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Heartwood in relation to age and
growth rate in Pinus sylvestris L.
in Scandinavia
P. GJERDRUM
Norwegian Forest Research Institute, Department of Economy, Technology and Processing, Högskoleveien 12,
N-1432 Ås, Norway
E-mail: [email protected]
Summary
Knowledge about the transformation of sapwood into heartwood contributes to the understanding
of the nature of pine trees and should be considered prior to the conversion of sawlogs to produce
timber of prescribed properties and optimal revenue. In this study, heartwood formation was
ascribed to the joint effect of ageing and growth rate. Observations of heart- and sapwood in 1656
trees and sawlogs of Scots pine (Pinus sylvestris L.), sampled throughout Scandinavia, were analysed
using mixed models. The most important finding was expressed in the pine heartwood age rule:
heartwood age equals the square root of cambial age less three, to the second power. This global
formula was valid irrespective of environmental factors and location within the tree, and described
93 per cent of the variance in the sample. Transition rate increases from 0.6 rings a–1 at 50 years to
0.8 rings a–1 at 200 years. The spatial amount of heartwood might be influenced by the silviculture
through the annual ring width pattern. For samples missing sapwood, e.g. archaeological wood, the
results might be combined with dendrochronology in specimen dating. When the diameter and the
heartwood diameter of sawlogs were known, the mean annual ring width could be estimated with a
standard deviation of 0.5 mm a–1. The simplicity, consistency and high correlation of the pine
heartwood age rule confirms the importance of age as the main factor in heartwood formation.
Introduction
Softwood timber is an excellent material for a
variety of applications. As far back as the fourth
millennium BC, King Gilgamesh of Uruk decided
that timber from old trees was the best material
for the construction of his palace: ‘I selected . . .
until I had found a fully mature pine’ (Dalley,
1991). Traditionally, mature pine has been
associated with heartwood. Pine heartwood and
© Institute of Chartered Foresters, 2003
pine sapwood can, in several respects, be
considered two different timber products. In
comparison to heartwood, sapwood possesses an
open wood structure, which is penetrable to
moisture, solvents and air, and is vulnerable to
biological degradation. Sapwood is light in
colour and is easily painted, impregnated or
otherwise treated. Heartwood, on the other
hand, may be used to obtain timber less disposed
to moisture fluctuations and, hence, more
Forestry, Vol. 76, No. 4, 2003
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dimensionally stable and durable. Knowledge
about the occurrence of heart- and sapwood
must, therefore, be considered prior to the conversion of sawlogs to produce timber of prescribed properties and optimal revenue.
When present, heartwood is consistently
located in the older part of the stem. All xylem is
initially formed as sapwood to act as a duct for
the fluid sap between the root and the crown. As
part of the natural ageing of the tree, the oldest
rings in the core will be irreversibly inactivated
and transformed into heartwood (Wagenführ,
1989; Tsoumis, 1991). This conversion includes
the termination of all physiological functions of
the parenchyma and epithelial cells, a decrease in
moisture content, the aspiration of the pits, and
the impregnation of the wood with resinous and
phenolic components. These components prevent
degradation and make the heartwood more susceptible to photochemical darkening than
sapwood.
Pilz (1907) set the stage for modern heartwood
research, by incorporating in his paper most of
the claims still being discussed. The relative
amount of heartwood produced in pine has been
shown to vary widely within, as well as between,
trees and stands (Hillis, 1987; Björklund and
Walfridsson, 1993; Climent et al., 2002). Variation is attributed, in part, to genetic inheritance
(Ericsson and Fries, 1999; Paques, 2001), albeit
these investigations are restricted to trees less
than 50 years old. The influence of silvicultural
efforts, like thinning and fertilization (Mörling
and Valinger, 1999) or pruning (Bergström,
2000), seems to be limited. As indicated already
by Pilz (1907), the ‘pipe-model theory’ has been
well established, irrespective of tree species, in the
field of tree physiology (Shinozaki et al., 1964;
Gilmore et al., 1996; Mencuccini and Bonosi,
2001). In this model a proportional relationship
is assumed between the magnitude of the transpiring crown and the duct capacity of the stem,
expressed as the cross-sectional area of the
sapwood. As the need for duct capacity in the
part of the stem below the living crown is
constant, this model seems to imply a constant
sapwood area below the living branches. A
slightly different interpretation used by
Kellomäki et al. (1999) is to assume a direct ring
to ring connection between annual rings in the
sapwood and in the living branches. This model
implies a constant number of sapwood rings
below the living crown. Neither a constant
sapwood area nor constant number of sapwood
rings, respectively, below the green crown has
been verified. Age, therefore, remains the most
substantial factor when describing sap- and
heartwood relationships (Pilz, 1907; Hillis, 1987;
Wagenführ, 1989; Tsuomis, 1991; Björklund and
Walfridsson, 1993; Sellin, 1996). Björklund
(1999) has presented a model for the number of
heartwood rings as a function of cambial age,
based on analyses of 198 trees from a Swedish
‘pine stem bank’. A second-degree polynomial
was chosen, indicating an increasing speed of
heartwood formation for greater cambial age.
Recently, functions for predicting sap- and heartwood diameter have been incorporated in
complex models for wood properties in Scots
pine and Norway spruce (Picea abies (L.) Karst.)
(Wilhelmsson et al., 2002) and in Atlas cedar
(Courbet et al., 2002). The use of thermo-photographic analysis seems promising for automatic
separation of heart- and sapwood in fresh sawlog
cross-cuts (Gjerdrum and Höibö, 2003). Observations of sap- and heartwood diameter might be
included in models for estimating mean ring
width and other wood quality parameters.
At present, knowledge of pine heartwood is of
interest to both scientists and professionals, from
tree physiologists, chemists and archaeologists to
carpenters, wood scientists and sawmillers. This
study is an attempt to add to the existing knowledge and increase the understanding of the
change of sapwood to heartwood in Scots pine.
The amount of heartwood in any cross-cut was
considered as the sum of all annual rings in the
heartwood. Tree ageing and width of the rings
(growth rate) both influence the amount. These
two elements were analysed separately. The main
aim of the study was to determine a model for
predicting the amount of heartwood in commercial forest stands, individual trees and sawlogs of
Scots pine in Scandinavia.
Materials and methods
Sampling for observation of heart- and sapwood
in the radial and longitudinal direction of the
trunks was conducted throughout Norway and
Sweden (Figure 1). Three different sub-samples of
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415
Figure 1. Location of sites for each sub-sample.
observations were incorporated: stands, sawlogs
and felled poles. The joint sample consisted of
observations from a total of 1656 individual trees
(from 142 locations): 1104 standing trees (138
locations) + 544 felled sawlogs (3 locations) +
8 felled poles (1 location). The sample came from
a variety of commercial and natural sites and had
been subjected to a wide range of silvicultural
treatments. According to general practice, most
trees are expected to have regenerated naturally.
A description of the observations is given in
Table 1.
In order to provide contrast and validation,
other data were included in the analysis: 44 data
sets reported by Pilz (1907) and 24 observations
from three stands in Luxembourg.
Stands
Core samples were taken at breast height from
1104 individual trees. Eight trees were selected
from each of the 138 forest stands (locations) in
order to examine the widest possible variety of
tree and stand characteristics (Table 1). Stands of
different maturity were chosen. Latitude, longitude and elevation varied from southern Norway
to north of the Arctic Circle and from the Norwegian West Coast via mountain forest to the
Gulf of Bothnia and Gotland. Site index, soil and
slope varied from fertile to poor, from flat, sandy
deposits or peat land to moraines and steep hills,
grown within homogeneous pine stands or alternating mixed forests of spruce and broadleaves.
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Table 1: A summary of the stands, sawlogs and poles used in the study
Stand variables (138 locations)
Site index, H40 (m)
Altitude (m)
Latitude (E)
Longitude (N)
Slope (inclination) (%)
Stand basal area (BA) (m2 ha–1)
Tree mixture, pine in % of total BA
Stands: tree variables at breast height (1104 individual trees)
Radius under bark, RC (mm)
Heartwood radius, RH (mm)
Heartwood radial fraction, RFH
Cambial age, AC (years)
Heartwood age, AH (years)
Mean ring width, RWC (mm a–1)
Height of tree (m)
Crown length, CLF, fraction
Cross-cuts (544 individual sawlogs from three locations)
Radius under bark, RC (mm)
Heartwood radius, RH (mm)
Heartwood radial fraction, RFH
Cambial age, AC (years)
Heartwood age, AH (years)
Mean ring width RWC (mm a–1)
Stems (128 observations from eight individual stems)
Butt end* Cambial age, AC (years)
Heartwood age, AH (years)
Small end Cambial age, AC (years)
Heartwood age, AH (years)
Mean
SD
Range
12
230
11° 00´
60° 40´
9
22
85
–
–
–
–
–
–
–
5–20
20–800
7° 05´–21°15´
56° 0´–68° 50´
0–60
4–42
20–100
115
63
0.53
99
49
1.3
18
0.50
34
28
0.14
40
29
0.55
4.5
0.14
30–242
2–194
0.03–0.85
21–308
3–235
0.4–4.4
4–33
0.13–0.95
112
62
0.53
73
32
1.8
29
27
0.14
32
24
0.65
55–207
2–126
0.02–0.88
15–219
2–166
0.5–5.1
111
63
67
29
28
27
22
20
71–160
25–106
41–112
15–71
* 80 cm above basal cross-cut of the trunk.
In each stand, dominant as well as co-dominant
and suppressed trees were selected, randomly
within each group. Three dead trees and 20 living
trees that showed severely reduced foliage and
vigour due to resin-top disease (Peridermium pini
(Pers.), Cronartium flaccidum (Alb. and Schw.))
were also included in the sample.
For each tree, diameter with bark at breast
height together with bark thickness, height to
lowest green branch, and total tree height were
recorded. Cambial age and radial width refer to
the core samples, which were taken at arbitrary
directions.
Sawlog cross-cuts
Five hundred and forty-four sawlogs were chosen
for examination from the ordinary log supply at
three locations (two Norwegian and one Swedish
sawmill). The larger end of butt logs was
avoided, so all cross-cuts were located ~4 m
(length of shortest log) or more above ground
level. It was unlikely that any two logs originated
from the same tree; however, no further information about the origin of the logs was recorded.
Cambial age and radial width refer to one
random radius, measured outwards from the
pith. In addition, 52 of the cross-cuts were
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measured for radius in 12 equally spaced directions to analyse within-cross-cut variation.
Stems
Eight commercially felled poles (trunks) were
sampled to analyse the within-stem variation.
Sixteen core samples, equally spaced along the
stems and in the same radial direction, were
taken from each pole, for a total of 128 cores.
Cambial age and radial width refer to each core.
Due to significant autocorrelation between observations from the same stem (r = 0.73, P < 0.01),
only observations at approximately breast height
were included in the joint sample.
Observations, calculations and symbols
The number of annual rings (A) and the radial
width (R) were identified in all samples, specified
for both heart- and sapwood. When necessary,
the boundary between sap- and heartwood was
identified using a mixture of sodium nitrite
(NaNO2) and sulphanil acid (C6H7NO3S)
(Wagenführ, 1989). Conventionally, the term
cambial age (AC) denotes the total number of
annual rings in a specimen. Accordingly, the
terms heartwood age (AH) and sapwood age (AS)
refer to the number of rings in the heartwood
and sapwood, respectively. For the sake of consistency, the subscripts C, H and S are used even
for radii and ring width; the symbols are listed
below.
In commercial forestry, mature stands are regularly harvested while in natural stands ageing
trees regularly die. Therefore, older stands and
biologically mature trees appear less frequently
than young trees, particularly on fertile sites.
Consequently, the distribution of tree age shows
considerable deviation from the normal distribution (skewness = 0.66, P < 0.01). To ascertain
reliability in the statistical analyses and for
reasons that will be dealt with later, the square
root transformation of cambial age (√AC: mean
9.7, standard deviation 2.1) and for the number
of annual rings in heart- (√AH) and sapwood
(√AS) were chosen. The distribution of the transformed values was found to be not significantly
different to the normal distribution (√AC:
skewness 0.02, P = 0.36).
Based on initial inspection, a linear relationship
417
between the transformed variables was assumed
(Figures 2–5). Model (1) was set up for statistical evaluation. As will be dealt with in formula
(4), the simultaneous model for heart- and
sapwood age can be substantially simplified if
parameter a vanishes.
A H = _1 + a i $ A C + b
+ !j c j $ predictorj + ε j
(1)
In samples of the actual size used, even weak
relationships could turn out to be significant. To
avoid any weak relationships masking more
important findings, a confidence level of 99 per
cent was chosen and residuals were extensively
examined before a parameter was accepted,
following Montgomery and Peck (1992).
General linear mixed models were used for
statistical estimation, with location as a random
factor and, when applicable, sub-sample as a
fixed factor. The location factor, thus, includes
any effect on the model related to any single
stand or sawmill. However, as will be shown,
there was no indication that within-location
correlation could invalidate the calculations. All
statistical analyses were performed applying
standard procedures, such as the General Linear
Models (GLM) of the Statistica software
(StatSoft, 2002).
Symbols
Basic observations:
AC, AH, AS: no. of annual rings in xylem, heartand sapwood, respectively
AC = A H + A S
RC, RH, RS: radial width (mm) in xylem, heartand sapwood, respectively
RC = R H + R S
Derived variables:
RFH = RH/RC: radial fraction of heartwood
RWC = RC/AC: mean ring width for xylem (mm a–1)
Forest stands:
CLF: crown length, fraction of tree height
H40: site index, mean height of the 100 trees ha–1
with the greatest diameter at breast height by
40 years
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Statistical symbols:
a, b, c: model parameters to be estimated
εi: residuals, observation no. i
predictorj: any independent variable in a statistical model
RMSE: root of mean squared errors
se: standard error of a model
Results
Pine heartwood age rule
For the joint sample of all 1656 trees, coefficient
a was found to be insignificant (t = –2.2, P = 0.03)
and was omitted from the consecutive calculations. No significant difference in b between the
stands and the sawlog sub-samples could be
found (t = 1.8, P = 0.07), indicating the congruence to b = 3.0 (se = 0.02) for all stands as well
as height levels within the trees (Figures 2 and 3,
equation 2). Observations along single trunks
from the stems sub-sample are shown in Figure 4.
Observations from each stem, as well as the sum
of all stems, fitted the same model (equation 2);
no significant difference from b = 3.0 was found
(t = –1.4, P = 0.2, n = 8).
A H = AC - 3.0
(2)
For the stands sub-sample, an augmented
analysis was performed, applying partly tree and
partly stand characteristics. By incorporating
four or more additional predictors, the RMSE
decreased from 0.55 to 0.53, which is equivalent
to a reduction in standard error for the retransformed model (see equation 5) from
7.7 years to 7.4 years for a 100-year-old
specimen. Thus, none of these predictors was
found to contribute consistently or considerably
to the model for √AH. Dead trees and trees with
resin-top disease demonstrated identical test
results compared with sound trees. Consequently,
a joint estimation for all observations could be
performed. The location factor only represented
1.1 per cent of the variance. After estimation, the
equation was re-transformed to (natural) cambial
age, resulting in equation (3):
A H = a AC - 3.0 k
2
14
12
10
8
6
4
2
0
0
2
4
6
8
(3)
This finding can be expressed in the pine heartwood age rule: heartwood age, at arbitrary height
in the tree, equals the square root of cambial age
less three, to the second power. The model is
global and, consequently, valid for all Scandinavian samples. The model (93.1 per cent of the
variance in AH) explained 93.7 per cent of the
variance in √AH. For validation, the model was
compared with the observations from continental
16
√(Heartwood age)
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12
14
16
18
√(Cambial age)
Figure 2. Pine heartwood age rule applied to the stands sub-sample; observations at breast height.
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419
16
14
√(Heartwood age)
12
10
8
6
4
2
0
0
2
4
6
8
10
12
14
16
18
√(Cambial age)
Figure 3. Pine heartwood age rule applied to sawlogs; observations taken at arbitrary height in trees.
16
14
12
√ (Heartwood age)
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8
6
4
2
0
0
2
4
6
8
10
12
14
16
18
√(Cambial age)
Figure 4. Pine heartwood age rule applied to stems sub-sample; equi-spaced observations along stems. Eight
stems, each with different symbol.
Europe (Figure 5). For all sub-samples, including
the validation data set, the pine heartwood age
rule was found to explain equivalent fractions of
the variance and to be consistent with respect to
residuals (Table 2).
The implicit formula for number of rings in the
sapwood (equation 4) was evaluated and found
adequate in the whole range of cambial age.
A S = AC - A H = 6.0 $ AC - 9.0
(4)
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F O R E S T RY
16
14
12
√(Heartwood age)
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10
8
6
4
2
0
0
2
4
6
8
10
12
14
16
18
√(Cambial age)
Figure 5. Pine heartwood age rule applied to the observations from continental Europe, Luxembourg
stands (circles) and Pilz 1907 (triangles).
Table 2: Pine heartwood age rule applied on each sub-sample
Sub-sample
Bias in AH
(years)
Explained variance
(R2)
Residuals se (AH)
(years)
Stands
Cross-cuts
Stems
Continent
Joint
0.0
0.7
3.1
–3.1
0.3
0.926
0.928
0.931
0.936
0.931
8.0
6.5
6.4
7.3
7.5
The residuals (se(√AH) = RMSE) for equation (2)
amounted to 0.55. The residuals were stationary;
no age bias in any part of the range from 30 to
>240 years cambial age could be detected. For the
re-transformed model (3), however, the residuals
increase with age (Figure 6). The expression for
the standard error of the re-transformed estimate
(se(AH)) was obtained by derivation of the
transformation equation (5a). Equation (5) could
then be obtained by taking the standard errors as
differentials and substitution by equation (2):
d _ AHi = 2 $ AH $ d a AH k
s e _ A H i = 2 $ a AC - 3.0 k $ 0.55
= 1.1 $ a AC - 3.0 k
(5a)
(5)
Formula (5) was verified by comparing it with
estimates calculated directly for the residuals
given in Figure 6 after assigning the observations
to discrete age classes. se(AH) evaluates to
3.7 years at cambial age 40 increasing to
13.7 years at 240, equivalent to 9 per cent
decreasing to 6 per cent of the cambial age. Only
a minor portion of the residuals might be attributable to variation within cross-cuts (2.1 years in
medium-aged samples) or inaccuracy in method
of observation.
The expression for the annual transformation
from sap- to heartwood (equation 6) was calculated by derivation:
J
N
3.0 O
∆A H = K 1 $ ∆AC
(6)
K
AC O
L
P
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40
30
20
Residuals, years
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0
-10
-20
Stands
Cross-cuts
Stems
-30
-40
0
50
100
150
200
Cambial age, years
250
300
350
Figure 6. Residuals of the pine heartwood age rule by sub-sample. One standard error (formula 5) is indicated by the curves.
The transition rate increases from 0.6 rings a–1 at
60 years cambial age to 0.8 rings a–1 at 220 years.
Heartwood radial fraction
Heartwood radial fraction (RFH) depends on two
elements: ring width pattern, and the heartwood
to cambial age relationship. Observed RFH at
breast height for the stands sub-sample varied
from 0.03 to 0.85 (Table 1). However, an analytical approach for modelling RFH, based on
general assumptions of ring width development
by age, failed. A model (equation 7) for RFH for
single trees in the stands sub-sample was therefore statistically estimated.
knowledge of log radius (or diameter) and heartwood radial fraction. Through substitution of the
estimated cambial age into the denominator of
the formula for ring width (equation 8), a simultaneous model for mean ring width (RWC) and
cambial age (AC) was obtained:
RWC =
RC
=
AC
RC
_ 2.6 + 0.013 $ R C + 8.0 $ RFH i
2
(8)
The model was verified against observed values:
52.7 per cent of the variation in mean ring width
was explained (se(RWC) = 0.50 mm a–1). No bias
in any part of the two-dimensional RC–AC range
could be identified.
RFH = - 0.57 + 0.19 $ ln a R C $ AC k
(7)
- 0.26 $ CLF - 0.0065 $ H40
No other characteristics of stand or tree were
found significant, resulting in only 44 per cent of
the variance being explained. According to this
model, timber rich in heartwood was found in old,
thick trees with short crowns located at poor sites.
Ring width in sawlogs
A model for the square root of cambial age was
estimated in the sample of cross-cuts, assuming
Discussion and conclusion
All analysed observations and sub-samples
complied with the pine heartwood age rule given
in equation (3) and reflected uniformity with
respect to the level of correlation and residuals.
This consistent relationship is the most important
finding of this study. The simplicity of expression,
the rather high correlation for a biological
property, and the validity for all growth and
stand conditions, as well as within trees, make
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this an outstanding model, in the vicinity of
characterization as a physiological law. An
implicit formula for sapwood age (equation 4)
was found adequate and added support to the
justification for using the square root transformation. The superiority of the age model compared
with spatial models used in this study confirms
the assumption that heartwood formation is
intricately tied to ageing and indicates a kind of
internal ‘timer’ triggering the physiological transformation. The nature of this ‘timer’ mechanism
might be interpreted as an inherent property of
the Scots pine, leaving minor, if any, importance
for environmental influences. Recent experiments
(Bergström, 2000) could not verify any change in
heartwood production when the magnitude of
the crown was manipulated, adding support to
the idea of age as the dominant factor in heartwood formation.
The investigation and sampling were performed
with the purpose of analysing heartwood in
mature stands and sawlogs. Establishing the age at
which heartwood formation is initiated, was
beyond the scope of the study. Even so, the following points should be considered. In young samples
lacking heartwood, the time remaining before
transition can be considered a ‘negative heartwood
age’, the magnitude of which remains unknown.
Accordingly, such samples might be omitted (as in
this investigation) or set at zero, both introducing
a sampling bias. Consequently, even if the mathematical interpretation of equation (3) indicates
initiation at 9 years, no decisive conclusion about
the age at which transition commences should be
made from this investigation.
Mature stands are regularly harvested and
hence stands older than 150–200 years are
rather infrequent. Those that are found are
usually from infertile site classes or located at
high elevations, resulting in potentially biased
samples. As has been shown, however, no substantial influence of environmental factors was
identified and no bias in the residuals could be
detected. Also, the model for transition rate
(equation 6) asymptotically approaches 1 for
very old trees, which seems sensible. Thus, the
square root approach applied in the pine heartwood age rule model seems to be fairly robust,
and there is no indication that this model should
suffer invalidity in any part of the range up to
~240 years.
The remaining variance in the age model might
be attributed to several factors. One factor was
shown to be the boundary of heartwood–
sapwood undulating through several annual
rings. Some variance is inevitably associated with
observational inaccuracies. As already discussed,
recent investigations (Fries and Ericsson, 1998;
Ericsson and Fries, 1999) indicate genetic differences. Albeit these experiments were performed
on quite young trees in Scandinavian terms and
dealt with the spatial amount of heartwood
rather than age, some genetic variation in heartwood age relationships might be present.
However, as far as can be analysed from this
study, the nature of the residuals remains stochastic.
The modelled values for AH agree fairly well
with the result reported by Björklund (1999) in
the range up to cambial age ~110 years. For older
stands, the new findings point to a less accelerated transformation from sap- to heartwood.
Transition rates (equation 6) are (Björklund’s
results in brackets): AC = 50 years: 0.58 rings a–1
(0.53); AC = 100: 0.70 rings a–1 (0.74); AC = 150:
0.76 (0.95); AC = 200: 0.79 (out of range).
A relatively high amount of heartwood was
found under the following conditions (equation
7): old and thick trees with small crowns located
on poor sites. Thus, a high level of heartwood
might be obtained by a suitable silvicultural
regime allowing trees to grow rapidly in their
youth, subsequently increasing the stand density
before harvesting the trees in their old age. Only
44 per cent of the variance in heartwood fraction
was, however, explained by this model. The
objective of estimating quantitative models for
growth rates is known to be quite cumbersome
(e.g. Blingsmo, 1984; Huang and Titus, 1995),
typically obtaining models explaining ~60–70
per cent of the variation even when employing
complex models with a large number of predictor variables. Models for relative heartwood area
typically explain 50–60 per cent of the variance
(Björklund and Walfridsson, 1993; Sellin, 1996).
Thus, models for the spatial amount of heartwood are inferior to the pine heartwood age rule
in this respect.
The spatial amounts of heart- and sapwood
are related to the age model through the annual
ring width. Environmental factors, as well as
silvicultural variation, heavily influence the ring
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width pattern. Small crowns provide a proportionally low potential for producing xylem,
resulting in narrow rings and low sapwood production. The connection to the pipe-model
theory is obvious; the question is which is considered to be the generating factor, transpiration
or cambial age.
In dating archaeological wood by dendrochronology, the sapwood might be decayed while
the heartwood remains. The year of felling for
such specimens might be estimated by adding a
suitable period for the sapwood, calculated by
the pine heartwood age rule.
The close relationship between cambial age
and heartwood age offers the possibility of estimating mean ring width in sawlogs (equation 7),
providing information about local diameter and
heartwood diameter. For a given diameter,
cambial age was found to increase with increasing radial fraction of the heartwood. While the
diameter is regularly measured in a log scanner at
most sawmills, the use of thermo-photographic
analysis has proved promising in separating
heartwood from sapwood (Gjerdrum and Höibö,
2003). Further investigations will be necessary,
however, to verify such findings and to establish
more reliable information regarding these
relationships, e.g. the connection between ring
width and timber quality.
Acknowledgements
Feedback provided by Olav Høibø throughout the
progress of the study is greatly acknowledged. I would
like to thank Eva Gjerdrum, Mette M. Espelin, Börje
Ohlsson and Geir Fuglum, who all provided valuable
assistance in the completion of this work. Lynell
Chvala revised the English text. The industrial support
of Norske Skog ASA in the completion of this investigation and the financial funding provided by the Norwegian Research Council are appreciated.
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Received 30 April 2002