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CHAPTER 5

WOOD QUALITY: IN CONTEXT

SHAKTI CHAUHAN1, ROBERT DONNELLY2, CHIH-LINHUANG3, RYOGO NAKADA4, YIN YAFANG5 AND JOHN

WALKER

1 Institute of Wood Science and Technology, Malleswaram, Bangalore, India;2 Donnelly and Associates, Curitiba, Brazil; 3 orporate Research, Weyerhaeuser

Inc, Federal Way, Washington, USA;4

Tohoku Regional Breeding Office, ForestTree Breeding Centre, Takizawa, Iwate, Japan; Research Institute of Wood

Industry, Chinese Academy of Forestry, Beijing, China

1. INTRODUCTION

Context is everything. It informs, clothes and gives shape to the simplest idea. Inisolation wood quality has little meaning; only where set against a particular set ofend use requirements is it possible to categorize the desired wood characteristics and

properties that a particular product needs.The market imposes acute constraints on ood products. First a tenuous thread

linking wood characteristics to wood properties, to product specifications, andfinally to consumer desires and needs generates weak or confusing signals as towhat the market requires. Second, this is complicated by the long time, from 5 to 50years, between establishment and commercialisation. However, it is an exaggerationto see a long time horizon as being unique or relevant to current forest practice. Forexample, it can be 10 to 30 years between first discovering a mineral resource and

bringing that mine to production. Further, forestry is resolutely, if incrementally,moving to shorter rotations. Pulpwood crops on seven year production cycles are

routine in a number of countries in the southern hemisphere, while one should setthe bar at no more than 20 years for a short rotation sawlog regime.

Discussion of the influence of time fo forest practices is deferred until later. Atthis stage it is useful to consider further this tenuous thread linking the forestresource to consumer desires and needs, as examination brings clarity or at leastsystematic order to the concept of wood quality.

2. MARKET PULL OR PRODUCT PUSH

2.1. Buying tomatoes and selling wood (Dickson and Walker, 1997)

In supermarkets the fruit and vegetable section remains the last bastion ofcommodity trading. No one has succeeded in growing and supplying branded, fresh,

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122 SHAKTI CHAUHAN ET AL

Figure 5.1. Selection strategies for fresh tomatoes and wood: genetic modified fruit (marketpull) contrasted with classical breeding for tree form, vigour and health (product push).

flavoursome produce all year round. Consequently it is interesting to juxtapose the best efforts of the suppliers of fruit and of wood in order to understand thosefeatures in common and contrasting strategies (Figure 5.1).

To supply field-grown tomatoes, the essential input requirements are disease andherbicide resistance, as well as lower production and harvesting costs; passed onthrough the intermediary where shelf-lif , firmness and solids-content (for ketchup)

are critical: and so to the consumer who seeks good-looking fruit, together withretention of freshness, taste and texture. It does not end there: the buyer is thinkingof eating the fruit or delighting a com anion with its succulence. In responding tothis market pull the breeder has sought primarily to neutralize the genes responsiblefor softening. This allows harvesting to be deferred while the tomatoes develop fullflavour and texture before passing ripe but firm fruit down the supply chain withminimal risk of spoilage. Juxtapose this with the current commodity practice of

premature picking of hard, tasteless, green tomatoes, trucking in cooled containersbefore finally ripening with ethylene gas. No wonder consumers are willing to pay ahuge premium of 50-250% for fully flavoured fruit over the price for the generic

product. In this context disease and herbicide resistance, as well as lower productionand harvesting costs, are merely essential prerequisites for a successful enterprise.The real goal is to meet the consumers desire for flavour, freshness and texture.

Contrast the tomato on a dinner-plate with a stick of lumber in the DIY store(which itself is a recent global phenomenon). Of course this comparison involves anelement of exaggeration. Modern plantation forestry is just 25-150 years old,depending on which species, industry and country one considers. Historically, thetree breeder focussed on the interests of the forest grower and the complaints of thesawmiller. It was reasonable that stem straightness, vigour and health should be

prime candidates in the first round of genetic improvement. However, althoughhighly appropriate for the forest manager and essential prerequisites to a


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successful forest enterprise these attributes are irrelevant to the desires and needsof the consumer, who is more likely to be interested in colour, odour and attractivegrain. Market demand is about aesthetics reflecting local culture, beliefs and

preferences as well as the ephemeral fashions of the time.

Maybe Figure 5.1 is a slight exaggeration, but it is only in recent years that treebreeders have even contemplated increasing density or modifying wood chemistryand structure. Commercial deployment of trees with improved intrinsic woodqualities is in its infancy.

2.2. The wood quality chain (Walker and Nakada, 1999)

Table 5.1. Traditional perspectives of forest and wood quality do not identify with the pull ofthe market. Industry must switch from being production-driven to being consumer-led inwhich individual preferences matter. This table ignores non-technical issues such as cost,

sustainability and eco-labelling.

onsumer desires Market concept

Housing

Furniture

Newsprint

Selling an investment: status, style and substance

Selling a dream: furniture is a statement of your personality

Selling advertising, sport, sex and politics

Wood characteristics Wood/paper properties Desired attributes and product

specifications

Density, i.e.

cell diameter and wallthickness

Microfibril angleReaction woodSpiral grain

PermeabilityHeartwoodExtractives

Tracheid lengthCoarsenessMore celluloseLess lignin

Stiffness

Longitudinal shrinkageStability/warp-free

Machinability/finishFigure/grain/textureHardness

Ease of dryingColourOdour

Tear strengthBrightness (lessbleaching)

Construction machine stress

grades: requiring stiffness,straightness and stability

Furniture quality finish: calling forhard surfaces; tight joints and littlemovement in service; ability to stain

Newsprint high speed printing,resilience, adequate brightness,opacity, low-cost

Paperboard and packaging requiring good handling andserviceability (burst, wet-strength),clear print

A hierarchical succession of steps marks out the wood quality chain, beginning withthe intrinsic characteristics and features of the wood cell (Table 5.1). The spatialarrangement hints at the relationships between characteristics, properties and

product specifications, but they are less obvious than is usually acknowledged. For

furniture one would expect adequate density (revealed in properties such as stiffness,strength and hardness), straight grain, absence of reaction wood and tight knots



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(easier to dress and finish) or conversely cross-grain (harder to machine but withmore figure), extractives or heartwood (distinctive colour) etc. The desired productspecifications for furniture are exemplified by surfaces that dont mark easily(requiring hardness), in tight joints and little movement (low shrinkage), and an

ability to stain uniformly (porosity of the surface) etc. Even when one selects amaterial that meets such criteria time and markets may decide otherwise silvicultural strategies are prey to future uncertainties. Wood (solid, veneered or

panel) is only one possible platform competing to satisfy the aspirations of theconsumer glass or metal may be in fashion. Who specifically needs preferswants warp-free wooden mouldings, stiff timber studs or even real solid wood forthat matter? The consumer presumes an acceptable and functional level of woodquality and focuses instead, and rightly, on what is wanted style, status and(financial) substance; for colour, odour, a fresh feel and creativity. Inescapably,most woods are commodities.

The paradox is that while industry may seek improved product specifications, thedelivery requires attention at a more fundamental level in the selection for improvedwood characteristics and properties. Selection implies choice and a focus on only afew characteristics. But first, one needs to understand how these key characteristicsand properties relate to one another.

3. INDUSTRY REQUIREMENTS

Each industry has its own distinctive set of requirements for the type and quality ofwood and each has to contend with a very variable resource. However, a quality

resource for chemical pulp is not the same as that sought for particleboard or evenfor mechanical pulp. Fortunately this allows each industry to compete for thatsegment of the wood supply that it can use best. The prices offered, as aconsequence of their differing assessments of the quality of a particular resource,determine who purchases that material. A sawmill will pay more for a large log thanfor an equivalent volume of smaller wood, because lumber can be cut moreeconomically from large logs and generally a better grade is obtained. The fibres ofthe discarded slabwood from a large log differ little from those in the adjacent sawntimber and are ideal for making strong paper by kraft pulping. Fortuitously, the pulpmill buys these slabwood chips at a fifth-to-tenth of the price paid for the sawlog

from which they are derived. Small top logs are very satisfactory for mechanicalpulp and particleboard manufacture and can also produce adequate kraft pulp. Eachindustry applies its particular selection criteria, e.g. log diameter and length, branchdistribution and size, density etc. for sawlogs; cell wall thickness, fibre length andfibre content for pulp logs. Price is synonymous with quality only if identicalselection criteria apply across a range of end uses. On the contrary, even softwoodlog grading rules for lumber, which classify material according to its appropriatenessfor that purpose, distinguish between board grades with desired visual features andstructural grades which emphasize stability, stiffness and strength. Consequentlysawmills cutting board and structural grades evaluate logs differently.


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The wood quality of a fast grown plantation species differs markedly from thesame species growing, often to a much greater age, in its natural environment (Baoet al., 2001). Plantation timber is a new timber, in terms of its changedcharacteristics and properties, for example the trade in New Zealand has always

distinguished between old-growth Oregon imported from North America and thelocal, plantation-grown Douglas fir.

In Brazil the principal eucalypts are E. grandis, E. saligna, E. urophylla andseveral others. However, more than half the plantations owned by the papercompanies are hybrids, most notably E. grandis x E. urophylla ( E. urograndis).Since there has been so much breeding to develop specific characteristics and

properties and so much deliberate hybridisation, at times it may be more appropriateto discuss density groups of eucalypts rather than species. An example of the hugevariability can be seen at Aracruz, who developed clones ofE. grandis as well as ofthe hybridE. urograndis with a wide range of basic density. Aracruz Wood Products

advertises Lyptus lumber (eucalyptus) available in densities ranging from 450 to750 kg m3

4. SPATIAL DISTRIBUTION WITHIN TREES

Traditionally, around the world the terms juvenile wood and mature wood have beentaken to relate to cambial age, i.e. juvenile wood is the wood surrounding the piththat is formed by the young ( juvenile) cambium. Confusingly, in some SouthernHemisphere countries the terms corewood and outerwood refer to the same radialgradient in wood quality. To avoid or add to (?) any potential confusion, this text

follows the new convention proposed by Burdon et al. (2004) that has yet to achievebroad consensus.

4.1. Corewood and outerwood

Wood quality varies within trees both in the radial and axial directions. Burdon et al(2004) propose a two-dimensional framework with the radial variations described interms of corewood and outerwood and the axial variations described in terms of

juvenile and mature wood. Arbitrarily, corewood has been described as a cylindricalzone enclosing the first few growth rings around the pith. Typically, for fast grown

pines this zone around the pith is considered to be of poor quality, having a numberof undesirable features (Zobel, 1975):

A low basic density in the corewood, primarily a consequence of thin cellwalls and the formation of relatively little latewood, means that the timber is lessstrong. A high moisture content (before heartwood formation) and a low basic densityin the corewood means the green density of young thinnings or top logs exceedsthat of mature butt logs and harvesting costs are high per tonne of oven-dry fibre. Longitudinal shrinkage is greater (> 1%) making sawn timber and plywoodless stable products. This is a consequence of both a larger microfibril angle (30-

50 ) in the S2 layer of the wall and spiral grain.



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There is a tendency for fast grown corewood to contain above averageamounts of compression wood. Production from a chemical digester is reduced as the lower basic densitymeans that the amount of oven-dry wood fibre within the digester is reduced.

Furthermore, corewood has a lower percentage of cellulose. Fibres are shorter than in outerwood, giving chemical pulps that have lowertear strength.

The properties that most prejudice the use of corewood as lumber are its lowstiffness and strength, together with poor stability. In turn these are a consequence ofthe low density, large microfibril angle and spiral grain, all of which are quitestrongly heritable. Thus there are prospects for improving the quality of corewood infast grown softwoods by selection and tree breeding.

Any definition of the corewood zone is arbitrary. Jane (1956) observed that the

period during which corewood is produced varies amongst trees and even inindividuals of the same species, but, in general, it is safe to assume that wood afterabout the 50th growth ring will possess the structure of outerwood. Jane wrote at atime when wood production was largely from abundantly available, old-growthtimber. More recently Harris and Cown (1991) discussing plantation grown radiata

pine noted that for sawn timber, its [corewood] most damaging features will beconfined to the first 3-5 annual growth layers from the pith yet outerwood inwhich all wood properties including density have stabilized may not be developeduntil after the 25-30 growth layers. For a 25 yr-old radiata pine it has beenconvenient to describe corewood as occupying a cylindrical zone enclosing the first

10 rings; this means that the proportion of corewood in the log increases from asignificant 35% in the butt log to 50%, 60%, 75% and 90% in other logs further upthe stem (Cown, 1992), i.e. 50% of the merchantable timber is corewood.

Figure 5.2. Typical density profile for pine. The introduction of a transition zone is not

particularly useful.


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The defining feature of corewood is that wood characteristics and properties arechanging rapidly, whereas in outerwood the changes are gradual at most: but forDouglas fir the corewood zone is quite prolonged and changes more gradually whencompared with that forPinus radiata in New Zealand.

As the rate of change diminishes it is questionable whether the wood is stillcorewood, resulting in the curious proposal of introducing an indefinite transitionzone with equally arbitrary boundaries of its own (Zobel and van Buijtenen, 1989).In practical terms the defining feature of corewood is the steep initial gradient inwood quality over the first few growth rings (Figure 5.2). Based on graphic solutionmethods, the corewood zone of loblolly pine ranges from 4-8 years in the coastal

plain to 10-12 years in the piedmont areas of the southeastern US (Clark andSaucier, 1989).

4.2. Juvenile and mature wood

Biological logic argues for a two-dimensional characterization of wood properties:juvenile vs mature for the progression up the stem, and corewood vs outerwood forthe radial progression (Burdon et al., 2004): wood quality improves from pith-to-cambium (from corewood to outerwood); and it improves from ground level upthe stem (from juvenile to mature wood) but most obviously over the lowest 3-5metres. The variation in wood quality up the stem, while less prominent than the

pith-to-cambium variation, still has important implications, most acutely for the buttlogs of fast grown pines. Changes from juvenile to mature wood appear largely to beexpressed in progressively more moderate microfibril angles on moving up a fewmetres from the base of the tree. This generates a series of conic sections tapering

upwards over the first 3-5 metres, above which each conic section becomes acylindrical section extending further up the stem: this 3-5 metre juvenile zone isreflected in the butt-swell and the propensity to form compression wood (Burdon,1975) in young trees. Such juvenile-mature wood changes match the classicalconcept of maturation (foliage, stem morphology and onset of reproduction). Thereis limited evidence that where cuttings or grafts from mature trees are planted theyoung trees express mature wood features from the outset.

While the juvenile zone only extends upwards for a few metres, this zone isencompassed by the butt log that, by tradition, is valued because of its size. As withthe corewood-outerwood boundary, there are tree-to-tree and species-to-species

variations in the vertical extent of juvenile wood and in the rate of change in woodquality toward the tree top. Cuttings from older branches lack juvenile responses tosilvicultural practices and the benefits of physiologically aged cuttings are improvedform and stiffness, i.e. minimizing the juvenile core but at the cost of some loss ofvigour.

4.3. Plantations and natural forests

The exhaustion of natural forest resources is forcing industry to come to terms withthe corewood of plantation timber. As its properties are better understood industry isfinding appropriate end uses that reflect the intrinsic properties of the individual



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piece of lumber. There is enormous variability in wood quality between trees so thatcorewood of one tree can be of better quality, e.g. of higher stiffness, thanouterwood of another tree.

The psychological significance of corewood should be appreciated. Historically

the softwood industry of the Pacific Northwest of North America relied on old-growth, and more recently second-growth, Douglas fir. In the former case the basicdensity is about 15-25% greater than that found in fast grown plantations (< 30 yearsold), which contain a very high proportion of corewood. Sawmillers have had toaccept the fact that high density material, > 500 kg m3, coming from stands 75years or older is being replaced by lower density material, < 450 kg m3, from fastgrown intensively managed stands that are only 40-50 years old.

ndeed in some parts of the world plantation-grown trees harvested forcommercial uses are composed entirely of juvenile wood, examples being 10-15 yr-oldPaulowania andPopulus sp. in parts of China (Bao et al., 2001). Fortunately the

differences between corewood and outerwood, and between the juvenile wood andmature are not nearly as obvious in hardwoods (or maybe they are less wellcategorized).

5. DENSITY

Even today, to many people density is synonymous with wood quality: this is a truththat misses the point. Certainly improving wood quality through selection of higherdensity material was justified in the 1950s to 1970s and is justified still whenscreening a new plantation species. For both hardwoods and softwoods density is

strongly heritable which favoured its inclusion in early tree improvement programmes. Further, the genotypex environment interaction is often low so thatany improvement should be sustained across a variety of sites.

However, plantation forestry today no longer resembles the one that gave rise tothat original insight. For example, discussion on wood quality often revolves arounddistortion, instability and stiffness, mirroring the preoccupations of the sawmillingsector. Yet none of these properties is affected by density in the manner so popularly

presumed. Density does not predispose lumber to behave in a particularly way, itmerely magnifies the effects of other intrinsic wood quality characteristics, whethergood or bad. Density describes the quantity of matter, not its intrinsic qualities.

Today density has become a concave/convex fairground mirror that, while revealing,also distorts the reflection.

For almost all softwoods and low-to-medium density hardwoods increasingwood density has been ranked above all other desirable objectives in traditionalwood quality improvement programmes. Basic density provides an index of woodquality to which all end users are able to relate. To the sawmiller a high densityindicates the timber will be stiff and strong; to the pulpmill it indicates that a givenvolume of wood will yield more pulp than would a low-density timber. But too higha basic density (> 600 kg m3) more a problem with some hardwoods is asundesirable as too low a basic density (< 400 kg m3) more a problem with

softwoods (alsoPaulownia and Populus sp.). For woods above 600 kg m3, furniture


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and building timbers are uncomfortable heavy, often unnecessarily stiff and strong,and joints can open if under-dried. For pulp it means that the dense fibres willrequire prolonged beating before the lumens collapse to give a dense, well-bondedsheet of paper.

Obviously basic density of wood relates to the amount of dry matter per unit ofgreen volume. The compound characteristic of density (Figure 5.3) means that twosamples can have the same density but very different wood and fibre properties.Differences are due to interactions between wall thickness and cell diameter, the

proportion of the cell wall that is occupied by the S2 layer, the amount of cellulose inthe wood (age related) and the microfibril angle in the S2 layer (Cave, 1969).Compression wood has a large microfibril angle and inflated density wherecompared with those of normal wood of the same age and the same height. For agiven density, samples with compression wood have poorer mechanical properties,and its stiffness is not related to density (Dohr, 1953). For severe compression wood,

there is a slightly negative relationship between density and stiffness.Further basic density is complicated by the presence of extractives, which vary

from less than 1% of the oven-dry mass in sapwood to well over 10% in theheartwood of some species. Extractives increase the weight of a wooden memberwithout contributing to its strength, and consume chemical without contributing tothe pulp yield. In rigorous studies the extracted (extractive-free) basic density may

be needed in order to compare pulp yields or mechanical properties between samplesor species.

Density has proved to be a useful, surrogate indicator for many properties ofwood and paper. Its other virtues are that it can be measured reasonably quickly and

cheaply. In essence an increment core taken at breast height is used to predict themean density of the tree.

Figure 5.3 Both examples have the same basic density, but one has four-times fewer fibreswhose walls are 50% thicker.



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The principal sources of variation in wood density relate to:

Within-ring variations; Within-tree variations;

Between trees on the same site; Between populations of the same genotype growing in different regions.

5.1. Within-ring variations

Most species, apart from Araucaria sp. and diffuse-porous hardwoods, showcontrasting differences in wood density across the growth ring. This is a response toseasonal climatic variations and the formation of latewood. The density variationacross a growth ring far exceeds the density variation between trees. As an extremecase, Harris (1969) cites the contrast between latewood (870 kg m3) and earlywood

(170 kg m3

) in adjacent growth rings in the outerwood of a sample of Douglas fir, Pseudotsuga menziesii. A more typical within-ring and between-ring variation forDouglas fir is shown in Figure 5.4. Douglas fir is used as a furnish for fibreboard soany mixture of species having widely different densities should be as acceptable.

The coarse texture of Douglas fir creates problems in wood use. It is onlymoderately easy to work. Care is needed in planing because the soft earlywood may

be compressed/crushed rather than cut, recovering slowly to create a corrugatedsurface. The strong contrast in hardness between earlywood and latewood makes foruneven wear. The surface does not paint well, and early failure on the broadlatewood bands of flat-sawn material is sometimes experienced. Nailing can cause splits.

Differential glue absorption between earlywood and latewood can cause starvedjoints. Differential shrinkage between earlywood and latewood can cause latewoodbands to shell out on weathered flat-sawn surfaces.

With all these shortcomings it is surprising that Douglas fir has gained such wideacceptance as one of the finest softwoods in the world (Harris, 1993). However, itsavailability in large sections and long lengths, its excellent strength properties(modulus of elasticity c. 13 GPa), its stability in use, and moderate durability out ofcontact with the ground, have won it widespread acceptance for structural use at alllevels from domestic to heavy industrial. Except where knots are large or growthrings are wide, properties of Douglas fir stiffness and straightness are acceptable:

unlike many pines the microfibril angle of its corewood is generally less than 35o

Considering some of the difficulties outlined above with respect to machining andgluing, the outstanding position of Douglas fir plywood on world markets may alsoappear somewhat surprising. Here, too, it seems that strength and axial stability areits strong points, but in addition technology has played a major part, particularly indefining the log characteristics suitable for veneering, and in developing appropriategluing and manufacturing systems. It is questionable whether Douglas fir willcontinue to be perceived so favourably when supply is largely from plantations.

Pinus caribaea, P. merkusii andP. oocarpa in Malaysia produce little latewoodduring the first two to four growth layers, but latewood develops strongly thereafter.

Even with subsequent latewood formation the contrast between earlywood and


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Figure 5.4. Within and between-ring variations in basic density with distance from the pith

for Douglas fir (Weyerhaeuser, unpublished).

latewood density is low, approximately 1:1.5, whereas the corresponding ratios for Pinus radiata, P. taeda and Douglas fir are 1:1.8, 1:2.3 and 1:5.0 respectively(Harris, 1973). The modest differences in density between earlywood and latewoodmeans that these tropical pines and P. radiata can be described as even textured,while their wide growth rings would classify them as coarse grained. Northernspecies such as spruce and hemlock are much sought after for certain pulps onaccount of their uniformity of density: with these species the difference in density

between earlywood and latewood is comparatively small and the transition from

earlywood and latewood is gradual.

5.2. Within-tree variations

For the hard pines, Douglas fir and some other, but by no means all softwoods, thewood adjacent to the pith is of lower density and is of poorer quality than the woodin the rest of the tree. Many important plantation species are found in these groups.In contrast, for Cryptomeria sp., true fir, hemlock and spruce the basic densitydecreases for the first few annual rings from the pith before levelling off orincreasing moderately toward the cambium.



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Figure 5.5. Within-tree variations in density for New Zealand radiata pine (Cown, 1992).(a) Basic density in the tops of old trees is similar to that from 10 yr-old trees. (b) Basicdensity increases with physiological age: the butt log of an old tree has proportionately moreouterwood and has a higher basic density. (c) Green density is highest in top logs where thebasic density is lowest and the moisture content corresponding to full saturation is very high.


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or hardwoods, the situation is complex.

All possible patterns of wood density variation appear in hardwoods. The middle tohigh density diffuse-porous hardwoods generally follow a pattern of low basic densitynear the pith and then an increase, followed by a slower increase or levelling off towardthe bark. The low density, diffuse-porous woods, such as Populus, seem to have asomewhat higher density at the pith, although some have a uniform density from pith to

bark [while Populus deltoids increases in density from pith to bark (Shukla et al.,1994)]. The ring-porous hardwoods tend to have a high density at the centre, whichdecreases and then increases to some extent toward the bark (Zobel and Buijtenen,1989).

As just noted, for many plantation species the corewood is of lower density thanis the outerwood. Further Figure 5.5 indicates that there is little difference in basicdensity between the corewood-mature wood zone in the topmost part of the stemand the corewood-juvenile wood in the butt log that had formed years earlier whenthe green crown of the younger tree was much lower.

On progressing up the tree there is proportionately more corewood, and the basicdensity of the stem-section decreases while both green density and moisture contentincrease. Thus young thinnings, top logs and whole trees when grown on shortrotations contain less biomass and more water, such that young radiata pine logsweigh over 1000 kg m3but contain only 400 kg m3 of oven-dry fibre (Figure 5.5).

This (arbitrary) transition from corewood to outerwood occurs between the fifthand thirtieth growth ring from the pith depending on the species and thecharacteristic or property being examined (Zobel and Buijtenen, 1989). Theysuggest that the corewood zone so far as basic density is concerned coincides withthe first 5-6 rings forPinus elliottii, P. caribaea andP. radiata, the first ten rings for

P. taeda and twenty rings or more forP. ponderosa. With New ZealandP. radiatabasic density increases quite markedly for the first 10-15 rings changing only slowlythereafter (Figure 5.6) but by convention corewood is taken to be the first 10 rings.But equally there is logic in defi ing the outerwood as beginning when the basicdensity exceeds 400 kg m3, in which case corewood would be restricted to only 5rings in low latitudes but would extend out to 15 rings in the south of New Zealand.

These corewood-outerwood density trends, when compounded by the highproportion of corewood in fast grown, short rotation plantations, have given rise tothe mistaken perception that fast growth per se is detrimental, whereas it is the

preponderance of poor quality corewood in the fast grown tree that is the critical

feature. Corewood can account for 50% of the stemwood of 25 yr-old well thinned,fast grownPinus radiata.

Growth rate has little effect on the wood properties of diffuse-porous hardwoods.These have approximately the same proportion of vessels across the annual ring,regardless of the growth rate. However growth rate has a noticeable influence on thedensity of ring-porous hardwoods, which usually produce denser wood when fastgrown. The volume of low density vessel tissue produced early each year in a ring-

porous hardwood is constant regardless of the total radial growth during each growthperiod: therefore the wider the growth ring the smaller the proportion of earlywoodwith its vessel tissue and the greater the proportion of denser, vessel-free latewood.



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Figure 5.6. Within-tree variations in extractive-free basic density of radiata pine from variousregions in New Zealand (Cown, 1992). Temperature and rainfall are major influences onwood density, outerwood being more sensitive than corewood: outerwood density decreases

by 7 kg m3 per degree increase in latitude and for each 100 m rise in altitude .

Bhat and Indira (1997) and Bhat (1999) observed that faster growth (6.00 mm/yrvs.2.8 mm/yr in the control) in 5 yr-old teak trees with the application of fertilizerresulted in wood with a lower vessel percentage and an 8% increase in density.

5.3. Within-stand variations

Regardless of species or where the forests are established, the variation in wood properties between trees is considerable. For a typical stand of Pinus radiata the

range of basic density is shown in Figure 5.7. Here the unextracted basic density in

the corewood rings 5-10 is 348 22 kg m3, whereas in rings 18-22 it is 430 30 kgm3 (both with a coefficient of variation of c. 7%). Where tree improvement

programmes emphasise high basic density the between-tree variations will bereduced and the distribution will centre on the medium-to-high density range shown(Figure 5.7). Unfortunately the absolute variation in corewood density is less thanthat of outerwood so an equivalent increase in corewood density is harder toachieve, in which case the within-tree variability may actually increase.

Another example of within-stand variability relates to Douglas fir (Figure 5.8).The basic density of the cross-section decreases on ascending the stem, but, because


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Figure 5.7. Variations in extracted basic density within selected stems from a typical 24 yr-old stand of radiata pine in the central North Island of New Zealand (Cown and McConchie,1983). Ten trees were chosen after assessing outerwood density at breast height in 193 stems

using increment cores (unextracted densities of 430 30 kg m3 with a range from 357 to

512 kg m

3). For the low, mean and high density stems the mean whole-tree basic densities(unextracted) were 354, 380 and 395 kg m

, while the corresponding outerwood basic

densities (unextracted) were 375, 433 and 494 kg m

.

the between-tree distributions are so broad, the section density at the top of one treemay exceed the section density at the base of another tree. For a comparable sample(same age, same site, etc.) a between-tree variation in average cross-sectional basicdensity of 15 to 25 kg m3 can be expected. High density trees tend to have bothmore latewood and higher average earlywood and latewood densities. Unfortunatelyin the short term only a 5% overall increase in wood density of Douglas fir is likely

from genetic improvement which will do little to offset the effect of lower densitydue to the shorter rotations envisaged in the Pacific Northwest (McKimmy, 1986).

5.4. Inter-regional variations

The environment exerts strong control over average basic density. Figure 5.6illustrates the variation for radiata pine plantations across New Zealand. Forindustries merely seeking biomass that is a compelling reason for locating to lowerlatitudes (Auckland) where, for example, the wood of ring 20 is some 20% greaterthan that of the southerly forests (Southland). The general trend is one of higher

basic density with lower altitude or latitude of the site. The effect is less obvious inthe first 5-10 growth rings.



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Figure 5.8. Frequency distributions at several points up the stem illustrating the variations incross-sectional basic density amongst sixty-four Douglas fir trees in a 55 yr-old stand. Arrowsindicate the distribution means (Megraw, 1986).

Figure 5.6 provides the broad conceptual approach. Like all big pictures itoversimplifies. The line for each region is constructed by collecting data from treesin 5-ring increments (the cross-sectional averaged increment core density for alltrees, taking rings 1-5, 6-10, 11-15, 16-20, 21-25, 26-30, 31-35 and 36-40) and

averaging again for all trees. This introduces uncertainty. Downes et al. (2002) have


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Figure 5.9. Density of radiata pine shows no latitudinal gradient in Chile; the average age of

the stands is 22 (Delmastro et al., 1982).

noted that true underlying trends can be obscured by such clumping of data. Thus apredetermined decision to define corewood as the first 10 rings results in samplingin 5-ring increments or more correctly the methodology predetermined thedefinition of corewood. Raymond and Anderson (2005) observed that forPinusradiata in New South Wales basic density is constant over the first 8 growth rings

before abruptly increasing. This contradicts the traditional picture of pine corewood.While the north-south trend of declining density appears so logical, it is worth

noting that same north-south trend for the same species in Chile (Figure 5.9). The

provinces/forests are spread along a latitudinal distance of over 1000 km, yet there isno discernable density gradient. One explanation is that the rainfall gradient (drier inthe north) counters the temperature gradient; equally Chile has two mountain ranges

parallel to the Pacific Ocean with great east-west influences over temperature andrainfall; and locally there are different conditions of water availability during the

period of latewood formation. The desire to generalise should not deny the need toparticularise. Decartes axiom e omnibus dubitandum(doubt everything) is usefuladvice.

The wide distribution of Douglas fir may be of more general interest. Naturalpopulations of coastal Douglas fir include the lower elevations of British Colombia

through to the higher elevations in northern California; whereas east of the Cascaderange, inland Douglas fir grows in a warmer, drier environment. Across these sitesthere are significant differences in the density of Douglas fir (Figure 5.10).

5.5. Density: a pragmatic convenience masquerading as insight

Many have claimed that density is the most important characteristic indetermining wood properties. For example, Zobel and van Buijtenen (1989) argue:

[basic density] is of key importance in forest products manufacture because it has amajor effect on both yield and quality of fibrous and solid wood products and because itcan be changed by silvicultural and genetic manipulation. Therefore, [basic density]

380

400

420

440

460

480

500

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Basicdensity[kg/m3]



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largely determines the value and utility of wood and overshadows the importance ofother wood properties.

The basis for such claims lies in the general observation that denser timbers arestiffer and stronger. In Figure 5.11 mean whole-tree stiffness and density are

plotted against one another species-by-species using North American and Europeandata from Table 4-11a in the USDA Wood Handbook (1999); and Table 2 in Lavers

Figure 5.10. Variations in the mean basic density of Douglas fir growing in various parts of

the United States (USDA, 1965).


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139

Figure 5.11. Mean whole-tree values of stiffness and density for hardwoods and softwoods.Superimposed is the range in values expected of lumber fromPinus radiata and Eucalyptusnitens. Variations within species are greater than differences between mean values for species.

(1974). Expressing the relationship in the form MOE = k (density)n, then n is either

0.7 or 1.01 for hardwoods and 0.85 or 0.82 for softwoods according to the US andEuropean data sets. Using the mean values for species, the data indicates that, at

best, if you double the density you dont even double the stiffness: there is noleverage.

Whole-tree data ignore variations in density and stiffness within trees. Stiffer andstronger outerwood happens to be denser than corewood. However, it is necessary todistinguish between the density of a wood, which is mere mass (the quantity ofmatter), and the intrinsic characteristics of the cell wall (the quality of matter). Thuson going from pith to cambium in pine the mean density might increase by 50%(Figure 5.6) and so one would conclude on the basis of mere mass that the

outerwood would be 50% stiffer, yet the increase in stiffness is typically three-fold.Fujisawa et al. (1993) compare density trends in a number of sugi clones. Sugi,

Cryptomeria japonica, is one of those softwoods where density decreases withdistance from the pith (Figure 5.12). Again for sugi, Hirakawa and Fujisawa (1995)compare density and stiffness in corewood and outerwood. They observe that thecorewood is denser and less stiff than the outerwood for fast and slow growingclones (Table 5.2). There is a negative correlation between density and stiffness.

Finally, a simple study by Simperingham (1997) demonstrates that theassumption that low density corewood produces wood of low elastic modulus is notnecessarily valid. Here 100 x 100 mm members were cut enclosing the pith, i.e. all



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Figure 5.12. Clonal variation in air-dry density of sugi, Cryptomeria japonica, with ringposition (Fujisawa et al., 1993).

Table 5.2. Diameter at breast height, density and clearwood stiffness (MOE) for select 30 yr-old clones of sugi, with high and low stiffness clones being compared in three growthcategories (Hirakawa and Fujisawa, 1995).

Clone DBH, cm Air dry density, kg m3 Youngs modulus, GPa

Corewood Outerwood Corewood Outerwood

Fast growing Takahagi 16 36 351 293 4.9 5.8Kooriyama 1 35 317 272 2.9 3.2Medium growth

Taga 6 26 421 354 6.8 7.0Usui 3 27 380 350 3.3 3.6Slow growing

Takahagi 15 19 413 375 7.3 7.5 Nakoso 1 16 322 314 3.5 4.8

the pieces were corewood with no more than five growth rings. The members wereripped into two pieces, kiln-dried, dressed and tested in bending (both on edge andon face) and their air-dry densities measured. The interesting feature was that thewith-pith lumber varied in stiffness by a factor of three (from 3 to 10 GPa) whereas

the air-dry density ranged from 320 to 500 kg/m

3

. In this study there was nocorrelation between stiffness and density (Figure 5.13).


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141

Figure 5.13. Modulus of elasticity (MOE) and unextracted air-dry density for with-pithradiata pine (Simperingham, 1997).

Perhaps the last word should be that of Uprichard (2002):

There appears to be a general assumption, based on little evidence, that a tree suitablefor pulping and papermaking will be unsuitable for timber, since softwood trees of veryhigh density are unsuitable for papermaking. In the context of radiata pine, with itslimited density range [italics added for emphasis], such arguments appear irrelevant.

The purpose of this section has been to emphasise that density describes thequantity of matter and not the intrinsic qualities of matter. Both vary within and

between trees. Generally one can find some statistical relationship between the two.Thus there is an indirect utility for line managers who can be exact in theirrequirements for density, e.g. sawmills may divide their wood supply into densitysorts to meet different processing issues (drying) and market needs. However,density does not predispose lumber to behave in a particularly way, it merelymagnifies the effects of intrinsic wood quality behaviour, whether good or bad. Forexample outerwood is stiffer and more stable, not because the wood is denser but

because the microfibril angle is lower. Indeed as will be discussed in Chapter 6, it ismore efficient biologically for the tree to change the microfibril angle in the cellwall to achieve superior stiffness than to achieve the same end result merely by

having more cell wall material.



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Product specifications can set desirable minimum and maximum thresholdvalues for density. This was anticipated in a more nuanced interpretation by Nichollsand Dadswell (1965) of the consequences in selecting for density.

The criterion associated with wood density is subject to qualification. Though an

increase in density is associated with better strength properties in sawn timber, and, inthe case of pulping processes, produces greater yield of pulp for a given volume ofwood, high basic density may not always be desirable. Because a close correlationexists between percentage latewood and basic density in coniferous timbers, and aslatewood fibres are observed to have much thicker cell walls than those of theearlywood of the same growth ring, high basic density will be associated with a

preponderance of thick-walled cells. Not only may a high latewood content beunsatisfactory in a utility softwood from the point of view of uniformity of texture, andnailing and working quality, but it can also adversely affect wood properties. Pulpsmade only from latewood fibres have different paper-making properties to those

produced from earlywood alone, and it has been suggested that forP. radiata less than20% of latewood would be satisfactory for general purposes (Watson and Dadswell,

1962). The criterion for basic density selection, therefore, is not straight forward, and itmay well be that improvement in uniformity is more important than high or low basicdensity. Zobel (1963) is of the opinion, in fact, that not only could a much more uniformwood be produced for breeding, but that improved uniformity alone could repay the

breeding costs many times over. Fielding and Dadswell (1961), speaking specifically ofP. radiata, thought that little purpose would be served by attempting to breed for greaterstrength in this wood, but that greater tree-to-tree uniformity of strength is desirable.

6. TREE FORM: SIZE, COMPRESSION WOOD AND KNOTS

Rightly, initial tree improvement programmes placed particular emphasis on fast

growth, forest health and good tree form (st aightness of stem; and the lightness andfrequency of branching).

Fast growth reduces the time needed to produce a commercial log of the desiredsize. Fast growth per se does not greatly influence wood quality (except for ring-

porous hardwoods). However, in a fast grown stem of particular diameter theproportion of corewood is greater than in a slower grown tree and it is this feature ofplantations that has the greatest downside influence on wood properties.

Improved tree form increases the value of the log and reduces harvesting andprocessing costs. Stem straightness and light branching improve wood quality in thatthere is less reaction wood in the stem (especially near the pith), small flat-angled

branches are less likely to have ingrown or encased bark trapped at their upperjunction with the stem, small branches are less resinous, and the volume of reactionwood associated with the knot is reduced. Fortunately severe reaction wood isnegatively correlated with stem straightness that is under strong genetic control, soselection for straight stems reduces the severity of reaction wood.

With softwoods the average knot volume in a stem is generally between 0.5-2.0%, although the volume of wood affected by knots is much greater. Thealignment of the axial tracheids is disturbed as they sweep around the knot and thevolume of disturbed wood tissue is 1-3 times that of the knot itself. Knot volume is

proportionately greater in young trees and in open grown stands ofPinus sylvestris


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143

Table 5.3. Percentage of knotwood within the stem of 44 yr-old Scots pine, Pinus sylvestris,on various sites in Sweden (Nylinder, 1958; see Timell, 1986, p. 924).

Spacing of trees in the stand [metres]

0.75 x 0.75 1.5 x 1.5 3.0 x 3.0

Position as apercentage of

stem height Percentage of knot wood

90-100 1.55 1.92 1.9880-90 1.77 1.94 2.2670-80 1.63 1.69 1.9760-70 1.14 1.09 1.5150-60 0.74 0.88 1.2340-50 0.52 0.53 0.9830-40 0.34 0.44 0.9720-30 0.18 0.30 0.7510-20 0.11 0.25 0.710-10 0.11 0.20 0.55

Average 0.44 0.52 0.92

(Nylinder, 1958) where the trees are more vigorous, resulting in larger branches andless branch mortality or suppression (Table 5.3). Wide initial spacing and thinningfavour fast growth, a large corewood zone, stem taper and large branches that areslow to self-prune. Branches grow until they experience canopy closure and the timeto closure is one factor determining branch and knot size. Knot volume increases upthe stem as the taller dominant trees acquire growing space at the expense of theirsuppressed neighbours. Similarly branch size increases with the quality of the site,

which promotes vigorous growth. The deeper green crown in thinned standspromotes the incidence of larger branches.In Nylinders study the average diameter of knots at breast height was only 9.7

mm where the spacing was 0.75 x 0.75 m as against 19.6 mm at 3 x 3 m spacing.Knots are small compared with those in fast grown softwoods in warmer climates.For example, the average knot size in 30 yr-old New Zealand grownPinus radiataranges from around 25 mm to over 80 mm, mainly as a result of spacing and site.Close spacings between trees are needed to prevent branches, and so knot size, from

becoming excessive. In addition the proportion of knot wood in the butt log is quitelimited because at these spacings conifer branches only increase rapidly in diameter

for the first two or three years after which they suffer suppression and growthvirtually ceases, whereas radial growth of the stem continues year after year (VonWedel et al., 1968). At the same time close spacings offer the benefits of a smallercorewood zone, and less stem taper. However growing trees too close together

produces small stems and requires a long rotation if large diameter logs are required.Increasing spacing not only results in larger knots, it leads to more stem

malformation. Malformed and less vigorous stems can be removed in a productionthinning or thinning-to-waste operation. The loss of merchantable volume due tothinning is compensated for by concentrating timber production on fewer stems.Clearfelling costs are reduced as there are fewer logs to harvest and these are larger.

Further, the loss in wood volume may be illusory (Bunn, 1981) as mortality and



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stem breakage on clearfelling in heavily stocked stands must be set against theearlier loss of volume when thinning-to-waste.

Large knots drastically reduce strength and are a major cause of downgrade inlumber. They are undesirable in fibre products. Knots are very dense, some 2-3

times as dense as stem wood and are frequently above 1000 kg m3

This is aconsequence of compression wood formation and heavy resinification aftermortality or pruning: resin can account for 30% or more by weight of knot wood.Knots are hard to penetrate with chemicals and are inadequately pulped, while inmechanical pulping knots are esistant to defibration.

The significance of branches and knots in lumber is out of proportion to the percentage of stem volume that they occupy. First, trees experience asymmetricloading from heavy branches such that compression wood is observed often in theimmediate vicinity of knots (usually in streaks extending below the branch). Thesecond effect arises from the stresses in the immediate vicinity of the branch or knot.

Intuitively, one recognizes that the swelling of the stem around branch whorls isthe means of accommodating the bending forces introduced by heavy branches. It isalso a defence mechanism against severe wind, although where trees break insteadof being uprooted, failure occurs in the immediate vicinity of a large branch.

Lumber has no such defence mechanism as it is straight edged. The knot and thecross-grain in its immediate vicinity is a far more serious defect in the lumber thanthe branch is in the tree, and grading rules place limits on knot size. For this reasonforest growers have to be mindful in selecting intensively for and keeping branchdiameters small. The implications for lumber strength are discussed in Chapter 10.

7. SOFTWOOD PLANTATION SILVICULTURE

There is an inevitable tendency to assume that the silvicultural systems with whichone is familiar have some general validity. More often they are determined as much

by cultural, fiscal and political forces as by silvicultural or physiological insight. Thearguments presented in this study of New Zealand forestry may have a generalapplicability and validity and yet they provide also a warning of presumption andsingle-mindedness. Ideas should never be writ on stone. New Zealand forestry wasdriven by production and growth and form; and not by wood quality. Where

prospective markets had been identified the ability to supply those markets with

quality wood was presumed. The most significant aspect of plantation silviculture isthe effect on wood properties of the reduction in the age of clearfelling. The youngerthe trees are the greater the proportion of corewood and the poorer the overall woodquality. All other factors are secondary.

Natural mortality of untended stands places boundaries on practical options inthe management of forests. Competition within heavily stocked stands is intense.First the canopy must close, restricting branch growth at the base of the greencrown, to be followed by branch mortality. Eventually the smaller suppressed treesdie. The first major plantings of Pinus radiata in the central North Island of NewZealand (1925-35) were not thinned. Galbraith and Sewell (1979) observed that


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145

Figure 5.14. Natural mortality in untended radiata pine as a function of stocking and age(Galbraith and Sewell, 1979). In tended stands it is desirable to thin with sufficient intensityto anticipate natural mortality so the final crop trees do not experience competition.

irrespective of the initial stocking the final stocking, arising from natural mortality,came down to the same figure of around 300 stems ha1 by age 45 (Figure 5.14).There is nothing inevitable about this figure. Stockings forPinus radiata as high as1500 stems ha1 in 30 yr-old stands have been observed in Chile (Delmastro et al.,1982). This may be due to differences in climate and site, and the presence of fewer

pathogens. However, in these early New Zealand plantings, the loss of stemwoodvolume due to mortality was considerable, being up to one-third of the totalincrement by the age of 35 (Sutton, 1984). These stands had to be maintained onextended rotations, typically 45-50 years to smooth out a major discontinuity in

plantings to provide a constant wood supply over a prolonged period.The end result was a heavily stocked final crop whose stem characteristics

included live knots near the pith and dead, bark-encased knots in the outerwood inthe lower half of the stem (Walker, 1984). The trick in processing such material wasto minimise the influence of defects, first by zoning and then by sawing each zone to

best advantage. Thus acceptable framing timber was cut from the outside of the buttand second log: despite the presence of dead, bark-encased knots, this is a zone ofstiff, high density wood and not overly large knots. The same loose knots preventedthe sawing of board grades from the outerwood of these two logs, but somemoderate quality boards was cut from near the pith where the branches would have

been alive when the wood was laid down, and the knots were sound and intergrown.Short, clear lengths cut from shop or factory grade were best taken from theinternodal regions between branch whorls of the second log. At this height in the



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stem the internodes are long since the tree would have been growing mostvigorously at that stage. Further up the tree stem-cone holes are too numerous toyield the better board grades, although some low-grade boards could be cut from thesecond and third logs provided the tree was of reasonable diameter and the branches

were still alive. Unlike most softwoods, radiata pine is prone to stem cone formationin the upper parts of the stem: cone formation occurs both as separate cone whorlsand at existing branch whorls. The best hope for recovery from the fourth log was tocut framing from the pithy corewood by enclosing the pith within the centre of thesawn section. Top logs and thinnings provided low density corewood fibres havingthin-walled cells. This chipwood was acceptable for particleboard, fibreboard andmechanical pulp. High density, long fibres from the slabwood was well suited forstrong high-tear kraft pulps used for linerboard and kraft sack paper.

Such trees provided a versatile mix of lumber and fibre, although the top boardgrades could be met only by finger jointing and the proportion of No. 1 framing was

disappointingly low (30%), because of the difficulty of keeping knot diametersbelow the specified 33 mm limit for that grade. High stockings, no thinning and longrotations to produce framing, boards with sound knots and pulpwood was nevergoing to be an economic proposition.

Since the 1950s most New Zealand plantations have been under some form ofactive management. Initially high stockings (2500 stems per hectare) ofPinusradiata were advocated for the following reasons:

To control branch size in unthinned stands. However, if stocking is to be thetool to control branch size the initial spacings between trees needs to be around

1.8 x 1.8 m on some sites (Table 5.4). High stockings ensure early canopyclosure and branch mortality as the green crown moves up the tree (pine andlarch are less shade tolerant than are spruce and fir). The same problem of branchcontrol is found with Picea sitchensis growing in Britain, where an initialstocking of 2500 stems per hectare (2 x 2 m spacing) and no subsequent thinninghas been advocated to ensure that a significant proportion (> 75%) of the sawntimber is of the better framing grades (Brazier et al., 1985). However theconsequences are stark. Expect either small logs or long rotations. To allow for selection of the best trees. Selection criteria include vigour,straightness and uniform local spacing after thinning. The first major plantings in

New Zealand (1925-35) were established with unimproved seed. In these youngunmanaged stands 37-42% of stems had multiple-leaders (Macarthur, 1952) andan acceptable final crop was achieved only because the high initial stockingresulted in heavy natural mortality (generally of the less vigorous and malformedstems) or the deliberate thinning of stands. Successive tree improvement

programmes mean it is no longer necessary to remove four out of every fivestems and initial planting (550-1100 stems per hectare) can be as low as three ortwo times the final stocking (300-400 sph) while still achieving a final crop withall trees of good form and vigour. The contrast between the original stands andthose planted with select seed 50 years later is dramatic.


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147

While it is technically possible to reduce knot size and to minimise the volumeof the corewood by restricting growth through high initial stockings, productivethinning must be delayed until the green crown has receded up the tree, at a standheight of 14-18 m. Further, any control of branch growth due to high stocking is lost

once the stand is thinned and the branches in the live crown will be greater thanwould have been the case had the stand not been thinned. Production thinning ofsuch stands yields expensive wood since the piece size and the volume per treeextracted is small, and the lumber is of low grade. Delaying production thinning toincrease the piece size and the volume of extracted wood brings with it the risk ofwindblow. Indeed there are considerable costs in growing such a forest, the mostsignificant being that the rotation age has to be extended a further 5-10 years beforethe piece size in the surviving stems makes final harvesting economic (age 35+).

Log size has a major impact on forest economics. The percentage of the log thatcan be recovered as lumber declines quite noticeably once the log size drops below

about 400 mm while harvesting, transport and handling costs escalate. Lower initialstockings and subsequent thinning shorten the rotation length but it is doubtful thatthe control of knot size would be sufficient to produce a significant proportion offraming timber. Here experience in Australia, Chile and the Cape Province of SouthAfrica on the one hand and New Zealand and Kenya on the other diverge. Adequatecontrol of branch size is achievable in many Australian radiata pine regions by

judicious manipulation of stocking levels through commercial thinnings. Moreespecially, coarse branching is not a feature of slower grownPinus radiata stands onlower rainfall sites of lower fertility. Some slash x caribaea hybrid clones have verysmall branches and narrow crowns yet they can be good volume producers.

As already discussed wide initial spacings result in large knots, but the effect inpractice depends on the grading rules that apply. The knot volumes and sizes noted by Nylinder (1958) in Sweden are small (Table 5.3) and the sawn outturn yieldsquality board and structural timbers. This contrasts with the faster growing, heavier

Table 5.4. Effect of initial spacing on branch size at various sites forPinus radiata in NewZealand (Sutton, 1970): branch size is the mean of the 16 largest knots from four quadrantsand at four heights. Historically for No. 1 framing, knots should not exceed one-third of thecross-section, which in a piece of 100 x 50 mm corresponded to a knot of 33 mm. This wasachieved only with the stockings shown in the shaded zone.

Initial spacing (m x m)Site (Forest)

1.8 x 1.8 2.4 x 2.4 3.0 x 3.0 3.6 x 3.6 4.8 x 4.8

Rotoehu 40 47 43

Kaingaroa 33 7 50

Gwavas 38

Ashley 31 34 39

Woodhill 25 25 33 37 46Golden Downs 25 28 33 35

Eyrewell 24 27 33 36 39



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limbed softwoods in warmer climates. With these faster growing species small knotssizes cannot be as easily achieved through close stocking and subsequent thinnings,and the knotty timber is invariably of poor quality (Table 5.4). A partial solution tothe larger knots endemic to fast grown pine in New Zealand was to use larger

framing members (100 x 50 mm green and 90 x 45 mm dry-dressed compared to 90 x35 mm dry-dressed pine framing in Australia) and to cut framing from the strongerouterwood; but now New Zealand has adopted the smaller section size. The inabilityto reduce knot size through initial spacing provided the impetus to use early pruningand thinning as a tool to improve wood quality, especially in the large butt log offast growing softwoods. In New Zealand pruning has increased the cost of the logdelivered to the mill by only 10-15%, principally because harvesting and transportcosts loomed so large (Fenton, 1972). The quandary is whether to plant at widerspacing and to both thin and prune the butt log (yielding very low grade corewood);or to adopt a close initial spacing (c. 2500 stems per hectare) and to control knot size

in the bottom two logs through early canopy closure, only thinning (to about 350-400 stems per hectare) much later (at 14-16 m) at the cost of a prolonged rotation.

Where quality lumber is sought there is a clear incentive for having low finalstockings. The basic premise must be to plant as few trees as possible (550-1100stems per hectare) while still allowing for some selection when thinning-to-waste to remove those trees that lack vigour or are of poor form. This concentrates themerchantable wood on the final 300+ sawlog trees. The wider initial spacing is

possible with reliable, improved planting stock. However the forester must now prune the butt log as there is no control over branch growth at these wide initialspacings. Further it is essential to prune as early as possible to minimise the size of

the knotty occluded core and to maximise the valuable clearwood in the butt log(Figure 5.16b). The aim is to confine the occluded core to a cylinder of no more than150 mm in diameter. Most of the value in the tree resides in the pruned butt log (c 60%) while the industrial pulpwood contributes very little (c. 2%). Silviculturaltreatments need to be timed precisely as a delay of 12 months from the prescribed

pruning date results in an enlarged knotty core that would delay clearfelling by 3years if the same proportion of clearwood is to be obtained. More likely the rotationlength will be kept short (age 25-30) and the proportion of clearwood will be muchreduced (Bunn, 1981).

Thinning is an integral part of the pruning operation as it gives the final trees the

growing space to put on greater diameter growth and produce more clearwood. Anunfortunate consequence of wider spacings is the larger branch size (> 50 mm)

Figure 5.15. Brazil: in a class of its own but not for long. (a) On the left is a 3 yr-old standofPinus taeda with a target of 34 m /ha/yr over bark at age 20 for sawlogs: on the right is a 3yr-old stand of Eucalyptus grandis with a target of 84 m3/ha/yr over bark at age 7 forpulpwood; 54 m3/ha/yr is the commercial average (International Paper do Brasil Ltda). (b) Astand of 10 yr-old Eucalyptus urophylla x grandis hybrid, planted at 5 x 2.4 m, and now 35-40 m tall. It is atypical in that pulpwood is grown for 7 yrs but this stand was planted early inanticipation of the 900 000 tonne bleached eucalypt kraft pulp mill commencing production in2005. Height/dbh can reach 150:1 and in a gentle breeze entire stands will sway in unison

(forest operations of Veracel Celulose S.A., a joint-venture between Aracruz and Stora Enso).


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149WOOD QUALITY: IN CONTEXT


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above the pruned log, although some of the deleterious effects of these largeintergrown knots are offset by the larger diameter of the second log (compared tothat in a more heavily stocked stand of similar age). In managed plantations thecosts of silvicultural operations compound over time. This more than anything

drives down the age of clearfelling and thereby increases the proportion ofcorewood. The same argument applies to efforts to improve the site, to reducecompetition and to the use of fertilisers and trace elements to boost growth. Theremay be a slight drop in basic density during the first year or two after fertilising, butthis is offset completely by faster growth. Indeed the principal effect of the use offertilisers is to reduce the time to harvesting and it is the shorter rotation rather thanany drop in basic density associated with fertilization that is of major significance.

Since clearfelling the first plantations from the 1920s-1940s there is evidencethat intrinsic wood quality of subsequent crops has deteriorated, in particular thewood is less stiff and has shorter tracheids. Until now this has been attributed to the

younger age of clearfelling, down from 45-50 years to 25-30 years. However,Lasserre et al. (2004) observe that for 11 yr-old pines corewood stiffness wasreduced from 6.7 GPa at high stockings (2500 sph as in early plantings) to 5.0 GPaat low stockings (833 sph as practiced today). If the negative relationship betweenDBH and stiffness big trees are less stiff is included as a covariate the adjustedstiffnesses are 6.4 GPa (2500 sph) and 5.3 GPa (833 sph) respectively, so the effectof stocking is reduced to 1.1 GPa. One is then comparing the effect of stocking

between trees of equal DBH. The study included ten clones. With clones (genotype)the difference between the best clone and the worst clone was 1.3 GPa for bothstockings so the difference between the mean and the best of the clones was only

0.65 GPa. However these clones had not been bred for improved wood properties, sothe future benefits are greatly understated. This work implies that both initialstocking and genotype should be used as complementary approaches for improvingcorewood stiffness. Wind and tree sway are likely factors in these findings.

However, only hardwoods have the potential to produce the wood quality neededfor sawlogs when grown on very short rotations, that is 10-12 yrs, a fact that haslittle to do with the growth rate (Figure 5.15).

Perhaps New Zealand pine forestry was not as smart at it thought. There is acurrent trend to switch back to higher initial and final stockings and to longerrotations: maybe with a commercial thinning. These self-defeating changes seek to

increase the proportion of outerwood ahead of any future genetic gain to bedelivered by the breeders. Extended rotations do not offer a long term solution andare, at best, an acknowledgment of past failures to attend to wood quality. To datethere has been no will to diversify, to contemplate planting fast growing hardwoodswith better corewood and superior wood qualities.

There is a tendency forPinus radiata in the central North Island to form one tofive branch clusters in a growing season, with c. 70% having either three or four

branch clusters. The number of whorls decreases on moving from the north to thesouth of New Zealand, with a corresponding increase in the length of the internodes:only 12% of the sawlog length yields clear lengths of 0.6 m or more in the north but

almost 50% in the south (Bannister, 1962; Cown, 1992). The prospect of cuttingshort clear lengths for componentry and finger-jointing is much greater in the south.


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151

Figure 5.16. Some management options. (a) Selecting the best species and provenance is akey to successful plantation forestry. ThePinus ponderosa in the foreground is the same age,45 yr-old, as theP. radiata in the background. Seed for this ponderosa pine stand came fromBritish Columbia. Poor growth was partly due to selecting too northerly a provenance that didnot correspond to the southern latitude of New Zealand. Even with better selection it isdoubtful whether its growth would be comparable to that of radiata pine. (b) Early thinningand pruning increase the diameter of final crop trees and allow the production of clearwoodfrom pruned logs. No more than half of the live crown is removed on pruning at age 5-7.(c) and (d) In unpruned stands clearwood is limited to the internodes. Only a limited amountof cuttings is greater than 0.6 m long. By breeding select long-internode trees it is possible to

obtain long-internodal lengths (> 0.6 m) from over half the internodes on some sites(Kininmonth and Whiteside, 1991).



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Table 5.5. Silvicultural practices that influence wood properties.

Silvicultural practice Influence on wood quality

Planting stock Species: clones or select/control-pollinated seed;

rooted cuttingsInitial spacing The onset of mortality and log size of final crop trees

The control of branch and hence knot sizeThinning Growth rate and hence rotation length

Affects indirectly whole tree density by reducing therotation length

Increases knot size in the live crownPruning Early pruning eliminates large knots otherwise present

in butt logsRotation length Affects mean age of wood and hence basic density

The branching habit of radiata pine has suggested two separate tree improvement programmes: that of breeding a more multinodal tree with lighter, more frequentbranching for most purposes (easily-pruned clearwood, framing and pulpwood), anda long internode type which would allow for the cutting of short (0.6-1.8 m) lengthsof clearwood from between the large knot whorls of the unpruned trees (Figure5.16c,d). Because multinodality is much more frequent it has proved easier to findand select genotypes that combine the multinodal form with vigorous growth andstraight stems than it is to find long-internode genotypes with desirablecharacteristics (Carson, 1988).

Perhaps the surprising feature of the New Zealand breeding programme is thatthe next generation of trees will be of lower density than the first generation. Theoverall drop of 20-30 kg m3 arises from the reduced rotation length (from 45-50years to 25-30 years) and the weak negative correlation between growth rate and

basic density (Cown, 1992). Although basic density is considered a highly heritabletrait, selection of the appropriate high density families will rectify the situation onlyslowly. A more immediate alternative is to vegetatively propagate superior clonalstock displaying high density (or, better, displaying high stiffness).

Sutton (1984) emphasises two general silvicultural principles: that no aspect ofsilviculture can be considered in isolation, and that for any silvicultural practice

both yield and tree quality are largely predictable. To which he added that tree qualityis very largely determined by the early silvicultural treatments, e.g. late pruning is awaste of time as the pruned envelope will be too narrow to provide much clearwood.The principal practices controlling wood properties are summarised in Table 5.5.

Figure 5.17 is an example from the southeastern US of contrasting silviculturalprescriptions in two locations. In the piedmont early slow growth is accelerated byaggressive thinning, e.g. from 1000 to 225 sph at age 15. On the coastal plain whereearly growth is better and where thinning ought to be earlier at age 12 thinningtoo late, e.g. from 1000 to 275 sph at age 21, resulted in forgone outerwood growth.

The viability of any intensive silvicultural programme is tied closely to therotation length and the premium to be paid for quality material, whether that be for


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clearwood or structural lumber. As rotation lengths are extended the compoundingeffect on the costs of managing the stands in their early years becomes so great thatthese operations cannot be justified unless enormous premiums are to be paid for the

higher grades of lumber that are produced as a consequence of that early silviculture.For this reason the management of temperate forests established in less benignclimates than that of countries such as New Zealand has tended to emphasise highinitial stockings to achieve effective branch control. However, high initialestablishment costs associated with high initial stockings and the extended rotationlength suggest that the issue has been only partially addressed.

A review of silvicultural systems forPicea sitchensis in the UK (Macdonald andHubert, 2002) provides further examples of product push with low valued pallets,

packaging and fencing absorbing two-thirds of production and Sitka spruce isclassified as non-durable and resistant to treatment.

Ideally the market for lumber and its interaction with species, silviculture andsite determine the appropriate management policies in any local situation, ratherthan the tax regime or the various subsidies that all too often distort managementdecision making. New Zealand has considered itself to be a good example of

plantation forestry, offering an internal rate of return of about 6-8% above that ofinflation. However the sale of the State Forests in the 1980s and 1990s suggests thatfor the people of New Zealand the profit from exotic forestry has been largelyillusory. Forests were not established solely to maximise the return on investment,

but for a plethora of worthy reasons such as soil protection, regional employment,and in the private sector, favourable tax treatment. Some State Forests were too

small to offer economies of scale, were unnecessarily dispersed and located at adistance from both mills and export ports. Further there was a lack of consistency:many stands were thinned and pruned too late, negating the benefits of suchsilviculture. Ironically the State Forest Service, having advocated enthusiastically amost intensive programme of thinning and pruning to produce clearwood, whentransformed to a self-funding State Owned Enterprise drastically reduced thenumber of stands being pruned (down 46% in its first year of operation) and beingthinned (down 20%) in order to increase its cash flow and provide a skinny dividendto its shareholder (the Government). Even that failed to satisfy the philosophical and

political objectives of the Government. These commercial forests have been sold to

private enterprise. Bilek and Horgan (1992) provide an excellent commentary.


Figure 5.17. Pith-to-bark increment cores forPinus taeda growing in the southeastern US.


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Tax incentives, tax breaks and government meddling distort forest managementdecision-making throughout the world, and by an Alice-in-Wonderland logic can

justify many plantings where such schemes are patently inappropriate. Manysilvicultural and management systems have their own internal logic and consistency,

but in reality are cocooned by the prevailing cultural, economic and political ethos.One purpose of this brief review is to emphasise that rational managementdecisions are at best shape-shif ing rafts of logic floating on a sea of partialvariables: there can be no rules, only principles.

Leadership in short rotation forestry lies in the sub-tropics. Table 5.6 represents atypical regime forPinus taeda in Brazil, i.e. all trees are pruned in the first lift and

potential final crop trees only in the second lift. Improved genetics and management

have raised average productivity of pine plantations from 18 23 to 33 m /ha/yr inthe successive decades, in the 1970s, 80s and 90s respectively. Whether corewoodquality issues have been addressed remains to be seen.

Table 5.6. Typical silvicultural system for loblolly pine, Pinus taeda, in Brazil on a highproductivity site, with an initial stocking of 1111 stems/hectare.

Age Pruned height [m] Pruned stems Remaining trees

3 2.5 1111 1111

4.5 750 1111

5 5.0 750 1111

6 Thinning-to-waste 750

7 6.0 400 750

10 Commercial thinning 400

20 Clearfell

This New Zealand study makes little mention of wood quality as it was largelyoff the agenda. Little effort was spent on improving important properties likestability, stiffness and strength all received virtually no practical attention. Nowonder the principal markets for UK spruce remain low valued commodities. Thefailure to address this issue is examined in Chapter 6. However, this failure should

be placed in context. Current understanding of wood behaviour, properties and

structure is reminiscent of that facing metals in the 1940s: the time lapse reflects thevastly more complex ultrastructure of the wood cell wall and the difficulty insecuring molecular structural information compared to the simple crystallinestructure of metals (Entwistle and Walker, 2005b). However, that means that theopportunities are enormous, once one sheds ridiculous, nave suppositions such as increasing wood density has been ranked above all other desirable objectives ofany wood quality improvement programme (Walker, 1993 p. 155). The basicsciences are available (Chapter 2), future prospects are visible at least in outline(Chapter 6), and opportunities for genetic improvement of wood quality are there to

be grasped. What so often is lacking is an industry willing to embrace substantial

change change that may foreshadow greater emphasis on hardwoods for marketsthat have been traditionally the preserve of softwoods.


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8. EUCALYPTS FOR WOOD PRODUCTION (DONNELLY ET AL., 2003)

With hardwoods opportunities for creative thinking are even greater. By 2010eucalypts are projected to account for about 25% of global non-coniferousroundwood production. Yet in 2000, eucalypt accounted for only a very small part,c. 2%, of sawnwood production, i.e. the major market for eucalypt is pulp, with themarket for lumber being relatively insignificant. Further, Donnelly et al. (2003)observe that although Australia is the home of almost all eucalyptus species, the 3.1million m of sawlog production comes almost entirely from native forests. Thisroughly matches the estimated 2.9 million m coming from plantations worldwide(with 37% from Brazil alone). Indeed in Australia the plantation area is only about15% of that in Brazil, which has c. 3 million hectares.

It would be hard to overstate the importance of South America and Brazil inparticular. By 2010 South America is projected to account for 55% of the worldssupply of eucalypt plantation roundwood, followed by Asia with 20%. Donnellyet al. (2003) estimate that eucalypt sawlogs from plantations will increase to 10.6million m by 2015, with a total pruned log supply of 1.4 million m Intensivelymanaged silvicultural systems for eucalypts are in their infancy.

The considerable demand for eucalypt as domestic fuelwood is not examined.However, the charcoal market supplying Brazils iron and steel industry is of moreinterest: 42% of Brazils 50-60 million m /yr of eucalypt production is for charcoal,with another 46% for pulp. Prior to the industrial revolution, smelting of iron oreused charcoal and was responsible for the deforestation of considerable areas innorthern Europe. Notably in China, cast iron and coal firing were known as early at

the 5

th

century BC, while they may have discovered how to smelt ore with coke bythe 12th century. However in Brazil the use of charcoal continues to be favouredover the use of coke in part because of the absence of sulphur in wood charcoal andthe very low net carbon emissions. Cheap, poor quality imported coal, low incalorific value and high in ash (30%), provides a (questionable) basis forsubstitution against charcoal and even then only when the exchange rate isfavourable. Under the Kyoto protocol charcoal is carbon neutral, an advantagecompared to coke.

The reduction of the ore is carried out in a blast furnace where the coke/charcoalis both a reducing agent and a source of energy It must supply a porous structural

support for the iron ore so that the hot air and released CO/CO2percolate up throughthe ore while the molten iron flows down to the hearth of the furnace The reaction atthe surface of the charcoal/coke is:

2C + O2 = 2CO + energy, (1)

and with the iron ore

3CO + Fe2O = 2Fe + 3CO2 (2)

Coal cannot be used directly because of its structural properties, the volatiles andits impurities hence the use of metallurgical coke or charcoal. After smelting the



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Bessemer converter refines the iron to steel by oxidizing out impurities such assulphur and surplus carbon arising from the reduction of the iron oxide.

In Brazil, the other principal industrial market for eucalypt has been pulp.Optimal pulpwood crops follow a 7-yr cycle for pulp production incorporating two

coppice rotations before finally replanting after 21 years. However continuouslyimproving genetic gain is making earlier replanting more attractive.

Coppicing can be attractive. In San Paulo State, Brazil, there are stands ofE. globulus that have been continuously coppiced for five rotations (50-60 yrs). Thechoice is one of cash flow. Small companies often cannot afford the much highercosts of fresh planting ($800/ha) ersus coppicing ($100/ha for thinning coppiceshoots and weed control). They accept the loss of vigour (c. 15% less volume) andthat the stand becomes progressively less uniform as openings develop within thestand. Equally, they cannot afford the high costs of clonal breeding that can deploynew clones that out-perform the older coppicing clones (c. 5-10% more volume). On

the other hand the inherent maturity of coppice implies improved corewood woodquality from aged material that must be set against uncertain (confidential)improvement in the intrinsic wood quality of newer clones.

The largest eucalypt pulpwood operations in Brazil are gradually reducing theirreliance on coppicing (down from 50% to 20% or less). Currently retaining somecoppice remains attractive for those companies that are having to expand rapidlytheir forest lands ahead of a sharp increase in pulp production increases of up to amillion tonnes of pulp per annum with new land having first call on availableclonal stocks.

While routine (conservative) operational growth rates in the region of 30-50

m3

/ha/yr are taken as normal, it is worth remembering that 30-40 years ago growthwould have been around 15 m3/ha/y

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