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IAWA Journal, Vol. 26 (2), 2005: 211220

EFFECTS OF HEARTWOOD EXTRACTIVES ON

MECHANICAL PROPERTIES OF LARCH

Michael Grabner1, Ulrich Mller1, Notburga Gierlinger2& Rupert Wimmer1

SUMMARY

The genusLarixis exceptional for its high content of extractives in the

heartwood, with the dominant component arabinogalactan found abun-

dantly in cell lumens of tracheids. On parallel samples prepared from

20 European, Japanese and hybrid larch trees (Larix decidua Mill., L.

kaempferi Carr., andL. decidua L. kaempferi, respectively) extractive

contents and mechanical parameters were measured. The hot-water ex-tractives in the heartwood had a signicant effect on transversal compres-

sion strength and Youngs Modulus. In heartwood, increasing extractive

content went hand-in-hand with better mechanical properties in the trans-

verse direction. The extraction procedure led to negligible changes in the

sapwood. Anatomically the extractive-lled tracheids showed a tendency

of being arranged radially, closely to wood rays. The extractive arabi-

nogalactan in larch heartwood has multiple effects on different aspects

of wood quality, among which is lateral mechanical enforcement.

Key words:Extractive, arabinogalactan, compression strength, stiffness,heartwood,Larix, biomechanics.

INTRODUCTION

Besides the cell wall macromolecules cellulose, polyoses and lignin numerous compo-

nents are categorized as accessory or extractive material of wood. The term extractive

commonly refers to the organic compounds only (Fengel & Wegener 1989), although it

can also refer to inorganic materials. An important aspect of extractives is their solubil-

ity, and with regard to analysis it is useful to distinguish between substances that are

soluble in water or in organic solvents.

Wood extractives may darken the heartwood and give species such as those within

the genusLarix their characteristic colour. Macro- and micro-distribution of extractives

within the tree stem vary greatly, with changes radially and vertically on the one hand

(DeBell et al.1999; Gartner et al.1999; Gierlinger & Wimmer 2004), and different cell

locations (Hillis 1971) and positions within earlywood and latewood (Ct et al.1966)

on the other.

1) BOKU University of Natural Resources and Applied Life Sciences Vienna, Department of

Material Sciences and Process Engineering, Peter Jordan Strasse 82, A-1190 Vienna, Austria.Corresponding author: M. Grabner [[email protected]].

2) Max Planck Institute of Colloids and Interfaces, Department of Biomaterials, D-14424 Pots-

dam, Germany.

Associate Editor: Barbara Gartner


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IAWA Journal, Vol. 26 (2), 2005212

While the role of heartwood extractives for natural durability has received consid-

erable study, possible effects of extractives on the mechanical performance of wood

have been very little documented (Taylor et al.2002). Any signicant differences in

mechanical performance between sapwood and heartwood are usually attributed to the

radial changes in wood density or anatomical structure and not to whether the sample

is heartwood or sapwood,per se(Panshin & DeZeeuw 1980). Among the rare reports,

Arganbright (1971), Kuo and Arganbright (1980) and Grabner (2002) presented evi-

dence for a direct inuence of extractives on the modulus of rupture and the modulus

of elasticity, in addition to their effect on wood density.

The genusLarixis known for its exceptionally high content of extractable compo-

nents in the heartwood, with the dominating component arabinogalactan, usually within

the wide range of 530% (Ct et al.1966; Fengel & Wegener 1989; Dix & Roffael

1994; Gierlinger et al.2002a). Besides other smaller components, up to 3.5% of avo-noids can also be found in the larch heartwood (Babkin et al.2001). Arabinogalactans

are water-soluble and heavily branched polysaccharides, they belong chemically to the

hemicelluloses, and they are present in all softwood species at levels no more than 1%,

exceptingLarix(Ct et al.1966; Karcsonyi et al.1984; Willfr et al.2002). Un-

like all other hemicelluloses, at least 90% of the arabinogalactans in larch are located

outside the cell wall (Ct et al.1966; Sjstrm & Westermark 1999), primarily lling

the lumens of tracheids that are close to ray cells. Extractive contents in larch gradually

increase from pith to bark, reaching the highest contents at the heartwood/sapwood

boundary, followed by an immediate drop to almost zero in the adjacent sapwood (Ctet al.1966; Hillis 1971; Gierlinger & Wimmer 2004).

Because larch has such a high heartwood extractive content the question of interac-

tions with mechanical performance becomes prominent. This study presents data from

axial and transversal compression tests obtained with small, clear samples, located at

either side of the heartwood / sapwood boundary, unextracted and extracted. The objec-

tive was to explore a possible biomechanical role of extractives in larch heartwood.

MATERIAL AND METHODS

The larch wood investigated in this work originated from the European funded project

Towards a European Larch Wood Chain (FAIR CT98-3354; Gierlinger et al.2002b;

Grabner 2002). Wood was sampled from sites that were part of the second IUFRO

provenance trial established around 1960 (Schober 1985). In total, 20 larch trees (L.

deciduaMill.,L. kaempferiCarr.,L. deciduaL. kaempferi)were selected from the

project sample collection encompassing 300 trees grown across Europe (see Table 1 for

details). The selected trees originated from young plantations as well as old-growth high

elevation alpine sites.

Trees were harvested in 1999, and 80 samples were obtained from quartersawn

boards. From the air-dried boards samples were split off at either side of the heartwood /sapwood boundary (40 in sapwood, 40 in heartwood) using a wedge-shaped blade that

guaranteed zero grain deviation. Beams with a cross section of 7 7 mm and an axial

length of 120 mm were machined with the wood rays strictly parallel to the edges.

From each sample wood particles were collected during trimming and spectra were


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Special care was taken that samples had no deformation prior to the compression tests.

Compression tests (both axial and loaded in the radial direction) were performed at

constant displacement rates on the samples using a Zwick/Roell Z100/SW5A uni-

versal testing machine. Sample deformation was monitored through the movement of

the crosshead, and stress/strain curves were computed during the tests. From these

curves we calculated maximum crushing strength (MCS) and Youngs modulus (MOE).

To analyse mechanisms during loading perpendicular to grain the crosshead was halted

when the force dropped more than 5% from its maximum. This allowed an observation

of single earlywood tracheid failures, as shown by Mller et al.(2003).

Extracted and unextracted samples showing apparent post-compressive deformation

were imaged with scanning electron microscopy (SEM). The SEM samples were gold

sputtered at 1 kV and 20 mA before placement in a Philips, XL30 ESEM, at lament

voltages between 10 and 15 kV.

RESULTS

Overall, contents of total extractable substances in heartwood ranged between 4.0 and28.5% dry weight, and in sapwood between 0.6 and 4.4%. By far the highest contribu-

tion was given by the hot-water extractives, which varied between 3.3 and 25.1%.

Figure 1 shows X-ray graphs of a sample prior and post extraction. In Figure 1A

extractive-lled tracheids are clearly seen. It is also seen that the lled tracheids have

the tendency to be radially arranged along the wood rays, beside of being randomly

distributed across the annual ring (Fig. 1 & 4). After hot-water extraction all substances

located in the tracheid lumens have disappeared (Fig. 1B).

Two-way ANOVAs were performed with position (sapwood or heartwood) and the

extraction treatment (unextracted or extracted) as the main effects (Table 2, Fig. 2) fortransverse and axial MCS and MOE. Most variance was explained for transverse MCS

(R2=0.55), followed by transverse MOE (R2=0.33) in a second ANOVA. The ANOVAs

for MCS in the axial direction had a considerably lower degree of explanation (R2=

0.18), and no variance was explained for axial MOE (Table 1).

A B

Fig.1. X-ray densitometry images of (A) unextracted and (B) hot-water/ acetone extracted larch

cross sections.


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215Grabner et al.Mechanical properties of Larch

Table 2. Two-way ANOVAfor transversal and axial MCSand MOEas dependent variables,

with position (heartwood or sapwood) and treatment (unextracted or extracted) as main

effects (df = degree of freedom).

Dependent Variable: transversal MCS (maximum crushing strength)

Source Type III Sum of Squares df Mean Square F Sig.

Corrected Model 702.7 3 234.2 31.4 0.00

Intercept 3620.1 1 3620.1 485.8 0.00

Position 295.6 1 295.6 39.7 0.00

Treatment 248.7 1 248.7 33.4 0.00

Position x Treatment 138.7 1 138.7 18.6 0.00

Error 566.3 76 7.6

Total 5002.7 80

Corrected Total 1269.0 79

Adjusted R2= 0.54

Dependent Variable: transversal MOE (modulus of elasticity)


Corrected Model 871484.3 3 290494.8 13.7 0.00

Intercept 7876201.1 1 7876201.1 371.3 0.00

Position 286440.6 1 286440.6 13.5 0.00

Treatment 460703.5 1 460703.5 21.7 0.00


Error 1612188.7 76 21213.0

Total 10530910.2 80Corrected Total 2483673.0 79

Adjusted R2= 0.33

Dependent Variable: axial MCS (maximum crushing strength)


Corrected Model 2733.5 3 911.2 7.22 0.00

Intercept 260319.7 1 260319.7 2062.75 0.00

Position 889.6 1 889.6 7.04 0.10

Treatment 1132.3 1 1132.3 8.97 0.04


Error 10348.4 82 126.2Total 280569.3 86


Adjusted R2= 0.18

Dependent Variable: axial MOE (modulus of elasticity)


Corrected Model 48234.2 3 16078.1 0.25 0.86

Intercept 91733053.9 1 91733053.9 1399.70 0.00

Position 0.0 1 0.0 0.00 0.99

Treatment 30591.1 1 30591.1 0.47 0.50 Position x Treatment 23184.3 1 23184.3 0.35 0.55

Error 5374109.4 82 65537.9

Total 98412759.9 86


Adjusted R2= 0.01


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An interaction effect is a change in the main effect of one variable over levels of the

second. Position treatment interaction was signicant for transverse MCS and MOE.

This means that the position treatment interaction is a change in the main effect of treat-

A B

A B

Fig. 4. Cross-sectional SEM image of a high extractive sample (unextracted) after transversal (ra-

dial) load. (A) Continuous rows of lled tracheids (arrow) that served as mechanical reinforce-

ment. (B) Buckling of the empty tracheids initiated transverse compression failure (arrows),

which limited transverse compression strength.

Fig. 2. Estimated marginal means for transverse maximum crushing strength (MCS) (A) and

Youngs modulus (MOE) (B) of adjacent larch sapwood and heartwood samples, unextracted and

extracted (n = 80).

Fig. 3. SEM micrographs of earlywood tracheids containing arabinogalactan before the transverse

compression load was applied (A). Differences in plastic deformation of the wood cell wall versus

the more ductile arabinogalactan (B) are visible.

14 -

12 -

10 -

8 -

6 -

4 -

2 -

0 - unextracted extracted unextracted extracted

Sapwood Heartwood

TransversalMCS

(N/mm2)

A

TransversalMOE

(N/mm2)

600 -

500 -

400 -

300 -

200 -

100 -

0 - unextracted extracted unextracted extracted

Sapwood Heartwood

B


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ment over levels of position, or vice versa. Sapwood extraction had a minor effect on

mechanical properties, while in heartwood the effect was great (Fig.2). This afrms

that removal of high extractive contents ceteris paribus had a signicant consequenceon transversal MCS and MOE, while the extraction procedure itself had little impact.

We also looked at anatomical evidence of this nding. Figure 3 shows an SEM of ex-

tractive-lled earlywood tracheids. Prior to transverse compression loading (Fig. 3A)

the interfaces between cell walls and extractive substances were barely visible and ap-

peared more or less as a continuum. As transverse forces deformed the samples (halted

as force dropped 5% from its maximum), the ductile hot-water extractives lling the

lumens separated from the more visco-elastic cell walls (Fig. 3B).

Heartwood samples were arranged by their original extractive contents and plotted

along with the corresponding transversal MCS and MOE data (Fig. 5). Although trends

are clearly seen, the mechanical data showed considerable scatter with increasing ex-

tractive content.

25

20

15

10

5

0

1000

800

600

400

200

0 0 5 10 15 20 25 30

Total extractive content (%)

0 5 10 15 20 25 30

Total extractive content (%)

YoungsModulus(MP

a)

tran

s.

(MPa)

A

B

Fig. 5. Compression strength (A) and Youngs modulus (B) of heartwood samples and initial

total extractive content. $= unextracted and O= extracted values; := differences between

unextracted and extracted samples (n = 20).


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DISCUSSION

The hot-water extractives in the larch heartwood have a signicant effect on transverse

maximum crushing strength (MCS) and Youngs modulus (MOE). Increasing extractivecontent goes hand-in-hand with better mechanical properties in the transverse direc-

tion, while the same extraction procedure had minor changes in the sapwood. These re-

sults were seen primarily in the transverse direction, the mechanical properties in axial

direction did not show changes of the same magnitude.

Larch heartwood contains high amounts of extractives (Dix & Roffael 1997) and

the major part is attributed to arabinogalactan, a water-soluble and heavily branched

polysaccharide, usually found within similar ranges as those reported here (530% dry

weight, Ct et al.1966). The high amounts of arabinogalactan are specic to larch,

mainly located in the cell lumen, and their direct role is still unclear (Ct et al.1966).In brown-rot decay it may enhance fungal growth by being a nutrient resource (Srini-

vasan 1999). On the other hand it could also serve as a mechanical barrier for fungal

hyphae penetrating the wood structure laterally. The current work has shown that water-

soluble extractives, i.e. arabinogalactans, act as a mechanical enforcement in the wood

structure laterally. This might go hand-in-hand with a mechanical barrier function for

fungi penetrating wood laterally. Besides arabinogalactan, up to 3.5% avonoids are

found in larch heartwood (Hegnauer 1962; Giwa & Swan 1975; Babkin et al.2001),

which plays a direct role in decay resistance.

Anatomically the extractive-lled tracheids showed the tendency to be arranged radi-ally with wood rays. Similar results were reported by Kuo and Arganbright (1980), who

related presence and degree of extractive deposits in tracheids with respect to alignment

and distance to ray parenchyma. Ct et al.(1966) suggested that arabinogalactans are

formed in the living ray cells at the sapwoodheartwood boundary, similar to other ex-

tractives. From the ray cells arabinogalactan penetrates into the tracheids, which explains

the higher likelihood of lled tracheids being close to wood rays.

In the lateral direction high spread of MCS and MOE values were observed with

increasing extractive content, which is associated with the radially oriented, extractive-

lled tracheids rows: the rows of extractive-lled tracheids are frequently interruptedby single empty tracheids, which limit compression strength by the critical buckling

load of their cell walls (Fig. 4). As a consequence, MCS of softwoods perpendicular

to grain is determined by the critical stress of these empty cell walls (Kollmann 1959,

1982; Mller et al.2003), a fact that seems to be more prone in smaller samples due

to the stochastic occurrence of uninterrupted radial, extractive-lled tracheids rows.

Buckling of the weakest cell walls initiates transversal compression failure (Mller et

al.2003), while samples with uninterrupted lled-tracheid rows may have signicantly

higher compression strength in the transverse direction.

Because arabinogalactan is more deposited in earlywood than latewood (Ct et al.1966), the reinforcement effect of the wood ray accompanying lled tracheid rows is

assumed to be higher in the initial tree-ring portion. Mechanical loading perpendicular

to grain in the radial direction and elastic deformation of the tissue (bumping out, start

of buckling) are relevant for Youngs modulus. Because of the lled tracheids, mechani-


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cal stresses and the likelihood of buckling are reduced, resulting in higher values of

Youngs modulus.

The available samples represented a range of hot-water extractive contents in larchheartwood. In practical saw-milling, larch is known to be cumbersome, as the resin

(i.e. arabinogalacatan) tends to built up on saw-blades causing loss of straightness of

the sawn boards. As a common strategy the blades are water-sprinkled during sawing,

reducing the negative effects. This serves as more evidence that arabinogalactan in

larch has multiple effects on wood quality, including workability, glueability, drying

behaviour, and warping.

ACKNOWLEDGEMENTS

This study is in part funded by the European Union, Fair project CT98- 3354 Towards a EuropeanLarch Wood Chain, coordinated by Dr. Luc Paques, INRA.

REFERENCES

Arganbright, D.G. 1971. Inuence of extractives on bending strength of Redwood (Sequoia

sempervirens). Wood Fiber Sci. 2: 367372.

Babkin, V.A., L.A. Ostroukhova, Y.A. Malkov, D.V. Babkin, N. A. Onuchina & S.Z. Ivanova.

2001. Isolation of biologically active compounds from larch wood. 11th ISWPC, International

Symposium on Wood and Pulping Chemistry, Nice: 119122.

Ct, W.A., A.C. Day Jr., B.W. Simson & T.E. Timell. 1966. Studies on larch arabinogalactan.

I. The distribution of arabinogalactan in larch wood. Holzforschung 20: 178192.

DeBell, J.D., J. J. Morrell & B.L. Gartner. 1999. Within-stem variation in tropolone content and

decay resistance of second-growth western redcedar. Forest Science 45: 101107.

Dix, B. & E. Roffael. 1994. Extraktstoffgehalt von Lrchensplint- und -kernholz. Holz Roh.

Werkst. 52: 336.

Fengel, D. & G. Wegener. 1989. Wood. Chemistry. Ultrastructure. Reaction. De Gruyter, Berlin,

New York.

Gartner, B.L., J. J. Morrell, C.M. Freitag & R. Spicer. 1999. Heartwood decay resistance by ver-

tical and radial position in Douglas-r trees from a young stand. Can. J. For. Res. 29: 1993

1996.

Gierlinger, N., S. Rosner, W. Gindl, M. Grabner, R. Wimmer, M. Schwanninger & L.E. Paques.2002b. Wood anatomical and chemical characteristics of different larch wood resources in

Europe. In : Improvement of Larch (Larix sp.) for better growth, stem form and wood qual-

ity, Proceedings, Gap, September 16-21: 388395.

Gierlinger, N., M. Schwanninger, B. Hinterstoisser & R. Wimmer. 2002a. Rapid determination

of heartwood extractives in Larix sp. by means of FT-NIR. J. Near Infrared Spec. 10: 203

214.

Gierlinger, N. & R. Wimmer. 2004. Radial trends of heartwood extractives and lignin in mature

European larch. Wood Fiber Sci. 36: 387394.

Giwa, S.A.O. & E.P. Swan. 1975. Heartwood extractives of a western larch tree (Larix occiden-

talis Nutt.). Wood Fiber 7: 216221.

Grabner, M. 2002. Relationships among wood quality indicators of Larch wood grown in Europe.

Diploma thesis, Universitt fr Bodenkultur Vienna, Austria.

Hegnauer, R. 1962. Chemotaxonomie der Panzen. Vol.1. Birkhuser Verlag, Basel, Stuttgart.

Hillis, W.E. 1971. Distribution, properties and formation of some wood extractives. Wood Sci.

Technol. 5: 272289.


10/10


Kollmann, F. 1959. Zur Frage der Querdruckfestigkeit von Holz. Holzforsch. Holzverw. 11: 109

121.

Kollmann, F. 1982. Technologie des Holzes und der Holzwerkstoffe. Springer-Verlag, Berlin,

Heidelberg, New York.

Kuo, M.L. & D.G. Arganbright. 1980. Cellular distribution of extractives in redwood and incense

cedar. Part I. Radial variation in cell-wall extractive content. Holzforschung 34: 1722.

Karcsonyi, S., V. Kovacik, J. Alfldi & M. Kubackova. 1984. Chemical and 13C-N.M.R. studies

of an arabinogalactan from Larix sibirica L. Carbohydr. Res. 134: 265274.

Lenz, O., E. Schr & F.H. Schweingruber. 1976. Methodische Probleme bei der radiographisch-

densitometrischen Bestimmung der Dichte und der Jahrringbreiten von Holz. Holzforschung

30: 114123.

Mller, U., W. Gindl, & A. Teischinger. 2003. Effects of cell anatomy on the plastic and elastic

behaviour of different wood species loaded perpendicular to grain. IAWA J. 24: 117128.

Panshin, A.J. & C. DeZeeuw. 1980. Textbook of wood technology. McGraw-Hill, Toronto,Ontario, Canada.

Schober, R. 1985. Neue Ergebnisse des II. Internationalen Lrchenprovenienzversuches von

1958/59 nach Aufnahmen von Teilversuchen in 11 europischen Lndern und den USA.

Schriftr. Forstlichen Fak.t Univ. Gttingen.

Sjstrm, E. & U. Westermark. 1999. Chemical composition of wood and pulps: basic constitu-

ents and their distribution. In: E. Sjstrm & R. Aln (eds.), Analytical methods in wood

chemistry, pulping and papermaking: 119. Springer Series in Wood Science.

Srinisvan, U., T. Ung, A. Taylor & P.A. Cooper. 1999. Natural durability and waterborne treat-

ability of tamarack. For. Prod. J. 49: 8287.

Taylor, A.M., B.L. Gartner & J. J. Morrell. 2002. Heartwood formation and natural durability A review. Wood Fiber Sci. 34: 587611.

Willfr, S., R. Sjholm, C. Laine & B. Holmbom. 2002. Structural features of water-soluble

arabinogalactans from Norway spruce and Scots pine heartwood. Wood Sci. Technol. 36:

101110.

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