Tree Physiology 27, 921–927
© 2007 Heron Publishing—Victoria, Canada
Radial patterns of carbon isotopes in the xylem extractives and
cellulose of Douglas-fir
ADAM M. TAYLOR,1,2 J. RENÉE BROOKS,3 BARBARA LACHENBRUCH4 and JEFFREY
J. MORRELL4
1
Department of Forestry, Wildlife and Fisheries, University of Tennessee, 2506 Jacob Drive, Knoxville, TN 37996, USA
2
Corresponding author ([email protected])
3
Western Ecology Division U.S. EPA/NHEERL, 200 SW 35th St., Corvallis, OR 97333, USA
4
Department of Wood Science and Engineering, Oregon State University, 119 Richardson Hall, Corvallis, OR 97330, USA
Received May 15, 2006; accepted August 24, 2006; published online March 1, 2007
Summary Heartwood extractives (nonstructural wood components) are believed to be formed from a combination of compounds present in the adjacent sapwood and materials imported from the phloem. The roles of local compounds and imported material in heartwood formation could have important
implications for the wood quality of species having naturally
durable wood. Stable isotope composition (δ13C) was analyzed
to assess radial variation in sapwood extractives, and to estimate the relative importance of adjacent sapwood extractives
and imported photosynthate in the formation of heartwood extractives. Cellulose and extractives from the outer 39 annual
rings of six Douglas-fir (Pseudotsuga menziesii (Mirb.)
Franco) trees were isolated and their δ13C composition determined. Although the extractives and the cellulose showed different absolute δ13C values, the patterns of change over time (as
represented by the annual rings) were similar in most cases.
Within an annual ring, carbon isotope ratios of extractives were
correlated with the cellulose isotope ratio (R 2 = 0.33 in sapwood, R 2 = 0.34 in heartwood for aqueous acetone-soluble extractives; R 2 = 0.41 in sapwood for hot-water-soluble extractives). These data suggest that some sapwood extractives are
formed when the wood ring forms, and remain in place until
they are converted to heartwood extractives many years later.
Sapwood extractives appear to be important sources of materials for the biosynthesis of heartwood extractives in Douglas-fir.
Keywords: δ 13C, heartwood, Pseudotsuga menziesii, sapwood, stable isotopes.
Introduction
Wood cell walls are composed primarily of cellulose, hemicelluloses and lignin, but xylem also contains extractives:
nonstructural substances that are soluble in organic solvents or
water (Sjostrom 1993). Sapwood extractives include materials
such as sugar, starch and lipids, which are assumed to serve as
energy reserves. Starch and sugar are absent from the heart-
wood, and the extractives there, such as phenols and terpenes,
are thought to contribute to a passive defensive system (Hillis
1987). Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco)
sapwood extractives also include phenolic glycosides and
other low molecular mass phenolic compounds that are
thought to be precursors of heartwood extractives (Dellus et
al. 1997a).
Extractives can have important effects on the properties of
wood products. The heartwood of species having naturally durable wood contains extractives that prevent attack by wooddestroying insects and fungi (Hillis 1987). Both the type and
amount of extractives present in heartwood vary among species, and within and among individual trees, but the factors
that control such variations are as yet unclear (Taylor et al.
2002). A better understanding of how environmental factors
influence heartwood extractive formation could lead to the development of silvicultural methods that enhance the production of naturally durable wood.
Heartwood results from the senescence of old sapwood tissue, beginning in the oldest part of a tree stem, branch or root
(i.e., surrounding the pith). It is believed that a new increment
of heartwood is formed each year (e.g., Nobuchi et al. 1984,
Magel 2000), but this pattern may vary among species. Heartwood extractives are formed at the heartwood–sapwood
boundary, at least partly from carbon-based compounds imported from the inner bark (living phloem) (Hillis and Hasegawa 1963). Because sapwood extractives differ from heartwood extractives, it is assumed that extractives in senescing
sapwood also contribute to the formation of heartwood extractives (Hillis 1987). The relative importance of these carbon sources (i.e., local compounds and imported material) for
heartwood extractive formation remains poorly understood
(Taylor et al. 2002).
The carbon sources that contribute to heartwood extractive
formation have yet to be identified because the biochemistry
underlying the formation of these compounds is uncertain
(Magel 2000) and the location of heartwood within the living
tree makes direct observation difficult. However, stable iso-
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TAYLOR, BROOKS, LACHENBRUCH AND MORRELL
tope analysis could allow for studies of the extractive materials
in individual growth rings. Stable isotopes have been used as
tracers; for example, fertilizer containing highly enriched
amounts of 15N was applied to trees and later detected in wood
tissues (Elhani et al. 2003). Radioactive carbon (14C) has also
been used as a tracer to monitor movement of materials within
trees (Hillis and Hasegawa 1963). These techniques have the
advantage that the presence of the tracer is easily distinguished
from the background radioactivity present in the wood tissue.
In addition, naturally occurring variation in the ratio of stable
isotopes in molecules has been used to study physiological
processes in trees. Variation in stable isotope abundance in tree
ring cellulose has frequently been made the basis of inferences
about environmental influences on trees.
Isotope fractionation occurs during photosynthesis in plants
(O’Leary 1993) and the relative natural abundance of the stable isotopes of carbon (13C/12C) in cellulose varies among the
annual tree rings in response to environmental changes (see review by McCarroll and Loader, 2004). Variation in δ13C in cellulose reflects variation in leaf physiology at the time of carbon fixation as described by the equation (Farquhar et al.
1982):
∆ = a + (b − a )
ci
ca
where ∆ is the discrimination against 13C, a and b are constants
for fractionation due to diffusion (4.4‰) and carboxylation
(~27‰), respectively, and ci / c a is the ratio of the carbon dioxide concentration ([CO2]) inside the stomatal cavity to the
[CO2] of ambient air surrounding the leaf. The ci / c a ratio is influenced by the rate of photosynthesis (A, which draws down
ci ) and stomatal conductance (g) which allows CO2 into the
leaf. Therefore, the ci / c a ratio is sensitive to many factors that
influence the A/g ratio including water availability, irradiation,
tree age and the position of the leaf in the crown (Francey and
Farquhar 1982, Leavitt and Long 1986, Livingston and
Spittlehouse 1996, Brooks et al. 1998, Barbour et al. 2002,
McDowell et al. 2003). Thus, the 13 C/ 12 C ratio in photosynthate changes over time in trees, and these changes are recorded in the increments of cellulose that are synthesized over
time from that photosynthate.
Just as annual variations in photosynthetic conditions are reflected in the carbon isotope composition of the cellulose,
there will be variation in the stable carbon isotope ratios of all
wood components because all are ultimately derived from
photosynthate. Extractives and cellulose made in the same
year may not have the same δ13C value, because of differences
in 13C discrimination along their biosynthetic pathways
(Schmidt and Gleixner 1998); however, the carbon isotope
values of these substances may be correlated over time if the
differences in discrimination along the various biosynthetic
routes are consistent from year to year. Thus, stable isotope
analysis may be a useful means of studying annual processes
such as extractive movement and heartwood formation in
trees.
The dynamics of extractives in trees has implications for the
stable isotope analysis of tree rings. If extractives are not removed, their isotopic signal will influence the signal of the
bulk sample. Even if the extractives are removed, they may influence the isotopic composition of the cellulose samples if
they are available as a carbon source for wood formed in
subsequent years.
In this study we used isotope analysis to determine which
carbon-based materials are precursors of heartwood extractives. It has been shown that material imported from the phloem
can contribute to heartwood extractives formation (Hillis and
Hasegawa 1963). However, the absence of sapwood extractives in heartwood suggests that sapwood extractives are a
source for the synthesis of heartwood extractives. We differentiated among: (1) newly assimilated (“new”) carbon that is
formed in the same year that the heartwood increment is
formed; (2) “old” carbon that has been stored in the annual
growth ring that is converting to heartwood; (3) a mixture of
these; or (4) other sources; and (5) a combination of both (3)
and (4). If heartwood extractives are derived mainly from
“new” carbon, there should be no correlation between the isotope ratio in the heartwood cellulose and the extractives of a
given wood sample, but heartwood extractives should correlate with cellulose in the newest sapwood present at the time of
heartwood formation. In contrast, if heartwood extractives are
made of “old” carbon, there should be strong correlation between heartwood cellulose and extractives within a given ring.
A mixture of both should have a weak correlation, if any, for
either. Our specific objectives were to determine whether there
is radial variation in the δ13C of extractives and cellulose in
Douglas-fir xylem, and then to examine the correlation between the isotope ratios in extractives and cellulose to infer the
carbon sources of heartwood extractives.
Materials and methods
Douglas-fir trees were sampled from one site located in the
Coast Range in western Oregon, USA (44°38′ N, 123°12′ W,
elevation 75 m) designated as Site Class III (McArdle et al.
1961). The stand, consisting of even-aged trees about 60 years
old (Table 1), was thinned in the fall of 2001. At the time of
harvest, the stand had a basal area of 33 m2 ha – 1, with a mean
diameter at breast height (DBH) of 50 cm (range 30–75 cm).
After thinning, a basal disk (30-mm thick) was removed from
each of six freshly cut stumps about 300 mm above the ground
line.
The disks were air-dried in the lab. A radial strip, 50-mm
wide (tangential) by 30-mm thick (longitudinal), was cut from
each disk with a band saw. The heartwood–sapwood boundary
was located by staining with alizarin red (Kutscha and Sachs
1962). Working from the cambium inward, sequential ring
groups, each group comprising three consecutive annual rings,
were separated from the strip with a chisel. Thirteen sets of
rings (39 annual rings in total) were taken from each disk.
The three-annual-ring samples were reduced to powder by
grinding in a ball-type tissue pulverizer mill (Kleco 4100,
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923
Table 1. Size and cambial age of the disks (300 mm above ground).
Tree
label
Disk age
(years)
Sapwood rings
(no.)
Mean radius
(mm)
Sapwood extractives
(% of dry wood)
Heartwood extractives
(% of dry wood)
Sapwood to heartwood
extractives ratio
A
B
C
D
E
F
61
60
60
53
59
62
20
12
19
19
18
16
334
306
303
244
218
219
6.0
7.4
6.9
6.1
5.8
5.7
15.2
14.6
13.7
13.7
12.6
11.7
0.40
0.51
0.51
0.44
0.46
0.49
Garcia Manufacturing, Visalia, CA) for 2 min. Samples of the
wood powder (0.15–0.20 g) were weighed and enclosed in
heat-sealable polyester filter bags (mesh size 25 microns,
ANKOM Technology, Macedon, NY). The bags were extracted with 25 ml per bag of 70% acetone (70:30 (v/v) acetone:water) (Dellus et al. 1997a) for 72 h on a rotary shaker table (100 rpm). The extract solution was condensed in a centrifuge evaporator to remove the acetone. The aqueous extract
solution was frozen and dried under vacuum. The resulting
dried extract was weighed into tin cups for isotope analysis.
Although this extraction technique did not remove all of the
soluble extractives, it was effective in providing sufficient
sample for subsequent analysis.
After extraction, holocellulose was isolated from the wood
powder samples by the method described by Leavitt and
Danzer (1993), which involves delignifying extractive-free
wood powder with sodium chlorite. Any extractives that may
have remained in the wood following the acetone:water extraction were removed by successive extraction steps with toluene:ethanol (2:1, v/v), 95% ethanol and hot water. The samples were delignified in a 70°C sodium chlorite solution acidified with acetic acid to pH ~4.0. Fresh sodium chlorite and
acetic acid were added over time to maintain the pH ~4.0 and a
bright yellow solution color. Delignification was continued
until the pH of the solution was stable (about 4 days). The bags
were rinsed with distilled water and dried. The cellulose residue in the bags was weighed into tin cups for isotope analysis.
Only one extractive and one cellulose isolation were performed for each wood sample.
Extractive and cellulose samples were analyzed for δ13C on
an isotope ratio mass spectrometer (Delta Plus, Finnigan, Bremen, Germany) interfaced with an elemental analyzer (ESC
4010, Costech, Valencia, CA) located at the Integrated Stable
Isotope Research Facility at the Western Ecology Division of
the US Environmental Protection Agency, Corvallis, Oregon.
All δ13C values (‰) were expressed relative to the Vienna
PeeDee belemnite carbonate (VPDB) standard as:
 Rsample 
δ13C = 
1000
 Rstandard − 1 
where R is the ratio of 13C to 12C atoms of the sample and the
standard VPDB. Measurement precision of the spectrometer
was 0.05‰ for δ13C, determined as the mean standard devia-
tion of replicate analyses of standard materials. Repeated measurements of five randomly selected extractive and cellulose
samples had a mean standard deviation of 0.07‰. Replicate
analyses of the wood flour samples were not performed. Values of δ13C of the cellulose and the extractives were compared
over the radial sequence for each tree.
To capture and analyze extractives that might remain after
acetone:water extraction, a follow-up analysis of the same
wood disks was conducted on the outermost 12 growth rings of
sapwood only. The methodology was the same, except that following the acetone:water extraction the re-dried wood samples
were further extracted in a beaker with 75 ml of distilled water
in a water bath at 95 °C for 8 h. Water was added as needed to
compensate for evaporation. After extraction, the aqueous extract was decanted, frozen and dried under vacuum. The hotwater-soluble extractives were analyzed for δ13C as described
above, as was cellulose extracted from the same samples. In
the subsequent data analyses, these samples were considered
separately from the above samples because of circumferential
variations in ring thickness and isotope signature.
Quantitative total extractive content of the sapwood and
heartwood from each tree was determined on adjacent radial
strips of wood (10 mm tangential × 30 mm longitudinal). Samples comprising the entire sapwood, or the number of rings of
heartwood included in the isotope samples, were ground in a
Wiley mill (Arthur Thomas Co, Philadelphia, PA) to pass a
20-mesh screen. Samples of the wood powder (~1.5 g) were
placed in weighed filter bags, oven-dried at 103 °C for 14 h
and reweighed. The bags were extracted according to the
ASTM Standard Method for the Preparation of Extractive-free
Wood D1105-84 (ASTM 1996), which involves successive extraction steps with toluene:ethanol (2:1, v/v), 95% ethanol and
hot water. The sample bags were then oven-dried at 103 °C for
24 h and reweighed. Total extractive content of the heartwood
and sapwood of each tree was determined as the mass lost from
the sample.
Previous chemical analyses of extractives from Douglas-fir
heartwood by acetone:water extraction showed that the extract
is nearly pure dihydroquercetin (Taylor et al. 2003), a biflavonoid that has been shown to be the principle heartwood
extractive in Douglas-fir (Dellus et al. 1997a). Carbohydrates
were not observed in the heartwood extracts and generally are
absent from heartwood (Hillis 1987). For this reason, an additional hot water extraction of the heartwood was not performed. The 13C NMR analysis of sapwood 70% acetone-solu-
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924
TAYLOR, BROOKS, LACHENBRUCH AND MORRELL
ble extracts obtained in this study confirmed the presence of
dihydroquercetin glucoside, procyanidins and pinoresinol
(Foo et al. 1992), which are probably precursors of the later
formed heartwood extractives (Dellus et al. 1997a). Simple
sugars were not a significant component of the sapwood 70%
acetone-soluble extracts. The 13C NMR analysis of the hot-water-soluble sapwood extracts indicated they were mostly carbohydrates such as the simple sugars glucose and xylose. This
is consistent with previous research on Douglas-fir sapwood
(Schowalter and Morrell 2002). These compounds may be part
of the energy reserve materials (starch, free sugars or lipids)
that are thought to exist in sapwood in general (Sjostrom
1993).
For all trees, linear regression models with an autoregressive covariance structure of the relationship between cellulose
and extractives δ13C values were created with the mixed procedure in SAS (Version 9.1.3, SAS Institute Inc., Cary, NC).
Results
The sample trees were relatively uniform in age and sapwood
depth but they varied in radial growth rates (Table 1). Sapwood extractive concentrations (~6% on average) were about
half those of the heartwood extractives (~14% on average) in
all trees.
The difference between the δ13C values of the 70% acetone-soluble extractives and cellulose was relatively consistent, with extractives having a δ13C value about 4–5‰ more
negative than cellulose (Figure 1). Within trees, the δ13C val-
ues of the cellulose varied over a range of about 2‰ over the
years, as represented by sampled rings, but there were no common trends over time (as represented by the growth rings sampled) or among trees. Within trees, the δ13C values of the 70%
acetone-soluble extractives varied over a maximum range of
about 2‰ over time, and the trends appeared to co-vary in
many cases with changes in the cellulose signal (Figure 1). Regressions between cellulose and 70% acetone-soluble extractive isotope ratios were not determined at the within-tree level
because sample sizes were small and highly autocorrelated.
Values of δ13C in cellulose and in 70% acetone-soluble extractives were moderately correlated when data were pooled
from the six trees for either sapwood or heartwood, with slopes
of 0.26 and 0.29, respectively (Table 2). In both cases, the
slopes of the regression lines were significantly less than 1
(P < 0.01), but significantly greater than 0 (P = 0.05) after accounting for autocorrelation. Cellulose and hot-water-soluble
extractives from the sapwood samples taken from different locations in the same trees were also significantly correlated
(Figure 2), but the slope of the regression equation (0.81) did
not differ significantly from 1 (P = 0.50).
Discussion
The observed variation in δ13C of cellulose among annual
rings likely reflects changes in the ratio of photosynthesis to
stomatal conductance (A/g ratio) that occurred over time
(Francey and Farquhar 1982). However, the annual pattern between trees was quite different, indicating that climate may not
Figure 1. Cellulose (䊏) and wood
70% acetone-soluble extractive
(䊊) δ13C values versus year of
wood formation for samples from
each of six Douglas-fir trees.
Disks are arranged in descending
order of diameter, reflecting radial
growth rate. Arrows indicate the
location of the heartwood–sapwood boundary at the time of harvest.
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925
Table 2. Generalized least squares estimates of the slopes from linear regression models with an autoregressive covariance structure of extractives
δ13C values on cellulose δ13C values for all trees together.
Extractive type
r2
Tissue
Slope
Standard error Mean square
of slope
error
P value
Slope = 0
Slope = 1
70% Acetone-soluble
Sapwood
Heartwood
0.33
0.34
0.26
0.29
0.13
0.10
0.46
0.33
0.05
0.01
< 0.01
< 0.01
Hot-water-soluble
Sapwood
0.55
0.81
0.28
0.31
0.01
0.50
have driven the variation in the A/g ratio. A pre-harvest examination of the trees was not made, so the reasons for the distinct
isotopic signatures of these Douglas-fir trees are unclear. The
trees grew on the same site and were of similar age with diameters ranging from 44 to 67 cm. Variations in the diameter of
the trees may reflect differences in vigor related to crown position, micro-site or other stand dynamic factors that also influenced A/g.
Differences in the stable isotope values among various
wood components within a year (extractives and cellulose in
this study) can reflect differences in the fractionation that takes
place in the biochemical synthesis pathways, or differences in
photosynthetic production over the growing season that reflect
climatic influences on A/g (Gleixner et al. 1993, Barbour et al.
2001, McCarroll and Loader 2004), or both. The finding that
there was no significant change in the offset between cellulose
and 70% acetone-soluble extractives in the sapwood compared
with the heartwood (P = 0.15 from a two-sample t-test) suggests that little fractionation occurs when sapwood extractives
are used as a raw material for the synthesis of heartwood
extractives.
The covariation of δ13C in extractives and cellulose (Figures 1 and 2) suggests that some of these sapwood extractives
remain in Douglas-fir annual rings for many years. The sapwood of these trees averaged 16 annual rings (Table 1). In particular, the hot-water-soluble extractives (sugars) had a nearly
1 to 1 correlation with cellulose (slope = 0.81, r 2 = 0.55), indicating that some of these extractives were likely formed during
the same year as the cellulose and stored in the sapwood for, in
the case of these trees, up to 16 years.
The occurrence of persistent, but potentially mobile, substances in wood rings over many years could be a factor to consider when modeling tree ring development based on isotopic
data. These extractives may be available as a carbon source for
Figure 2. Sapwood cellulose (䊏)
and hot-water-soluble extractive
(䊊) δ13C values versus year of
wood formation for the outermost
12 growth rings from each of six
Douglas-fir trees. Disks are arranged in descending order of diameter, reflecting radial growth
rate.
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TAYLOR, BROOKS, LACHENBRUCH AND MORRELL
(and thus could influence the isotopic ratio of) various tissues
formed in the years after the extractives were first formed. This
stored reserve could be a source of “error” in relationships between the carbon isotope ratios of wood components and environmental factors affecting photosynthesis. For example, the
use of persistent, “old” sapwood extractives in new wood formation may be a contributing factor to the autocorrelation that
has been observed among adjacent rings (Monserud and Marshall 2001). However, if these extractives were being used over
time by new growth, we would expect to see a decrease in extractives with age of the sapwood, but our data and previous
work with Douglas-fir and other species (Hoch et al. 2003,
Taylor et al. 2003) show no such decline. We therefore speculate that, in Douglas-fir, some sapwood extractives are persistent and serve as precursors in the formation of heartwood extractives. This is in contrast to other studies showing that, depending on species, sapwood extractives are either depleted
and restored annually (e.g., Sauter and van Cleve 1994,
Barbaroux and Bréda 2002), or gradually decrease in concentration with distance from the cambium (e.g., Saranpaa and
Holl 1989, Magel et al. 1994). Magel (2000) describes two
types of heartwood formation: a Robinia-type, in which phenolic materials are absent from the sapwood, and a
Juglans-type, in which phenolic, heartwood extractive precursors are found in the sapwood; however, the latter grouping is
based on work with two Juglans species (Burtin et al. 1998)
and Douglas-fir (Dellus et al. 1997b).
Extractives in a senescing sapwood ring are believed to be
raw materials for the formation of heartwood extractives
(Hillis 1987, Dellus et al. 1997a, 1997b). This is supported by
the finding that the δ13C values of the heartwood extractives in
our study trees covaried with the cellulose δ13C values in a way
that paralleled the correlation between sapwood extractive and
cellulose δ13C values. The slopes of these relationships between extractives and cellulose were similar for sapwood and
heartwood (0.26 versus 0.29), further supporting the idea that
sapwood extractives are converted to heartwood extractives.
These data also suggest that the contribution of extractive
compounds imported to newly formed heartwood from elsewhere in the tree is important because the correlation between
heartwood extractive δ13C values and those of cellulose had
slopes of less than 1 for heartwood, and because the concentration of heartwood extractives is twice that of sapwood extractives. Hillis and Hasegawa (1963) showed that raw materials
for heartwood extractive formation could be translocated radially. They injected labeled glucose into the phloem of Eucalyptus sieberiana F. Muell. trees and detected the label in
heartwood extractives a few weeks later. Such imported photosynthate, whether from the phloem, outer sapwood or other
storage locations, would be expected to have a different stable
isotope signature than the reserve compounds in the inner sapwood because it is formed in a different year.
One way to examine the importance of imported photosynthate to heartwood extractive formation is to compare the
most recent ring of sapwood cellulose δ13C with the most recent ring of heartwood extractives δ13C values, and to repeat
this comparison in the adjacent rings of sapwood and heart-
wood of increasing age. We found no significant positive relationship of this type between sapwood cellulose and heartwood extractives within trees (simple linear regression
P > 0.10, data not shown).
The wood samples we studied were from a separate investigation of the relationship of growth rate and extractive content
(Taylor et al. 2003). In that study, the heartwood extractive
concentration in a given ring appeared to covary with the radial growth during the year that the heartwood ring was
formed, suggesting that heartwood formation in a given year
(annual ring) is affected by the current physiological status of
the tree. Moreover, the heartwood extractive concentration
was roughly double that of the sapwood (Table 1), implying
that materials must be imported at the time of heartwood formation. However, in our study, the covariation in δ13C of extractives and cellulose within the same annual ring suggests
that the physiology of the tree as a wood ring forms affects
heartwood formation many years later when that ring is converted from sapwood to heartwood. Such complexity in the relationship between heartwood extractives and tree physiology
suggests that silvicultural manipulation of heartwood durability will not be simple.
In conclusion, variations in δ13C values of the cellulose of
Douglas-fir coincided with similar shifts in the δ13C values of
extractives located in the same ring, both in the sapwood and
heartwood. These data suggest that some sapwood extractives
persist within annual rings for many years, and that these substances are sources of heartwood extractives when the sapwood rings are eventually converted to heartwood. To the extent that tree physiology influences sapwood extractives, it
may also affect heartwood extractive formation, and thus the
natural durability of the wood. However, our data suggest that
this influence may be delayed for years. The persistence, and
ultimate fate, of sapwood extractives is also a factor that
should be considered in tree ring isotope analyses.
Acknowledgments and notes
Our thanks to Dr. Joe Karchesy (OSU College of Forestry) for useful
discussions of extractive analysis, to Dr. Mike Newton (OSU College
of Forestry) for advice and wood samples and to Bill Griffis (EPA Integrated Stable Isotope Research Facility) for processing the isotope
samples. We also thank Dr. Steve Leavitt who reviewed an earlier version of this manuscript. This work was supported by a USDA Special
Grant to Oregon State University for Wood Utilization Research, by a
Fellowship from the Weyerhaeuser Foundation and by the United
States Environmental Protection Agency. This manuscript has been
subjected to EPA peer and administrative review and has been approved for publication as an EPA document. Mention of trade names
or commercial products does not constitute endorsement or
recommendation for use.
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