the role of mechanized harvesting in the development of bluestain in pine

The role of mechanized harvesting in thedevelopment of bluestain in pine

Adnan Uzunovic, Joan F. Webber, Andy J. Peace, and David J. Dickinson

Abstract: The influence of mechanized harvesting on the development of bluestain was assessed by comparing pinelogs felled and trimmed with a chainsaw with those felled by a commercial harvesting machine. Corsican pine (Pinusnigra var maritima (Ait.) Melville) grown in the United Kingdom was cut in June and August, and the logs wereassessed for bluestain 3, 6, 9, and 12 weeks after felling. Mechanically harvested logs were in two groups: maximumdamage (mean amount of bark loss-35% of total cover) and minimum damage (-12% mean bark loss). However, allmechanically harvested logs were much more susceptible to attack by bluestain fungi than chainsaw-processed logs,which typically had <1% bark loss. Mechanically harvested logs had bluestain on-10% of the surface area of samplediscs compared with <1% in the chainsaw-harvested logs. Little bluestain developed if bark loss was <10%. The mostextensive stainers wereCeratocystis coerulescens(Münch) Bakshi andLeptographium wingfieldiiMorelet; otherfrequent bluestain fungi includedOphiostoma piceae(Münch) H. & P. Sydow,Sphaeropsis sapinea(Fr.) Dyko &Sutton, and aGraphiumspecies. Bluestain bark beetle vectors were excluded from the logs, but other arthropodsapparently acted as vectors. Using data from the study, a model was devised to predict of stain development followinga known amount of bark damage.

Résumé: L’influence de la récolte mécanisée sur le développement de la bleuissure a été évaluée en comparant desbilles de pin abattues et ébranchées à l’aide d’une scie à chaîne avec des billes récoltées par une abatteusecommerciale. Du pin noir de Corse (Pinus nigravar. maritima (Ait.) Melville) croissant au Royaume-Uni a été coupéen juin et août et les billes ont été évaluées pour la bleuissure 3, 6, 9 et 12 mois après avoir été coupées. Les billesrécoltées mécaniquement appartenaient à deux groupes : dommage maximum (perte moyenne d’écorce représentant~35% de la surface totale) et dommage minimum (~12% de perte d’écorce). Cependant, toutes les billes récoltéesmécaniquement étaient beaucoup plus vulnérables à l’attaque par les champignons responsables de la bleuissure que lesbilles récoltées à la scie à chaîne qui typiquement avaient perdu <1% d’écorce. Les billes récoltées mécaniquementavaient approximativement 10% de bleuissure sur la surface des disques servant d’échantillon comparativement à <1%pour les billes récoltées à la scie à chaîne. Il y avait peu de bleuissure sur les billes qui avaient perdu <10% d’écorce.Les champignons les plus fréquents étaient leCeratocystis coerulecens(Münch.) Bakshi et leLeptographiumwingfieldii Morelet; les autres champignons rencontrés fréquemment incluaient l’Ophiostoma piceae(Münch.) H. & P.Sydow, leSphaeropsis sapinea(Fr.) Dyko & Sutton et une espèce deGraphium. Les scolytes qui servaient de vecteurspour la bleuissure avaient été exclus des billes mais d’autres arthropodes ont apparemment servi de vecteurs. À l’aidedes données de cette étude, les auteurs ont élaboré un modèle pour prédire le développement de la coloration à partird’une quantité connue de dommages à l’écorce.

[Traduit par la rédaction] Uzunovic et al. 251

From the moment trees are felled they become the targetof primary colonizers such as moulds and bluestain fungi(Seifert 1993). Intact bark gives effective and prolonged pro-tection against fungal damage (Pearce 1996) so these early

colonizers commonly enter logs through the cut ends or anyother discontinuities that are created by animal activity.Bluestain fungi can also be introduced directly under barkvia breeding bark beetles, which act as vectors of thesefungi, and if temporary storage of logs coincides with peri-ods of bark beetle flight, it can result in extensive sapstaininfestations. In addition, the bark damage suffered by logsduring harvesting further increases the opportunities for sap-stain attack, and in the absence of beetle vectors it has beensuggested that this may lead to significant amounts of staindevelopment. The increasing use of mechanized harvesting,which is gradually superseding manual chainsaw felling inmany countries, including Britain (Greig 1997), may also beexacerbating the problem of bluestain, particularly in speciesof pine.

Despite observations that mechanized harvesting increasesthe incidence of bark damage, there have been relatively fewattempts to quantify the effect this may have on the amount

Can. J. For. Res.29: 242–251 (1999) © 1999 NRC Canada

242

Received June 17, 1998. Accepted November 12, 1998.

A. Uzunovic,1 J.F. Webber, and A.J. Peace.ForestryCommission Research Agency, Alice Holt Lodge, Farnham,Surrey GU10 4LH, U.K.D.J. Dickinson. Department of Biology, Imperial College ofScience, Technology and Medicine, Prince Consort Road,London SW7 2BB, U.K.

1Author to whom all correspondence should be addressed.Current address: Forintek Canada Corp., 2665 East Mall,Vancouver, BC V6T 1W5, Canada.e-mail:[email protected]

I:\cjfr\cjfr29\cjfr-02\X98-195.vpThursday, March 04, 1999 11:21:23 AM

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of degrade caused by bluestain fungi. Jackobsson (1976)reported that bluestain damage to logs occurred more rapidlyin logs processed using a mechanized harvester comparedwith those trimmed manually. In particular, he concludedthat the spiked feed rollers, often found on these processors,needed modification to minimize damage to logs. Söder-ström (1986) noted that stud holes produced by harvestersencouraged bluestain damage during storage, but he found itdifficult to quantify the influence of bark damage on theoverall development of bluestain. Similarly Helgesson andLycken (1988) showed that machine-processed timber de-veloped more severe bluestain compared with traditionallyharvested timber. These observations have led to the generalrequirement that mechanized harvesters should cause mini-mal bark damage and be capable of accurate delimbing andtrimming. Trials described by Lee and Gibbs (1996) in Brit-ain confirmed that bark removal was significantly greater inlogs harvested mechanically than with a chainsaw and thatthis was related to the amount of stain that developed after6–12 weeks. They also observed that there was significantlyless bark removal when rubber feed rollers were fitted to theharvesters instead of metal spiked rollers.

All these investigations have raised further questionsabout the connection between mechanized harvesting andbluestain attack. In particular, how and to what extent doescommercial harvesting cause damage, and how much doesthis influence the amount of bluestain that can occur inpine? Also, do certain types of damage predispose logs toincreased attack by staining fungi, and how does this interactwith season and storage time in the occurrence and develop-ment of bluestain? The study reported below attempts to an-swer some of these questions.

Trials were set up on 24 June and 3 August 1993 in ThetfordForest District in the southeast of Britain. As a part of routine com-mercial forestry practice, an Åkerman harvester H7 excavator basefitted with Lako 60 harvesting head (Fig. 1) processed logs be-tween rubber feed rollers with chains from a stand of 64-year-oldCorsican pine (Pinus nigra var maritima (Ait.) Melville). Logswere cross-cut into 4.2–5.2 m lengths with top diameters of15–25 cm. From several hundred available logs, 20 with severebark damage (harvester log treatment: maximum damage) and 20logs with small amounts of bark damage (harvester log treatment:minimum damage) were selected for both trials. An additional 20logs were cut with a chainsaw, trimmed, and extracted avoidingbark damage; these were the control treatment. Logs were carefullytransferred to a stacking site situated in narrow north–south orien-tated track, which was shaded on both sides by 45-year-old Cor-sican pine and remote from any areas of recent felling or otherdisturbance. A stack 1.5 m high was constructed on bearers withall the logs being aligned east–west. Logs were allocated at ran-dom to form batches comprising five logs from each of the threeharvesting treatments and then covered with a layer of extra Cor-sican pine logs. To further minimize the possibility of the logs be-ing attacked by the bluestain vector bark beetle,Tomicuspiniperda L., small decoy stacks of freshly cut Scots pine (PinussylvestrisL.) were placed nearby. The main flight period for thisspecies of bark beetle occurs during April and May, and it isknown to attack Scots pine in preference to Corsican pine (Daviesand King 1977; Evans et al. 1989).

Sampling of logsAt 3, 6, 9, and 12 weeks a batch of 15 logs (five of each treat-

ment) was removed for destructive sampling. Discs 3 cm thickwere cut, using a chainsaw, 20 cm from the top and bottom of eachlog and a further three cut at ca. 1.2 m intervals along the remain-der of the log. Discs were stored in plastic bags at 2–5°C and mea-surements taken within 3 days of cutting. The measurements couldbe grouped into three categories. The first category related to logparameters: disc diameter, bark thickness, and a number of annualrings in the outermost 2 cm of sapwood. The second category de-tailed harvester damage: the amount of bark removed and loosenedaround the disc circumference, the extent of the circumferencewith the outer bark removed leaving the living phloem exposed,and the extent of the circumference where the exposed wood hadbeen ripped and splintered. All the circumferential measurementswere made using a transparent template with etched radii 5° apartand concentric rings at 5-mm intervals. In this way outlined unitswere counted, and the measurements were expressed as a percent-age of total circumference. The third category of measurements es-timated the extent of bluestain colonization: the stained area on thedisc face (measured using a template etched in units of 0.25 cm2),maximum radial stain penetration (measured in millimetres on thecolumn of stain that penetrated deepest into the disc) and the ex-tent of stain visible around the circumference of the disc.

Isolation and identification of fungi and insectsThe sampled discs were examined using a stereomicroscope for

any signs of mycelium or fruiting. Where possible, fungal identifi-cation was made in situ, otherwise hyphae or spore drops were cul-tured onto 2% malt extract agar. In addition, the same agarcontaining 100µg·mL–1 actidione (cycloheximide) and 200µg·mL–1

streptomycin sulfate was used as selective media forOphiostomaspecies (Harrington 1981). Isolations were also attempted fromstained and adjacent areas by shaving the wood surface with sterilescalpel and then removing 2–3 mm3 wood chips from underneathand transferring to agar plates. The plates were incubated at 20°Cin dark, any mycelial outgrowth subcultured and examined. Identi-fication of the species was carried out partly on the basis of cul-tural characteristics, morphology, growth rate, and on comparisonwith reference cultures but principally on conidial morphology(Punithalingham and Waterson 1970; Upadhyay 1981; Wingfield etal. 1988; Wingfield 1993).

The majority of insects, both adult and larval stages, were col-lected from experimental logs at the 3 week sampling date after theJune felling. During general data assessment, every disc was exam-ined for any bark beetle activity. Some insects were additionallycollected from beneath the bark of sampled discs.

Statistical analysisThe data were analysed using analysis of variance (ANOVA).

The analysis firstly determined if the experimental logs used in thetwo experiments (which were removed in batches at 3-week inter-vals for sampling) were uniform in terms of diameter, bark thick-ness, and mean ring width. ANOVA was also used to explore theinfluence of harvesting treatment (chainsaw, maximum and mini-mum harvester damage) on the extent of bark damage and associ-ated bluestain development. In all ANOVA, harvesting treatmentwas considered as a fixed effect; nested within this were thebatches of logs removed for sampling and the discs cut from thoselogs, both of which were considered as random effects. Appropri-ate error terms were used in each analysis, and percentage datawere transformed to square roots.

The relationship between the stain area and bark damage wasanalysed using a general linear model. The effects of log parame-ters, season (June vs. August experiment) and the degenerative

© 1999 NRC Canada

Uzunovic et al. 243



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244 Can. J. For. Res. Vol. 29, 1999

Fig. 1. Machinery for mechanized harvesting; (a) Åkerman harvester operating in Thetford Forest, (b) detail of the Åkerman harvesterhead showing the cutting blades, chain-covered rollers for gripping and moving logs, and the measuring wheel.



effect of time on bark damage were all investigated within themodelling process. Statistical analysis were performed using SAS(SAS Institute Inc. 1994) and Genstat (Lawes Agricultural Trust1993).

Isolation and identification of fungi and insectsLittle or no breeding byTomicus piniperdaor any other

bark beetles was observed in the experimental logs, indicat-ing that any bluestain that was present had not been intro-duced by bark beetles. The collected insects were identifiedas species of Coleoptera (Anaspis spp., Arhopalus rusti-cus L., Hylobius abietisL., Pissodes piniL., RhizophagusferrugineusPaykull, Tharasnimus formicariusL., individu-als from the families Elateridae and Staphylinidae), variousCollembola, Isopoda, Diptera (Ceratopogonidae, Myceto-philidae), and spiders (Arachnoidea).

Of a total of 600 attempts from stained and stain-adjacentwood, bluestain fungi were only obtained from stained ar-eas. Often,Trichodermaspp. were also isolated from discol-oured wood, supporting the observation of Gibbs (1993) thatthis genus frequently replaces bluestain fungi in stainedwood, apparently acting as a secondary colonizer.Sphaer-opsis sapinea((Fr.) Dyko & Sutton) was the most frequentlyisolated bluestain fungus in the June experiment (Table 1).Ceratocystis coerulescens((Münch) Bakshi) was isolatedmore frequently from the August experiment then the June.Ophiostoma piceae((Münch) H. & P. Sydow), and one par-ticular Graphium sp. occurred to a similar extent in bothexperiments. The most commonly isolated species ofLepto-graphium was L. truncatum ((Wingf. & Maras.) Wingf.);other species comprised ofL. wingfieldii (Morelet), L. pro-cerum((Kendr.) Wingf.), and an unknown species ofLepto-graphium. Ophiostoma piliferum(Fries) H. & P. Sydow)was also isolated on few occasions. The darkest and mostextensive staining was produced byL. wingfieldii andC. coerulescens; the latter commonly formed large longitu-dinal columns of stain that developed from the log ends.

© 1999 NRC Canada

Uzunovic et al. 245

Fig. 2. Different types of bark and wood damage producedduring mechanized harvesting: (a) damage caused by chains onharvester feeding rollers; (b) disc cut from cracked log showingassociated bluestain; (c) punctures caused by the measuringwheel (arrows); (d) substantial harvester damage to a top logincluding bark loss ripped and splintered wood.

Total no. of successful isolations

IsolatesJune–Septemberexperiment

August–Octoberexperiment

C. coerulescens 4 14O. piceae 16 10O. piliferum 3 4Graphiumsp 13 21L. wingfieldii 3 3L. truncatum 7 4L. procerum 5 1Leptographiumsp. 7 3S. sapinea 28 14

Table 1. Species of fungi identified following positive isolationfrom sample discs.



Damage caused by the harvesting processIn addition to occasional cracks and splits, different track

patterns and types of damage caused by the chains, de-limbing knives, and measuring wheel of the harvester couldbe seen on the log surfaces (Fig. 2). However, attempts toidentify the track patterns on the sampled discs and relatethis damage later on with particular areas of stain on thediscs were unsuccessful. Damage that could be classifiedand consistently measured on the discs included completebark removal, loosened bark that remained attached to thelog, removal of the outer part of the bark leaving livingphloem tissue exposed, and splintering of exposed wood.Severe harvester damage was more common on top logs,i.e., those from the upper part of the trunk, as these usuallyhad live branches and were often pushed through the de-branching mechanism of the harvester more than once to en-sure complete branch removal; this caused extensive barkdamage.

Table 2 presents the mean values with standard errors (de-rived from the ANOVA) for log diameters, bark thickness,and annual ring width in the outermost 2 cm of sapwood.The ANOVA indicated these parameters were consistent be-tween the June and August experiments, but there were sig-nificant differences between the harvesting treatments. Logsin the maximum damage harvester treatment had a signifi-cantly smaller mean diameter (p < 0.05) compared withthose in the minimum damage treatment. Also, the bark wassignificantly thinner on logs which sustained maximum har-vester damage (p < 0.001), while tree ring increment was

significantly greater (p < 0.001). Thus, “maximum damage”harvester-processed logs had, on average, smaller diameters,thinner bark, and larger annual rings in the outer wood thanthe logs in the minimum damage treatment. This was consis-tent with their being obtained from higher up the tree.

Not surprisingly, there was also a significant difference(p < 0.001) in the amount of damage among the three treat-ments in both experiments, with very little damage of anykind visible on the chainsaw-harvested control logs. Com-parisons between the June and August experiments (Table 3)indicated significant differences in the amounts of bark re-moved (p < 0.01), in the amounts of loosened bark (p <0.01), and in the amount of phloem tissue exposed when theouter bark was removed (p < 0.01). For the experiment setup in June there was a negative correlation between barkthickness and bark loss in the maximum damage harvestertreatment (p < 0.001) despite substantial variation (R2 =0.137), suggesting that logs with thicker bark were lesslikely to suffer harvester damage. There was also a correla-tion between log diameter and the extent of wood ripping(p < 0.01), with logs of bigger diameter tending to suffer in-creased wood ripping once the bark was removed.

Harvester damage and development of bluestainBluestain damage was visibly greater in both sets of har-

vester-processed logs then in the chainsaw-processed controllogs. Indeed, there was little stain in the latter even after12 weeks of storage, and almost all the stain could be tracedto wounds made by the forwarder, which extracted and

© 1999 NRC Canada

246 Can. J. For. Res. Vol. 29, 1999

Experiment and treatmentStem diameter(cm)

Bark thickness(mm)

Annual ring width in theoutermost sapwood (mm)

JuneHarvester, maximum 22.5±0.47 9.6±0.53 2.5±0.16Harvester, minimum 24.0±0.57 12.4±0.63 1.6±0.12Motor manual 22.5±0.58 9.3±0.42 2.7±0.18AugustHarvester, maximum 22.0±0.43 8.8±0.56 2.6±0.17Harvester, minimum 23.6±0.70 12.4±0.76 1.6±0.12Motor manual 24.0±0.69 10.0±0.44 2.5±0.13

Note: Values are mean ± SE. Maximum and minimum refer to the amount of harvester damage to the logs.

Table 2. Stem diameter, bark thickness, and annual ring width in experimental logs.

Percentage of disc circumference

Experiment and treatmentWithoutbark

With loosenedbark

With rippedwood

With outerbark removed

JuneHarvester, maximum 34.8±3.40 10.6±1.01 4.8±2.11 5.9±1.54Harvester, minimum 12.5±2.73 6.0±1.34 0.5±0.29 5.9±1.54Motor manual 0.2±0.50 0.5±0.32 0.0±0.00 0.1±0.08AugustHarvester, maximum 29.0±2.77 7.8±0.78 6.9±1.52 3.4±0.64Harvester, minimum 6.7±1.34 3.7±0.93 1.5±0.40 1.3±0.35Motor manual 2.1±1.05 1.0±0.44 0.2±0.22 0.2±0.10

Note: Values are mean ± SE. Maximum and minimum refer to the amount of harvester damage to the logs.

Table 3. Mean values for the different types of bark and wood damages (measured at 3 and 6 weeks only).



© 1999 NRC Canada

Uzunovic et al. 247

transported the logs, to the cut ends of logs or to branchscars. Figure 3 shows the development of stain over time inthe two experiments, expressed as percent coverage of discarea (Fig. 3a) and percent radial penetration of disc radius(Fig. 3b). In the June experiment, the two harvester treat-ments resulted in similar amounts of stain during 9 weeks ofstorage, although beyond that time the amount of stain in-creased significantly in the maximum damage treatment af-fecting, on average, ca. 8% of the total disc area at 12weeks. In the August experiment there was little increase inthe amount of stain after 3 weeks, but logs with the greatestdamage always suffered the most bluestain.

To assist in interpreting the relationship between bark or

wood damage and the development of stain, logs from bothharvester treatments were assigned to 10 categories of in-creasing bark damage. These were divided at equal intervalsbetween 0 and 100% bark damage, and the stain was ex-pressed as a percentage of the disc area and as percentagepenetration of disc radius (Table 4). Stain data were takenfrom the 9 and 12 week sampling dates. The amount of staintended to increase with increasing damage, although thispattern was not entirely consistent. For both experiments,there was a marked increase in the area of stain when barkdamage was between 31–60 and 71–100% (Table 4), whereasradial stain penetration did not tend to increase with increas-ing bark damage. In both experiments, stain damage was

Harvester, maximal Harvester, minimal

Motor manual

June experiment (weeks)

June experiment (weeks)

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age

stra

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erce

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est

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August experiment (weeks)

August experiment (weeks)

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Fig. 3. Development of stain over time: (a) stain area expressed as percentage of disc area; (b) maximum radial stain penetrationexpressed as a percentage of disc radius.



only significantly reduced when bark damage was reducedto less then 10%.

The relationship between stain and bark damageinflicted during harvesting

To provide some guidance on the amount of stain thatmight result from damage during harvesting, the relationshipbetween stain and bark damage was analysed using a generallinear model. For the purposed of the model, stain area wasdefined as the percentage area of the disc with visible blue-stain, and bark damage was defined as the percentage of thedisc circumference where bark had been removed, loosened,or just the outer bark removed. Only data derived from logsassessed at 9 and 12 weeks after harvesting were used tomodel the relationship; these had maximum stain develop-ment but, unfortunately, were found to suffer extra bark lossfollowing the completion of harvesting as a result of insectand fungal activity. As a consequence of this, the initiallevel of bark damage at the time of harvesting had to derivedfor the logs sampled at 9 and 12 weeks. This was achievedby modelling the ordered level of damage visible on logssampled at 3 and 6 weeks against the ordered level of dam-age on logs assessed at 9 and 12 weeks. If there was no ad-ditional damage to logs over the 12 weeks of theexperiment, the relationship should appear linear with aslope of unity, and this was indeed the case for the Augustexperiment (slope = 1.02,R2 = 0.98). However, for the Juneexperiment, the data were more appropriately fitted by anonlinear model, as heavily damaged harvester logs sufferedcontinuing bark loss over the 12 weeks of the experiment(Fig. 4a). Estimates of initial bark loss were then derivedfrom this model.

The best fitting model for predicting the amount of stainlikely to develop following harvesting damage was a three-parameter model of initial bark damage and time of harvest-ing. This model has a common slope (Fig. 4b) and thedependent variable, stain error, was square root transformedto both standardize the error distribution and produce a lin-ear relationship with bark damage. The coefficients of thismodel confirm that any increase in harvester damage was as-sociated with an increased level of stain in both the June andAugust experiments. The model can also be used to predictthe amount of stain that might be expected with a knownamount of bark damage at the optimal time of year forbluestain development (June–August) compared with later inthe year (August–October) when colonization by bluestainfungi appears to be reduced.

A summary of fit for the model (R2 = 0.59) is as follows(details of the analysis are given in Table 5):

June experiment: (% stain area)0.5 = 0.64 (0.01)

+ 0.027 (0.002) × bark damage

August experiment: (% stain area)0.5 = 0.18 (0.06)

+ 0.027 (0.002) × bark damage

The standard errors of the coefficients are shown inparentheses.

© 1999 NRC Canada

248 Can. J. For. Res. Vol. 29, 1999

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The use of mechanized harvesters in many countries islikely to increase. It provides a safer working system, withreduced harvesting unit costs; felling is faster and usuallymore reliable (Warkotsch 1990; Uzunovi� 1997). However,the damage caused by mechanized harvesters to bark andwood can be extensive, and as this work has shown, evenwhen rubber fed rollers are used, debarking can readily be asmuch as 50% of total bark cover. In July the amount of barkloss was particularly striking compared with later in the yearprobably because the activity of the cambium in early sum-mer meant that the bark was readily detached from logs.Damage to wood and bark in general could be attributed to

various factors, the main ones being machine design and cal-ibration relating to knife profiles, the feeding rollers, andtheir closing pressure (Warkotsch 1994). The level of expe-rience of the harvester operator and the position of a logwithin a tree were also important factor. Logs with maxi-mum damage tended to be top logs, whereas those with lessbark damage usually originated from lower in the stem andhad thicker bark.

The study also highlighted the increased susceptibility ofmechanically harvested logs to attack by bluestain fungicompared with those cut manually by chainsaw. Certaintypes of damage, such as loosened, crushed, and puncturedbark, probably present ideal infection courts for mould andsapstain fungi as they are less subject to desiccation and UVradiation. Observation confirmed that the debarked and dam-aged areas on the logs were visited by numerous insectfauna, some crawling beneath the disrupted bark to feed orlay eggs. This was the case forHylobius abietis, which ap-peared to attack preferentially areas with exposed livingphloem. These areas were commonly infested byLepto-graphium procerum, which is known to be associated withthe H. abietis (Wingfield 1983). The importance of micro-fauna factor (mites) in dissemination of bluestain fungi insmall pine logs was first suggested Dowding (1970) and

© 1999 NRC Canada

Uzunovic et al. 249

Fig. 4. Statistical modelling of relationship between bark damage and stain. (a) Plot of bark damage at 3 and 6 weeks versus barkdamage at 9 and 12 weeks.n, June experiment;u, August experiment. Discs sampled at 3 and 6 weeks were ordered from high tolow levels of damage and plotted against similarly ordered discs sampled at 9 and 12 weeks. (b) Plot of the best fit for therelationship between stain damage and total bark damage. Solid line shows the June experiment, and broken line shows the Augustexperiment.

Source df SS MS F

Model 3 441.98 147.98 367.65Error 240 96.17 0.40Total 243 538.15

Table 5. ANOVA for Regression model to predict staindevelopment.



© 1999 NRC Canada

250 Can. J. For. Res. Vol. 29, 1999

more recently by Powell et al. (1995). The present work alsoemphasized the role of microfauna and other insects in trans-mission of stain to log stacks in the absence of bark beetlevectors.

The most commonly isolated staining fungi wereO. piceae,S. sapinea, C. coerulescens, and a species ofGraphiumthatwas a distinct biological species and not the anamorph ofO. piceae(Brasier and Kirk 1993). However, the species as-sociated with the most noticeable and most extensive areasof defacement were usuallyC. coerulescensand L. wing-fieldii, while O. piceae, which is widely regarded as a seri-ous cause of stain in lumber (Wingfield et al. 1993), causedonly very faint discolouration. This confirmed results of anearlier study, which examined the capacity of a range ofcommon bluestain fungi to colonize pine logs and cause dis-colouration (Uzunovi� and Webber 1998). The developmentof stain was also found to differ drastically between individ-ual logs within the same treatment. Unfortunately, it provedimpossible to decide if a particular type of damage was asso-ciated with more or less stain than other forms of damage.However, observations did suggest that stain was most de-veloped when associated with the edge of damaged areaswhere bark was stripped away. In contrast, stain developmentwas much less substantial in areas where large amounts ofwood were exposed. Also if the exposed wood was rippedand splintered, stain development appeared to be inhibited,not enhanced. Average amounts of stain defacement accountedfor <10% of the total disc area, even 12 weeks after harvesting.

Clearly, the extent of staining depended on various factorssuch as the presence of competitive organisms, how effec-tively spores of staining fungi were disseminated to reachthe infection courts, and climatic interactions. Nonetheless,within individual logs, stain did develop unexpectedly, andsome individual discs had as much as 23% of the area dis-coloured by bluestain. As the logs aged, whatever the har-vesting treatment, the amount of bluestain increased, but therate of degradation declined in logs harvested later in theseason, probably because of declining mean temperatures.The mean monthly temperature for Thetford in June 1993was 13.9°C and only 9.4°C in October.

The final conclusion must be that significant reduction ofstain only comes with very low amounts of bark damage(0–10%), and in practice this is not currently achievablewith mechanized harvesting. However, it is important tostress that, although mechanically harvested logs did sufferincreased bluestain, the average amount of stain was rela-tively small even though the experiment was conducted atthe optimal time of year for fungal growth. This suggeststhat, if bark beetle attack can be prevented, the stain damageis likely to extend to only a small proportion of the overalltimber volume. This is an encouraging result for the forestryindustry, particularly in the United Kingdom suggesting that,in the absence of bark beetle attack, Corsican pine logs canbe stored in the forest or at the mill during midsummer with-out developing any substantial stain problems. Other treespecies, however, could prove to be more susceptible.

This research was funded by the Forestry Commissionwith additional support from the British Council, British

Scholarship Trust for Yugoslavs., and the Soros GrantsProgramme for students from the former Yugoslavia. A.U.also gratefully acknowledges a studentship from the Impe-rial College of Science and Technology, London. We thankThetford Forest Enterprise for providing logs and technicalassistance, and special thanks go to Bob Brooker and MartinLipscombe. Insects were identified by courtesy of Tim Winter,Entomology Branch, and John Gibbs and David Wainhousecritically read the manuscript and gave valuable advice.

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