2 Fundamental Aspects

2.1Wood Anatomy

In wood decay, the wood structure of trees is very important, as well as theenzymatic potential of the fungi. Trees can differ not only in the anatomicalstructure of their wood but also down to structural differences of individualcell-wall layers. All possess differing 'attractiveness' for fungal enzymes tobreak them down, this being manifested by the diverse patterns of wooddecay observed. Beyond the purely visual changes, this has far-reaching con­sequences for the mechanical properties of the fungus-infected wood, such asits strength or stiffness.The extent to which a pathogen can invade a substrate, and the method it

uses to do this, will depend both on its ability to degrade different cell typesand cell-wall constituents and also on its adaptability to the other conditionsof the host.This chapter deals firstly with the structure of lignified cell walls. This

includes the degradation behavior of fungi, in order to work out the complexinteractions between wood structure and wood degradation. The structure ofthe cell wall will be demonstrated using a model, then the three types of rot(white rot, brown rot and soft rot) will be discussed; these exhibit some com­mon features, despite their different ways of decomposing woody material.

2.1.1Structure of the Lignified Cell Wall

Readers wanting more information on the wood anatomy of native broad­leaved and coniferous trees should refer to fundamental works by Grosser(1977), Schweingruber (1978) and Carlquist (1988). As detailed accounts of thestructure of individual tree species would increase the size of this bookexcessively, here we shall mention briefly only the principal differences be­tween conifers and broad-leaved trees.The wood structure of most conifers is relatively homogeneous. The tissue

consists mainly of tracheids and, to a small extent, of parenchyma cells. Thelatter are generally arranged as uniseriate xylem rays running radially in thewood. In addition to these, axially aligned parenchyma cells and resin canalssurrounded by epithelial cells also occur in the wood of various genera.Tracheids are elongated dead cells, which serve for water conduction andstrengthening, and thus have a dual function in the wood of conifers.The wood of the more highly developed broad-leaved trees is structured

much more heterogeneously and exhibits a functional division of work withspecial types of cells (Braun 1970,1988). Vessels (tracheae) have the function

F. W. M. R. Schwarze et al., Fungal Strategies of Wood Decay in Trees© Springer-Verlag Berlin Heidelberg 2000

6 Fundamental Aspects

of water conduction, fibers contribute to strength, and parenchyma cells pro­vide storage, conversion and transport of nutrients. The different types ofcells can often be distinguished by macroscopic features. In particular, thisapplies to those broad-leaved which exhibit powerful multiseriate xylem raystrees and/or a characteristic arrangement and concentration of the vessels orof the longitudinal parenchyma.Kerr and Bailey's scheme (1934) distinguishing five cell-wall layers

(Fig. 1A) is usually taken as the starting point for the structure of the lignifiedcell wall. These five layers are the middle lamella, the primary wall, and athree-layer secondary wall (Kollmann 1951; Liese 1957; Kollmann and Cclte1984; Fengel and Wegener 1989). This model is based on numerous histo­logical studies. A light micrograph of cell walls is shown in Fig. IE. With stan­dard staining, two defining cell-wall layers can be distinguished from eachother by color differences in the cross section (Fig. 1B). These are the middlelayer stained red with safranine, and the secondary wall stained with aura­mine and methylene blue. The cellulose, an important constituent of thesecondary wall, can be shown especially by its birefringence under polarizedlight (Fig. 1C). Cell-wall regions which possess no cellulose or only smallconcentrations of cellulose, e.g. the middle lamella, exhibit no birefringenceand appear dark (Fig. 1C, arrow). This technique is also used to determinecell-wall regions in which cellulose degradation by fungi has occurred. Inthese regions the birefringence is visibly reduced (Schulze and Theden 1937).The individual cell-wall layers described below differ mainly in their fine

structure or in the orientation of the microfibrils and their chemical compo­sition.

2.1.2Middle Lamella

Basically, in all plant tissues, neighboring cell elements are connected by amiddle lamella (Fig. 1B), which exhibits the following features: the middlelamella is largely isotropic and appears largely homogeneous under the scan­ning electron microscope, as it consists largely of amorphous, i.e. shapeless,substances like pectin and lignin (matrix). Because of its amorphous structureand lack of cellulose, it does not exhibit birefringence under polarized light,and appears dark (Fig. Ie}. Pectin is a constituent of the middle lamella, and asa cementing substance (ultra adhesive) it has the important task of connectingneighboring cells to one another (Liese 1981; Wagenfiihr 1989). Pectin is ahigh-polymer substance which contains building-blocks of galacturonic acidmolecules with carboxyl groups partially esterified by methanol. The thick­ness of the middle lamella ranges from a few tenths of a micrometer up to5 ~m in the cell-wall corners. For various reasons the middle lamella plus theprimary wall is also called the compound middle lamella, the former beinglaid down during the development of the middle lamella.The mechanical properties of the compound middle lamella impart com­

pression strength and stiffness to the cell wall (Brooker 1996). The highercompression strength derives from the smaller proportion of microfibrils

Wood Anatomy 7

Fig. lA-C A Conventional cell-wall model which distinguishes five cell-wall layers. These are themiddle lamella (ML), the primary wall (PW), and the three-layer secondary wall (5): outer (51)'middle (52) and inner secondary wall layer (53)' B Transverse section through spruce wood (early­wood tracheid). Standard staining clearly distinguishes the red (dark) middle lamella (ML) from theblue (light) secondary wall (51 to 5). The cell lumen (L) is not stained, as it is a cavity in which thewater and dissolved nutrients are transported in the conducting sapwood. C In the photo, the secon­dary wall exhibits birefringence under polarized light, whereas the middle lamella (arrows) appearsdark (xlOOO)

8 Fundamental Aspects

laid down in the matrix, and the associated increased degree of lignificationof the compound middle lamella. In birch wood, in which the middle lamellafills out the rounded cell-wall corners of the fibers as a cement, the ligninconcentration can exceed 80% (Fergus and Goring 1970).During maceration, i.e. the chemical dissolution of the middle lamella,

individual cells in the wood tissue are separated and changed in shape. Thusthe mechanical properties of the wood are altered. An identical effect can alsobe achieved by fungi. Preferential lignin degradation (selective delignifi­cation) by particular fungus species is repeatedly observed in nature, forexample, Ganoderma species from the group of white-rot fungi (Blanchette1984a,b). In contrast, other fungi either do not break lignin down or do soonly together with cellulose, this depending on different enzyme systems,living conditions and the particular host. The nature of the lignin itself alsohas a great influence on the degradation behaviour of fungi, for lignin is not awell-defined chemical substance but rather a heterogeneous class of com­pounds (Otter 1996). Of these, the two phenylpropanoid units guaiacyl andsyringyl are the most important monomers in the lignin of trees. Coniferwood lignin consists almost exclusively of guaiacyl monomers, whereashardwood lignin consists of approximately equal proportions of guaiacyl andsyringyl (Whetten and Sederoff 1995). The proportions of these monomersvary between individual cell types. For example, the vessels and the middlelayer in the cell walls have a very high concentration of guaiacyl, and thus areparticularly resistant to some soft-rot fungi, such as Ustulina deusta andother fungus species (Blanchette et al. 1988; Schwarze et al.1995b). Moreover,a strongly lignified cell wall becomes more dense and compact by ligninincrustation, so that enzymes with their relatively large molecules find itharder to penetrate. Therefore the strongly lignified corners of the middlelamella are preserved the longest in most wood-decay situations.

2.1.3Primary Wall

As already described, the primary wall is difficult to distinguish from theadjacent middle lamella under a light-microscope or an electron microscope,and therefore it is evaluated jointly with it from the biomechanical stand­point. Like the cell-wall layers still to be described, the primary wall exhibits aframework substance of cellulose fibrils, besides the matrix. This cell-walllayer is characterized by its cellulose forming only ca. 2.5% of the total, andthe fibrils run scattered (scatter texture), mainly transversely to the axis ofthe cell (Fig. lA; Braun 1982).

2.1.4Secondary Wall

The secondary wall forms the largest part of the cell wall (Fig. lA). It imme­diately adjoins the compound middle lamella and closes off the cell wall inthe direction of the lumen. In contrast to the compound middle lamella, here

Wood Anatomy 9

cellulose forms up to 94% of the defining chemical substance. Its biomechani­cal function is primarily to impart high tensile strengths to the cell.The secondary wall has a pronounced layer structure, usually in the form

of an outer (SI)' a middle (S2) and an inner (S3) secondary wall. Both thethickness of each layer and the arrangement of its cellulose fibrils differ asfollows:- The outer secondary wall (51 layer) lies next to the primary wall and is alsocalled the transitional lamella (Liese 1957). Its cellulose fibrils exhibit aweak parallel arrangement, being oriented approximately transversely tothe longitudinal axis of the cell. According to Meier (1955), their thicknessin spruce tracheids and birch fibers is ca. 0.2 11m.

- The central secondary wall (52 layer) is several micrometers thick, is thestoutest wall layer, and forms the bulk of the cell wall. In spruce tracheids itis between 1 and 5 11m thick (Meier 1955) and thus forms 74-84% of thewhole cell wall. The fibrils run parallel to each other in a shallow spiral(parallel helical arrangement) nearly in the direction of the cell's longitudi­nal axis. Thus, individual lamellae composed of cellulose, lignin and hemi­cellulose follow one another, together forming the S2 layer. The cell wall isthus built up in the form of concentrically arranged lamellae (Liese 1970;Kerr and Goring 1975; Ruel et al. 1978; Fengel and Wegener 1989). From thehigh content of cellulose in the S2 layer of the cell wall, it is clear that itplays a great part in the tensile strength of the wood. Also, the S2 layer,being very rich in carbohydrates (cellulose), is preferentially broken downby brown-rot and soft-rot fungi. It is brown-rot fungi especially whichdestroy the cellulose at the initial stage. This has the consequence that evensmall amounts of degradation at the initial stage will lead to a drasticreduction in wood strength (von Pechmann and Schaile 1950; Wilcox 1978;Schwarze 1995).

- The inner secondary wall (53 layer) separates off the cell wall from thelumen. It is relatively thin, and most cell-wall researchers believe it consistsonly of a single lamella. Jayme and Fengel (1961) found wall thicknesses of0.1-0.15 11m on spruce tracheids. The microfibrils are arranged in the terti­ary wall either parallel or slightly scattered, the texture resembling the pri­mary wall. On the basis of its chemical composition, the S3 layer certainlyoccupies a special position within the secondary wall. It has much lesscellulose than the S2layer, and in conifers it clearly exhibits a higher degreeof lignification. Moreover, especially in the tracheids of conifers, it posses­ses increased resistance to certain fungus species. This becomes particu­larly obvious in fungi which cause simultaneous rot (a form of white rot)and thus can degrade the cell wall only in the immediate vicinity of theirhyphae. If such hyphae are in the lumen of a cell, then they will automati­cally come into contact with the anti-fungal inner secondary wall, whichoffers them hardly any possibility of degradation. This works so selectivelythat such fungus species occur only very rarely in conifers. In contrast,brown-rot fungi have adapted better to the conifer wood substrate in theirontogeny. Admittedly the hyphae of the brown-rot fungi are also incapableof degrading the anti-fungal secondary wall directly, but it is penetrated by

10 Fundamental Aspects

the enzymes and/or similar substances, so that the regions of the secon­dary wall lying behind it (SJ can be preferentially broken down. Only insome exceptional cases is no S3 layer formed, for example in certain typesof tension-wood fibers of many broad-leaved trees and in the compressionwood of conifers.

2.1.5New Information on the Structure of the S2 Layer

The defining secondary wall layer (S2) is assumed to possess a lamellar struc­ture (Liese 1970; Fengel and Wegener 1989). In other words, we assume thatmany individual layers are superimposed upon each other, in their totalityforming the S2 layer and thus following the circumferential direction of thecell. Deviating from this model representation of the secondary-wall struc­ture, there have been repeated indications that other forms of structure of theS2 layer also exist. As long ago as 1938, Bailey described a preferred radialdirection of cell-wall constituents in the secondary wall. Recently there havebeen numerous reports on the presence of radial structures in the secondarywall of certain cell elements (Sell and Zimmermann 1993a,b; Larsen et al.1995; Zimmermann and Sell 1997). In particular, using a field-emission scan­ning electron microscope (FE-SEM), Sell and Zimmermann (1993a,b) andZimmermann and Sell (1997) show on transverse fracture surfaces of longi­tudinally stressed softwoods and hardwoods a preferential radial orientationof the cellulose microfibrils relative to the longitudinal axis of the cell andtransverse to the middle lamella. In their studies on broad-leaved trees, radialfibril orientations were found only in the secondary wall of the strengtheningtissues, i.e. in fibers and tracheids, but not in the secondary wall of vessels orparenchyma cells.Apart from purely academic interest, cell-wall structure has far-reaching

consequences for the way in which fungi degrade cell-wall substances.Understanding of the patterns of wood decay will develop with increasingknowledge of the forms and chemical composition of the cell walls.Using wood-decay fungi as a tool offers a further possibility of elucidating

the structure of the cell wall. The degradation processes of wood-decay fungion naturally and artificially inoculated wood reveal radial cell-wall sub­stances (Fig. 2; Schwarze and Engels 1998, Schwarze and Fink 2000).Unusually, in this wood decay extensive delignification of the cell wall pre­

cedes the formation of cavities in the S2 layer. This is clearly seen by stainingwith safranine and astra blue (Srebotnik and Messner 1994; Fig. 2A). Withthis method the cellulose stains blue (dark) only in the absence of lignin, soprevious delignification of the cell wall has been achieved. A further featureof this wood decay is the fact that no hyphal growth is observed in the cellwall, although numerous cavities occur in the secondary wall. The latter areseparated from each other by radial structures running perpendicularly tothe middle lamella (Fig. 2B).The radial structures extend from the outer (S,) to the inner secondary

wall (S3)' and like the S3 they exhibit increased resistance to degradation by

Wood Anatomy 11

With axialpressure the52 swells inthe transversedirection1

1AXIAL ;RESSJ'RE .5' Inner stiffening:3 prevents inward

buckling of 52

Radial structures

5: Outer bracing, " prevents

outwardsbuckling of 52

Fig.2A-D. Transverse sections of spruce wood (early-wood tracheids). A Starting from the hypha (H)growing in the lumen on the S3layer, the cell wall is discolored from red (light) to blue (dark). This isattributable to the preferential degradation of the lignin in the cell wall. BAt the advanced stage, indi­vidual cavities occur as a result of cell-wall degradation, which are separated from each other byradial structures (arrows, xlOOO). CAn alternative, idealized model of the cell wall, based on degrada­tion patterns of wood-decay fungi. In addition to lamellar structure of the S2layer, cell-wall constitu­ents also have a preferential radial direction. DWith axial compression the S2 layer acts like a torsionbrake, outward buckling being prevented by the SI layer and inward buckling by the S3 layer. The tor­sion path is increased by tough axial structures

wood-decay fungi (Schwarze and Engels 1998). If these results are transferredinto a cell-wall model, a preferential radial direction of cell-wall constituentscan be detected as well as a lamellar structure in the secondary wall. It should

12 Fundamental Aspects

be noted here that much more research is still needed on cell-wall structure.The more we know about the structure of lignified cell walls, the greater willbe our understanding of the ways fungi break wood down and what decaypatterns they cause.If we try to evaluate these radial structures as regards their appropriaten­

ess for the individual cell, then they are presumably fulfilling a biomechanicalfunction, e.g. increasing the safety margin of a cell against buckling (Booker1996). This could also be why parenchyma cells (which have a storage func­tion) or vessels (which have a water-conducting function) possess an over­whelmingly lamellar-concentric structure.The different structure of the secondary walls in different cell elements has

a significant influence on the wood-decay pattern of different fungus species.Interestingly, Courtois (1963) reports that soft-rot fungi cause most damageto tracheids and fibers, and that the cavity width increases with increasingcell-wall thickness.Soft-rot fungi usually break lignin down later (Eslyn et al. 1975). They can

penetrate more easily into the cell wall e.g. via radially oriented structuresrich in hemicelluloses and celluloses. The observation by Courtois (1963) thatcell-wall thickness influences cavity width accords with the observations ofSell and Zimmermann (pers. comm.) and Engels (1998). They observed thatthin cell walls exhibit a smaller and much more uniform orientation of radialstructures than thicker cell walls. This could be the reason why soft-rot fungiare found more rarely on radially running cell-wall structures that are harderto degrade, and consequently form larger cavities.The presence of radial structures influences not only wood decay by soft­

rot fungi but also that by brown-rot and white-rot fungi. The absence ofradial structures in the secondary walls of vessel and parenchyma cell wallspresumably makes degradation by brown-rot fungi more difficult.In contrast to 'normal' wood fibers, there are no radial structures in

tension-wood fibers. The structure of tension-wood fibers, e.g. in beech, isstrictly concentric and shows a lamellar arrangement of S\+S2+S3+a gelati­nous layer or S\+a gelatinous layer (Preston 1947; Kocon 1988). The alignmentof the cellulose microfibrils in tension-wood fibers is strictly parallel (Pres­ton 1947; Kocon 1988). A further feature of tension-wood fibers is the occur­rence of numerous irregular spirally running clefts arranged at right anglesto the grain. According to Chow (1947), these clefts are associated with inci­pient tension failure of the cell wall. However, Sachsse (1965) believes that theclefts are caused by increased compression stress and a contraction of the cellwall. Irrespective of the biomechanical explanation, the clefts running atright angles represent an entry way offering the least resistance to the hyphaepenetrating the cell wall (Schwarze and Fink 1998). Therefore hyphae ofMeri­pilus giganteus, for example, penetrate into tension-wood fibers of beech viaclefts arranged at right angles to the grain (Fig. 3).These results underline the fact that the structure of the lignified cell wall

significantly affects the structural changes caused by wood-decay fungi, andthis has far-reaching effects on the mechanical impairment of the particularhost.

Mechanical Model for Wood 13

oFig.3A-D. Tangential section of beech wood, artificially inoculated with Meripilus giganteus (xIOOO).A In the tension-wood fibers, fine penetration hyphae (arrows) penetrate into the cell wall via clefts atright angles to the grain. B Enzymes are released at the tips of the hyphae (arrows) and the cell wall islocally dissolved. C Rectangular bore-holes (arrows) within the cell wall are produced by cell-walldegradation by the special structure of the tension-wood fibers. D Schematic diagram of the degrada­tion process described

2.2Mechanical Model for Wood

As already described, the main constituents of wood possess differingmechanical functions. Thus the cellulose microfibrils are flexible frameworkfibrils with high tensile strength surrounded by a dense rigid filler, the lignin.Accordingly, the lignified cell wall can be compared to reinforced concrete(Troll 1959; Sitte et al. 1991), in which the steel corresponds to the cellulosefibrils and the concrete to the lignin.

14 Fundamental Aspects

On the basis of this, plus recent biomechanical information, Mattheck andSchwarze (1994) have devised a modified mechanical model to illustratewood structure in a better way. In this model the compound middle lamellais conceived as a 'chimney-like brickwork' of 'lignin bricks', because lignin isthe defining substance of this cell-wall layer. In contrast, the secondary wall isshown as a system of 'hollow cellulose ropes', because cellulose predominatesin this layer (Fig. 4A).

p

Tangential compressivestress against crackformation along the rays

X~lem ra~lJ :."'--'---'---

A Lignin chimney

Annual ring

B cFig. 4A-C. A In the mechanical model for wood the compound middle lamella is represented by ligninbricks and the secondary wall by hollow ropes of cellulose. Inherent tensile stresses act in the longitudi­nal direction on the tree's surface against fiber buckling. Lateral compression stresses prevent the xylemrays becoming cracks, as observed during the drying of wood. B Compression wood has thick ligninchimneys resistant to pressure, and only thin hollow ropes of cellulose. C Tension wood has thick hol­low ropes of cellulose resistant to tension, and only a few stiff tension-sensitive lignin chimneys

Mechanical Model for Wood 15

Fig. SA,B. Representation of wood-decay phenomena in light-microscopy and in the wood-decay model.A Selective delignification: in the wood model the lignin chimneys are broken down, leaving the softtough hollow ropes of cellulose. In the light-microscope picture, individual cells are separated by thedegradation of the lignin chimneys out of their matrix. Moreover, the discolorations of the secondarywall show that lignin was also broken down there (Heterobasidion annosum on spruce). BSimultaneousrot: here, the hollow cellulose ropes are broken down from within. Numerous bore holes also weakenthe lignin bricks. The two result in a brittle transverse fracture (Fornes fomentarius on beech,xl000)

This mechanical model is still further extended by giving the xylem rays amechanical importance in addition to their biological function. This could beshown using common ash as an example.The simplified model illustration certainly harbors some inaccuracies, but

it does help us understand the effects on the tree of wood decay by fungi.

16 Fundamental Aspects

Moreover, this model also provides illustrations of the importance of specialtissues in wood, such as compression wood and tension wood. In comparisonto normal wood, the compression wood of conifers can be thought of accord­ing to this model as a 'chimney' with thicker brickwork and weaker celluloseropes (Fig. 4B). In contrast, the tension wood of broad-leaved trees containsno 'chimney walls' of lignin bricks or only quite thin ones, but it does containthick cellulose ropes swelling inwards like gelatin (Fig. 4C).Wood decay and its effects on wood structure can also be illustrated using

this model. For example, selective lignin degradation is characterized by thepreferential destruction of the 'chimneys' and the persistance of the inter­nally hollow ropes of cellulose (Fig. SA).In contrast, simultaneous rot causes a progressive degradation of the

secondary wall from the lumen towards the middle lamella, the latter beingsimultaneously perforated by bore-holes (Fig. SB).In contrast, in brown rot and soft rot the hollow ropes of cellulose are

mainly destroyed, and the lignin brick walls remain as a stiff but brittlebrickwork. Numerous clefts occur in the secondary wall during the degra­dation of cellulose by brown rot (Fig. 6). Moreover, the primary wall is alsobroken down, so that the lignin of the middle lamella crumbles like powder.In contrast, in soft rot caused by Ustulina deusta the whole compound middlelamella remains preserved (Fig. 7).

2.3Types of Wood Decay

Various methods of analysis are available to evaluate fungal degradationprocesses on wood. These are based on the fact that fungi cause chemicaland structural changes. These changes have long been used by scientists toclassify wood decays. Usually three groups are distinguished: white, brownand soft rots. These are discussed below, and are presented in Table 1.

2.3.1Brown Rot

Brown rot is a kind of wood decay caused exclusively by fungi of the Basi­diomycetes. This class contains many families, though the overwhelmingmajority of the brown-rot fungi belong to the family Polyporaceae. Inte­restingly, only 6% of all the known wood-decay fungi are now known tocause a brown rot. Moreover, they are overwhelmingly associated with coni­fers, whereas white-rot fungi are associated with broad leaves (Gilbertson1980;Watling 1982).Cellulose and hemicelluloses are broken down in the wood substrate, while

lignin remains preserved in a slightly modified form (Rayner and Boddy1988; Green and Highley 1997). Because of the preferential degradation ofcarbohydrates, the decayed wood acquires a brittle consistency, breaks up likecubes and finally crumbles into powder. The modified lignin remaining givesthe decayed wood its characteristic color and consistency.

Types ofWood Decay 17

Fig.6A-(. Representation of the wood-decay phenomena in light-microscopy and in the wood-decaymodel. A and C Brown rot: at the initial stage of brown rot longitudinal clefts are formed in the hol­low ropes of cellulose (Fomitopsis pinicola on spruce, xIOOO). BAt the final stage of brown rot the soleremaining lignin chimneys can be rubbed to powder between one's fingers

The degradation of cellulose and hemicellulose takes place at different stages.It is assumed that hydrogen peroxide is probably formed in a pre-cellulolyticphase, and easily penetrates into the cell wall and, together with iron ions,overcomes the lignocellulose matrix by oxidative depolymerization (Koenigs

18 Fundamental Aspects

Fig. 7A-(' A The soft-rot of Ustulina deusta first produces holes in the hollow ropes of cellulose. B Atthe final stage the hollow ropes of cellulose are completely broken down; only the stiff brittle ligninchimneys always remain preserved. C At the late stage of wood decay a rigid framework (lignin chim­neys) remains, consisting of the compound middle lamella and the cell walls of the vessels (Ustulinadeusta on beech,xI000)

1974a,b). This assumption seems necessary, as cellulose-decomposing en­zymes are relatively large and the much smaller cell-wall capillaries cannotbe simply penetrated without further ado (Keilisch et al. 1970). Clearly, thehemicelluloses surrounding the cellulose are also affected, so that the cellu-

Types of Wood Decay 19

Table 1. Characteristic features of brown rot, white rot, and soft rot

Brown rot

Host

Fungi

Degradation

Consistency

trength

• Especially in coniferous trees

• Ba idiomycetes• Especially from the family of the Polyporaceae

• Cellulose and hemicellulose degradation

• Fragile, powdery, brown• Cracks and clefts

• Drastic reduction of bending and impact strength

White rot

Simultaneous rot Selective delignification

• Broad-leaved trees and conifers

• Brittle fracture

• Broad-leaved trees,• but seldom in conifers

Basidiomycetes and Ascomycetes

• Cellulose, lignin and • Fir t lignin and hemicellulose,hemicellulose later cellulo e also

• Brittle • Fibrous (stringy)

Less drastic than in brown rot

• Ductile fractureat the initial stage

• Slight increa ein impact bending strength

• Great reductionof impact bending strength

Fungi

Degradation

Host

Con i tency

Strength

Soft rot

Conventional picture ew information

Between brown and white rotHigh stiffnessBrittle fracture

• Extensive decay in living broad-leavedtrees

• Basidiomycete

• Cellulose and hemicellulose• Lignin trongly

Brittle

• Broad-leaved trees and conifers• Especially on wooden structures

• Deuteromycetes• Ascomycetes

• Cellulo e and hemicellulose• Lignin slightly

Degradation

Host

Fungi

Consistency

Strength

lose then becomes accessible for cellulases, the actual enzymes. Then followsan indiscriminate cleaving of the cellulose chain molecules (starting at manyplaces), quickly forming many individual cellulose chain fragments (Cowling1961).The combination of the preferential and indiscriminate degradation of

cellulose is closely associated with a drastic loss of bending strength afteronly a very short period of degradation (von Pechmann and Schaile 1950;Wilcox 1978; Schwarze 1995). According to the latest studies by Green et al.(1991) and Winandy and Morrell (1993), this strength loss is mainly attribu-

20 Fundamental Aspects

table to hemicellulose degradation, because far-reaching cellulose degrada­tion could not be determined at the very early stages of degradation.However, these extreme strength losses in brown-rot wood could have

other causes. As shown in Section 2.1.5, there are numerous indications fordeviations from the general cell-wall structure in the form of radial structu­res in the S2layer. In brown-rot wood these radial structures are preferenti­ally broken down at the early stage of wood decay, and then numerous fineclefts and cracks occur in the secondary wall, extending from the S3 to the S1(Fig. 8). Degradation of these radial structures could be closely associatedwith the strength losses, as the resistance of the cell wall against bucklingunder compression stress in the grain direction is greatly reduced.Moreover, radial structures could allow the mycofibrils formed by brown­

rot fungi to penetrate into the S2 layer of the secondary wall more quickly, asthey can better overcome the anti-fungal S3layer. The S3layer is very resistantto the enzymes of brown-rot fungi, so that hyphae cannot destroy the ligni­fied cell walls in their immediate vicinity (Liese 1970). For this reason, cell­wall degradation occurs not in the immediate vicinity of the hyphae or outfrom the lumen, but the ectoenzymes of the brown-rot fungi must first dif­fuse very deeply into the cell wall through the S3 layer in order to degrade thecellulose-rich S2 layer (Liese 1963). Thus the radial structures presumablyrepresent the path of least resistance for the fungus. The possibility of degra­ding cellulose enzymatically at a great distance from the hyphae via radialstructures would permit even a few hyphae to cause far-reaching destructionof the cell wall (Meier 1955).In this context, if we also consider the colonization strategy of Laetiporus

sulphureus on Robinia, then we can recognize a preferential degradation ofcertain wood zones within the annual ring (Schwarze 1995; Sect. 3.3). Thereduction of birefringence indicated a preferential degradation of cellulose inthe early-wood fibers at the early stage of the decay, while neither longitudi­nal parenchyma nor vessels exhibited structural changes. There are two setsof explanations for this phenomenon:1. The degree of lignification of longitudinal parenchyma and vessels is hig­her, or qualitatively different, to that of fibers, so that degradation by fungitakes place more slowly and 'reluctantly'.

2. In contrast to fibers, the secondary walls of parenchyma cells and vesselsexhibit no radial structures. This possibly limits the ability of the fungus tooccupy the cell wall quickly and extensively, resulting in delayed degrada­tion.

The ontogeny of wood decay by brown-rot fungi is very uniform, apart froma few exceptions such as Fistulina hepatica (Schwarze et al. 2000b). Thereason for this is presumably the adaptation of these fungus species to therelatively simply structured softwood of conifers, and also their restrictedability to degrade lignin. Therefore, most brown-rot fungi can be consideredrather inflexible. In contrast, white-rot fungi, which preferentially occur onbroad-leaved trees, exhibit an extraordinary diversity in the ontogeny ofwood decay. The adaptation of white-rot fungi to the much more hetero-

Types ofWood Decay 21

).,..-y', ; 1

{

".. JJ I/

'1-. _.' '--'" I I

\ I

I ;i

, / \,/1: I\ " I \

" l/\

-----..I

r-

.y', .,

~'.: ..... :~:.

A B

r-- f-/

I

I/

"' /-

, ,

/'

- r--

o

=0(L--.-..........-.....-a.JJ~.\

cFig. SA-D. Development stages of a brown rot. A At the early stage enzymes penetrate radially into thecell wall from hyphae growing in the lumen. B With an increasing degree of degradation, enzymeshave penetrated into the entire secondary wall, involving extensive degradation of hemicellulose andcellulose. C The cell-wall volume decreases because of the cellulose degradation described in B, andmany cracks and clefts appear. D At the late stage these close up because of the continuing loss of cell­wall volume, and a framework of modified lignin remains

geneously structured wood of broad-leaved trees, plus their ability to de­grade all the cell-wall constituents extensively, leads to a multiplicity of diffe­rent patterns of wood decay. In contrast to brown-rot fungi, white-rot fungiare very adaptable and can therefore be called plastic.

22

2.3.2White Rots: the Whole Spectrum of Diversity

2.3.2.1White Rot

Fundamental Aspects

The fungi causing white rot are represented in all the main groups of theBasidiomycetes and in some of the Ascomycetes, namely the Xylariaceae(Sutherland and Crawford 1981). In common usage, the term 'white rot' hasbeen traditionally used to describe forms of wood decay in which the woodassumes a bleached appearance and where lignin as well as cellulose andhemicellulose is broken down. The relative rates of degradation of lignin andcellulose and other cell-wall constituents differ, sometimes considerably,depending on the species of fungus and the conditions prevailing in thewood. Despite the great diversity in wood decay caused by white-rot fungi,two important forms of white rot are generally recognized, viz. selective deli­gnification and simultaneous rot (Blanchette 1984a,b; Adaskaveg and Gil­bertson 1986; Rayner and Boddy 1988).

2.3.2.2White Rot: Selective Delignification

During selective delignification, at the early stage of decay, lignin is brokendown more than hemicellulose or cellulose. Various kinds of selective lignindegradation have been described in the literature. These include white pocketrot, which some fungi cause in wood. Macroscopically this is recognized bylight patches, as preferential lignin degradation in places causes lighter zoneswith pure cellulose to appear. A typical example of this wood decay is causedby Phellinus pini (Thore:Fr.) Pilat (Liese and Schmidt 1966; Hartig 1878; Blan­chette 1980) or Grifola frondosa (Sect. 3.3).The different patterns of cell-wall degradation during selective delignifica­

tion can be observed under the light microscope. Firstly, a degradation of themiddle lamella occurs in conjunction with extensive lignin degradation inthe secondary wall. At the late stage, individual cells become separated fromtheir matrix (Fig. 9; Hartig 1878; Blanchette 1984a). According to Hartig(1878) and Peek et al. (1972), lamellar collapse of the secondary wall (S2) intosubmicroscopic layers is also possible. Moreover, extensive delignificationmay occur in the S2 layer, leading to the accentuation of radial structures(Schwarze and Engels 1998; Fig. 10).White-rot fungi break the lignin down by oxidative processes by means of

the phenoloxidases formed and released by the hyphae, such as laccase, tyro­sinase and peroxidase. Cellulose is broken down more slowly than in brownrot and soft rot, so that the reduction in wood strength properties is lessdrastic. One reason for this could be the fact that glucose or cellobiose is splitoff only from the ends of the cellulose chain molecules (Manion 1991), andthus large coherent cellulose fibrils are preserved longer. Another reasoncould be the longer persistence of radial structures in the S2layer which con-

Types otWood Decay 23

A B

ocFig. 9A-D. Different stages of selective delignification. A,B At the early stage of the decay enzymes(shown as dots) diffuse into the secondary wall from hyphae growing in the lumen. This causes lignindegradation within the secondary wall, which also extends to the middle lamella. C,D At the late stagethe preferential lignin degradation leads to individual cells being separated from one another'smatrix. The cellulose at first remains intact during this decay

tribute to preserving the strength properties of a cell (Engels 1998; Schwarzeand Engels 1998).As already mentioned, the cellulose remains relatively unchanged during

selective delignification, at least at an early stage of decay. Due to dissolutionof the lignin-rich middle lamella and the separation of individual cellelements from their matrix, the consistency of the decayed wood becomes

24 Fundamental Aspects

:() .:Mf!flfl/~r'

:,J I;,~F

Fig. lOA-D. Different stages of wood decay leading to the accentuation of radial structures in the cellwall. A The start of selective delignification of the secondary wall. B Fine clefts appear because of theincreasing cell-wall degradation. C Because of the local degradation of hemicellulose and cellulose,individual clefts enlarge into cavities, which are separated from one another by radial structures run­ning perpendicular to the middle lamella. DAt the late stage the S31ayer is also broken down

increasingly fibrous or stringy, and it loses stiffness and compressionstrength, although its toughness remains at first. Accordingly, if the decayedwood has a fibrous consistency, we can deduce selective delignificationmacroscopically. The longer preservation of the cellulose fibrils has far­reaching biomechanical consequences. Because of the preferential degrada­tion of the middle lamella and the associated reduction in stiffness, thetracheid cell walls easily kink under mechanical loading (Schwarze 1995). Thebuckling in of the cell walls is based on the fact that after their separation, thecellulose-rich secondary walls can still be heavily stressed under tensile load­ing but easily fail under compression loads. When fiber buckling occurs inwood, it has the consequence that the cellulose microfibrils, which run in ashallow spiral to the cell-wall axis, straighten out and thus can be stressed upto 20% more under tensile loading (Gordon 1976).Data from investigations on delignified birch wood confirm that its tensile

strength in the air-dry state is considerably greater than that of the initialwood (Klauditz 1957). This can be explained on the one hand by the increasedbulk density of the delignified wood, and on the other hand by a higher con­tent of tension-resistant cellulose as a result of degradation of the lignin(Klauditz 1957). A slight increase in impact bending strength was also foundat the early stage of decay on spruce wood naturally infected by Heterobasi­dion annosum (Pratt 1979).Wood decay near the surface, caused by fungus species such as Heterobasi­

dion annosum and Ganoderma spp. for example, is often accompanied by atypical symptom at the base of the stem, called bottle butt. This defect sym­ptom on trees may be associated with a selective delignification of the wood.Konig (1958) showed that local fiber buckling can lead to a bulge formationeven in sound wood. Clearly, a wood zone with partially buckled fibers is sof-

Types of Wood Decay 25

ter and behaves mechanically like wood that has lost some load-bearingstrength as a result of selective delignification (Mattheck and Breloer 1994).The cambium reacts to the soft decay and the increased bending moment ofthe stem by increasing its rate of division, leading to the formation of un­usually wide annual rings and to externally recognizable bulges, either localor embracing the whole stem. Moreover, the increased bending moment ofthe stem may lead to a change in the appearance of the outer bark. Thesedefect symptoms are important in the VTA (visual tree assessment) methoddeveloped in recent years by Mattheck and Breloer (1994) for the visualmonitoring and detailed expert inspection of trees.As already explained elsewhere, no increase in the bending moment

occurs with brown rots and other relatively brittle kinds of wood decay, e.g.soft rot caused by Ustulina deusta, and with the large group of white-rotfungi causing simultaneous rot. The consequence is that with these kinds ofwood decay there are generally no externally recognizable defect symptomsin the form of bulges or local fiber buckling (Schwarze 1995; Schwarze et al.1995b; Engels 1998). From the biomechanical viewpoint, selective delignifica­tion is the direct counterpart to brown rot.Here, we must again stress that even when extensive selective delignifica­

tion does occur in a tree, in the later course of the decay cellulose degrada­tion does take place. This has the result that besides the initial change instiffness and compression strength, later on the tensile strength of the woodis also very severely modified (Pratt 1979; Schwarze 1995), finally leading totree failure.Although selective delignification is usually associated with cellulose

degradation in wood, extreme forms of selective delignification are wellknown. From the temperate rainforests of southern Chile, as long ago as 1893Phillipi reported a wood decay which was called 'palo podrido'. This is aname for decayed tree stems which are used as cattle fodder in southernChile (Phillipi 1893; Gonzalez et al. 1986). Chemical analyses have shown thatthe wood of some decayed tree stems consists of 97% cellulose and only 0.9%lignin (Agosin et al. 1990). Native peasants in southern Chile use the term'palo blanco' for this astonishingly white wood, whereas 'palo podrido' is ageneral term for delignified wood.

2.3.2.3White Rot: Simultaneous Rot

In the course of simultaneous white rot, the lignin, cellulose and hemicellu­lose are broken down at approximately the same rates. The hyphae grow inthe lumen on the S3 layer, and the cell wall is broken down in the immediatevicinity of the hyphae, which leads to the formation of erosion furrows. Thehyphae sink into the cell wall like a river in its bed.Wood-decay fungi which cause a simultaneous rot occur overwhelmingly

on broad-leaved trees. This decay is characterized by the fact that the en­zymes released by the fungal hyphae can degrade all the main constituents ofthe lignified cell wall (Liese 1970; Rayner and Boddy 1988; Eriksson et al.

26 Fundamental Aspects

1990). The term simultaneous rot refers to the fact that degradation of cellu­lose, hemicellulose and lignin takes place approximately equally. However, itshould be noted that this group of species are generally called white-rot fungi(Nilsson 1988).With simultaneous rot cell-wall degradation takes place in theimmediate vicinity of hyphae (Fig. 11).This localized cell-wall degradation is caused by a slime coating around

the hyphae through which the enzymes gain closer contact with the wall sub­stances. As a consequence of many erosion furrows merging together, thethickness of the lignified cell walls gradually decreases from the inside(lumen) outwards (middle lamella; Liese 1970; Schwarze 1995).In contrast to selective delignification, simultaneous rot leads to a brittle

fracture of the infected wood because of the progressive degradation of thecellulose-rich secondary wall. This conclusion was reached from the resultsof studies on the dynamic impact bending strength of wooden rods decayedto various degrees by Fomes fomentarius and Ganoderma pfeifferi. In contrastto the simultaneous rot by Fomes fomentarius, Ganoderma pfeifferi causedselective delignification in the wood.Investigations on the impact bending strength of artificially inoculated

rods of beech wood after 4, 6 and 8 weeks of incubation showed that theimpact bending strength was reduced much more by Fomes fomentarius thanby Ganoderma pfeifferi (Schwarze 1995). Light-microscope studies showedthat this difference is attributable to the two different forms of the white rot(simultaneous white rot and selective delignification) caused by these parti­cular fungi.The cellulose remains largely preserved during the selective delignifi­

cation by Ganoderma pfeifferi. This has the consequence that the fracturepattern of the test rods shows enormous toughness and fibrous failure, notbrittle failure, under loading (Schwarze 1995). In contrast, the beech woodinfected by Fomes fomentarius exhibited high stiffness and a very brittle frac­ture behavior, even at a late stage of decay, because of the intact middlelamellae (Schwarze 1995).The phenomenon of brittle fracture of fungus-infected wood has beenknown for a long time, but was mainly associated with those fungus speciespreferentially occurring on wooden structures such as masts, poles, coolingtowers and railway sleepers, causing a soft rot there.

2.3.3Soft Rot

The characteristic feature of soft rot is the preferred growth of the hyphaewithin the secondary wall, which manifests itself in the form of typicalcavities oriented longitudinally to the cell axis. This striking kind of decaywas first described by Schacht (1863). Nearly a 100 years later, Savory (1954)described the ontogeny of this wood-decay phenomenon, and proposed thename 'soft rot'. The name 'soft rot' is used because of the soft consistency ofthe wood which is decayed by Ascomycetes and Deuteromycetes. There arealso soft-rot fungi which make the wood brittle, e.g. Ustulina deusta. As

..~ ..'..'"

tf

{ift

I

Types ofWood Decay

A

c

27

B

D

Fig. llA-D. Different stages of simultaneous rot. AAt the early stage of wood decay cell-wall degrada­tion occurs in the immediate vicinity of the hyphae. B The cell wall is progressively broken downfrom the inside (lumen) outwards. Individual hyphae penetrate into the cell wall at right angles to thecell axis. C The cell wall becomes increasingly thinner, and numerous bore-holes appear between twoneighboring cells. D At the late stage delayed degradation of the compound middle lamella and cellcorners takes place .

Ustulina deusta is the most important soft-rot fungus in practical arbori­culture, its pattern ofwood decay is shown schematically in Figure 12.Hyphae involved in soft rot grow within the cell wall in the direction of the

cellulose microfibrils (Savory 1954; Courtois 1963; Liese 1964). In contrast tobrown rot, but analogous to simultaneous rot, in soft rot the destruction ofthe cell walls always takes place in the immediate vicinity of the hyphae. This

28

o

Fundamental Aspects

Fig. 12A-D. Wood decay pattern ofUstulina deusta. A Penetration of hyphae into the lignified cell wall.B Branching and orientation of the hyphal canal parallel to the direction of the cellulose microfibrilsin the 52 layer. C Enzymatic degradation of the wood cell wall around the hyphal canal leads to theformation of cavities with conically shaped ends. DAt the late stage of wood decay by Ustulina deustathe secondary wall is nearly completely broken down, whereas the compound middle lamella remainspreserved

leads to the formation of cavities within the cell wall, which in cross sectionappear as small circular to oval holes in the secondary wall.Soft-rot fungi cause two different kinds of wood degradation, which are

generally distinguished as type 1and type 2 (Courtois 1963; Corbett 1965; Haleand Eaton 1985a,b). Some fungus species have the ability to cause not just theone type of soft rot or the other but both jointly (Nilsson 1973; Schwarze 1995).

Types ofWood Decay 29

Soft rot of type 1 is characterized by the formation of a series of successivecavities with conically formed ends which follow the direction of the micro­fibrils within the S2layer (Savory 1954). Hale and Eaton (1985a,b) investigatedthe way in which the cell walls of type 1were degraded, by studying hyphalgrowth under the microscope by means of time-lapse photography. On thebasis of these studies, different stages of cavity formation were distinguished.Hale and Eaton (1985a,b) showed that the process of development of the

hyphal canal exhibits a fluctuating growth pattern. After a period of longi­tudinal increment of the hyphal canal its growth stops and a cavity formsaround the hypha. The hypha in the cavity increases in size and then a newhyphal canal is formed from the pointed end of the cavity, a process whichcan be repeated an indefinite number of times.One explanation for this degradation mechanism presumably is that

nutrient supply for the development of the hyphal canal must take place fromthe mother hypha located in the cavity. With a lack of available nutrients orsenescence of the mother hypha the transport of nutrients is abruptly inter­rupted. The change-over from enzyme release at the hyphal tip to enzymerelease along the whole hyphal surface takes place at different times (Haleand Eaton 1985a,b). When a new cavity is formed the hypha supplies itselfwith nutrient; by enlarging the cavity until it has sufficient reserves availableto form a new hyphal canal (Hale and Eaton 1985a,b).Soft rot of type 2 resembles a localized simultaneous rot, the degradation

proceeding outwards from the lumen by the formation of small erosionfurrows in the form of V-shaped notches (Courtois 1963; Corbett 1965). Thiskind of soft rot occurs only very seldom in conifers, this presumably beingattributable to the greater resistance of the S3 layer against wood decay.From the biochemical viewpoint, because of preferential cellulose degra­

dation and the relatively small lignin degradation, soft-rot fungi have greatersimilarity to brown-rot fungi than to white-rot fungi (Liese 1963; Nilsson etal. 1989).For a long time it was thought that only fungus species of the Deutero­

mycetes and Ascomycetes (e.g. Chaetomium spp.) had the ability to cause softrot, but not fungus species of the large taxonomic group of the Basidio­mycetes. Moreover, scarcely any attention was paid to wood decay caused bysoft-rot fungi in living trees. These were usually regarded only in the contextof superficial parts of wood exposed directly to the weather and resultingfrom injury, e.g. removal of large branches or water pockets (Rayner andBoddy 1988; Blanchette 1992). The main importance of soft-rot fungi in theecosystem is presumably in accelerating natural branch shedding in theliving tree (Butin and Kowalski 1983)Detailed investigations in recent years on this form of wood decay have

provided much important new information going well beyond conventionalthinking, and this will be discussed below.

30

2.3.3.1Soft Rot in Living Trees Caused by an Ascomycete

Fundamental Aspects

It is known that various Ascomycetes, especially members of Xylariaceae (e.g.Hypoxylon species) can cause wood decay in living trees, though smaller inextent than Basidiomycetes. Ustulina deusta is an extraordinary member ofthe Ascomycetes which is capable of degrading wood very extensively even inthe central parts of the tree. In all its hosts Ustulina deusta causes a characte­ristic pattern of decay, in which many thin black boundary lines (pseudo­sclerotic layers) are formed in the decayed wood (Wilkins 1936, 1939a,b, 1943).On the basis of chemical analyses, wood decay by Ustulina deusta has been

classified as white rot (Campbell and Wiertelack 1935). However, recentstudies have shown that Ustulina deusta definitely causes a soft rot at theearly stage of wood decay on various hosts (Schwarze et al. 1995b).The ability of Ustulina deusta to cause an extensive soft rot in living trees

is of great interest, as for a long time it had been assumed that this kind ofwood decay does not occur in living trees. On the other hand, this discoveryis not really surprising, as Ustulina deusta belongs to the Ascomycetes, a largegroup of fungi which are known to cause soft rot in wooden structures or inwood exposed to the weather. In this connection a far more surprising dis­covery was made, viz. that under certain conditions wood-decay Basidio­mycetes can also cause a soft rot.

2.3.3.2Basidiomycetes as Facultative Soft-Rotters

Although the ability to form cavities within the cell wall had long beenascribed exclusively to Ascomycetes and Deuteromycetes, many structuralchanges by Basidiomycetes which resemble a soft rot have been described inthe past. There are numerous reports on the occurrence of conically pointedcavities within the cell wall of brown- and white-rot wood (Liese 1963,1964;Courtois 1965; Liese and Schmid 1966; Foster and Marks 1968; Nilsson andDaniel 1988). Rigidoporus crocatus can serve as an example; it usually occursas a brown-rot fungus on conifers, but in the wood it forms cavities in thedirection of the cellulose microfibrils, analogous to typical soft rot (Duncan1960). Only recently have structural changes by a Basidiomycete been foundon pine and birch wood artificially inoculated with Oudemansiella mucida(Schrad.:Fr.) Hohn, which can definitely be classed with those produced bysoft-rot fungi (Daniel et al.1992).A soft rot caused by a Basidiomycete in living trees was proven for the first

time in the case of wood decay by Inonotus hispidus (Schwarze et al. 1995b),and this was also confirmed in studies with Meripilus giganteus (Schwarzeand Fink 1998) and Inonotus dryadeus (Engels 1998).These results underline the fact that soft rot should be accorded equal

importance to brown rots and white rots in practical tree care. There are tworeasons for this. Firstly, because this form of wood decay can also be causedby Ascomycetes in extensive parts of a tree, and thus involves biomechanical

Types ofWood Decay 31

consequences. Secondly, it must be remembered that Basidiomycetes areclearly capable of this form of wood decay, and it is known that they candecay trees extensively.In our studies we have shown that many white-rot fungi (e.g. Ustulina

deusta, Inonotus hispidus) do cause a soft rot. As lignin is also broken downalong with cellulose and hemicellulose during soft rot by these two fungi, it isunderstandable that they were classed as white-rot fungi on the basis ofearlier chemical analyses, and at that time soft rot had not yet been describedas an independent form ofwood decay. The hyphal growth of the two fungusspecies within the cell wall (first described by us) and the associated struc­tural changes are characteristic for soft-rot fungi.

Inonotus hispidus is a fungus species which has a dual wood-decay stra­tegy. There may be ontogenetic reasons why many white-rot fungi also causea soft rot as well as their typical kind of wood decay. Finally, some authorspresume that white-rot and brown-rot fungi have arisen from the soft-rotfungi (Worrall et al. 1997) and therefore still possess some genes which en­able them to cause a soft rot under certain conditions.