Tree Physiology 26, 459–468
© 2006 Heron Publishing—Victoria, Canada
Carbon content variation in boles of mature sugar maple and giant
sequoia
SABAH H. LAMLOM1 and RODNEY A. SAVIDGE1,2
1
University of New Brunswick, Faculty of Forestry and Environmental Management Fredericton, NB, E3B 6C2, Canada
2
Corresponding author ([email protected])
Received April 7, 2005; accepted July 8, 2005; published online January 16, 2006
Summary At present, a carbon (C) content of 50% (w/w) in
dry wood is widely accepted as a generic value; however, few
wood C measurements have been reported. We used elemental
analysis to investigate C content per unit of dry matter and observed that it varied both radially and vertically in boles of two
old-growth tree species: sugar maple (Acer saccharum Marsh.)
and giant sequoia (Sequoiadendron giganteum (Lindl.) Bucholz). In sugar maple there was considerable variation in tree
ring widths among four radii for particular annual layers of xylem, revealing that the annual rate of C assimilation differs
around the circumference and from the base of each tree to its
top, but the observed variation in C content was unrelated to diameter growth rate and strongly related to the calendar year
when the wood was formed. Carbon content in sugar maple
wood increased in an approximately linear fashion, from < 50
to 51% from pith to cambium, at both the base and top of the
boles. In giant sequoia, C was essentially constant at > 55%
across many hundreds of years of heartwood, but it declined
abruptly at the sapwood–heartwood boundary and remained
lower in all sapwood samples, an indication that heartwood formation involves anabolic metabolism. Factors that may be responsible for the different C contents and trends with age
between sugar maple and sequoia trees are considered. Treering data from this study do not support some of the key assumptions made by dendrochronology.
Keywords: Acer saccharum, annual rings, cambium, crossdating, heartwood, partitioning, sapwood, secondary growth,
Sequoiadendron giganteum, wood, xylem.
Introduction
The amount of carbon (C) stored in trees has often been stated
without reference to the method of calculation or the underlying knowledge foundation (Matthews 1993). Global approximations of C content (w/w) in forest biomass on a dry-mass
basis have been estimated as both 45% (Whittaker and Likens
1973, Ajtay et al. 1979) and 50% (Kinerson et al. 1977, Brown
and Lugo 1982, Harmon et al. 1990, Dewar and Cannell 1992,
Schroeder 1992, Houghton 1996, Karjalainen 1996, Krankina
et al. 1996, Maclaren 1996, Price et al. 1996, Fearnside 1997).
Data from Atjay et al. (1979) indicate that the uncertainty attending the actual C content of forests could be as much as
10%; however, 50% C has been the most widely promulgated
generic value in forest modeling (Wenzyl 1970, Atjay et al.
1979, Karchesy and Koch 1979, Sedjo 1989, Dewar and Cannell 1992, Hollinger et al. 1993, Matthews 1993, Thuille et al.
2000).
Few measurements of wood C content have been reported
(Mingle and Boubel 1968, Reichle et al. 1973, Chow and
Rolfe 1989, Yu-Jen et al. 2002, Elias and Potvin 2003,
Lamlom and Savidge 2003) and the tendency has been to look
at C in oven-dried wood. However, when oven-dried wood
powder was compared with wood powder dried at ambient
temperature over a desiccant, the oven-dried woods had significantly lower C contents, indicating that considerable volatile
C had been driven off (Lamlom and Savidge 2003). Thus, past
data on C content in oven- or kiln-dried woods are likely inaccurate as a measure of the C content of forests. Yu-Jen et al.
(2002) found that C content of commercial hardwoods in Taiwan can be several percent less than 50% and Elias and Potvin
(2003) documented varied C concentrations in trunk woods of
32 neotropical species. Lamlom and Savidge (2003) found
North American woods contain 46–55% C by dry mass, depending on species. There was 1–2% variation within bole
wood, depending on the year in which the wood was produced
and where in the stem the sample was taken, with earlywood
invariably having a higher C content than latewood (Lamlom
and Savidge 2003).
All woods are chemically complex (Kaar and Brink 1991,
Sjöström 1993) and the wood of each species is chemically
unique (Savidge 2000). Wood is composed of molecular components, each containing a particular proportion of C (Table 1), resulting in variation in C content among tree species
(Savidge 2000, 2003). For example, cellobiose (the building
block of cellulose) contains 42.1% C and the monosaccharides
common to hemicelluloses contain about 40% C. The C content of heterogeneous substances such as lignin varies considerably depending on what particular kind(s) of material are
present. In addition, the proportions by mass of lignin, lignans,
polyphenolics, extractives and other compounds making up
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LAMLOM AND SAVIDGE
Table 1. Examples of carbon percentage (C%; by mass) of some molecular components in heartwood.
Compound
C%
Cellulose (as cellobiose)
Hemicelluloses
Lignin
Sequoyitol
myo-Inositol
D-Pinitol
Sequirin-E
Sequirin-F
Sequirin-G
α−Pinene
Limonene
δ–Cadinene
Sabinene
Pinocarveol
Cinnamaldehyde
α−Terpineol
Borneol
β−Eudesmol
42.10
40–44
63–72
43.3
39.9
43.3
69.75
67.36
65.33
88.17
88.17
88.17
88.17
78.83
81.79
77.86
77.86
81.02
wood can vary over a wide range (Savidge 2001a). To account
accurately for total C stored in forest biomass, C content in
each tissue of each tree species merits individual investigation.
For example, the C content of leaves (about 42%) is considerably lower than that of woody or root material (47–52%)
(Ajtay et al. 1979). In addition to the importance of knowing C
contents of the various tissues and their relative contributions
to total tree biomass, more accurate estimation of total stand
volume or biomass is necessary (Savidge 2001a). Research in
this area will improve the accuracy and precision of C estimation in forests, assist in resolving ongoing inconsistencies in
estimating global C cycles (Vogt 1991, Caspersen et al. 2000,
Schimel et al. 2000, Elias and Potvin 2003) and enhance overall forest management.
The total C in wood or the proportion of C locked up in any
particular macromolecular class can be thought of as the phenotype determined by an interaction between genes and environment (Savidge 2001b, 2003). Environmental (e.g., climatic) changes have occurred over the last century and, as
indicated by the sugar maple data reported below, C assimilation and metabolic partitioning by cambium appear to have responded to such changes. Reforestation with “improved” or
genetically modified trees could alter C-based metabolism, but
to what extent remains uncertain (Savidge 2003). Accurate
baseline data are necessary to determine what, if any, change
in forest biomass C storage is occurring in response to changes
in the environment and the genetic diversity of the forest.
Lamlom and Savidge (2003) provided methods for accurate
and precise determination of C in wood and began work to establish such a baseline. We continue with that aim here.
The purpose of this study was to determine if C content of
wood within a species has varied over the last millennium, to
observe if there is a correlation between C content in wood and
cambial growth rate, i.e., radial and circumferential increment,
and to examine the extent of within-tree variations by assessing C content both radially and vertically. Two old-growth tree
species, a hardwood and a softwood (sugar maple and giant sequoia, respectively), were investigated.
Materials and methods
Tree species and sample preparation
Three Acer saccharum Marsh. trees growing wild on the same
site in an unevenly aged upland forest were felled in winter
2000 near Napodogan, New Brunswick. Discs from both the
top and base of each log were removed from each tree, labeled
and transferred to the laboratory. Radial sectors extending
from pith to cambium of the stumps of three Sequoiadendron
giganteum (Lindl.) Bucholz trees that grew short distances
apart in the Mountain Home Demonstration State Forest, California, were also obtained. Giant sequoia Trees 1 and 2 were
felled in 1995 following a fire and the bole of Tree 3 snapped
in 1980 during a severe winter storm. All material investigated
in this research consisted of sound sapwood and heartwood
with no signs of wounding, decay, insect attack or fire scarring.
Wood from both the base and the top of boles of A. saccharum was investigated. An electric planer was used to create
clean surfaces for four radii of each stem disc (Figure 1). In
some cases, where it was difficult to distinguish growth rings,
a power-disc sander with 110- followed by 220-grit sand paper
was used.
Triangular-shaped radial sectors from the flat surfaces of
stumps (1.5 m aboveground) of three S. giganteum trees were
removed with a chainsaw and transported to New Brunswick.
The radial sectors were trimmed to 7.0 (transverse) × 2.5 (radial) cm dimensions with a band saw. A power sander with
80-grit paper was used to remove the rough transverse surface,
which was then further sanded (110- and 220-grit sandpaper)
to enhance the visibility of tree rings. Trees 1, 2 and 3 were
about 106, 1600 and 2300 years old, respectively.
Tree-ring analysis
Radial widths of the annual rings were measured at the University of New Brunswick Wood Technology Laboratory, Tweeddale Center, in Fredericton, New Brunswick. Beginning beside
the pith, the width of each annual ring was determined across
each of four (A. saccharum) radii or a single (S. giganteum) radius. To measure annual ring widths, each stem piece was
mounted vertically and imaged on a computer monitor by
means of a mobile optical sensor (Pansonic Model WVCP410) providing 16× magnification and a centralized crosshair. The optical sensor was programmed to advance vertically
(i.e., parallel to the radial files of wood elements in the annual
growth layers) at a rate of 0.1 mm s – 1 and could be stopped as
necessary. Four calibration tests were carried out before the
acquisition of each data series with a digital testing gauge
(Digimatic Height Gage, Mitutoyo; Model No. 192-605). The
TREE PHYSIOLOGY VOLUME 26, 2006
C CONTENT VARIATION IN BOLES OF MATURE TREES
461
Figure 1. Sugar maple Tree 1 bole top,
with the sapwood–heartwood boundary
arrowed. Wood powder for elemental
analysis was obtained by drilling several adjacent locations of every 20th
ring within a planed radius, as shown.
A–D indicate the four radii that were
investigated. Numbers on the scale bar
are in cm.
degree of precision in measuring the vertical displacement of
the optical sensor was determined to be ± 0.01 mm. During the
collection of raw ring-width data, when the operator determined that the monitor crosshair was exactly at a latewoodearlywood imaged boundary, the vertical position of the optical sensor was stored as a linear displacement datum using
software developed in-house (M. Albright, personal communication, University of New Brunswick, Canada). Care was
taken to keep a consistent angle of visual interpretation of the
monitor when viewing the crosshair and imaged wood.
C-content analysis
Every 20th annual ring was marked, beginning with the outermost annual layer and working inward across the four radii of
each sugar maple and across the one radius of each giant sequoia. Wood was removed as a fine powder from the marked
annual ring positions with an electric drill with a small diameter bit, which was carefully cleaned between each sample. As
shown in Figure 1, holes were drilled at a number of locations
in each 20th annual layer of wood across each planed radius
and the powder combined to provide sufficient material for
analysis. The wood powders were dried in a vacuum dessicator
as previously described (Lamlom and Savidge 2003). Three or
more weighed replicates (about 1 mg) of each wood powder
were subjected to elemental analysis as earlier described, using a calibrated Carlo Erba NA 1500 elemental analyzer and
determining C content from a standard curve based on L-leucine (Lamlom and Savidge 2003). Carbon content was ex-
pressed as a percentage of wood dry biomass (C% = (mass of
C/dry mass of wood)100).
Statistical analysis
For both species, the mean and standard deviation of at least
three replicates per sample were calculated. Correlation analysis was carried out to determine whether C content and diameter growth rate were statistically correlated. For S. giganteum
trees, C content as a function of calendar year of wood formation was examined by linear regression analysis. The significance of differences in mean C content of sapwood and heartwood of giant sequoia trees was assessed by the Student's t
test. For each A. saccharum tree, variation in C content between the stem top and base was evaluated by repeated measures analysis of variance (ANOVAR), with radius as a random factor nested within height, and comparing the corresponding year of wood formation (i.e., the same layer of xylem)
between top and base (Table 2). Regression analyses were performed to test how C changed over time in sugar maple trees.
Results
For sugar maple, data from the time series of ring widths over
four radii at 0.5 m aboveground (bole base) and at a height of
18 m (bole top) revealed wide variation in each tree in annual
ring widths across any one radius and among the four radii of
the same disc (Figures 2 and 3). Ring width differed markedly
from the base to the top of the bole within the same layer of xy-
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462
LAMLOM AND SAVIDGE
Table 2. Summary of analysis of variance (ANOVAR) of carbon content of dry wood (g g – 1) of the three Acer saccharum trees. Asterisks
indicate statistical significance at the 1% probability level. Abbreviation: df = degrees of freedom.
df
F
P value
Tree 1
Height
Year
Height × year
1
6
6
108.70
14.06
1.43
< 0.001*
< 0.001*
0.2307
Tree 2
Height
Year
Height × year
1
3
3
3.69
12.25
0.89
0.1031
< 0.001*
0.4648
Tree 3
Height
Year
Height × year
1
4
4
0.00
18.01
2.01
0.9875
< 0.001*
0.1320
lem, demonstrating a lack of correspondence between radii,
heights and trees in annual cambial growth rate (Figures 2
and 3). The lowest C content was found near the pith and increased gradually over the life of the tree at both the top and
the base of all three sugar maple trees (Figures 2 and 3). The
ANOVAR data revealed that, among all three sugar maple
trees, the year of xylem layer formation consistently influenced C content (Table 2). An interaction between height and
year of formation was not detected, indicating that the effect of
calendar year of wood formation on C content did not differ
significantly between the base and top (significance level, α =
0.01). Figure 4 depicts variable C allocation at different positions around the circumference of the three sugar maple trees.
Based on linear regressions, C content and year of wood formation were strongly related in all three trees. In one of the
sugar maples, the top of the bole had a significantly lower C
content than the base (Figure 4, bottom graph). No such difference was apparent in the other sugar maple trees.
Figure 5 profiles radial widths (mm) of successive annual
rings in the trunks of giant sequoia trees at about 1.5 m aboveground. Annual ring width varied significantly over the life of
the trees. In Tree 1, growing beneath a canopy, cambial growth
occurred at a moderate rate in the early years and accelerated
strongly at about age 70. In Trees 2 and 3, cambial growth was
Figure 2. Ring widths and carbon percentage (C%; (g g – 1 of
dry wood) × 100) versus the
year of ring formation in sugar
maple Tree 1, at the top of the
trunk (upper panel) and 0.5 m
aboveground (lower panel).
The C% was measured at
20-year intervals over four radii (see Figure 1). The error
bars represent standard deviations.
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C CONTENT VARIATION IN BOLES OF MATURE TREES
463
Figure 3. Ring widths and carbon percentage (C%; (g g – 1 of dry wood) × 100) of sugar maple Trees 2 and 3 versus the year of ring formation at
0.5 m aboveground (lower panels) and at 18 m aboveground (upper panels). The C% was measured every 20 years over four radii (see Figure 1).
The error bars represent standard deviations.
most rapid early in the life of the tree (Figure 5). Thereafter,
Trees 2 and 3 underwent cycles of relatively strong and weak
cambial growth; however, these cycles were not synchronous
in the two trees. Ring width declined gradually with age in
Trees 2 and 3 (Figure 5). Carbon content in the three studied
trees exhibited little variation (Figure 5). Based on linear regressions (level of significance, α = 0.01), the relationship between C content and cambial age was weak in Trees 1 and 2 for
both the sapwood and heartwood (Tree 1 (heartwood) r 2 =
0.333%, F = 1.50, P = 0.308 (%C year – 1 slope: 0.0020); Tree 2
(sapwood) r 2 = 0.038%, F = 0.24, P = 0.642 (%C year – 1 slope:
0.0003); Tree 2 (heartwood) r 2 = 0.044%, F = 3.30, P = 0.073
(%C year – 1 slope: < 0.0001)). There was a statistically significant association between C content and cambial age in Tree 3,
but only for heartwood (Tree 3 (sapwood) r 2 = 0.40%, F =
2.00, P = 0.252 (%C year – 1 slope: 0.0020); Tree 3 (heartwood)
r 2 = 0.233%, F = 32.78, P = < 0.001 (%C year – 1 slope: <
0.0001)).
Figure 5 shows that an abrupt change in C content occurred
at the sapwood–heartwood boundary. The student’s t test con-
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464
LAMLOM AND SAVIDGE
Figure 4. Wood carbon as a percentage
of dry wood mass (C%) versus year of
formation in four radii (A, B, C and D)
at the top and base of three sugar maple boles. Error bars (standard deviations) are omitted for clarity, but the
ranges were as follows: Tree 1 top,
0.1–0.4%; Tree 1 base, 0.02–0.5%;
Tree 2 top, 0.04 –0.4%; Tree 2 base,
0.04 –0.3%; Tree 3 top and base,
0.03–0.3%. The bottom panel shows
C% versus year of wood formation at
both the upper and lower ends of the
bole of sugar maple Tree 1, with standard errors of the means for the four
radii indicated except for years 1860
(bole top) and 1760 (bole base) where
the error bars are standard deviations
(triplicate analyses of single wood
powder samples taken from near the
pith— cf. Figure 1).
firmed a statistically significant difference between the mean
C percentages of heartwood and sapwood (t = –21.31, P <
0.001).
Discussion
Our study provides an indication of environmental effects on
the variability of C allocation within trees, recognizing that a
single tree has the same genotype throughout its vegetative tissues over its entire life span and that the wood formed within a
tree tends to be heterogeneous because local site and climatic
variability affect the environment within the stem (Panshin
and de Zeeuw 1980, Savidge 2001b, 2003). Because of the
large number of samples collected, this study was limited to
three trees of each species. The primary aim was, therefore, to
discover the extent of variation in C content within rather than
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C CONTENT VARIATION IN BOLES OF MATURE TREES
465
Figure 5. Radial widths and wood carbon as a percentage of dry wood mass (C%) of giant sequoia annual rings. Measurements were made on every 20th growth ring at 1.5 m height. Error bars (standard deviations) are omitted for clarity, but their ranges were as follows: 0.02–0.1%,
0.02–0.2% and 0.01–0.3% for Trees 1, 2 and 3, respectively.
between trees, although our data also permit some comparisons between genotypes and species.
Based on correlation analyses, no relationship was found
between C content and growth rate (narrow versus wide rings)
in either species, but our observations nevertheless indicate
that C content in sugar maple and giant sequoia are influenced
by different factors. The percentage of C in an annual layer of
secondary xylem increased over the life of the sugar maple
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LAMLOM AND SAVIDGE
trees but not over the life of the giant sequoia trees. Heartwood
C content remained constant in giant sequoia but not in sugar
maple, regardless of ring width, indicating that C allocation
during secondary wall formation (including lignification, protoplasmic autolysis and conversion of sapwood into heartwood) and cambial growth rate are independently controlled.
Sugar maple differed from giant sequoia in displaying a radial trend of rising C content from pith to bark. The same trend
was found at both the top and the base of all three boles and
ANOVAR indicated that C content of sugar maple wood was
strongly influenced by the calendar year of wood formation
(Table 2). If C content changes regardless of growth rate and
there is no anatomical change in the wood, then there must be a
change in the chemistry of one or more wood components. We
did not investigate the proportions of labile reserves (e.g.,
starch) as a function of radial position, but earlier studies have
demonstrated qualitative as well as quantitative changes in ray
cell reserve material with increasing distance from the cambium (Höll 2000, Sauter 2000). Thus, it is possible that the observed trend toward reduced C content with radial depth may
be explained by a decrease in amount of storage reserves. In
addition to radial variation, there is seasonal variation in quantity of stored materials (starch, sugars, fat and specific vegetative storage proteins) in woody rays (Sauter and Witt 1997,
Höll 2000, Sauter 2000) and, therefore, the observed C contents could have been different if trees had been harvested at a
different time of year.
Many biochemical pathways contribute to C utilization and
sequestration during secondary growth (Savidge 2000, 2001b)
but the primary explanation for variability in the amount of C
incorporated into woody biomass must reside in variation in
the microenvironment within cambium and wood. Our C%
data for sugar maple indicate that metabolism in the tree stem
varied both vertically and circumferentially as well as being
influenced by genotype (Figure 4).
In giant sequoia heartwood, C content remained constant
above 55% until declining abruptly at the sapwood–heartwood
boundary. Heartwood formation occurs at the transition zone
between existing heartwood and sapwood and involves increased accumulation of secondary products such as condensed tannins, terpenes, flavonoids, lignans, stilbenes and
tropolones (Hillis 1987, Croteau et al. 2000). All of these possess much higher percentage C than the carbohydrate molecules from which they are biosynthesized (Croteau et al. 2000)
and all are retained in the heartwood. Anabolism by definition
involves the conversion of simple substances, such as glucose
derived from starch, into more complex compounds and it is
concluded, therefore, that the higher C content in giant sequoia
heartwood is indicative of an anabolic process. It is widely believed that heartwood formation is controlled primarily by living ray cells, but the biochemistry of heartwood formation is
not well understood and it is possible that enzymes that remain
active in cell walls of “dead” xylem elements and fibers may
also participate (Savidge 2001b). The more or less constant C
content found across millennia of giant sequoia heartwood annual rings is an indication of fairly constant metabolism during
secondary growth. This interpretation is supported by the ob-
servation of Craig (1954) that giant sequoia wood contains a
fairly constant C isotope composition (13 C/ 12C), showing no
systemic trend with time. Evidently, the biochemistry in cambium and wood determining both isotopic discrimination and
the net sequestration of C into mature heartwood does not
change systematically with age in giant sequoia. Our database
is limited to three trees from a single location, but it suggests
that S. giganteum heartwood could serve as a reference standard in support of future C investigations in general.
In some tree species, an association between increasing extractive content and increasing tree age has been noted, with
older trees containing more extractives in the heartwood
(Hillis 1962, 1985, 1987). However, variation in C content between heartwood of young and old giant sequoia was not evident (Figure 5). Nevertheless, there is little doubt that biosynthesis and retention of extractives is a major factor
contributing to the 1% increase in S. giganteum heartwood C
content relative to sapwood C content. Table 1 illustrates the
variations in C percentage of some of the many different molecular components found in giant sequoia heartwood. Almost
all extractives including tannins, terpenes, essential oils, fats,
resins, waxes, lignans, flavonoids and other polyphenols have
high C contents. The only exception being the cyclitols (e.g.,
sequoyitol, myo-inositol and pinitol), but these carbohydrates
make a minor contribution to the overall content of extractives
(Anderson et al. 1968).
The substantial difference in C content that we observed between giant sequoia and sugar maple is consistent with earlier
studies that concluded that softwoods, in general, have a
higher C content than hardwoods (Lamlom and Savidge 2003)
and that this difference should be considered in accounting for
overall forest C content (Savidge 2001a). The wood of most if
not all trees has features that change from the pith outward,
from the base to the top, within an annual ring and sometimes
even on different sides of the tree (Dadswell 1958, Sluder
1972, Savidge 2003, Burdon et al. 2004). Given the immense
variation in both wood chemistry and quality and the many different kinds of wood found in nature, as well as the difficulties
attending estimation of the biomass of wood in forests
(Savidge 2001a), the amount of C presently tied up in global
woody biomass can be approximated only crudely .
Our observations on tree rings have relevance to dendrochronology. When wood from differently aged trees of the
same species growing on the same site are examined at a fixed
tree height, a rough pattern of cambial growth rate in relation
to tree aging may sometimes be found, beginning with large
radial increments in the small diameter shoots followed by an
exponential decline as the circumference increases (Fritts
1963). However, our data show that this is not always the case.
Exponential decline is superficially evident in the two older
S. giganteum trees, but this interpretation does not stand up to
close scrutiny (Figure 5). All three giant sequoia trees were
tall, evidently healthy, specimens. Tree 1 grew beneath a canopy of larger giant sequoias, such as Trees 2 and 3. In contrast
to Trees 2 and 3, the diameter growth of Tree 1 was flat over its
first 60 years, but began to increase exponentially after 1950
(Figure 5). Shaded trees such as Tree 1 usually begin their
TREE PHYSIOLOGY VOLUME 26, 2006
C CONTENT VARIATION IN BOLES OF MATURE TREES
growth slowly and later accelerate and such trees are common
in wild stands. Thus, the idea in dendrochronology that master
chronologies need to be “standardized” by applying a smoothing curve to the overall growth trend is not supported by the
natural variation shown in Figure 5. Genetic variability is as
important, if not more so, as environmental variability in determining the intrinsic environment and a tree’s ability to grow
(Savidge 2001b); thus, it could be argued that standardization
of tree chronologies is only useful when considering clonal
stock. Standardization of variation in a number of wild-type
specimens may serve to provide an indication of an “average”
tree, but it undoubtedly ignores substantial uncertainty about
the natural variation in populations of trees.
In wild old-growth sugar maple, the pattern of ring widths
over radii varies significantly from one cardinal direction to
the next at any particular height, and when xylem layers of the
same year are compared, there are obvious between-tree differences in ring width patterns (Figures 2 and 3). Thus, it
would be a serious error to assume that any single radius from
a bole is highly representative of the trunk as a whole, although
such assumptions are routinely made in dendrochronological
studies. Figures 2, 3 and 5 also make it evident that these singularly wide or narrow annual rings, which are considered in
dendrochronology to be environmentally sensitive indicators
of abrupt environmental change and therefore useful for
cross-dating purposes, cannot be relied on to be present in every tree, even when the trees have grown in close proximity.
This suggests that attempts to estimate the year of annual ring
formation from year-to-year differences in ring width may involve a large degree of uncertainty.
Acknowledgments
The Libyan Cultural Section provided S.L. with Ph.D. studentship
funding. We are indebted to Dave Dulitz, former manager of the
Mountain Home Demonstration State Forest, for providing the Sequoiadendron samples and we thank David Boyle for transferring the
same from California to UNB. Larry Richard of Napodogan Veneer
Products provided samples of mature sugar maple trees. At UNB,
Dean McCarthy provided expert technical assistance with sample
preparation and Professors W. Knight, R. Turner and D. Keppi provided helpful statistics discussion. Funding for performance of the research was provided RAS by the Natural Sciences and Engineering
Research Council of Canada.
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