Juvenile Wood Characteristics, Effects and Identification ... · Juvenile Wood Characteristics, Effects and Identification Literature Review Prepared for the Forest & Wood Products

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Juvenile Wood Characteristics, Effects and Identification Literature Review

© 2003 Forest & Wood Products Research & Development Corporation All rights reserved. Publication: Juvenile Wood Characteristics, Effects and Identification Literature Review The Forest and Wood Products Research and Development Corporation (“FWPRDC”) makes no warranties or assurances with respect to this publication including merchantability, fitness for purpose or otherwise. FWPRDC and all persons associated with it exclude all liability (including liability for negligence) in relation to any opinion, advice or information contained in this publication or for any consequences arising from the use of such opinion, advice or information. This work is copyright and protected under the Copyright Act 1968 (Cth). All material except the FWPRDC logo may be reproduced in whole or in part, provided that it is not sold or used for commercial benefit and its source (Forest and Wood Products Research and Development Corporation) is acknowledged. Reproduction or copying for other purposes, which is strictly reserved only for the owner or licensee of copyright under the Copyright Act, is prohibited without the prior written consent of the Forest and Wood Products Research and Development Corporation. Project no: PN02.1907 Researchers: J. Ilic, R. Northway and S. Pongracic CSIRO Forestry and Forest Products Private Bag 10, Clayton South VIC 3169 Final report received by the FWPRDC in January 2003 Forest and Wood Products Research and Development Corporation PO Box 69, World Trade Centre, Victoria 8005 Phone: 03 9614 7544 Fax: 03 9614 6822 Email: [email protected]: www.fwprdc.org.au

The FWPRDC is jointly funded by the Australian forest and wood products industry and the Australian Government.

Juvenile Wood Characteristics, Effects and IdentificationLiterature Review

Prepared for the

Forest & Wood Products Research & Development Corporation

by

J. Ilic, R. Northway and S. Pongracic

1

Executive Summary:

Objective The objective of the literature review is to provide a background to explain how the extent and effect of the juvenile wood core can affect softwood processing with an emphasis on product straightness, stiffness and stability. The principal species to be considered will include Pinus radiata, and southern pines grown in Queensland (P. elliottii and P. caribaea).

Key Findings The key findings of this review are: § Juvenile core wood does exhibit different properties to outer wood (details in

introduction) § There is no agreed scientific definition as to what constitutes the extent of

juvenile wood § Improving the properties of juvenile wood is more financially beneficial than

improving the properties of outer wood § Detecting the extent to juvenile wood in a standing tree would be valuable to

processors § Much more research is required to quantify the extent of juvenile core wood,

particularly with respect to wood properties such as spiral grain, in combination with density and stiffness.

Application This literature review provides the background for the “Effect of juvenile core on softwood log processing: Product straightness, stability and stiffness and wood characteristics” project. The information contained in this review will assist in the analysis of data from this project.

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Table of Contents Executive Summary:....................................................................................................1

Objective ....................................................................................................................1 Key Findings ..............................................................................................................1 Application.................................................................................................................1 Further work...............................................................................................................1

Table of Contents .........................................................................................................2Introduction..................................................................................................................3Characteristics of juvenile wood.................................................................................5

Tracheid characteristics..............................................................................................5Spiral grain.................................................................................................................6Wood density .............................................................................................................7

Characteristics affecting processing and end-use.....................................................8 Compression wood.....................................................................................................8Timber stiffness and strength...................................................................................10Distortion resulting from drying ..............................................................................10

Twist.....................................................................................................................11 Spring and bow ....................................................................................................12

Extent of the juvenile core .........................................................................................13Assessment techniques and technologies .................................................................13 References...................................................................................................................15

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Introduction Juvenile wood is formed during the early stage of tree growth near the pith. Larson (1969) recognised that juvenile wood is formed in the proximity of the crown (close proximity of the foliage). Different terms have been used for juvenile wood depending on position or tree development, or tree maturity. Some of the common terms include: juvenile wood, core wood, crown-formed wood and crown wood. Some of these terms can be somewhat confusing when they are used interchangeably. The specific application of the terminology has been discussed in relation to growth, age and position by Amarasekara and Denne (2002). They recommend that the term ‘core wood’ should be used as a general purpose term to indicate the central region of a log where the structure and properties are variable and differ from those of the outer wood. However, because of its high acceptance in the timber industry, in this report, the term ‘juvenile wood’ will be used to describe such wood as the zone surrounding the pith. Such wood is regarded as low-quality for many uses (Senft et al. 1985) as it typically has the following general characteristics:

• wide growth-rings; • high grain spirality; • low density and stiffness; • thin cell walls and short fibres; • lower cellulose:lignin ratio (Bendtsen 1978); • high microfibril angle; • high longitudinal shrinkage (Figure 1); • low transverse shrinkage (Figure 1); and • usually high levels of compression wood.

Figure 1 Typical variation of tangential shrinkage (across the grain) and longitudinal shrinkage (along the grain) with successive growth rings in radiata pine (CSIRO data)

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The main aim of this review is to provide a background to explain how the extent and effect of the juvenile wood core can affect softwood processing with an emphasis on product straightness, stability and stiffness. The principal species to be considered include Pinus radiata, and southern pines grown in Queensland (P. elliottii and P. caribaea). The juvenile wood zone contributes significantly to the overall variation in wood properties Dadswell (1958). Juvenile wood is particularly prominent in fast grown softwoods and may comprise the bulk volume of small-diameter thinnings and even short rotation final-crop logs. Rapid growth rates obtained from intensively managed trees permit crop rotations much shorter than are possible in natural stands, thus greatly increasing the quantity of juvenile wood being produced. The timber industry may experience processing problems if it has developed a preference for, or systems to cope with, either high or low proportions of juvenile wood (Cown 1992). Cown (1992) indicated that from the wood processors point of view, the reduced recovery of products , and changed wood quality resulting from the juvenile core, forms an unfortunate association with small-diameter logs. Some desirable characteristics of juvenile wood in solid wood processing are lower variability (lower contrast) between earlywood and latewood, lower transverse shrinkage (from lower density) and greater collapsibility of the thin walled fibres for papermaking. Juvenile wood is usually identified by assessments of the number of growth rings from the pith at which an important wood property (sometimes taken as density or fibre length or another characteristic) changes. However wood properties vary not only with distance from the pith, but with growth rate, stem height, between trees and between sites. In some pines (e.g. slash and caribaea) the change from juvenile wood to mature wood can be abrupt, whereas in other pines including radiata and loblolly, it can be more gradual. Since different characteristics change at different rates, the change to mature wood is complicated and extends over several years (Zobel 1980). The relationships commonly used to illustrate the changing wood properties with age are shown in Figure 2 (Hillis and Bachelard 1981).

Figure 2 Effect of age on wood characteristics of typical pine tree. Note that most of the changes occur in the juvenile wood core

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Characteristics of juvenile wood The age of the stem has a major effect on the variability of wood anatomical and thereby physical and mechanical characteristics in pine (specifically the age of cambium when growth rings are formed). Wood properties change in the stem, ring by ring - from the pith outwards. So the wood formed in the first 5-6 years of growth out from the pith at almost any height shows similar wood properties ignoring the effect of heartwood (Cown 1980). Growth rings observed at the butt-end of a log represent the entire growth of the stem in which the inner rings were formed first and the outer rings were added later. Wood from the top of the stems contains fewer newly formed growth rings. While the anatomical characteristics of the wood from the juvenile zone are similar at different stem heights, butt logs also contain low moisture-content heartwood and secondary enrichment of resin (Harris 1965). As the wood structural characteristics change gradually from the centre of the stem they also show different patterns, so the demarcation between the juvenile wood and the mature wood is unclear. However, the wood properties are usually sufficiently different to cause problems in processing and in service. Walker and Nakada (1999) have reviewed the characteristics and properties of the juvenile core within logs and trees of mostly pine wood, these included density, MFA, fibre length and chemical composition. They indicated that the poor perception by industry of juvenile wood is partly a consequence of the variability of the intrinsic characteristics.

Tracheid characteristics Tracheids are wood cells and the basic building blocks (sometimes called fibres) of conifer woods. They usually make up about 90% of the whole volume of the the wood. Consequently their properties are very important in any consideration of wood quality. Broad patterns of variation of tracheid length with tree age were initially described by Nicholls and Dadswell (1960) and studies of variation within growth rings were carried out. The findings were generally substantiated by most other studies. The general trend is for increasing length in the juvenile zone up to approximately 20 growth rings. The length more than doubles in this region from approximately 1.5mm to 3.5mm. In older wood the length may increase further but it usually stabilises and fluctuates about a mean value. The longest tracheids are usually found in the outer wood some distance above the base of the tree. Earlywood tracheids were shown to be shorter than latewood tracheids. Tracheid diameter and tracheid wall thickness are also important anatomical characteristics that are related to wood density. Tracheid diameter increases from the pith outwards by up to 20% in the first 20 rings in both earlywood and latewood (Kininmonth and Whitehouse 1991). Tracheid wall thickness also shows a similar trend. However, within growth rings, the radial wall thickness increases from earlywood to latewood following a similar trend to density (Harris 1981). Often the variation within growth rings is greater than that between them (Bamber and Burley 1983).

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Besides changes in cell width, wall thickness and length, other anatomical changes occur in juvenile wood. The alignment of cellulose microfibrils in the S2 layer of the cell wall (microfibril (MFA)) changes markedly from an angle of approximately 45-50o to the axis of the fibre in growth rings near the pith to an angle of 10-20o or less in wood outside the juvenile zone Dadswell (1958). An inverse relationship has been found between cell length and MFA (F), L = a + b cot (F), where L = cell length, and a and b are constants. This relationship is discussed in Preston (1974). However, it is still not certain whether or not there is a causal relationship between L and F since fibre length is largely governed by the cambial- initial length. Although, the exact mechanisms of deposition and alignment of fibrils angle are still uncertain (Brazier 1985), the S2 is laid down a well after cell length has been established. Donaldson (1992) studied pith to bark variation of MFA at different heights in 22-year-old trees of radiata pine. Angles declined curvilinearly from pith to bark and ranged from 9o-55o, the highest angles occurred in the juvenile wood and MFA declined rapidly with tree height, reaching a more or less constant value above 7 m height with small increases in the core of the top log.

Spiral grain On the macro scale, the alignment of fibres, or the grain inclination in the wood changes from the pith outwards. While there are a variety of patterns in softwoods, the general trend is: straight at the pith, developing a marked left-handed spiral (S - shaped) over the next few growth rings, thereafter straightening as the stem ages; sometimes the stem may develop a right-handed spirality (Z – shaped) (Noskowiak 1963, Harris 1989). Harris (1969, and 1989) provided a physiological explanation for the change of grain alignment postulating that wood fibres are oriented parallel to the flow of metabolites. Bamber and Burley (1983) and Kininmonth and Whitehouse (1991) both suggest that in radiata pine the spiral grain reaches a peak by the second or third growth ring decreasing thereafter. However, Harris (1989) indicated that the maximum value is often reached by the second or third growth ring, but that values may increase up to 12 years or more. Bamber and Burley (1983) indicated that mean value of spiral grain of nearly 5o and a maximum value of about 10o was typical of NSW radiata pine, and Balodis (1972) gives a range of spiral grain between –9o (S or left-handed spiral) to +6o (Z or right-handed spiral) for five, 28-year-old thinnings of Pinus elliottii and P. taeda grown in Queensland. Little information is available on differences of spiral grain angle between the earlywood and latewood within growth rings. Within ring variation in growth rings close to the pith can be seen clearly in Pinus patula from South Africa, (Burley et al. (1967); Harris (1989) reported little difference in spiral angle within rings of radiata pine.

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Figure 3. Spiral grain variation with successive growth rings in two trees of Pinus radiata. Note that the upper trace indicates a tree with high levels of spiral grain with outer wood having higher spirality than the peak of the on the second tree (lower trace) (CSIRO data). Cown et al. (1991) obtained data on spirality of grain in young trees of radiata pine and showed from drying studies that spiral grain angles in excess of 5o are likely to cause problems with twist even under favourable conditions of drying. The extent of the influence and hence the “danger zone” of the spiral grain would be within the first 10 growth rings. Furthermore, extremes of spiral grain from individual trees are likely to cause significant problems for the conversion of young logs, but that there may be scope for selective breeding to reduce the impact of the spiral grain on sawn wood products. Figure 3 provides an example of the wide variation in spiral grain between trees. Haslett et al. (1991) also showed strong evidence that spiral grain can contribute very significantly to the economics of processing.

Wood density Among the basic wood characteristics, wood density is considered important because of its influence on a wide range of fibre and solid wood products. Zobel and Jett (1995) consider that it is widely recognized that density is the single best predictor of wood performance. The wood density is affected both by the amount of earlywood and latewood, and the thickness and composition of the individual cells. Cown (1999) indicated that most workers agreed that, within a species, the latewood component is highly sensitive to environmental and site effects. Furthermore, he showed that latewood percentage was the most sensitive to such effects in both juvenile and mature wood. X-ray densitometry (Cown and Clement (1983), Davis et al. (1993)) has been used to measure density variations. Much work has been done to classify density variations in radiata pine in New Zealand. General variation of wood density and its effect on wood quality was provided by Bamber and Burley (1983); and Kininmonth and Whitehouse (1991). Additional data is available from Cown and Ball (2001); Cown

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and McConchie (1983); Cown (1992); Nicholls (1984). The general relationship between strength and density is well established in wood, and it is not difficult to see why density is considered to be important in wood qua lity. However, as Bamber and Burley (1983) pointed out from studies on the effect of tree age on wood strength, strength increased more with age than can be accounted by similar increases in density pointing to the importance of other characteristics. Cown (1992) quoted that 400 kg/m3 would be a reasonable minimum clearwood density to ensure satisfactory in-grade performance based on Australian Standard 1720.1 (1988).

Characteristics affecting processing and end-use There are several important characteristics that have an affect on product straightness, and its stability and stiffness (strength). These arise from the macro and micro characteristics of the fibres and their variation with age. Management practices also influence these characteristics.

Compression wood Reaction wood is formed in stems reacting to external stresses or stimuli e.g. a tree on the side of a hill trying to correct a lean, or reorienting its position to get access to more light. The wood known as “compression wood” produced in softwoods forms on the under side or compression side of the stem and exhibits different characteristics from normal wood. Dimensional changes on drying, typically higher longitudinal shrinkage lower transverse shrinkage in juvenile wood compared with mature wood are explained in terms of changes in the microfibril orientation and cell wall alignment. A high incidence of reaction wood - compression wood – can be a feature of juvenile wood as well as outer wood. The presence of higher levels of reaction wood in young stems may be attributed to low resistance to imposed stress in rapidly growing wood. This is likely to occur in trees grown in a condition with ample growing space without the protection of nearby stems. In pine, compression wood is not only observed in leaning stems but can be seen in relatively straight stems where at some stage of growth there were large deviations from vertical resulting from overcorrection to external stimuli.

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Figure 4. Cross-section of a leaning stem showing eccentric pith and wide growth rings of the darker coloured compression wood Compression wood characteristics usually lead to very undesirable processing and end-use properties such as reduced strength, distortion in apparently clear timber, and high lignin levels in comparison to normal wood. Harris (1977) stated that compression wood is, in many respects, the most insidious defect commonly encountered in sawn radiata pine. Distortion arising from compression wood is particularly evident when different parts of a board contain compression wood and normal wood. Westing (1965; 1968) and Timell (1986) provided a good review of the formation and function of compression wood in conifers. Generally the formation of compression wood can be regarded as a mechanism for correcting stem lean in response to gravitational stimulus. Compared with normal wood, compression wood characteristically has (Harris 1977):

• Shorter fibres (tracheids); • Higher density; • Darker colour woods with brownish or reddish-brown tinge, greater opacity to

light in green wood; • Eccentric pith (Figure 4); • Rounded cell cross-section with intercellular spaces; • Tracheids with missing S3 layer; • Helical slits or striations in S2 layer of cell wall; • Higher lignin and galactan content; • Larger microfibril angle than normal tracheids; • Lower transverse and greater longitudinal shrinkage; and • Different relationship between microfibril angle and shrinkage.

Harris (1977) also pointed out some of the characteristic components of compression wood noted above were dependent on the severity of the compression wood. Intercellular spaces and absence of S3 layer were sometimes not evident in less severe

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forms of compression wood. The visual appearance of compression wood in the juvenile core was not a good guide to its shrinkage behaviour (Harris 1989). When large a microfibril angle (MFA) is observed in the growth ring with compression wood and from opposite wood (wood on the other side of the tree from the compression wood zone), the longitudinal shrinkage is greater in the compression wood. However, when MFA is small in the outer wood, compression wood will show large longitudinal shrinkage only when the average MFA is greater than 30o.

Timber stiffness and strength The average modulus of elasticity (MOE) increases with cambial age (increasing number of growth rings from the pith). Mature wood from pine is stiff enough for most industrial uses, but current trends of harvesting trees at less than 30 years result in high proportions of low stiffness juvenile wood. However, there is evidence that improving juvenile density may not be sufficient to fully address the low stiffness (Jayawickrama 2001). Kininmonth and Whitehouse (1991) have drawn attention to the disadvantage that low stiffness radiata pine faces on world markets, particularly as such material comes from the knotty juvenile core. Considerable losses arise from the juvenile core as the low stiffness material from this zone barely makes minimum structural grade (MGP10, [F5]). Better financial returns will be expected by making improvements to raise the proportion of lower grades (i.e. MGP10) than by attempting to improve higher grades such as MGP12 (F8) further. Addis Tsehaye et al. (1997) indicated these increases to be of the order of $100/m3 and $20/m3 respectively. Consequently, greater benefits will arise from selection and manipulation of the stiffness of the juvenile core. A comparison between density and stiffness as a property for selecting trees for structural timber indicates that stiffness is the better parameter for selecting superior trees (Addis Tsehaye et al. 1997). Addis Tsehaye et al. (1997) challenged that the traditional belief that density is the principal determinant of mechanical properties. Addis Tsehaye et al. (1997) quoted Cave (1969), Bendsen and Senft (1986) and Cave and Walker (1994) to illustrate how stiffness changed from the juvenile core to mature wood, that MFA decreased 5-6 times while density only changed by approximately 0.4 times indicating that the inc rease in density was not sufficient by itself to account for the observed increases in wood stiffness. They concluded that comparisons between density and stiffness for selecting trees for structural timber indicated that stiffness is a better parameter for selecting superior trees within the natural population of a forest stand and that MFA remains the key to interpreting stiffness of timber in radiata pine. Walker and Butterfield (1995) and Booker et al. (1997) also support these findings. Addis Tsehaye et al. (1997) measured spiral grain and confirmed the known distribution within the juvenile core, but unfortunately they did not consider the relationships between spiral grain and the other measured characteristics including: MFA, density, tracheid length, and other chemical factors.

Distortion resulting from drying One of the most serious problems in drying and utilisation of young plantation grown conifers is warp in the form of twist, spring (crook) and bow (Johansson et al. (1994); Kliger (2001); Johansson and Kliger 2002). The influence of wood characteristics on

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warp has been investigated extensively (Balodis, (1972); Mackay (1973); Fridley and Tang (1993); Mishiro and Booker (1988); Perstorper et al. (1995); Simpson and Tschernitz (1998); Taylor and Wagner (1996); Cown et al. (1996); Wagner et al. (2002)).

Figure 5. Excessive warp due to presence of juvenile wood in a dried stack of southern pine Mackay (1973), Arganbright et al. (1978) and Simpson and Tschernitz (1998) studied how warp increased with decreasing moisture content and showed that the method of drying affects the magnitude of the warp developed.

Twist

Factors that have the greatest influence on the amount of twist include spiral grain and distance from the pith (Kloot and Page (1959); Brazier (1965); Balodis (1972); Shelly et al. (1979); Mishiro and Booker (1988); Perstorper et al. (1995); Cown et al. (1996)). The influence of other wood characteristics including density, compression wood, ring width and the presence of knots seem to have little effect on twist (Perstorper et al. (1995) and Beard et al. (1993). Cown et al. (1996) studied drying distortion from14 and 27-33-year-old radiata pine. They concluded that log diameter was the most influential log property and that spiral grain was also important due to its influence on twist during drying. Furthermore they pointed out “spiral grain is a little known feature of plantation pines and that it is only now getting the attention it deserves”. Most problems involving twist result from material containing juvenile core wood, as the spirality declines in older wood, the corresponding twist also declines. However, mature logs of Norway spruce with spirality in excess of 3o under bark result in 86% yielding studs with an unacceptable amount of twist (Kliger 2001). Whether or not this also applies to pines requires investigation. Figure 3 shows an example of outer wood of short rotation material that could contain substantial levels of spiral grain

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much greater than 3o. This indicates much greater levels of spiral grain in the inner core are likely to cause severe warping in dried material.

Spring and bow

Johansson and Kliger (2002) indicated that the causes of spring and bow arise from residual stresses and uneven longitudinal shrinkage. Bow and spring can be seen at sawing (Archer (1987); Mishiro and Booker (1988)) as release of growth stresses. The other major cause of spring and bow stems from uneven longitudinal shrinkage. It is well known that a substantial longitudinal shrinkage on one edge of a board will result in spring or a greater contraction of the edge with the higher longitudinal shrinkage (Simpson and Gerhardt (1984); Megraw et al. (1998). The causes of uneven longitudinal shrinkage arise from large MFA in fibres often found in the juvenile core and in compression wood either in the juvenile core-wood zone or from the mature wood zone (Shelley et al. (1979); Perstorper et al. (1995)). Du Toit (1963) and Harris (1977) indicate that characteristics other than of longitudinal shrinkage are of little consequence.

Growth ring number (bark = 1...)

Long

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0.2

0.6

1.0

1.4

1.8

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5 6 7 8 9 10 11 12 13 14 15 16 17

Figure 6. Spring in a pine board resulting from compression wood from rings 13-17 indicated by longitudinal shrinkage above 1% - growth rings shown on wood sample (CSIRO data) Johansson and Kliger (2002) provided a summary of the literature dealing with the mathematical modelling of warp. They indicated that the models could explain up to 65% of the variation in twist, but that they are less accurate for spring and bow. An analytical model was also presented by Stevens and Johnston (1960) and Balodis

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(1972). One of the difficulties encountered when modelling bow and spring arises from lack of adequate data corresponding to the variation of longitudinal shrinkage along the length of the board (Johansson et al. (2001); Johansson and Kliger 2002).

Extent of the juvenile core Juvenile wood is often defined in terms of the rate of change in wood properties from the pith to the outside; it has been convenient to describe its location in terms of number of growth rings from the pith. Cown (1992) indicated that most researchers consider that anywhere from the first five to 20 growth rings can be classified as juvenile, although usually the first-10 growth rings can be considered as juvenile, on the basis of known density patterns. Massey and Reeb (1989) also indicated that the first 10 growth rings is used as a rule-of-thumb in Texas for Pinus taeda. They described a method based on that developed by Booker (1987) which uses a celluloid template with concentric rings of known curvature for matching growth ring orientation and hence the position of the juvenile core. Additional descriptions of ways of determining the extent of the juvenile core in southern pines based on percentage latewood are provided in Larson et al. (2001). Cown and Ball (2001) later showed the extent of the juvenile wood zone for 10 families of radiata pine growing on the extremes of sites in New Zealand to be 5-20 growth rings based on a nominal density of 400 kg/m3 as an indication of mature wood. Walker and Nakada (1999) suggested four to 20 growth rings on the basis of density and six to 20 years on the basis of tracheid length. The classification of the extent of the juvenile wood core is important because a large proportion of the stem is likely to have juvenile wood characteristics, and it is important to understand their implications. The effects of low density, high levels of spiral grain, coupled with knots, high longitudinal shrinkage and compression wood have given young logs a poor reputation in sawmills (Cown 1992). Two problems arise with the use of the first 10 rings as a definition of the juvenile core. One is that the properties are changing gradually (density), and it provides little indication of the absolute properties, which vary greatly between sites, and seed sources. Cown (1992) suggests that the first-10 rings definition of the juvenile wood has severe limitations for radiata pine, where the properties of the inner rings varies markedly between stands. He suggested that an alternative approach may be to identify the most important juvenile characteristic(s) (none other than density were indicated), and define an appropriate juvenile/mature wood transition, based upon measurable end-use requirements.

Assessment techniques and technologies Addis Tsehaye et al. (1995) studied differences between wood density and wood stiffness as indicators of wood quality, and concluded that the quantity of structural timber can be increased if trees were selected on the basis of wood stiffness. They suggested that some means of in situ stiffness measurement of logs would be of great benefit and that this would enable decisions to be made for allocating logs to structural or other products. The same argument was suggested for the selection of

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trees for breeding Pinus radiata, indicating that MFA was the most suitable characteristic (Walker and Butterfield 1995). Jayawickrama (2001) reviewed the literature in relation to breeding pines for wood stiffness and also provided a good summary of the various acoustic technologies that are currently available for the evaluation of stiffness. Stress waves and vibrational techniques have been used for a long time for the non-destructive evaluation of wood properties. Ross and Pellerin (1994) presented a comprehensive review of techniques spanning early work, including vibration, stress-wave and ultrasonics from Jayne (1959), Japanese researchers, and others. Much of this early work was used for the assessment of small clear wood samples and board sized timber and in a few cases, logs (Arima et al. 1990). However, these have not been reviewed for use in logs or trees. Ridoutt et al. (1999) compared the use of pilodyn penetration method (indirect indicator of wood density) and longitudinal stress-wave velocity and found that a combination of sonic velocity and knot size gave the best improvement in structural grade recovery compared to processing unsorted logs. Pilodyn penetration was found to be useful for structural wood cut from the outer-wood (Ridoutt et al. 1999). The concept of juvenile wood was noted by Walker and Nakada (1999) to be imprecise because it is difficult to define the rapidly changing juvenile characteristics relative to the more stable outer wood. In addition, the effect of changes with height complicate the definition. Therefore they suggested that it is much relevant to identify logs that meet a minimum threshold quality requirement, and log sorting using acoustics provides a partial and valuable approach. It was further shown by Addis Tsehaye et al. (2000) that acoustic sorting of logs provides an opportunity to identify the best (high stiffness) sawlogs for the sawmill and that the financial returns from acoustic sorting of logs for structural grade wood appear attractive. Booker et al. (2000) evaluated various sonic and other approaches for measuring stiffness of green radiata pine logs and found that the best predictor of stiffness is based on a combination of resonant velocity, branch size index (size of knots), and small end stem diameter. An ideal solution would be to develop cost effective nondestructive evaluation tool that could assess the extent of juvenile core in standing tree - alternatively, a tool that can measure the stiffness of the standing tree.

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References Addis Tsehaye, A, Buchnan, Walker, JCF (1995) A comparison of density and stiffness for predicting wood quality. Journal Institute of Wood Science 13(6): 539-543 Addis Tsehaye, A, Buchnan, AH, Meder, R, Newman, RH, Walker, JCF (1997) Microfibril angle: determining wood stiffness in radiata pine. In: Butterfield, BG (ed) Microfibril angle in wood. University of Canterbury, Christchurch, pp 323-336 Addis Tsehaye, Buchnan, AH, Walker, JCF (2000) Sorting logs using acoustics. Wood Science Technology 34: 337-344 Amarasekara, H, Denne, MP (2002) Effects of crown size on wood characteristics of Corsican pine in relation to definitions of juvenile wood, crown formed wood and core wood. Forestry 75(1): 51-61 Archer, RR (1987) Growth stresses and strains in trees. Springer-Verlag, Berlin Germany. Arganbright, DG, Ventturio, JA, Gorvad, M (1978) Warp reduction in young-growth ponderosa pine studs dried by different methods with top- load restraint. Forest Products Journal 28(8): 47-52 Arima, T, Maruymura, N, Maruyama, S, Hayamura, S (1990) Natural frequency of log and lumber hit with hammer and applications to production processing. Proceedings, International timber engineering conference; October 23-25, 1990, Tokyo, Jpan Balodis, V (1972) Influence of grain angle on twist in seasoned boards. Wood Science 5(1): 44-50 Bamber, KR, Burley, J (1983) The wood properties of radiata pine. Commonwealth Agricultural Bureaux pp. 84 Beard, JS, Wagner, FG, Taylor, FW, Seale, RD (1993) Influence of growth characteristics on warp in two structural grades of southern pine lumber. Forest Products Journal 43(6): 51-56 Bendtsen, BA (1978) Properties from improved and intensively managed trees. ForestProducts Journal 28(10): 61-72 Bendtsen, BA, Senft, J (1986) Mechanical and anatomical properties in individual growth rings of plantation grown eastern cottonwood and loblolly pine. Wood and Fibre Science 18(1):23-38 Booker, RE (1987) A method for recording annual ring orientation in boards. Technical note. Forest Products Journal 37(6): 31-33 Booker, RE, Harrington, J. Shiokura (1997) Microfibril angle: determining wood stiffness in radiata pine. In: Butterfield, BG (ed) Microfibril angle in wood. University of Canterbury, Christchurch, pp 296-311

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