ENVIRONMENTALLY INDUCED PHYSICAL CHANGES IN ANCIENT KAURI ( AUSTRALIS) Cassia Freedland Graduate Student1 Department of Forestry University of Wisconsin-Madison Madison, WI 53706 Roger M. Rowell Research Chemist USDA Forest Service Forest Products Laboratory2 One Gifford Pinchot Drive Madison, WI 53705-2398

and David Plackett Research Scientist Forest Research Institute Rotorua, New Zealand (Received February 1993)

ABSTRACT

The physical properties of 30,000-year-old and modem kauri (Agathis australis) were explored. The acidic burial environment had modified the ancient wood cell-wall structure, affecting strength prop- erties. Degradation of labile cell-wall polysaccharides resulted in strength differences in the ancient kauri both below and above the proportional limit (PL) of the wood. Significantly lower modulus of elasticity (MOE) and work to proportional limit (WPL) values, as compared with modem kauri, indicated that the ancient material was more readily deformed. Above the PL, the modulus of rupture (MOR) of the ancient wood was significantly lower and the work to maximum load (WML) significantly higher than the corresponding values for recently felled kauri, suggesting that the ancient material did not resist stress; rather, the structure gradually “gave way” under imposed stress. Scanning electron micrographs (SEM) of the fractured ancient kauri surfaces reveal degradation of the woody cell wall. Keywords: Agathis australis, ancient wood, buried wood, carbohydrates, lignin, scanning electron microscopy (SEM), strength.

INTRODUCTION material. Wood is chemically and structurally Ancient wood, recovered in a variety of deg- complex, and this complexity is compounded radational states, serves as a unique research by the diverse ways in which ancient wood can be changed by its environment over time. 1 Present address: Lecturer, Office of Environmental Chemical and physical changes in ancient wood Health and Safety, University of Virginia, Charlottesville, have been explored (Waksman and Stevens VA 22903. 1929; Mitchell and Ritter 1934; Gortner 1938; 2 The Forest Products Laboratory is maintained in co- Varossieau and Breger 1952; Kohara 1953, operation with the University of Wisconsin. This article was written and prepared by U.S. Government employees 1954, 1956; Wayman et al. 1971; Winandy on official time, and it is therefore in the public domain 1984; Hedges et al. 1985; Jagels et al. 1988). and not subject to copyright. One way to quantify physical changes in wood

Wood and Fiber Science, 26(1), 1994, pp. 5l-61 52 WOOD AND FIBER SCIENCE, JANUARY 1994, V. 26(1) is through standardized mechanical strength above the proportional limit (PL) has yielded tests. Unfortunately, there is great difficulty in more predictable results. Degraded ancient assessing the strength properties of ancient wood will often show lowered strength prop- wood. Because ancient wood is rarely found erties, translating to lowered MOR and work in either large quantities or in sound shape, to proportional limit (WPL) values (Winandy structural studies of ancient material are often 1984; Jagels et al. 1988). restricted to micrographs of selected cells Mechanical tests alone give an incomplete (Blanchette et al. 1990). If mechanical tests are picture of the extent of wood alteration over conducted, both test specimens and sample time. Changes in the chemical bonding char- population sizes are usually small (Winandy acteristics of wood are intricately linked to 1984; Jagels et al. 1988). physical disruption of cellular structure. Cel- The comparison of the mechanical proper- lular structure, in turn, influences the ability ties of ancient wood to the same species, re- for the composite to effectively transfer stress- cently felled counterpart can provide a picture es under load. Therefore, alteration of the of environmentally induced structural changes chemical and physical properties of wood will that have occurred over time. Severe alteration affect the strength properties of the composite. of wood structure by decay fungi will drasti- Strength is provided by the basic structural cally alter mechanical strength (USDA Forest units of the cell wall, the microfibrils. Losses Service, Forest Products Laboratory 1987). of hemicelluloses, which are thought to act as However, bacterial degradation will not al- packing material in the cell wall, are associated ways result in decreased strength properties with losses in strength (Schniewind 1964). The (Winandy 1984). Therefore, to elicit meaning- lignin-rich middle lamella serves as the cellular ful mechanical test results from scarce ancient glue, binding adjacent tracheids together. Deg- wood samples, it is essential to initially choose radation of lignin would result in loss of cel- an informative testing method. Previous work lular organization and decreased structural ri- with ancient wood has shown static bending gidity. tests to be a good overall test of strength changes Density measurements can also be used as over time (Jagels et al. 1988). a predictor of wood strength. Increasing spe- The behavior of ancient degraded wood un- cific gravity values correlate with increased der load cannot be easily categorized, as the strength properties (Winandy and Rowe11 type and extent of degradation will influence 1984), and a more dense material will be ex- mechanical strength. Values for the modulus pected to show the property of increased stiff- of elasticity (MOE), a measure of a material’s ness. For example, Kohara (1954) found that ability to withstand deformation under load, ancient cypress wood possessed a higher spe- typify such variation in ancient material. Ja- cific gravity than the modern same-species gels and others (1988) noted a loss of stiffness, or increased plasticity, in their wet and highly counterpart, and subsequent strength tests re- degraded wood. Kohara’s (1954) study of vealed that the ancient material was indeed 1,300-year-old cypress (Chamaecyparis obtu- stiffer than recently felled cypress. With de- sa) samples indicated that the MOE was higher terioration of wood structure, the density and for the ancient samples, as compared with strength properties of wood should corre- modern cypress. Rather than becoming more spondingly decrease. However, strength losses plastic over time, the ancient cypress was hard, are not always mirrored by density changes strong, and brittle. Winandy (1984) noted that (Schniewind 1990). If enzymatic attack re- 50-year-old live oak (Quercus virginiana) sam- sulted in depolymerization of cellulose, the en- ples, previously submerged and infected by suing strength loss might not be detected by bacteria, exhibited equivalent MOE values as density changes. Seifert and Jagels (1985) found compared to newer live oak. Behavior of wood specific gravity to be a poor predictor of wa- Freedland et al. -ANCIENT KAURI 53 terlogged ancient wood strength due to wide polystyrene standards. The Mark-Houwink density fluctuations. coefficients were K = 0.0053, a = 0.84. The To explore the interaction of the chemical methoxyl content of isolated Klason lignin was and physical changes in wood over time, determined by Galbraith Laboratories, Knox- 30,000-year-old and recently felled kauri wood ville, Tennessee. (Agathis australis) from New Zealand were used One hundred precut samples (2.5 cm × 2.5 in this study as test specimens. Preserved for- cm × 30.5 cm) of both ancient and recently ests of ancient kauri have been recovered from felled kauri (Agathis australis) were received acidic swamp environments in , at the Forest Products Laboratory in an air- New Zealand. Modem kauri forests can be dried state. Visible collapse of the tangential found in the same ecological space as the an- surfaces of the ancient samples indicated that cient material. Chemical and mechanical test- the material had been wet at the time of cut- ing was facilitated by the availability of large ting. All samples were acclimated under the quantities of both the ancient and recently felled identical environmental conditions (73 C, 50% kauri. Using recently felled kauri as a basis for RH) at the Forest Products Laboratory. The comparison, changes in the chemical compo- samples were rotated regularly to promote even sition of the ancient kauri were characterized conditioning. After 90 days, the samples through extensive chemical analyses (Freed- showed little loss in weight, and were deter- land et al. in press). Through static bending mined to be stable. The moisture content of tests and spectroscopic studies of cut and frac- each sample was calculated using the condi- tured cell surfaces, the interaction of chemical tioned weight at test and the oven-dry weight change and structural alteration over time could after test. be elucidated. For density measurements, the wood was cut into small samples and the volume mea- MATERIALS AND METHODS sured before and after drying according to Samples of recently felled kauri (Agathis ASTM D2395-83 (American Society for Test- australis) and 30,000-year-old kauri were ob- ing and Materials 1986). Density measure- tained from swampy regions around Kaitaia ments were calculated on an oven-dried basis, and Herekino, North Island by the New Zea- following two days of oven-drying at 103 C. land Forest Research Institute, Rotorua, New Clear samples of ancient and recently felled Zealand. Determination of ash content, pH, kauri wood were subjected to a center-point acetyl content, monosaccharides after acid hy- load static bending test as defined by ASTM drolysis, acid soluble lignin, and Klason lignin D- 143 (American Society for Testing and Ma- were conducted according to the established terials 1986). The specimens, 71 ancient kauri Forest Products Laboratory procedures (Eff- and 61 recently felled kauri samples, were cut land 1977; Pettersen and Schwandt 1991). The to 1.3 cm by 1.3 cm by 22.9 cm and tested on molecular weight of the derivatized cellulose an Instron Universal Mechanical Testing Ma- fraction was analyzed using the procedure of chine. Each sample was tested on the radial Wood et al. (1986). Analysis of molecular face with a head load speed of 0.13 cm/min. weight was conducted on a Spectra-Physics A load cell with a 453 kilogram capacity was SP8100 liquid chromatograph. Shodex KF803 utilized, and the deflection of the wood was and KF805 SEC columns were connected in measured with an LVDT to ± 1% accuracy. series. The THF eluent flow rate was 1 ml/ Cellular structure of gold-coated ancient and min. A UV spectrophotometer (Spectra-Phys- recently felled kauri samples (air-dried) was ics SP8400) was used at 235 nm. Molecular studied with a JEOL JSM-840 scanning elec- weight and retention volume correlation for tron microscope (SEM). Two sample prepa- the carbanilate was based on calibration using rations were examined: razor-planed sample a narrow molecular weight distribution of surfaces, as well as fracture surfaces following 54 WOOD AND FIBER SCIENCE, JANUARY 1994, V. 26(1)

bending tests. To detect mineral components restricted to cleavage of side chains and some in the wood, carbon-coated sections were sub- demethoxylation. The acid-hydrolyzing effects jected to X-ray analysis by a Tracor Northern of the burial environment could account for TN-5500 energy dispersive spectrometer these changes. (EDS). Anatomical studies RESULTS Kauri has a very simple anatomical struc- Chemical studies ture. The wood is composed of rounded to The ancient kauri had been buried in an irregularly polygonal tracheids bordered by acidic swamp environment for thousands of large intercellular spaces (Fig. 1), some axial years. Results of pH determinations revealed parenchyma, and rays (Butterfield and Meylan that the ancient kauri was more acidic (pH = 1980; Garratt 1924). There are no canals 3.7) than the modern wood (pH = 4.4), sug- (Meylan and Butterfield 1978). Growth rings gesting that the acidic swamp environment had are delineated by a gradual transition of nar- facilitated degradation of the waterlogged kau- row thickened bands of cells at the end of the ri. Chemical analyses (Table 1) indicated losses latewood tissue (Garratt 1924). Micrographs of labile carbohydrates (arabinose and galac- of cut surfaces demonstrate the remarkable tose) normally existing as susceptible branch- preservation of ancient kauri fibers. In cross- ing units on hemicellulose chains (Freedland sectional views, both the recently felled and et al. in press). Deacetylation was also evident, ancient kauri show a large amount of early- as well as degradation of the hemicellulosic wood (Fig. 2). backbone units (mannose, xylose). In addition, The tracheids and ray cells have a varied molecular weight studies of isolated a-cellu- pitting structure. In radial longitudinal sec- lose revealed depolymerization of cellulose tions, the intertracheary pits of the kauri should chains (Table 2). By contrast, the chemical be bordered by circular pit apertures (Meylan change in ancient kauri lignin appeared to be and Butterfield 1978). However, the tori and Freedland et al. -ANCIENT KAURI 55

FIG. 1. Transverse face of modem (A) and ancient (B) kauri (Agathis australis). Growth, rings, formed by narrow thickened cells at the latewood boundary, are difficult to detect in both the modem and ancient wood. Band = 100 µm.

FIG. 2. Transverse face of modem (A) and ancient (B) kauri (Agathis australis), showing the rounded to irregularly polygonal tracheids of the earlywood bordered by large intercellular spaces. Band = 10 µm.

FIG. 3. Radial longitudinal face of modem (A) and ancient (B) kauri (Agathis australis). Tori, absent on the intertracheary pits of the modem wood, are visible in the ancient wood. Band = 10 µm. 56 WOOD AND FIBER SCIENCE, JANUARY 1994, V. 26(1)

FIG. 4. Radial longitudinal face of modem (A) and ancient (B) kauri (Agathis australis). Cupressoid pits between tracheids and rays are collapsed in both the modem and ancient samples. Small tears in the cell walls of the ancient kauri could be the result of drying stresses. Band = 100 µm. surrounding tissue of both the modern and sample preparation. In addition, the rays of ancient kauri are in poor condition. While the the ancient wood (Fig. 5) are filled with sulfur- tori are absent in the recently felled kauri sec- rich particles (Freedland et al. in press). In the tions, intact tori are visible in the ancient wood tangential longitudinal section, cell-wall ma- sections (see Fig. 3). Tracheid to ray pits are terial is peeling away from the tracheids of the collapsed into angular slit-like openings in both ancient wood (Fig. 6). recently felled and ancient samples (Fig. 4). Micrographs of ancient and modern kauri Small tears in the cell walls of the ancient kauri fracture surfaces illustrated the relationship extend through the tracheid to ray pits, pos- between chemical changes and cell-wall sibly the result of drying stresses following strength (Fig. 7). Using the recently felled kauri as the basis for comparison, it can be seen that

the S2 layer of this recently felled material has fractured cleanly. The cell has separated at the

S1-S2 interface, causing the middle lamella to become structurally distorted. By contrast, the

middle lamella-S1 area of the ancient kauri is intact, while the carbohydrate-deficient S2 lay- er is structurally distorted and porous. In some cells, individual lamellar sheets can be differ-

entiated, and pinholes within the S2 layer rep- resent areas where-bundles of microfibrillar strands have been pulled from the surrounding matrix.

Mechanical studies FIG. 5. Radial longitudinal face of carbon-coated an- The results of all mechanical tests are tab- cient kauri (Agathis australis). EDS scans of the particles ulated in Table 2. The ancient material, which within the rays indicated a sulfur-rich material. These par- ticles were absent in the modem kauri wood. Sulfur com- was initially cut in New Zealand while still wet, pounds are abundant in swamp environments. Band = showed some collapse and warping along the 100 µm. radial and tangential faces. As determined by Freedland et al. -ANCIENT KAURI 57

FIG. 6. Tangential longitudinal face of modem (A) and ancient (B) kauri (Aguthis australis). Cell wall material is peeling from the ancient wood tracheids. Band = 100 µm. oven-drying, the ancient material had a higher ture content of the ancient kauri was reliably specific gravity (0.5 1) than the recently felled less than the moisture content of the recently kauri (0.47). felled kauri. Ideally, the wood should be tested when it The mean MOE value for the ancient wood has been conditioned down to a 12% moisture (0.9 × 106 psi; SE = 0.1 × 106) was contrasted content. The ancient kauri was conditioned to against the mean MOE value for the recently an average of 9.9% moisture content [Standard felled wood (1.4 × 106 psi; SE = 0.1 × 106). Error (SE) = 0.04], while the recently felled The ancient kauri MOE was significantly less wood exhibited an average moisture content than the recently felled kauri MOE [t(130) = of 10.5% (SE = 0.1). A statistical comparison 6.51, P < 0.001]. The results would support between these two groups was significant [t( 130) the conclusion that the ancient wood had a = 6.61, P < 0.00l], indicating that the mois- lower resistance to deformation. The mean

FIG. 7. Transverse fracture surface of the modem (A) and ancient (B) kauri (Agathis austrulis). The S2 layer of the modem material fractured cleanly, in contrast to the ragged fracture surface of the ancient wood. Loss of carbohydrates within the wood cell wall has altered the strength properties of the ancient kauri. Band = 10 µm. 58 WOOD AND FIBER SCIENCE, JANUARY 1994, V. 26(1)

WPL value for ancient wood was 1.3 in.-lb/ this collapse. In fact, air-dry moisture contents cu. in. (SE = 0.1), while the mean WPL value of ancient wood are often lower than for mod- for the recently felled wood was 2.7 in.-lb/cu. em material (Kohara 1956). In the case of the in. (SE = 0.1). The ancient kauri WPL was ancient kauri, however, the combination of low significantly less than the recently felled kauri moisture content and hemicellulose degrada- WPL [t(130) = 13.06, P < 0.001], meaning tion imparted lowered strength properties. that it would take less work to move the an- Results of static bending tests indicated that cient wood from the unloaded to the elastic the ancient and recently felled kauri exhibited limit of the material. differences in mechanical behavior both below The ancient kauri MOR (9547 psi; SE = 218) and above the PL of the static bending curve. was significantly less than the recently felled Below the elastic limit, MOE and WPL values kauri MOR (13469 psi; SE = 229); t(130) = for the ancient kauri were significantly lower 12.38, P < 0.001). The results substantiate than the recently felled kauri values. Not only that ancient wood had a lower ultimate was the ancient kauri more readily deformed, strength. The work to maximum load (WML) but not as much energy was needed to reach mean value for the ancient wood was 16.1 in.- the proportional limit of the wood. At the mo- lb/cu. in. (SE = 0.9) while the WML mean lecular level, movement of the polymers with- value for the recently felled wood was 13.5 in.- in the cell wall is controlled by steric hindrance lb/cu. in. (SE = 0.7). Again, the ancient kauri and bonding characteristics. For the ancient WML value was significantly greater than the kauri, degradation of carbohydrates had likely recently felled kauri WML value (t(130) = 2.22, altered the tightly bonded network of hydrogen P < 0.029). The findings suggest that more and covalent bonds within the cell wall. Not work was required to ultimately cause failure only had cellulose depolymerization reduced in the ancient wood. the number of covalent bonds within the mi- crofibrillar structure, but removal of hemicel- DISCUSSION luloses probably resulted in the cleavage of The mechanical behavior of nondegraded lignin-hemicellulose bonds. These structural wood can exhibit several predictable trends. changes could have facilitated the rotation and For instance, below the fiber saturation point twisting of molecular components within the (FSP), increasing moisture content will result cell wall, resulting in plastic deformation of in decreased stiffness. The stiffness of wood is the ancient kauri wood. controlled by interatomic bonds, such as hy- Beyond the PL, MOR and WML values in- drogen bonding between cellulose chains in dicated that the cell walls of the ancient ma- microfibrils. As water enters the cell wall and terial were less brittle than the cell walls of the pushes tightly packed cellulose chains apart, recently felled kauri. Because the flexible cells swelling occurs perpendicular to the microfi- of the ancient kauri had a lower ultimate brillar axis. The decrease in the degree of cell- strength (MOR), more work (WML) had to be wall packing results in the property of de- done to actually fracture the material. SEM of creased stiffness, as evidenced by the extreme the fracture surfaces revealed that cell-wall flexibility of wet ancient wood at test (Jagels fracturing behavior differed between the an- et al. 1988). cient and recently felled samples. Under load, By contrast, dry degraded wood exhibits stress is elastically transferred along the tra- variable behavior under load. Generally, dry- cheids and cell-wall material until a disconti- ing stresses can cause the hemicelluloses in the nuity is reached. At the site of the disconti- cell wall to collapse structurally, thereby im- nuity, stress will build and cracks eventually parting increased strength properties. Degra- develop (Winandy and Rowell 1984). At a cell- dation of cell-wall components, a common fea- wall level, the fracturing of wood can take sev- ture of ancient wood samples, can facilitate eral forms. Schniewind (1964) described the Freedland et al. -ANCIENT KAURI 59

cell-wall structure as a less rigid S2 layer tion. Since the microfibrils serve as the strength bounded by more rigid S1 and S3 layers. Lignin, component of the cell wall, the effect of this highly concentrated in the middle lamella, ef- depolymerization would be manifested in low- fectively holds the cells together under load. ered strength properties. The MOE and MOR Due to the strength of the lignin matrix, failure values for the ancient kauri wood were signif- will occur in the carbohydrate fraction of the icantly lower than the corresponding recently wood (Winandy and Rowell 1984). For this felled wood strength values. reason, failures usually occur within the cell By contrast, the ancient kauri lignin re- wall, rather than the lignin-rich region between mained essentially unaltered in structure or cells. chemistry. Serving as the stiffening material of This fracture behavior was noted in the re- the cell, the lignin continued to hold adjacent cently felled kauri samples. The secondary cell- cells together under load. However, the deg- wall fracture surface was cleanly broken, with radation of the other cellular components had cellular distortion at the S1-S2 interface. There- offset any advantages that the stiffening prop- fore, stress was distributed within the cellular erties of lignin could provide. matrix up to a critical point, at which time the Ancient kauri wood did not follow the spe- stress was rapidly released through a clean frac- cific gravity-stiffness trend. Despite a higher ture of the cell wall. By contrast, SEM of the specific gravity value than the recently felled fractured ancient kauri surfaces vividly re- material, the significantly lower MOE values vealed degradation of the woody cell wall with- for the ancient kauri revealed a loss of stiffness in the S2 layer. Because of carbohydrate deg- over time. This variation in the specific grav- radation in the ancient kauri, as noted in ity-stiffness trend may be explained by the nat- chemical analyses, the cell walls could not ural variation of wood structure within species. evenly distribute stresses. Degradation of In this manner, even with degradation of cel- packing material (hemicelluloses) from around lular components, the density of ancient wood the cellulose microfibrils loosened the cell-wall may still be higher than the same-species mod- structure. Note that some ancient kauri sec- em material (Schniewind 1990). ondary cell walls are divided into lamellae-like structures, indicating that cellular components CONCLUSIONS have been removed from between the micro- Chemical and physical tests revealed that fibrillar sheets (Fig. 7). the structure of the ancient kauri cell wall had This structural disorganization resulted in been modified. Because of carbohydrate deg- stresses building up at voids and discontinu- radation, structural disorganization prevailed ities within the cell wall, causing cellular dis- in the ancient material. Rather than the bonds tortion. With applied load, the microfibrils within the composite material imparting could shift in orientation and lose the ability strength properties, the ancient wood had a to effectively transfer and resist stress. Stress decreased ability to withstand deformation. was incrementally relieved by gradual tearing This increased plasticity is reflected in the sig- of this secondary cell-wall material, giving the nificantly lower MOE and WPL values of the fractured surfaces of the degraded ancient kau- ancient as compared to recently felled kauri ri a very porous, rough appearance. Pinholes wood. The amount of work needed to reach in the S2 layer are evidence of microfibrillar the PL is lowered, because the material is more strands being pulled from the surrounding ma- readily deformed. trix, a feature that was not evident at the re- Since depolymerization of the carbohy- cently felled kauri fracture surface. drates had already been chemically initiated In addition to the losses of hemicellulosic in the acidic swamp, the cell wall fractured material, chemical analyses revealed a de- more readily under load. The MOR of the an- crease in the degree of cellulose polymeriza- cient wood was significantly lower than the 60 WOOD AND FIBER SCIENCE, JANUARY 1994, V. 26(1)

MOR value for the recently felled kauri, in- dicating that flexible cells had a lower ultimate strength. However, because of this plasticity, more work must be done to actually fracture the material. The significantly higher WML value for the ancient wood indicated that the ancient material did not resist stress; rather, the structure gradually “gave way” under im- posed stress. Scanning electron microscopy of the frac- tured ancient kauri surfaces shows this loss of cellular organization. Whereas the secondary cell wall fracture surface within the recently felled kauri wood was cleanly broken, the frac- tured surface of the degraded ancient kauri ex- hibited a very porous appearance. Because the ancient kauri S2 layer was weaker than the re- cently felled kauri S2 layer, the stress was in- crementally relieved by gradual tearing of this secondary cell-wall material. However, serv- ing as the stiffening material of the cell, the essentially unaltered lignin continued to hold adjacent cells together at the middle lamella.

ACKNOWLEDGMENTS The authors would like to thank the New Zealand Forest Research Institute for obtain- ing samples. In addition, sincere thanks to G. E. Orbell, District Soil Scientist at the Division of Land and Soil Sciences, for providing soil information and detailed soil maps; Virgil Schwandt and Marty Wesolowski for chemical analyses; Tom Kuster for SEM pictures; Les Floeter and Mary Doran for contributing greatly to the mechanical tests and data anal- yses; and the reviewers for their helpful com- ments. This research was conducted at the U.S. Forest Products Laboratory, with funding pro- vided by a grant from the University of Wis- consin Graduate School.

REFERENCES Freedland et al. -ANCIENT KAURI 61