ALCES VOL. 45, 2009 HÄRKÖNEN ET AL. - WOOD QUALITY OF BIRCH AND MOOSE 67 WOOD QUALITY OF BIRCH (BETULA SPP.) TREES DAMAGED BY MOOSE Sauli Härkönen1,3, Arto Pulkkinen2, and Henrik Heräjärvi1 1Finnish Forest Research Institute, Joensuu Research Unit, P.O. Box 68, FI-80101 Joensuu, Finland; 2University of Joensuu, Faculty of Forest Sciences, P.O. Box 111, FI-80101 Joensuu, Finland ABSTRACT: European white birch (Betula pubescens) and silver birch (B. pendula) are important tree species for Finnish pulp and wood-products industries. Moose (Alces alces) damage, however, reduces the quality of butt logs intended for high-quality plywood and saw logs. In addition to flaws in stem form, pith discoloration and color change outside the pith reduce quality and value of logs irrespective of their end use. Our objectives were to 1) analyze the external and internal quality of birch trees damaged by moose, 2) measure whether the severity, type, and occurrence of damage dif- fered between silver birch and European white birch trees, and 3) evaluate visual criteria that would enable a forest-owner to assess damage and future value of moose-damaged birch trees prior to the first commercial thinning. We sampled 4 stands with a known history of moose damage; 18 trees per stand were classified by visual evaluation into 3 damage categories. The severity and type of damage lowering the internal quality of logs from sample trees were classified into 5 grades. The proportion of all visible color defects and/or decay was 74% in silver birch trees and 67% in white birch trees. Moose damage caused no visible color defect and/or decay in 35% of silver birch and 33% of white birch trees. The commercial quality and value of birch trees damaged by moose was reduced by the internal color defects and/or decay, even in certain trees without obvious external moose damage. Nev- ertheless, forest-owners can evaluate the internal quality of most birch trees in order to remove those of low-quality in the first commercial thinning by using external quality indicators of moose-damaged stems (e.g., stem form and clear curve at the point of stem breakage). ALCES VOL. 45: 67-72 (2009) Key words: Alces alces, Betula pendula, Betula pubescens, birch, browsing, damage, decay, discol- oration, moose, wood quality. European white birch (Betula pubescens) and silver birch (B. pendula) are the third and fourth most common tree species in Finland. The pulp industries and wood-products indus- tries used about 14.5 and 1.5 Mill. m3 of the production of 16 Mill. m3 of birch roundwood in 2006. Birch species are considered as medium-preferred browse of moose (Alces alces) and a high proportion of their annual browse consumption consists of birch owing to its widespread availability (Bergström and Hjeljord 1987). This browsing may cause substantial damage and financial loss in young birch stands. Moose population density has increased in Finland in recent decades (Torvelainen 2007). The post-harvest moose population peaked in 2001 when it was estimated at 139,000 (mean moose density of 3.3 moose/10 km2). Concurrently, increasing moose damage (i.e., twig-browsing, stem breakage, and bark strip- ping) was raising increased concern amongst forest-owners and associated forest industries. Concern is based on the fact that, as a long- term consequence, moose damage reduces the quality of butt logs (i.e., merchantable timber that is intended as high-quality plywood or sawn timber), especially when main stems are broken (Heikkilä et al. 1993, Ingemarson et al. 2007, Lilja and Heikkilä 2007). In addition to flaws in stem form, pith discoloration and color change outside the pith reduce quality, hence 3Present address: Hunters Central Organization, Fantsintie 13-14, FI-00890 Helsinki, Finland WOOD QUALITY OF BIRCH AND MOOSE - HÄRKÖNEN ET AL. ALCES VOL. 45, 2009 68 the value of logs irrespective of their end use. As a consequence of damage risk from a high moose population, plantations of birch trees have declined markedly, especially in southern Finland in the last decade (Viiri 2007). The objectives of this study were to: 1) analyze the external and internal quality of birch trees damaged by moose, 2) determine whether any difference in severity, type, and occurrence of damage exists between silver birch and European white birch, and 3) determine selection rules based on visual evaluation that would enable a forest-owner to decide whether to remove or retain moose- damaged birch trees in the course of the first commercial thinning. MATERIAL AND METHODS Data were collected in 2007 from 1 Euro- pean white birch stand and 3 silver birch stands with a known history of moose browsing. All 4 stands were in central Finland (Kannonkoski, Saarijärvi, and Viitasaari municipalities, ca. 63˚N, 25–26˚E) and had reached the growth stage for the first commercial thinning (Table 1). All stands had been planted and suffered from severe moose browsing damage at the sapling stage. The randomly selected sample trees (18 per site) were classified by visual evaluation into 3 damage categories: 1) 6 trees with no visible moose damage (trees were known to have had previous moose damage, i.e., stem breakage), 2) 6 trees with slight moose damage (i.e., slightly visible curve at the point of stem breakage), and 3) 6 trees with moderate moose damage (i.e., moderately visible curve at the point of stem breakage). Consequently, this classification excluded birch with severe visible moose damage. We justified this approach because it is known that the inner quality of birch trees is reduced or lost when moose damage is highly visible (Heikkilä et al. 1993, Lilja and Heikkilä 2007). Sample trees were felled and two, 2 m-long logs were cut from each sample tree, yielding a total of 144 logs. The stem form of each log was measured as a maximum deviation from the center line of the log. The logs were sawn into one, 20 cm-long bolt and six, 30 cm-long bolts. Next, each bolt was pith-centrally sawn into cants using a band saw. The severity and type of damage lowering the internal quality of log (i.e., color defects and/or decay) were classified into 5 grades: 1 = no damage (no visible color defects or decay); 2 = color de- fect in pith with a diameter <20 mm (slight color change); 3 = hard rot in pith with a diameter <20 mm (clear color change, usually as a result of chemical reaction or preliminary stage of decay); 4 = hard rot (the wood mate- rial dark but still hard, a condition caused by an infection related to a decaying fungus); 5 = soft rot (wood material is dark and soft, and wears away when scratched) (see Schatz et al. 2008). In addition, moose damage was separated from other damaging agents (i.e., insects, voles, and others) by visual evaluation. The spreading distance of the moose-caused color defects and/or decay in the stem wood was measured both vertically and horizontally. The maximum spreading distance was used in the calculations. All statistical analyses were performed Stand Tree species Planting year Density (stems/ha) Mean height (m) Mean dbh1(cm) A Silver birch 1991 1,725 15.8 12.0 B Silver birch 1987 900 15.9 12.5 C Silver birch 1987 1,450 14.0 11.1 D White birch 1987 1,950 10.7 10.6 Table 1. Stand characteristics of three silver birch stands and one white birch stand, central Finland, 2007. 1 Mean dbh = mean diameter at breast height (1.3 m). ALCES VOL. 45, 2009 HÄRKÖNEN ET AL. - WOOD QUALITY OF BIRCH AND MOOSE 69 with SPSS package. The parametric tests (ANOVA) were employed because the vari- ables had normal distributions. RESULTS When combining all damage categories, the proportion of all visible color defects and/ or decay was 74% in silver birch trees and 67% in white birch trees (Table 2). Moose damage alone did not cause any visible color defects and/or decay in 35% of silver birch and 33% of white birch trees (Table 3). In both species the most common damage type (>50%) was hard rot in pith with a diameter <20 mm; this exceeded or equaled the proportion of no damage (Tables 2 and 3). In practice, this means a clear color change is visible in the stem wood from a chemical reaction or preliminary stage of decay. The moose-caused color defects and/or decay were spread both vertically and hori- zontally in damaged trees, as well as trees with no visible damage (Table 4). There was no difference in the vertical spreading distance of moose-caused color defects and/or decay in the stem wood among different damage categories of moose-damaged silver birch trees (F = 0.17, P >0.05). In white birch trees, the vertical spreading distance increased considerably (>50%) with increasing damage level, but no difference was found (F = 1.20, P >0.05). All horizontal spreading distances were <40 mm and were not different (P >0.05). Vertical spreading distance averaged 160 cm in the moderately damaged white birch trees. In silver birch trees, the maximum deviation from the center line of the log increased with increasing damage level (F = 5.38, P <0.01). DISCUSSION Our data indicate that wood discoloration caused by different damaging agents remains at a lower level in European white birch than silver birch (Table 2), although this differ- ence was small. There was no moose-caused visible color defect and/or decay in 35% of silver birch and 33% of white birch trees based on the assessment of internal wood quality (Table 3). Thus, moose damage apparently does not always result in reduced wood qual- ity because all trees were damaged by moose at some point. On the other hand, these data indicate that from a forest-owner perspective, the commercial quality and value of birch trees damaged by moose was substantially reduced due to internal color defects and/or decay. This was also the case for birch trees that were evaluated visually to have no external moose damage (Table 4), hence, there was no indication of the compromised internal quality. Importantly, these trees would normally be retained in the course of the first commercial thinning, therefore, visual assessments alone prior to the first commercial thinning will probably result in some low-quality birch trees being retained until maturity. Such timber at harvest will necessarily be pulpwood with only 30–50% value in comparison to saw or plywood logs. Nevertheless, for the worst Tree species Damage type No damage (%) Color defect in pith (%) Hard rot in pith (%) Hard rot (%) Soft rot (%) Silver birch 25.9 11.1 35.2 18.5 9.3 (n = 54) White birch 33.3 5.6 44.4 5.6 11.1 (n = 18) Table 2. The proportion (%) of damage type measured in logs cut from silver and white birch trees identified as damaged by moose browsing. Logs were graded to the damage type that most lowered the internal quality (e.g., log with soft rot may also contain hard rot, hard rot in pith, or color defect in pith). These data reflect pooling of moose and other damage agents. WOOD QUALITY OF BIRCH AND MOOSE - HÄRKÖNEN ET AL. ALCES VOL. 45, 2009 70 cases forest-owners can visually evaluate the internal quality of moose-damaged birch trees with external quality indicators (e.g., stem form, clear curve at the point of stem breakage), and be able to remove the lowest-quality trees in the first commercial thinning. If the first commercial thinning is carried out properly, potential damage and economic loss caused by moose browsing will be reduced. The horizontal spreading distance of moose-caused color defects and/or decay was relatively limited in size (<40 mm in different damage categories), as reported previously (Heikkilä et al. 1993, Lilja and Heikkilä 2007). The vertical spreading distance of moose- caused color defects and/or decay was com- parable to that measured by Lilja and Heikkilä (2007). The maximum deviations from the center line of the logs were relatively low in both birch species, and probably reflected the exclusion of sample trees with severe visible damage. This also indicates that the diameter of broken main stems at the sapling stage might have been relatively small in both species (cf., Heikkilä et al. 1993). The internal quality of birch trees was reduced by factors other than moose (Table 2). It is known that insects (Annila 1979, Lilja and Heikkilä 2007) and voles (Henttonen et al. 1994) are potential damaging agents, and other related data and our observations suggest such also (S. Härkönen et al., Finnish Forest Tree species Damage type No damage (%) Color defect in pith (%) Hard rot in pith (%) Hard rot (%) Soft rot (%) Silver birch 35.2 11.1 33.3 14.8 5.6 (n = 54) White birch 33.3 5.6 44.4 5.6 11.1 (n = 18) Table 3. The proportion (%) of damage type measured in logs cut from silver and white birch trees identified as damaged by moose browsing. Logs were graded to the damage type that most lowered the internal quality (e.g., log with soft rot may also contain hard rot, hard rot in pith, or color defect in pith). Classification is based on the effect of moose damage; other damage agents were excluded but may have been present. Tree species Damage category Vertical (cm) Horizontal (mm) Deviation (mm) Silver birch None (7) 143±16 17±4 29±5a Slight (13) 157±28 33±7 41±6ab Moderate (15) 142±14 37±5 60±7b F 0.17 2.07 5.38 P 0.84 0.14 0.01 White birch None (3) 39±17 16±4 28±5 Slight (5) 95±39 13±2 40±11 Moderate (4) 160±72 26±6 32±8 F 1.21 2.54 0.45 P 0.34 0.13 0.65 Table 4. Damage categories, mean vertical and horizontal spreading distances of moose-caused color defects and/or decay in stem wood, and mean maximum deviation from the center line of the log in moose-damaged silver birch and white birch trees. The 3 damage categories were: None = trees with no visible moose damage, Slight = trees with slight moose damage, and Moderate = trees with moderate moose damage. Sample sizes are in parentheses; means (± SE) with the same letter are not different (ANOVA, P >0.05). ALCES VOL. 45, 2009 HÄRKÖNEN ET AL. - WOOD QUALITY OF BIRCH AND MOOSE 71 Research Institute, unpubl. data). In addition, pruning, sapping, and other wounds caused by careless thinning activities may also cause color defects in stem wood (Nevalainen 2006, Schatz et al. 2008). Hallaksela and Niemistö (1998) showed that planted silver birch trees may easily have stem discoloration from dead and broken branches, and that discoloration was connected with microbial invasion in 83% of their sample birch trees. We did not deter- mine the microbes associated with damage, but different basidiomycotina and stain fungi species have been isolated in moose-damaged birch trees (Heikkilä et al. 1993). It is evident that moose damage will lower the external and internal quality of birch trees. Thus, preventive measures may be required depending on the level of moose browsing and the desired timber product, and whether a forest-owner wants to ensure high quality birch trees at harvest. Various chemical re- pellents, visual and acoustic devices, and tree sheltering methods and devices have all been used to prevent moose damage in seedling and sapling birch stands. These methods are rather expensive, their effects are variable, and in many cases they have shown little promise for reducing moose damage on a large-scale or long-term basis. Thus, devel- opment of cost-effective mechanical and/or chemical preventive methods is still needed to reduce the risk of moose damage in young birch stands. Lower moose density may be the most cost-effective approach to reduce damage caused by moose, however, various moose-interest groups often have conflict- ing values and goals with respect to an ideal moose population density (Aarnio et al. 2008). We suggest that moderate moose population densities would provide for both sustainable and profitable forestry producing high-quality timber, as well as socio-economically accept- able management of moose. ACKNOWLEDGEMENTS We thank Mr. Pertti Hokkanen and Mr. Hannu Koivunen (Finnish Forest Research Institute, Joensuu Research Unit) for their skillful technical help during fieldwork and laboratory tasks, respectively. UPM Forest and two private forest-owners kindly pro- vided the sample trees for this study. We are also grateful to Kristine M. Rines, Pete Pekins, and 2 anonymous referees for their valuable comments on the manuscript. The Finnish Ministry of Agriculture and Forestry is gratefully acknowledged for their financial support. REFERENCES AArnio, J., S. Härkönen, L. Petäjistö, and A. selby. 2008. Hirvikannan nykyisen säätelyjärjestelmän ajanmukaisuus ja toimivuus riistanhoitopiirien hallitusten näkökulmasta. (The Finnish moose man-(The Finnish moose man- agement system from the perspective of board members of Game Management Districs.) Metlan työraportteja /Working papers of the Finnish Forest Research Institute 92. (In Finnish). AnnilA, E. 1979. 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