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TECHNICAL PUBLICATION

Publ ished 1961 (Bulletin)Revised 1964 (Technical Publicat ion)Revised 1974, 1977Reprinted 1979, 1981, 1983Revised 1987

ISBN 0 7240 2067 SISSN 0 1SS-7S48

Doe 811.5

Sapwood and Heartwoodby

R. K. Bamber

Sapwood

TransitionZone

Heartwood

(b)

Scanning electron micrographs ofsapwood/heartwood boundary in Eucalyptus sa/igna. (a) Transverse surface. X 400;(b) Radial longitudinal surface. X 90

\Wood Technology and Forest Research Division '

, PO Box 100 Beec roll. NSW 2119 Australia II .___ _ _ ..

In most trees two types of wood can be recognised,sapwood and heartwood. Sapwood is the outer, pale­coloured wood and heartwood the inner, mostly darker,wood. Because of its susceptibility to fungi and wooddestroying insects and lack of colour, sapwood isgenerally considered inferior to heartwood and is oftendiscarded during conversion.

However, by use of the appropriate preservativetreatment, sapwood can be made equal if not superior toheartwood of the same species in durability, thusreducing waste and in addition enabling the marketing ofspecies with little or no heartwood. It is possible, if theproperties of sapwood are well understood, to useuntreated sapwood of many species in numeroussituations with safety.

To assist in the best utilisation of sapwood, the propertiesof sapwood and heartwood are reviewed , with emphasisbeing given to the function, determination of sapwoodand heartwood, relative strengths, durability andutilisation.

FUNCTION OF SAPWOOD ANDHEARTWOOD IN THE LIVING TREE

Sapwood has three main functions in the living tree:support, conduction and storage. Support for the tree isprovided by the tracheids in the softwoods (conifers ,Figure I) and by the fibres in the hardwoods (floweringplants, Figure 2). These are the most abundant wood cellsand can constitute as much as 95 per cent of the totalwood volume. Conduction of water and mineral saltsfrom the roots to the leaves occurs in the sapwood bymeans of the vessels in hardwoods and tracheids in thesoftwoods. The sapwood is so well evolved forconduction that sawcuts can be made past the centre of atree from opposite sides at different levels without haltingthe flow of water to the leaves. The sapwood serves as astore for food reserves usually in the form of starch. Thesereserves originate from the sugars produced by thephotosynthesis in the leaves, the sugars beingtranslocated down through the phloem tissue and theninwards by the ray parenchyma tissue (Figure 3), and thentransformed into starch grains in the axial and rayparenchyma cells (Figure 4). The starch grains may bereconverted back into soluble sugars to satisfy foodrequirements for growth. The starch reserves are knownto be greatly depleted during periods of intense growth ofthe crown and can be completely resorbed when suchgrowth occurs after fire or insect defoliation. Once thetree is felled and the wood dried , the starch is retained inthe wood cells as a permanent deposit. No economicprocess for removing the starch deposits from wood hasbeen successfully demonstrated. Great variations in theamounts of starch occur both between and within species.

In contrast to the sapwood heartwood has no living cellsand neither conducts water or stores reserve materials.Heartwood does not appear to be essential for the growthand survival of trees. While it does provide the mainstructural support for most trees this role could have beenadequately provided if it had remained as sapwood .

It has been suggested that heartwood forms as arepository for waste metabolic products (polyphenolicsor tannins) or for surplus photosynthates. It is morelikely, however , that heartwood forms to keep the amountof sapwood at an optimum thus conserving thenutritional balance in the living part of the tree as it is well

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Figure I. Simplified drawing of a cube of softwood magnifiedabout 250 tim es. The cell wall pits have been omitted

.....,.renchy....

Figure 2. Simplified drawing ofa cube ofhardwood on the samescale as Figure 1. Cell wall pits again omitted

known that potassium, phosphorus, nitrogen and sulphurare resorbed from the sapwood as it is transformed intoheartwood. Elements not resorbed are lost until the treedies, decays and becomes part of the forest humus. Thelarger the tree the larger the amount of nutrients whichcould be locked up in th is way.

out.. ba,k ldeodl

Inne, ba,k lllvlnll'

camblurn

lOpwood lllvlnlll

hea,twood ldeadl

pith

1-1f----- ray parenchyma

Figure 3. Cross section of tree trunk showing the relationship ofthe various tissues

Figure 4. Photomicrograph of radial longitudinal section ofAngophora costata smooth-barked apple. Magnification X 270.Starch bodies can be seen as spherical bodies in the axialparenchyma

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FORMAnON OF SAPWOODA D HEARTWOOD

Wood cells are produced by a layer of cells between theliving bark (phloem) and the wood (Figure 3). This layeris called the cambium and can be seen only with amicroscope. Cells which form on the inside of thecambium become wood and those on the outside phloem.Once wood cells are laid down by the cambium, theyrapidly differentiate in size and shape until finally athickened wall is formed , the process being completedwithin a millimetre or so from the outer wood layers.After the thickened wall is produced, the cells remainlargely unaltered in shape for the life of the tree. Thewood cells thus produced by the cambium become thesapwood.

The ray and axial parenchyma cells remain alive in thesapwood, while the more specialised wood cells, thefibres, vessels and tracheids, lose their living contentssoon after differentiation and thickening of the cells arecompleted.

Changes in the nature of the sapwood occur with thedeath of the parenchyma cells and with the completion ofthese changes heartwood is produced. These changesoccur gradually through a layer of cells of variable widthknown as the transition zone. In this zone increasedamounts of so called tannins (polyphenolics) andcoloured materials of many types are deposited in the rayand axial parenchyma and these materials diffuse into thesurrounding tissue (Figure 5). The development ofheartwood colour is due to this deposition.

The development of heartwood colour is the mostspectacular change between sapwood and heartwood.While a great number of timbers do not produce colouredheartwood extractives the large majority of species showsome evidence of doing so. Heartwood colours rangefrom white to cream as in southern silver ash, alpine ashand European ash, to black in ebony, olive-green inlignum vitae, yellow-orange in osage orange, brown intallowwood and red in red ironbark. Most of the coloursundergo a change with age, becoming darker and morebrown. Frequently the heartwood substances areunevenly distributed giving rise to bands of wood ofdifferent colour. Such banded or striped effects areevidently related to the growth conditions as they occurconcentrically with the growth rings (Figure 6).

In hard woods, as well as the production of tannins andother materials, changes occur in the vessels whichbecome blocked on heartwood formation. This blockingof the vessels takes place by means of the pits. Pits occurabundantly in all wood cell walls and are well-definedthin regions in the wall through which solutions canreadily diffuse from one cell to another (Figure 7). Inspecies with larger pits, blockage of the vessels occurs as aresult of the growth of adjacent parenchyma cells throughthe pit forming balloon-like structures in the vessels.These growths are known as tyloses. In species with smallpits , blockage of the vessels occurs due to the secretion bythe ray parenchyma of tannin or gum-like materialsthrough the pits. This deposition of materials can alsooccur in the species with large pits along with tyloses.Tyloses are common in the eucalypts and the merantiswhereas tannin-like materials are common in themahoganies, true Australian red cedar and Queenslandmaple.

..-- PIT MEMBRANE

PRIMARYWALL

t

_ -- TO R I

SECONDARYWALL

++

In softwoods, blockage of the trach eids on heartwoodformation can occur either by closing of the pits(aspiration) or by the increased deposition of resins andtannins. The pit membrane of a trach eid is a specialisedstructure consisting of a thickened centre (to rus)-supported by a cellulose web, the web being flexible and

. allowing the torus to move and close (aspirate) the pitopening (Figure 7).

Aspiration is confined to the pits of the earlywoodtracheids, latewood pits rema ining open. Latewoodtracheids appear to be specialised structural cells notconducting cells. The y have narrow lumens and thick cellwalls and are formed near the end of the growing season.Earlywood trac heids have wide lume ns and thin cell wallsand are formed in the first part of the growing season .

Figure 5. Photomicrograph ofa transverse section ofExcoecariaparufolia. Magnification X 150. The deposition of colouredheartwood material in the parenchyma cells can be seen

OPEN CLOSED

Figure 6. Photograph of tree segment of Callitris columellariswhite cypresspine. The irregular line ofheartwood formation canbe seen on the end face

Figure 7. Diagrammatic representation of a pair of pits in thewalls of two adjacent tracheids approx imately. MagnificationX 2000 .

WIDTH OF SAPWOOD

The width of the sapwood depends primarily on thelongevity of the parenchyma cells. It is subject to greatvariation between species and minor variation betweenindividuals of the one species and even within theindi vidual. While the sapwood width seems regular alongthe grain , it is apt to show fluctuations around the growthring (Figure 6). In radiata pine the sapwood may be widerthan 200 mm , while in eucalypts it is typically less than 50mm. Table 1 lists measurements obtained from 14species .Some trees , such as flame kurrajong, do not have acoloured heartwood or blocked vessels. The fact thatstarch is frequently detected well into the heart of theselogs indicates that parenchyma cells are able to remainalive for long periods.Variation in sapwood width within a species has beenrelated to man y factors, some of which are:• Rat e ofgrowth. It seems fairly evident that the width of

sapwood would be wider in dominant than insuppressed trees .

• Environment. It also seems fairly evident that the widthof sapwood would be wider in the more favourableenvironment; however, it has been suggested thatheartwood formation is encouraged by favourablewater relations in rad iata pine.

(

;

4

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• Position in tree. Sapwood widths remain fairly constantwith height. It can be seen therefore, due to the naturaltaper of a tree, that the proportion of sapwood toheartwood increases with height.

• Age. Young trees have wider sapwood than old trees.

It is important to note that once the tree has commencedto lay down heartwood the volume of sapwood inproportion to heartwood becomes less as the treeb~comes older. Consequently the production of woodwith the l~ast am0l;lnt of heartwood such as required bythe paper industry is favoured by rapid growth and earlyharvestmg whereas the production of wood with aminimum volume of sapwood such as required forexterior uses is encouraged by slower growth and laterharvesting.

SUSCEPTIBILITY OF TIMBERSTO BORER ATTACK

Infestations by almost all timber borers are either limitedto the sapwood or else sapwood is infested in preferenceto heartwood. This is undoubtedly due to the presence offood materials such as starch and the absence ofunpalatable and perhaps toxic heartwood substances.

Two common groups of borers, lyctids and bostrychidsboth limit their infestation to the sapwood and arediscussed below.

The Lyctid BorerThese borers are members of the family Lyctidae whichare commonly known in Australia as powder postbeedles. They only attack partially or wholly seasonedsapwo?d of hardwoods. The susceptibility of timbers toattack is governed by several factors. The first factor is theability of the borer to obtain access into the wood foregglaying. For example the common lyctid borer of NewSouth Wales, Lyctus brunneus, cannot infest the wood ifthe diameters of the vessels of the timber are too small forthe beetle to insert its egg laying organ (ovipositor).

For practical purposes timbers with vessel diameters lessthan 0.09 mm are considered to be resistant to L.brunneus attack. Softwoods do not have vessels and areconsidered to be immune to lyctids. Attack on softwoodshas not been recorded in Australia.

Once access to the timber is made through the vesselsthen the degree of damage by the lyctids is largelydependent on the concentration of the starch deposits.Where starch is absent infestation does not occur andimmunity is assured.

Further information on the lyctid borer is continued inForestry Commission of New South Wales TechnicalPublication No. 18 Timber Borers of CommonOccurrel}~e. and Technical Publication No. 19 LyctusSusceptibility of the Commercial Timbers used in NewSouth Wales.

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The Bostrychid BorerThese borers may attack timber as soon as it is felled andduring seasoning. Seasoned timber and timber of lowmoisture content are not attacked. However ifbostrychidlarv~e are already present in such timber they maycontmue to feed and work and adult insects may lateremerge. Bostrychid infestation of the sapwood may befollowed by lyctid infestation when the timber isseasoned.

In New South Wales attack by bostrychid appears to beconfined to the sapwood of hardwoods. In contrast withlyctid infestation it is not limited by the presence in thewood of vessels of a certain size as the bostrychid eggsarelaid in tunnels in the sapwood. These tunnels, which areoften "L" shaped, are bored by the adult bostrychid.

DURABILITY

The greater durability of heartwood over sapwood isattributed to the presence of extraneous materials(described earlier) which are deposited during thetransition from sapwood to heartwood. Extraction ofthese extraneous heartwood substances with suitablesolvents reduces the resistance of heartwood to fungaldecay.

Sapwood is also more susceptible to attack by surfacemoulds than is heartwood. Similar measures to thoseused for the control of sap stain fungi will prevent thegrowth of surface moulds.

In the case of untreated poles, it is customary to removethe sapwood from the portion which enters the ground toabout 45 cm above ground level as the sapwood mayencourage termites or the growth of fungi. Also the loss ofthe sapwood while the pole is in the ground would allowitto loosen. The sapwood is frequently left on the upperpart of the pole and although it may ultimately be lost byinsect or fungal attack, it protects the outer heartwoodfrom the weather for such time as it remains. Becausesapwood is relatively free of deposits it can be readilyimpregnated with preservatives by means of pressuremethods. This allows the sapwood to be retained over thewhole tree.

Cypress pine flo~ring enjoys great popularity, partlybecal;lse of the resistance of the heartwood to fungi andt~rmites. However, because of the relatively smalldiameter of the trees and the fairly wide sapwood it isfound in practice that most cypress flooring contains aconsi~er~ble proportion of sapwood. If correct buildingpractice is followed the presence of sapwood in cypresspine flooring is quite acceptable and because of the colourcontrast may enhance the appearance of the floor.

The blocking of the vessels by tyloses or gum-likemat~rials which characterises heartwood appears torestnct the development of many fungi. The blocking ofthe vessels could mechanically impede the free progressof fungi or restrict the diffusion of gases.Fungal attack ondense timbers appears to be retarded due to thediminished oxygen and accumulated carbon dioxidearound the hyphae.

SEASONING

Despite the higher moisture content of the sapwood it isfound in practice that it seasons in much the same time asheartwood, the lack of deposits and tyloses allowing amuch more rapid loss of moisture than is possible inheartwood. In regard to the relative shrinkage ofheartwood and sapwood, there appears to be only a smalldifference between the two. While checking often appearsfirst on the outside of a log, this may be due to rapid waterloss there and by the radial checks becoming largertowards the outside with the increasing circumference.

STRENGTH

In most species there is no significant difference in thestrength of sapwood and heartwood. This is because thecell wall becomes fully thickened and lignified very closeto the cambium, as noted earlier. While sapwood mayexhibit some greater flexibility in the green condition,this will disappear on drying. It is well known, however,that brittleness is associated with older wood near thecentre of the tree, and so sapwood would be less liable tobrittleness than the heartwood. The outer heartwood andthe sapwood are normally free from brittle heart.

In certain uses, requiring high strength, for example, axehandles (hickory Carya spp.) and cricket bats (willowSalix alba, sapwood is preferred to heartwood. This islargely a "customer" preference for the even whiteappearance of the sapwood.

In some timbers which have large amounts of extractivesdeposited during heartwood formation, a small increasein the strength of the heartwood is found (e.g. redwoodSequois sempervirens). With redwood removal of theheartwood extractives of this species with solvent resultsin a loss of strength to values approaching that of thesapwood.

UTILISATION

Provided the sapwood is immune or resistant to lyctidborer and the situation does not require a durable timberthen sapwood is structurally equivalent to heartwood.

In situations requiring a pale-coloured timber free ofextractives and tainting substances, sapwood is preferredto heartwood; for example, wooden spoons, pulpwood,and food containers. Sapwood is also more desirablewhere complete impregnation with preservative isrequired. This allows the utilisation of smaller trees suchas thinnings for fence posts, poles and other uses wherehigh durability is a desirable feature.

Being pale in colour sapwood can be readily stained tomatch coloured heartwood. If the sapwood has beeninfested with sap stain fungi this will not be successful asthe grey colour imparted to the sapwood by the fungi ispermanent.

Sapwood is utilised extensively in the manufacture ofplywood. It peels and glues as satisfactorily as heartwoodand is readily penetrated by preservatives (dip diffusionprocess).

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DETERMINATION OF SAPWOODAND HEARTWOOD

Sapwood becomes heartwood with the death of theparenchyma. It is difficult however to show, even with themost sophisticated techniques, the precise zone wherethis takes place. It is possible by using some of the otherproperties which change from sapwood to heartwood toobtain a fairly reliable estimate of the sapwood/heartwood boundary. Some of these are described below:

• Colour. The development of heartwood colour in manyspecies gives a good indication, although in somespecies the transition zone tends to be wide, mergingslowly into heartwood.

• Starch. The presence of starch indicates that the wood issapwood; however, its absence does not indicateheartwood. Starch may be detected by the blue-blackcolour with iodine/potassium iodide solution. Refer tothe Forestry Commission of New South Wales Wood,Technology Leaflet No. 8 Preparation and Care ofIodine Solution for Starch Tests.

• Colour reactions of wood material. A number ofchemicals are known to give quite spectacular colourchanges between heartwood and sapwood. Some ofthese chemicals are listed in Appendix 1. Table 2 listssome reactions obtained using six chemicals.

• Acidity. In certain eucalypts the acidity increases frompH5 to pH4 from sapwood to heartwood.

• Penetration of stained solution. The differentialpenetration of a stained solution (for example, 0.5 percent alcoholic safranin) into the end grain of driedtimber allows reasonable detection, the stain beingabsorbed more readily by the sapwood than theheartwood.

• Tyloses. The presence of tyloses is easily detected oncleanly cut end grain by examination with a 10x handlens; however, it is possible for tyloses to formabnormally following certain stimuli such as axe-cuts,or chemical treatment of the tree. Its absence certainlyindicates sapwood in the species which normallydevelop tyloses.

Tests for heartwood such as colour reactions and aciditycan only be relied upon where the wood is unaffected byrot, leaching, of some other agency likely to alter thechemical constituents of the wood. It is also important torealise that the measurement of sapwood width may varysomewhat between methods.

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Table 1. Sapwood widths of some Australian Timbers

Table 2. Colour indicators for sapwood and heartwood (See Appendix I forformulations)

'These measurements have been obtained from a limited number ofsamples. . .

"These are timbers with which the reagents have been used wuh success.The reagents will no doubt be applicable to many other species.

APPENDIX IA number of these formulations are poisonous and it isrecommended that they be made up by qualified peopleand used with care.Potassium iodide - iodine solution

1.0 g potassium iodide0.5 g iodine crystals

98.5 ml waterDissolve the potassium iodide in a little of the water, addthe iodine and stir until dissolved and then add theremainder of the water. Additional information aboutthis solution is available in Preparation and Care ofIodine Solution for Starch Tests. Wood Technologyleaflet, W.T.8 of the Forestry Commission of New SouthWales.Sodium nitrite

10 per cent aqueous solutionFerric chloride (aqueous)

10 per cent aqueous solutionFerric chloride (methanolic)

10 per cent ferric chloride in a 50 per centsolution of methanol

Silver nitrate (ammoniacal)14 g silver nitrate

100 ml water6N ammonium hydroxide

Dissolve silver nitrate in the water, add ammoniumhydroxide until the silver oxide which precipitates outjust redissolves.

Vanillin sulphuric acid (alcoholic)Solution A

250 ml water436 ml cone. sulphuric acid425 ml absolute ethanol

Add the sulphuric acid to the water carefully and coolbefore adding to the ethanol.Solution B

5 per cent solution of vanillin in 100 percent ethanol

Mix 4 parts of solution Awith 1part of solution Bkeepingmixture at 2YC while being made.Bromcresol blue

0.1 per cent solution in 80 per cent ethanolDimethyl yellow

0.1 per cent solution in 80 per cent ethanol

Approximate widthof sapwood inmillimetres*

6 average3 average9-166-25

16-38

22-3012-3816-3825-5025-602575100

No heartwood known

Common name

MyallLancewoodWhite mahoganySydney blue gumGrey iron barkMessmate stringybarkTallowwoodRed blood woodSpotted gumSmooth barked appleCypress pineWhite birchMonterey pineFlame tree

ColourReagent Species"

Sapwood Heartwood

Bromcresol blue E. vtminalis Green YellowDimethyl yellow E. maculata Yellow RedVanillin sulphuric

acid E. viminalis Red Purple-brown(alcoholic) Shorea spp. Red Purple-brown

Ferric chloride(aqueous) E. viminalts Blue Blue-black

Ferric chloride(methanolic) E. vitninalis Green-blue Blue-black

Silver nitrate(ammoniacal) E. viminalis Brown Blue-black

Botanical name

E. microcorvsE. gummifeiaE. maculataAngophora costataCallitris spp.Schizomeria ovataPIlIUS radiataBrachychiton acerifolium

Forestry Commission of New South Wales photojilenumbers:Figure 1: SB 9608Figure 2: SB 9607Figure 3: SB 9609Figure 4: SA 3734/2Figure 5: MP 250Figure 6: SA 3739Figure 7: SB 9610

Acacia pendulaA. doratoxvlonEucalyptus acmenioidesE. salignaE. paniculataE. obltqua

This publication may be reproduced in full providedacknowledgement is made to the Forestry Commission.Extracts may not be published without prior referencetothe Forestry Commission ofNew South Wales.

Printed by Ambassador Press Pty Lld51 Good Street, GranvilIe 2142

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