proper-
ties of timber, surprisingly little attention has been paid to variation in the transverse
dimensions or frequency of libriform fibres in hardwoods. According to Chalk (1983)
"There is a general tendency for diameter to increase from the pith outwards" and
"There is some evidence that the wall thickness of fibres increases from the pith out-
wards", but there seems to be little quantitative information on within-tree variation of
these fibre dimensions in oak, or about the influence of growth rate upon them. A
number of authors have given data for libriform fibres as a percentage of ring area for
oak (summarised by Stokke & Manwiller 1994; Lei et al. 1996), but all those data
were means for entire growth rings, so would depend on the proportion of earlywood
to latewood.
The influence of growth rate on the dimensions and percentage of fibres in oak may
be difficult to distinguish from changes with cambial age, for in many trees both ring
width and density tend to decrease with ring number from the pith (Zhang et al. 1993,
1994; Aebischer & Denne 1994; Lei et al. 1996). The aim of the present work was to
analyse the variations in libriform fibre dimensions in oak, considering changes in fibre
diameter, wall thickness and proportion of latewood area in relation to both ring number
from the pith and latewood width, and to investigate the extent to which such varia-
tions in fibre parameters account for variations in latewood density. Data on vessel
area as a percentage of latewood area have also been included, as it seemed likely that
change in fibre percentage would be accompanied by change in vessel percentage. For
these analyses, breast height discs were sampled from ten trees from a German site,
and the latewood width, density and fibre parameters measured in every tenth ring
from pith to bark.
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MATERIAL AND METHODS
Materials
Five trees of Quercus robur L. were selected from each of two neighbouring stands
in the Kottenforst near Gottingen, in Germany. One stand was managed to a stocking
density of 172 trees ha- 1 and the other to 324 trees ha- 1, but they were similar apart
from their stocking density, and since analyses of variance revealed no significant
differences between stands in any of the parameters analysed in the present paper (at
p = 0.05), they have here been treated as a single sample of ten trees. The trees were
dominant or co-dominant within the stand, with a lean ofless than 10° from the verti-
cal, and a clear bole of at least 8 m. They were 87 years old at the time of felling.
Width of latewood
All measurements were made on a disc taken at breast height (1.3 m above ground
level) from each tree. A strip 1 cm wide was cut across the disc, 12.5° to the right of its
widest diameter. The transverse surface of each strip was cleaned with a razor blade to
obtain a clear view of the growth ring boundaries. Earlywood and latewood widths of
all rings were then measured to 0.01 mm using a travelling microscope. Data present-
ed here are for latewood only, for every tenth growth ring from rings 9 to 69 from the
pith.
Latewood density
Variation in density across individual growth rings was determined by computer-
ized axial tomography (CAT-scanning), using the Siemens Somatom DRH body scan-
ner at Ysbyty Gwynedd Acute Hospital Unit, Bangor, Gwynedd, U.K. These scans
were done on strips taken from just above those used for the ring width measurements.
The strips were conditioned to 12% moisture content before scanning.
The data given here are means of density calculated as described below, from the
latewood only, across the same growth rings as measured for latewood width.
CAT-scanning has been developed as a method of measuring variation in density
within slices of a solid object, originally for medical purposes, but more recently for
scanning the density and moisture content of wood (Hattori & Kanagawa 1985; Lindgren
1991; Lindgren et al. 1992). Lindgren et al. (1989) found that wood could be measured
to an accuracy of +/- 2 kg m- 3 using CAT-scanning. The data output by the scanner
was in the form of a trace in Hounsfield units. These were converted to 'CT-density'
by the formula:
CT-density = 1000 - Hounsfield value
In a parallel investigation which compared CT-density with density of the same growth
rings determined by the Archimedes method, CT-density was found to be consistently
higher than that determined by the Archimedes method, so the following correction
factor (derived from the linear regression between CT-density and density from the
Archimedes method) was used:
density = 679.33 + (CT-density x 0.379)
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Anatomical measurements
Fibre diameter, wall thickness, and percentages of fibres or vessels by area, were
determined in the latewood of each of the growth rings measured for ring width and
density. Transverse sections (30 11m thick) were cut from each strip. Groups of four
libriform fibres along a radial line were taken at random about halfway across the
latewood of each of the sampled growth rings from each disc, and measurements re-
peated in 12 such groups (i.e. 48 cells from each ring of each disc). Mean radial fibre
diameter and mean radial lumen were measured with a SEESCAN image analyser, and
double wall thickness calculated by subtracting lumen diameter from fibre diameter.
The following formula was used as an estimate of the proportion of wall material to
lumen in libriform fibres:
fib re waII area per umt. fib re area (fibre diameter)2 - (lumen diameter) 2
(fibre diameter) 2
Percentages of libriform fibres and vessels per unit area were obtained by point sam-
pling. An II-point micrometer scale was placed tangentially within the latewood and
the type of cell under each numbered point on the scale was identified and recorded.
The total score for each cell type was obtained from eight to ten adjacent II-point
sequences. In narrow rings (less than I mm wide) this series of measurements was
placed halfway between the earlywoodllatewood boundary and the end of the ring.
In rings of medium width (1- 2 mm) and wide rings (over 2 mm wide) two and three
such series of measurements were made respectively, evenly spaced across the latewood.
RESULTS
Variation in cell parameters and latewood density with ring number from the pith
As shown in Figure la, mean latewood width did not change significantly with ring
number from the pith across discs from this site (from analysis of variance, p 0.05).
Similarly, ring width did not vary significantly across these discs (data not shown here).
In contrast, latewood density decreased significantly (p 0.001) with ring number
from the pith (Fig. 1b).
Latewood libriform fibre diameter and wall thickness both increased significantly
with ring number from the pith (Fig. Ic, d), with no significant changes in these dimen-
sions from rings 29-69 (p 0.05). Ho wever, analysis of variance indicated the calcu-
lated value for fibre wall material per unit fibre area did not vary significantly (p
0.05) across all ring numbers from the pith (Fig. 2).
For brevity, proportions of libriform fibres (or vessels) per unit area of latewood
cross section will here be termed fibre (or vessel) percentage. Fibre percentage de-
creased significantly (from analysis of variance p 0.01) with ring number from the
pith (Fig. 3a), implying a decrease in numbers of fibres per unit area since individual
fibre cross-sectional area increased with ring number (as indicated by change in fibre
diameter shown in Fig. Ic). Conversely, latewood vessel percentage increased signifi-
cantly (p 0.001) with ring number from the pith (Fig. 3b). Though the inverse cor-
relation between fibre and vessel percentages was highly significant (R 2 = 0.397,
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Fig. 2. Variation in calculated value for latewood libriform fibre wall area per unit fibre area with
ring number from the pith, showing mean often oak discs. - Vertical lines indicate twice standard
error on each side of the mean.
p 0.00 I), the increase in vessel percentage did not fully compensate for the decrease
in fibre percentage, implying an increase in percentage of other tissues (such as ray or
axial parenchyma) with ring number from the pith.
Variations in cell parameters and latewood density with width of latewood
As shown in Figure 4a, latewood density tended to increase with increase in late-
wood width in rings 39 to 69 from the pith (here taken to represent mature wood). The
linear regression from those rings showed a significant (p = 0.006) but low value for
the coefficient of determination (R2 = 0.189); these data fitted slightly better to the
curve derived from the second order polynomial equation (R2= 0.227, P = 0.010), the
density reaching a maximum at latewood width about 2 mm. Given this indication that
the relationship was curvilinear, latewood width data were transformed to the recipro-
cal (Fig. 4b), using a formula comparable to that used by Zhang et al. (\ 993) for relat-
ing mean ring density to ring width and cambial age of oak. Linear regression of these
transformed data gave a value of R 2 = 0.227 (p = 0.002), which was identical to that for
the second order polynomial in Figure 4a.
Regression analyses for rings closer to the pith (rings 9 to 29) showed no significant
correlations between latewood density and latewood width (R2 = 0.00 I for linear and
for polynomial equations). Comparably, Zhang and Zhong (1991) found no significant
correlation between whole ring density and growth rate in juvenile wood of Quercus
liaotungensis.
There were no significant variations in fibre diameter, wall thickness or fibre wall
area per unit fibre area with latewood width, either using data from mature wood rings
39 to 69 from the pith (equations 1-3 in Table I), or from all rings measured. Fibre
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Fig, 4, Variation of latewood density with latewood width in mature wood of discs from ten oak
trees, Latewood density in relation to: a: latewood width ; b: I lIatewood width, - The straight
line shows the line of best fit for the linear regression, and the curve shows that of best fit for the
second order polynomial.
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co136 IAWA Journal, Vol. 18 (2), 1997
Table 2. Simple and multiple regression equations to indicate influence of fibre parameters
and fibre or vessel frequency on latewood density. Data from ring numbers 39 to 69 from
the pith.
latewood density = 341.9 + 3.9 fibre %** R2 = 0.164**
2 latewood density = 653.1 - 10.3 vessel %*** R2 = 0.456***
3 latewood density = 604.4 - 3.0 ring number*** + 60.9 latewood width*** R2 = 0.381***
4 fibre % =47 .2 - 0.16 ring number ns + 6.03 latewood width*** R2 = 0.231**
5 vessel % = 10.0 + 0.16 ring number** - 4.6 late wood width*** R2 = 0.384 ***
6 latewood density = 331.2 - 2.8 ring number** + 51.5 latewood width*
+ 1.6 fibre % ns R2 0.400***
7 I late wood density = 678.8 - 1.84 ring number* + 26.6 latewood width ns
- 7.4 vessel %** R2 0.526***
ns = not significant at p 0.05; * = significant at p 0.05; ** = significant at p 0.01; *** = signifi-
cant at p 0.00 I.
latewood density was more closely correlated with vessel % than with fibre %. Trans-
formation to the reciprocal of latewood width gave little or no improvement to the
values of R2 in any of the regressions shown in Table 2 (transformed equations not
shown).
DISCUSSION
The relative uniformity of latewood width across discs of the present trees has the
advantage that changes in wood structure or density with cambial age (i.e. with ring
number from the pith) can here be considered independently from changes with latewood
width. Though both the diameter and the wall thickness of latewood fibres increased
with ring number from the pith, there was little variation in fibre wall material per unit
fibre area with cambial age, for it seems that the changes in wall thickness were offset
by those in fibre diameter. However, proportions of the different cell types did vary
with ring number from the pith, percentage late wood fibres tending to decrease, and
percentage latewood vessels to increase with cambial age; both those trends are likely
to account for at least some of the decrease in latewood density which occurred with
ring number from the pith. Such a decrease in fibre proportions would be expected to
decrease the mechanical properties of the latewood produced as the cambium ages, but
possibly the need for additional strength becomes less crucial as the stem increases in
diameter. The influence that the changes in vessel proportions have on the hydraulic
conductivity of the latewood will depend on the extent to which variation in vessel
percentage reflects change in vessel diameter or vessel number, and that remains to be
determined.
From the present work, the variation in latewood density with increase in growth
rate appears to be curvilinear, with maximum density when latewood width was about
2 mm; given an average earlywood width of about 0.5 mm this would correspond to a
total ring width of about 2.5 mm. Such fitted curves must obviously be treated with
caution, especially when, as in the present analysis, they are derived from data from a
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limited number of samples with a wide range of variability, but it is interesting to note
that maximum density was at a ring width comparable to that shown by Zhang &
Zhong (1991) and Nepveu (1993).
The variation in late wood density implies that whole ring density of oak is influ-
enced by variation in latewood structure as well as by changes in the proportion of
latewood to earlywood. However, contrary to the hypothesis with which this present
work begun, variations in latewood density (with ring width, or with ring number from
the pith) appear to be more closely associated with change in vessel percentage than
with dimensions or frequency of libriform fibres. A similar result was reported for East
Liaoning oak by Zhang & Zhong (1992). The present data suggest that simultaneous
changes in proportions of other cell types (such as an increase in the amount of ray
and/or axial parenchyma with distance from the pith) also tended to affect variation in
density with cambial age and with growth rate. Clearly more data are needed to con-
firm that conclusion, especially from trees with wider growth rings.
ACKNOWLEDGEMENTS
We thank Professor G. Becker and his colleagues from the University of Gottingen, Germany, for
supplying the oak discs, and are grateful to staff of the Acute Hospital Unit at Ysbyty Gwynedd,
Bangor, for their co-operation and assistance with the CAT scanner. The financial support of the EC
Forest Programme for contract number MA26-CT90-000 5001 is gratefully acknowledged. Dr. R.Y.
Rao thanks the Director General, ICFRE, Dehra Dun, India, and the Director, Institute of Wood Sci-
ence and Technology, Bangalore for sponsorship, and O.D.A. and the British Council for a fellowship
at the University of Wales, Bangor.
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