267
 www.metla.fi/silvafennica · ISSN 0037-5330
 The Finnish Society of Forest Science · The Finnish Forest Research Institute
 SILVA FENNICA Silva Fennica 45(2) research articles
 Saw Log Recovery and Stem Quality 
of Birch from Thinnings in Southern 
Finland
 Harri Kilpeläinen, Jari Lindblad, Henrik Heräjärvi and Erkki Verkasalo
 Kilpeläinen, H., Lindblad, J., Heräjärvi, H. & Verkasalo, E. 2011. Saw log recovery and stem quality 
of birch from thinnings in southern Finland. Silva Fennica 45(2): 267–282.
 The objective of this study was to examine the timber quality of silver birch (Betula pendula 
Roth) and European white birch (Betula pubescens Ehrh.) trees in the first and second thinnings 
in southern parts of Finland, from the viewpoint of sawing of small-diameter, short logs, in 
particular. The average stem volume of birch was 0.140 m3 in the first thinning stands and 0.206 
m3 in the second thinning stands. In planted stands, the trees were larger in the first thinnings 
but slightly smaller in the second thinnings, compared with naturally regenerated pure birch 
stands or mixed stands of Norway spruce and birch species. Almost 60% of the harvested 
and 35% of the remaining stems that could provide saw logs were graded as pulpwood for 
timber quality due to the occurrence of stem defects. The most common stem defects were 
multiple crooks and middle crooks. Only minor between-stratum differences were detected 
in the numbers of defects. Depending on the bucking option, the total percentage of saw and 
plywood logs from the total birch recovery in the thinning of the sample stands varied between 
11.7 and 18.2. The recovery of saw logs was clearly higher in the second thinnings, 12–19%, 
than in the first thinnings, 8–14%. Of the stand types, saw log recovery was the highest in 
planted birch stands, 12–19%, but lower in naturally regenerated pure birch stands and mixed 
stands of Norway spruce and birch. The highest share of saw logs was in the second thinning 
of planted stands, 17–25%. This study shows that the harvesting recoveries of end-use based 
timber assortments can be estimated in different kinds of thinning birch stands. Based on 
tree and log dimensions and stem squality, silver birch firstly from plantations and secondly 
from mixed stands should be the most interesting source of raw material for the saw milling, 
furniture and interior product sectors.
 Keywords Betula pendula, Betula pubescens, European white birch, silver birch, thinning, 
saw logs, stem quality, stem defects, bucking, recovery
 Addresses Finnish Forest Research Institute, Eastern Finland Research Unit, P.O. Box 68, 
FI-80101 Joensuu, Finland E-mail henrik.herajarvi@metla.fi
 Received 17 August 2010 Revised 15 April 2011 Accepted 10 May 2011
 Available at http://www.metla.fi/silvafennica/full/sf45/sf452267.pdf
268
 Silva Fennica 45(2), 2011 research articles
 1 Introduction
 1.1 Background
 Timber from the first and second thinnings of both 
naturally regenerated and cultivated birch stands 
(silver birch, Betula pendula Roth; European 
white birch, B. pubescens Ehrh.) has a consider-
 able and increasing potential as a raw material for 
saw mills in northern Europe (e.g., Luostarinen 
and Verkasalo 2000, Heräjärvi 2002). According 
to the few previous studies available (Rantanen 
et al. 2000, Lehtimäki et al. 2002, Lindblad et al. 
2003), approximately 10–20 per cent of the birch 
thinning removal, on average, should be appropri-
 ate for sawing in stands particularly selected for 
saw log harvesting.
 The aim in the commercial harvesting of birch 
saw logs from thinnings was first to provide raw 
material for furniture industries manufacturing 
sound-knotted products, such as home tables and 
shelves (Luostarinen and Verkasalo 2000, Lind-
 blad et al. 2003). The industries started in Finland 
(and Sweden) during the very late 1990’s, but 
moved thereafter to eastern countries of Baltic 
Sea Basin (e.g., Verkasalo et al. 2007). Similar 
products can be manufactured from selected parts 
of birch trees from final cuttings, but certain 
visual, physical and mechanical properties may 
be then different in the final product (knottiness, 
grain angle, colour, hardness, density, strength 
and stiffness) (Luostarinen and Verkasalo 2000, 
Heräjärvi 2002, Lindblad et al. 2003, Möttönen et 
al. 2004, see also Cameron et al. 1995, Dunham 
1996, Dunham et al. 1999). Later, new product 
innovations were applied for indoor flooring and 
panelling, partly with physical modification using 
heat-treatment or pressure-treatment (e.g., Verka-
 salo and Heräjärvi 2009).
 In Finland, the pulp industries are the largest 
user of small-diameter birch, consuming more 
than 10 Mill. m3 of pulpwood annually (Yli-
 talo 2010). Birch consumption of saw mills is 
only some per cents of the consumption of pulp 
industries. Most saw logs are rather large in diam-
 eter, i.e., more than 18 cm over bark. Despite of 
the apparent raw material shortage at the birch 
using saw mills, according to the timber produc-
 tion analyses, the thinning potential of birch is 
currently inadequately used in many districts in 
Finland (Hynynen et al. 2002, Verkasalo 2002, 
Ylitalo 2010). Birch timber balance is similar 
in other European countries with birch supply, 
such as Sweden (Ekström 1987, Johansson 2002), 
Norway (Vadla et al. 1982) and Estonia (Kiviste 
and Uri 2002), see also Verkasalo et al. (2007).
 Birch is mainly managed with the intention to 
get high-quality and high-value logs from the final 
fellings for the plywood and veneer industries. It 
is well known that birch species, especially silver 
birch, are sensitive to the timing and intensity of 
thinning in order to maximise the growth, yield 
and quality of the trees (e.g., Niemistö 1991, 
1995a,b, Niemistö et al. 1997; see also Fries 1964, 
Erken 1972, Raulo 1979, 1981, Mielikäinen 1980, 
1985, Langhammer 1982, Denne et al. 1994, 
Cameron et al. 1995, Dunham 1996, Valkonen 
and Valsta 2001). Most of the birch timber is 
and will be obtained from naturally regenerated 
birch stands or mixed stands of conifers and birch 
in Finland. In addition, the area of cultivated 
birch stands that should be thinned for the first 
or second time will still increase for some dec-
 ades. Although these facts are commonly known, 
little attention has been paid on the potential of 
thinning-aged birch timber for uses other than 
pulping or firewood.
 1.2 Objective
 The objective of this study was to examine the 
timber quality of European white and silver birch 
trees in marked stands of first and second thin-
 nings in southern parts of Finland, from the view-
 point of commercial sawing of small-diameter, 
short logs, in particular. Saw logs were defined 
as follows: all logs meeting the minimum top 
diameter of 11 cm (over bark) and the fixed length 
of 2.2–3.3 metres or 3.0 metres depending on the 
used bucking option. Dimensions, stem form, 
branch characteristics and technical defects of the 
birch trees in planted, naturally regenerated and 
mixed stands were studied. Finally, the recovery 
of different timber assortments and their grade 
distributions were investigated in harvesting trials 
of commercial stands. Based on measurements 
of standing trees, the outcomes of three different 
theoretical bucking options were compared using 
the bucking-to-value simulator, which maximises 
269
 Kilpeläinen et al. Saw Log Recovery and Stem Quality of Birch from Thinnings in Southern Finland
 the value of each stem based on the dynamic pro-
 gramming approach (see Näsberg 1985, Ahonen 
and Mäkelä 1995).
 2 Materials and Methods
 The study material was sourced from 48 birch-
 dominated stands (28 first thinning and 20 second 
thinning stands) in central and eastern Finland, 
where a considerable proportion of cutting poten-
 tial of birch from thinning stands will be in the 
future in Finland (Fig. 1). The structure of the 
study material is consistent with the future’s birch 
resources, due to the stratification to the prov-
 inces according to the cutting potentials of birch, 
estimated by the MELA group of Finnish Forest 
Research Institute (Metinfo 2011). 
The sites were fertile OMT (Oxalis-Myrtillus type) 
and medium fertile MT (Myrtillus type) mineral 
soil types (see: Lehto and Leikola 1987, Hotanen 
et al. 2008). The study stands were subjectively 
selected according to the current silvicultural status 
and the possibility to extract at least a moderate 
volume of small-diameter saw logs in commercial 
harvesting operations. The data were furthermore 
divided into three strata: 26 planted birch stands, 
12 naturally regenerated pure birch stands and 10 
mixed stands of birch and Norway spruce (Picea 
abies Karst.) (Table 1). At least one third of the 
number of stems in mixed stands contained both 
birch and spruce (Lindblad et al. 2003). In the case 
of planted stands, few white birch trees observed 
were naturally regenerated among the planted silver 
birch. In naturally regenerated pure and mixed birch 
stands, white birch was more frequent than silver 
birch. Although the stands were also classified into 
the first and second thinnings, this classification 
was not necessarily unambiguous in some of the 
study stands.
 Depending on the size and homogeneity of 
the stand, from one to four sample plots were 
established in each stand. The size of a single 
sample plot in birch dominated stands varied 
from 24 × 30-metre rectangles in the first thinning 
stands to 24 × 40-metre rectangles in the second 
thinning stands; larger sample plots were needed 
in the second thinning stands in order to include a 
representative number of trees in the plot. Unlike 
Fig. 1. Locations of the study stands in south-eastern Finland.
270
 Silva Fennica 45(2), 2011 research articles
 in birch dominated stands, circular sample plots 
with 15-metre radius were established in mixed 
stands. Establishing right-angled sample plots in 
the birch dominated stands was based on the idea 
that the effect of skid roads should be taken into 
account in the estimation of harvesting removal. 
The same system was planned to be applied in 
the mixed stands, as well. However, it showed 
that in mixed stands the prospective locations 
of skid roads could not be estimated reliably. 
Therefore, circular sample plots that are clearly 
less laborious to establish and measure, were 
finally used in mixed stands. The harvested trees 
in each sample plot were selected and marked in 
the forest by experienced field workers according 
to the prevalent Finnish thinning regimes that are 
based on basal area of tree stock prior to and after 
the thinning. The procedure combined thinning 
from below and thinning for quality, aiming to 
increase the dimensions of the remaining stock 
and developing the technical quality of the butt 
logs (Recommendations for good… 2007).
 All standing birch trees from the sample plots 
were detected for birch species, and measured 
and graded for dimensions and saw log quality. 
The trees assessed to be harvested were bucked 
into timber assortments (plywood logs, saw logs, 
and pulpwood) according to their dimensions 
and exterior defects. Location of each assortment 
along the tree height was recorded. All trees were 
also classified into log, pulpwood and waste-
 wood stems. This was done using the bucking-to-
 value simulator developed at the Finnish Forest 
Research Institute by Kilpeläinen (2001). Here, 
the stem defects were neglected, and only the 
dimensions defined the stem class.
 The following characteristics were measured 
or estimated from the birch trees in sample plots: 
diameter at breast height (dbh), diameter at six-
 metre height (d6), height of the tree, height of 
the lowest dead and living branch, all technical 
defects and their effective locations along the 
stem, and, as a result, quality of the tree graded 
into timber assortments by section. Defects 
were classified into two groups: defects inhibit-
 ing sawing (class 1) and defects decreasing the 
quality of products (class 2). Defects in class 1 
included sweep, crook in the butt, multiple crooks, 
and other stem form defects (incl. crooks in the 
middle section of stems, forks and large vertical 
branches). Defects in class 2 included branchi-
 ness (dry or rotten branches bigger than 3 cm in 
diameter, grouped branches and small vertical 
branches), decay, and surface defects such as 
scars and checks. Defects were detected only from 
stem sections more than 10 cm in diameter.
 Altogether 7768 trees from 109 sample plots 
were measured from the total area of 8.74 hec-
 tares. Their stem volumes were calculated using 
taper curve models based on the tree species, dbh, 
d6 and height of the tree (Laasasenaho 1982). Of 
all sample trees on the plots, 3921 trees were 
assessed to be removed in the thinning. Those 
trees comprised the data for the computer-based 
Table 1. Characteristics of the study materials.
 Stratum N of N of Total area of N of sample trees Total volume, m3
  sample stands sample plots sample plots, ha
  B. pendula B. pubescens B. pendula B. pubescens
  Planted stands
 1. First thinning 19 37 2.76 2941 62 445.5 4.6
 2. Second thinning 7 20 1.78 1410 0 275.0 0.0
  Naturally regenerated stands
 3. First thinning 4 8 0.62 34 810 7.0 87.2
 4. Second thinning 8 18 1.49 57 1144 20.7 243.7
  Mixed stands
 5. First thinning 5 11 0.78 28 490 4.4 56.4
 6. Second thinning 5 15 1.31 142 650 35.6 97.5
 All 48 109 8.74 4612 3156 788.1 489.3
271
 Kilpeläinen et al. Saw Log Recovery and Stem Quality of Birch from Thinnings in Southern Finland
 bucking. It must be kept in mind that these trees 
represented the harvesting removal, i.e., the small-
 est trees or the poorest quality trees from the 
entire material of 7768 trees. Due to the practice 
of selective thinning, the remaining 3847 trees 
represent the highest quality. Fig. 2 presents the 
dbh series commonly for the entire data, but sepa-
 rately for the trees that were assessed to remain 
and those to be harvested. After the thinning, the 
proportion of silver birch stems was 65% of all 
trees, whereas before the thinning the correspond-
 ing percentage was 59.
 For harvested sample trees, volumes of differ-
 ent timber assortments were calculated using the 
bucking-to-value simulator (Kilpeläinen 2001). 
In the bucking software, several bucking alterna-
 tives are simulated for each stem, and the value 
output of each alternative is calculated according 
to the dynamic programming approach. Finally, 
the optimal alternative is selected based on the 
stem value. The software calculates the volumes 
and values for each bolt, stem, sample plot and 
stand. It takes into account the dimensions and the 
defects in the measured trees. The quality require-
 ments for saw and plywood logs that were applied 
in this study were presented, e.g., by Heräjärvi 
and Verkasalo (2002). However, the diameter of 
sound knots was not restricted in this study, and 
the diameter of black knots was limited up to 
30 mm. In the simulation software, the possible 
diameter–length combinations of logs, as well 
as the quality requirements and values need to 
be defined for each timber assortment as input 
parameters. The stem parts that contain defects, 
including jump cuts, off-cuts and top-cuts, are 
bucked into pulpwood or waste wood. Stem parts 
that do not meet the requirements for the dimen-
 sions of any commercial timber assortment are 
classified as waste wood.
 Bucking the stems was executed according 
to three alternative combinations of logs and 
pulpwood (Table 2) in order to study how dif-
 ferent dimension requirements of logs and 
pulpwood affect on the relative proportions of 
timber assortments. Bucking was steered in the 
software according to the approximate prevalent 
Fig. 2. Dbh series for remaining and harvested birch trees. The “harvested trees” were 
selected by experienced field workers, but no real harvesting was done.
 0
 2
 4
 6
 8
 10
 12
 7 9 11 13 15 17 19 21 23 25 27 29
 Diameter class (cm)
 )
 %(
  s
 mets
  f
 o
  reb
 m
 u
 n
  lat
 ot
  f
 o
  egat
 necreP
 Remaining trees
 Harvested trees
 Table 2. Timber assortments of birch allowed in simu-
 lated bucking options 1–3.
 Timber Minimum top Log Bucking options
 assortments diameter, cm lengths, m
  1 2 3
 Plywood logs 18 3.1–5.5 × × ×
 Saw logs 11 2.2 ×  ×
   3.0  × 
  3.3 ×  ×
 Pulpwood 7 2.7–3.3 × × 
  2.5–5.0   ×
272
 Silva Fennica 45(2), 2011 research articles
 unit prices of different timber assortments (e.g., 
Ylitalo 2010).
 Stand-level mean values were compared 
between the strata (planted, naturally regener-
 ated and mixed stands) by using one-way analysis 
of variance (ANOVA) to identify which char-
 acteristics (incl. defects in stems) were signifi-
 cantly different in various types of birch stands. 
Characteristics between the remaining and har-
 vested trees were compared using paired samples 
t-test, and those between the first and second 
thinning stands using the independent samples 
t-test. To normalize the distributions of variables 
(x), proportional values (p) of trees containing 
defects in individual stands were formulated by 
using transformation: p = arcsin (√(x)) (Ranta et 
al. 1994). The stratum wise characteristics, i.e., 
the harvesting recoveries, were evaluated based 
on observations only. This was reasoned by the 
smaller number of study stands in some of the 
strata (see Table 1). Statistical tests or analysis 
were not performed on the proportions of timber 
assortments, because they were obtained as a 
result of bucking simulation.
 3 Results
 The characteristics describing the quality attributes 
of the sample stands and all trees in the stands 
are presented in Table 3. The mean diameters 
at breast height (dbh), mean height and average 
tree volume in all sample stands were 15.4 cm, 
17.6 m and 0.168 m3, respectively. In silver birch 
dominated planted stands, the trees (average stem 
volume 0.168 m3) appeared to be slightly smaller 
than in naturally regenerated stands (0.187 m3). 
Table 3. Mean characteristics of the sample stands and trees by stratum and birch species. The figures are based 
on measurements of all trees in the sample plots, i.e., both harvested and remaining trees.
 Stratum Dbh Height Height of the Height of the Average N of stems Volume
    lowest dead branch lowest living branch tree volume per ha in stand
  cm m m % of m % of dm3  m3/ha
     tree height  tree height
  Planted stands
 1. First thinning 14.7 17.5 3.5 19 7.9 44 152 1103 164
 2. Second thinning 17.0 19.3 4.2 22 7.6 39 213 809 164
 B. pendula 15.4 18.0 3.7 20 7.8 42 169 1005 162
 B. pubescens 12.3 16.1 3.4 20 8.9 52 89 95 1
 All 15.3 18.0 3.7 20 7.8 42 168 1024 164
  Naturally regenerated stands
 3. First thinning 13.2 15.8 3.2 20 7.7 47 113 1341 148
 4. Second thinning 17.4 19.1 5.5 28 9.5 50 224 837 183
 B. pendula 21.1 19.7 4.4 21 9.2 46 325 44 14
 B. pubescens 15.7 17.9 4.7 25 8.8 49 179 961 158
 All 16.0 18.0 4.7 25 8.9 49 187 1005 171
  Mixed stands
 5. First thinning 14.3 15.2 2.4 16 6.7 45 117 679 80
 6. Second thinning 15.6 17.1 4.2 24 7.8 44 168 621 103
 B. pendula 18.2 17.7 3.3 18 7.7 43 224 70 15
 B. pubescens 14.5 15.9 3.3 20 7.2 45 132 580 77
 All 14.9 16.1 3.3 20 7.3 44 143 650 91
  All stands
 First thinning 14.4 16.8 3.3 19 7.7 45 140 1061 147
 Second thinning 16.8 18.7 4.7 25 8.4 44 206 773 156
 B. pendula 17.1 18.3 3.8 20 8.1 43 212 570 94
 B. pubescens 14.6 16.8 3.9 22 8.2 48 145 371 56
 All 15.4 17.6 3.9 21 8.0 44 168 941 151
273
 Kilpeläinen et al. Saw Log Recovery and Stem Quality of Birch from Thinnings in Southern Finland
 However, the difference between the strata was 
insignificant (ANOVA: df1 = 2, df2 = 45, F = 1.829, 
p = 0.172). The average stem volume was 0.140 
m3 in the first thinning stands and 0.206 m3 in the 
second thinning stands.
 In planted stands, the birch trees were on aver-
 age larger in the first thinning, compared with 
naturally regenerated pure birch stands or mixed 
stands of Norway spruce and birch. As an excep-
 tion, in the second thinnings the birch trees were 
slightly larger in naturally regenerated stands 
than in planted stands. Silver birch trees were 
larger than white birch trees in all strata; this 
affected also the differences between the types 
of regeneration.
 Naturally regenerated stands were slightly 
better self-pruned and had smaller proportion of 
living crown. The difference in the average heigth 
of the lowest dead branch between the strata was 
significant (ANOVA: df1 = 2, df2 = 45, F = 3.482, 
p = 0.039). The differences were insignificant in 
case of the heigth of the lowest living branch and 
the proportion of living crown. 
According to the measurements of standing 
trees, smaller trees were more often defect-free 
than larger trees (Table 4). This is partly reasoned 
by the fact that the defects were recorded only 
from the stem sections with more than 10 cm 
in diameter. In comparison to the smaller dbh 
classes, larger trees were more seldom rejected 
in to pulpwood due to the defects. In small-sized 
stems, already one defect most often totally pre-
 vented the bucking of saw logs. The percentage of 
log-sized stems rejected in to pulpwood decreased 
along with the increment of the dbh up to the 
dbh of 18 cm. From this dbh on, the percentage 
remained relatively constant at a level of 14–29% 
for all stems and 28–46% for the harvested stems 
in all sample stands.
 In case of silver birch, 3.1% of harvested log-
 sized stems were defect-free, and 49% of them 
were downgraded in to pulpwood. For white birch, 
the respective percentages were 2.8 and 66.
 The harvested log-sized stems appeared to 
have only slightly poorer overall quality than 
the remaining ones in the study material. Of 
the remaining log-sized stems, on average 4.5% 
were totally defect-free, whereas the respective 
percentage in the harvested stems was 2.8. Almost 
60% of the harvested and 35% of the remaining 
log-sized stems were graded in to pulpwood for 
timber quality due to the occurrence of different 
stem defects. Differences were clear especially in 
larger diameter classes.
 Defects inhibiting sawing (class 1) were more 
common than defects decreasing the quality of 
saw log (class 2). The most common defects in 
birch stems were multiple crooks and crooks in 
the middle sections of stems (Table 5). The per-
 centage of trees with multiple crooks in individual 
birch stands varied from 4 to 94 (mean 62%). 
Crooks in the middle sections of stems were 
detected in 13–99% of trees, on average (mean 
53%). Other common defects in the trees were 
grouped branches and crooks in the butt section, 
they influenced in 17% and 15% of the sample 
trees, respectively. Sweep, forks, decay, surface 
defects, oversized and vertical branches were 
recorded in less than 10% of the trees.
 Only minor between-stratum differences were 
detected in the numbers of defects. Multiple 
crooks were more common in the sample trees 
in naturally regenerated stands than in planted 
and mixed stands (80%, 50% and 70%, respec-
 tively). Differences between these strata were 
statistically significant according to the ANOVA 
(df1 = 2, df2 = 45, F = 11.496, p = 0.000).
 More surface defects were detected in naturally 
regenerated stands (5% of the sample trees) than 
in planted and mixed stands (3% and 2% of the 
trees, respectively) (ANOVA: df1 = 2, df2 = 45, 
F = 3.829, p = 0.028). On the other hand, crooks 
in the butt section were more common in silver 
birch dominated planted stands (19% of the 
sample trees) than in white birch dominated natu-
 rally regenerated (9%) and mixed stands (13%) 
(ANOVA: df1 = 2, df2 = 45, F = 5.142, p = 0.010). 
Other defects were equally numerous in the dif-
 ferent strata.
 Branchiness and crooks in the middle sec-
 tions of stems were more common defects in the 
remaining trees than in the harvested trees, partly 
due to the bigger stem size (Fig. 2). Depending on 
the stratum, branchiness was estimated as a defect 
in 27–40% of the remaining trees (average 35%), 
but only in 15–24% of the harvested trees (aver-
 age 20%) (t-test: t = –10.810, df = 47, p = 0.000). 
Crooks in the middle sections of stems were also 
more common in the remaining trees (59% of the 
stems) than in the harvested trees (46%) (t-test: 
274
 Silva Fennica 45(2), 2011 research articles
 t = –7.410, df = 47, p = 0.000). On the other hand, 
sweep, crook in the butt, and decay were more 
common in the harvested than in the remaining 
trees. Only 0.8% of the remaining trees had a 
visible decay, whereas the respective percentage 
in the harvested trees was 2.6. The difference was 
significant (t-test: t = 6.704, df = 47, p = 0.000). 
Although the remaining and harvested stems 
seemed to be rather equally sweeped, 3.5 and 4.5 
per cent, respectively, the difference was also sig-
 nificant (t-test: t = 2.295, df = 47, p = 0.026). The 
same concerned butt crooks, which were recorded 
in 14% of the remaining and 16% of the harvested 
stems (t-test: t = 2.011, df = 47, p = 0.050). Occur-
 rence of other defects did not differ between the 
remaining and harvested stems.
 On avarage, the proportions of defect-free stems 
were at the same level (6% of all stems) on both 
birch species, but differences in single defect types 
on silver and white birch stems were detected. 
Table 4. Numbers and average percentages of defect-free stems and stems with defects in all sample trees grouped 
by dbh class. Only log or pulpwood sized stems a) are included b). 
Dbh class, cm All stems Log sized stems  Pulpwood sized stems 
 N Defect-free, % N Defect-free, % Defected, % Rejected in  N
       to pulpwood
       due to defects, %
 8.0–9.9       
All stems 444 19 - - - - 444
 Harvested stems 400 19 - - - - 400
 10.0–11.9       
All stems 1135 11 60 9 13 78 1075
 Harvested stems 904 11 43 6 14 80 861
 12.0–13.9       
All stems 1409 5 1357 5 27 68 52
 Harvested stems 859 4 826 4 24 73 33
 14.0–15.9       
All stems  1547 5 1547 5 45 50 -
 Harvested stems 692 3 692 3 40 57 -
 16.0–17.9       
All stems 1378 3 1378 3 58 38 -
 Harvested stems 509 3 509 3 51 46 -
 18.0–19.9       
All stems 910 2 910 2 69 29 -
 Harvested stems 281 1 281 1 57 42 -
 20.0–21.9       
All stems 494 1 494 1 73 26 -
 Harvested stems 126 1 126 1 53 46 -
 22.0–23.9       
All stems 228 1 228 1 73 25 -
 Harvested stems 53 - 53 - 66 33 -
 24.0–25.9       
All stems 83 2 83 2 84 14 -
 Harvested stems 19 - 19 - 72 28 -
 26.0–       
All stems 73 1 73 1 71 28 -
 Harvested stems 12 - 12 - 63 38 -
 All stems 7701 6 6130 4 52 45 1571
 Harvested stems 3855 6 2561 3 39 58 1294
 a)
  Log and pulpwood sized stems were determined in bucking according to the dimension requirements of sawlogs and pulpwood without any 
detected defects. At least one saw log was bucked from log sized stems and one pulwood bolt but no saw logs was bucked from pulpwood 
sized stems.
 b)
  Defects were recorded from the stump to the height where the stem diameter was 10 cm.
275
 Kilpeläinen et al. Saw Log Recovery and Stem Quality of Birch from Thinnings in Southern Finland
 Most common defects in silver birch stems were 
multiple crooks (73% of stems), crooks in the 
middle sections of stems (40%) and brachiness 
(18%). In white birch stems, crooks in the middle-
 sections of stems were the most common defects 
(55% of stems). Other major defects included 
multiple crooks and branchiness (49% and 30% 
of white birch stems, respectively). 
Birch species also differed from each other in 
branchiness. Grouped branches were identified 
in 24% of all white birch stems and 5.4% of 
silver birch stems. On the other hand, dead or 
rotten branches bigger than 3 cm in diameter were 
detected in 8.1% of silver birch stems and 2.9% 
of white birch stems.
 Irrespectively of the bucking option, the har-
 vesting recoveries were between 53.4 and 54 m3 
per hectare.
 The average total percentages of saw and plywood 
logs from the harvesting recovery in all sample 
Table 5. Occurrence of defects in harvested (H) and remaining (R) trees in the 
sample stands grouped by stratum. One or more defects were defined in each 
tree from the stump to the height where the stem diameter was 10 cm.
 Defect Planted Naturally regenerated Mixed stands
  H R H R H R
  Proportion of sample trees, %
 First thinnings Class 1: Defects inhibiting sawing
 Sweep 5 5 4 3 5 1
 Crook in the butt 20 20 10 9 21 12
 Multiple crooks 52 51 75 81 72 79
 Other form defects a) 61 75 35 57 54 64
  Class 2: Defects decreasing the quality of sawn timber
 Branchiness b) 22 38 12 26 13 30
 Decay 3 1 2 1 1 1
 Surface defects c) 5 2 6 6 2 1
 Defect free trees 6 5 9 2 11 5
 Second thinnings Class 1: Defects inhibiting sawing
 Sweep 2 2 4 3 8 3
 Crook in the butt 19 14 9 8 10 6
 Multiple crooks 56 36 81 80 67 65
 Other form defects a) 49 57 50 63 48 52
  Class 2: Defects decreasing the quality of sawn timber
 Branchiness b) 32 47 16 27 16 28
 Decay 1 0 5 1 2 1
 Surface defects c) 1 2 4 6 3 1
 Defect free trees 5 9 4 4 6 8
 All stands Class 1: Defects inhibiting sawing
 Sweep 4 4 4 3 6 2
 Crook in the butt 20 19 9 8 15 9
 Multiple crooks 53 47 79 80 69 72
 Other form defects a) 58 70 45 61 51 58
  Class 2: Defects decreasing the quality of sawn timber
 Branchiness b) 24 40 15 27 15 29
 Decay 2 1 4 1 2 1
 Surface defects c) 4 2 5 6 2 1
 Defect free trees 6 6 6 3 9 7
 a)
  Other form defects impeding sawing, such as crooks in the middle section of stems, and forks.
 b)
  Branchiness includes dead or rotten branches bigger than 3 cm in diameter, grouped and vertical 
branches.
 c)
  Surface defects include surface scars and checks.
276
 Silva Fennica 45(2), 2011 research articles
 Table 6. Average harvesting recoveries by different assortments of birch in alternative bucking optionsa) in study 
stands grouped by stratum and thinning type. Percentages from the harvesting recovery in parentheses.
 Bucking option  Plywood logs Saw logs Pulpwood Harvesting Waste Total
     recovery wood cutting drain
  Volume, m3/ha
 Bucking 1   First thinning stands
 Planted 0.2 (0.4) 8.3 (15.2) 46.1 (84.4) 54.6 5.5 60.2
 Naturally regenerated 0.0 (0.0) 2.3 (5.1) 42.0 (94.9) 44.2 9.0 53.3
 Mixed 0.6 (1.7)  3.2 (9.6) 29.5 (88.7) 33.2 4.6 37.9
 All 0.3 (0.5) 6.5 (13.3) 42.5 (86.2) 49.3 5.9 55.2
    Second thinning stands
 Planted 2.7 (4.3) 14.7 (23.7) 44.5 (72.0) 61.9 3.8 65.7
 Naturally regenerated 2.2 (3.3) 10.6 (15.8) 54.5 (80.9) 67.4 4.4 71.8
 Mixed 0.6 (1.5) 6.6 (15.7) 34.8 (82.8) 42.0 3.7 45.7
 All 2.0 (3.4) 11.0 (18.7) 46.1 (77.9) 59.1 4.0 63.1
    All stands
 Planted 0.9 (1.6) 10.0 (17.7) 45.7 (80.7) 56.6 5.1 61.7
 Naturally regenerated 1.5 (2.5) 7.8 (13.1) 50.3 (84.4) 59.7 6.0 65.6
 Mixed 0.6 (1.6) 4.9 (13.0) 32.1 (85.4) 37.6 4.2 41.8
 All 1.0 (1.8) 8.4 (15.8) 44.0 (82.4) 53.4 5.1 58.5
 Bucking 2   First thinning stands
 Planted 0.2 (0.4) 5.3 (9.8) 49.0 (89.8) 54.6 5.6 60.2
 Naturally regenerated 0.0 (0.0) 0.9 (2.0) 43.4 (98.0) 44.3 9.0 53.3
 Mixed 0.6 (1.7) 1.4 (4.3) 31.2 (94.0) 33.2 4.7 37.9
 All 0.2 (0.5) 4.0 (8.1) 45.0 (91.4) 49.3 5.9 55.2
    Second thinning stands
 Planted 2.7 (4.3) 10.7 (17.3) 48.5 (78.4) 61.9 3.8 65.7
 Naturally regenerated 2.2 (3.3) 5.9 (8.8) 59.4 (87.9) 67.5 4.3 71.8
 Mixed 0.6 (1.5) 4.1 (9.7) 37.3 (88.8) 42.0 3.7 45.7
 All 2.0 (3.3) 7.1 (12.1) 50.0 (84.6) 59.1 4.0 63.1
    All stands
 Planted 0.9 (1.5) 6.8 (12.0) 48.9 (86.5) 56.5 5.1 61.7
 Naturally regenerated 1.5 (2.5) 4.2 (7.1) 54.0 (90.4) 59.7 5.9 65.6
 Mixed 0.6 (1.6) 2.7 (7.3) 34.2 (91.1) 37.6 4.2 41.8
 All 1.0 (1.8) 5.3 (9.9) 47.1 (88.3) 53.4 5.1 58.5
 Bucking 3   First thinning stands
 Planted 0.2 (0.4) 8.9 (16.0) 46.3 (83.6) 55.3 4.9 60.2
 Naturally regenerated 0.0 (0.0) 2.5 (5.4) 43.1 (94.6) 45.6 7.7 53.3
 Mixed 0.6 (1.7) 3.3 (9.9) 29.9 (88.4) 33.8 4.0 37.9
 All 0.2 (0.5) 7.0 (13.9) 42.9 (85.6) 50.1 5.1 55.2
    Second thinning stands
 Planted 2.7 (4.3) 15.5 (24.9) 44.1 (70.8) 62.3 3.4 65.7
 Naturally regenerated 2.2 (3.3) 11.1 (16.3) 54.5 (80.4) 67.8 4.0 71.8
 Mixed 0.6 (1.5) 6.8 (16.1) 35.0 (82.4) 42.4 3.3 45.7
 All 2.0 (3.4) 11.6 (19.4) 46.0 (77.2) 59.6 3.6 63.1
    All stands
 Planted 0.9 (1.5) 10.6 (18.6) 45.7 (79.9) 57.2 4.5 61.7
 Naturally regenerated 1.5 (2.5) 8.2 (13.6) 50.7 (83.9) 60.4 5.2 65.6
 Mixed 0.6 (1.6) 5.1 (13.3) 32.4 (85.1) 38.1 3.6 41.8
 All 1.0 (1.8) 8.9 (16.4) 44.2 (81.8) 54.0 4.5 58.5
 a)
  Log lengths in alternative bucking options:
  Bucking 1: saw logs 2.2 and 3.3 m, pulpwood 2.7–3.3 dm (approximate length).
  Bucking 2: saw logs 3.0 m, pulpwood 2.7–3.3 m (approximate length).
  Bucking 3: saw logs 2.2 and 3.3 m, pulpwood 2.5–5.0 m (free length).
277
 Kilpeläinen et al. Saw Log Recovery and Stem Quality of Birch from Thinnings in Southern Finland
 stands were 17.6, 11.7 and 18.2 in bucking options 
1, 2 and 3, respectively (Table 6). The recovery of 
saw logs was higher in the second thinning stands 
(12–19%) than in the first thinning stands (8–14%). 
Of the stand types, the percentage of saw logs 
was the highest in silver birch dominated planted 
stands, varying from 12 to 19, depending on the 
bucking option. Saw log recoveries were, on the 
other hand, at their lowest, 7–14%, in white birch 
dominated naturally regenerated stands and mixed 
stands of Norway spruce and birch. The highest 
share of saw logs, 17–25%, was obtained from 
the second thinning of planted stands.
 In the birch dominated thinning stands, the 
recovery of plywood logs was marginal, and 
almost constant irrespectively of the bucking 
option. In the entire material, 1.0 m3 of plywood 
logs were obtained per hectare, the observed 
recoveries being 2.0 m3 per hectare in the second 
thinning stands and as low as 0.2 m3 per hectare 
in the first thinning stands. Based on average 
recoveries, the highest share of plywood logs was 
obtained from the naturally regenerated stands, 
still not more than 2.5% of the total harvest-
 ing recovery. In the planted and mixed stands, 
the respective percentages were 1.5–1.6. In the 
second thinnings, the percentage of plywood logs 
was the highest in the planted stands, but in the 
first thinnigs in the mixed stands (Table 6).
 The largest volumes of saw logs, 8.9 and 8.4 
m3 per hectare were bucked in options 3 and 1, 
respectively. Option 2 provided only 5.3 m3 of 
saw logs per hectare due to only one available saw 
log length (3.0 m). The highest share of saw logs 
was obtained from planted stands, for example, 
ca. 18% of the total hectare wise recovery in 
bucking options 1 and 3.
 4 Discussion and Conclusions
 The purpose of this study was to determine the 
potential of birch thinning stands to provide saw 
logs. In addition to the actual measurements in 
sample stands, harvesting recoveries were studied 
by simulating in accordance with three different 
bucking options. 
Generally, the material of 48 stands and almost 
8000 measured sample trees can be considered 
representative for the purpose of this study. How-
 ever, in mixed stands of spruce and birch, the 
circumstances for timber quality development 
are so variable depending on site fertility, rela-
 tive proportions of species, silvicultural manage-
 ment history, and other reasons that they limit 
the possibilities to generalise the results. In case 
of pure birch stands with rather straightforward 
management regime, the results can be applied 
more safely. 
Geographically, the materials represent a major 
cutting potential of birch in Finland. However, 
some relatively important birch growing areas in 
southern, western and central Finland were not 
included in this material. There are distinctive dif-
 ferences between the two birch species in growth 
rate, stem form, branchiness, ability to utilise the 
fertility of the ground, as well as the silvicultural 
management recommendations. These features 
have been described and analysed widely in the 
literature (e.g., Heiskanen 1957,1966, Verkasalo 
1997, Heräjärvi 2001,2002, Heräjärvi and Ver-
 kasalo 2002, Lehtimäki et al. 2002, Lindblad 
et al. 2003, Kaurala et al. 2004, Arponen et al. 
2008). In case of silver birch, the planted trees 
represent genetically bred materials that differ 
from the natural trees in their branchiness, stem 
form and growth rate. In our material, the number 
of silver birch trees representing the natural origin 
was very small compared to the other strata. This 
deficiency sets limits to the possibilities to gener-
 alise the results from this part.
 The principle in selecting the removed trees 
in thinning was a combination of thinning from 
below and thinning for quality. However, in the 
second commercial thinning, also thinning from 
above can be considered. This procedure would 
increase the thinning recoveries of saw and ply-
 wood logs to some extent, but also postpone the 
final felling for some years. Thinning from above 
has positive impacts at least to the branchiness of 
the remaining trees.
 Data collection included some subjective esti-
 mation of characteristics of timber quality from 
standing trees, classification for straightness 
being the most challenging of them. However, the 
coherent training and long-term experience of the 
field groups of the organisation strongly dimin-
 ished this drawback. Only stem sections with 
diameter over 10 cm were assessed for quality. 
278
 Silva Fennica 45(2), 2011 research articles
 In case of remaining trees that are still growing 
rapidly, this does not provide sufficient informa-
 tion concerning the prospective quality of the 
trees in next thinning or final felling. Evaluation 
of standing trees for the suitability for mechanical 
wood processing has also the weakness that inter-
 nal defects affecting the processing value partly 
remain unknown. For birch, the largest uncer-
 tainty lies in the detection of heart decay, brown 
streaks and internal checks. However, only brown 
streaks caused by larval tunnels of Phytobia betu-
 lae and discoloured or firm-decayed wood caused 
by different mammals are frequent in younger 
trees such as those in this data (Niemistö et al. 
1997, Hallaksela and Niemistö 1998, Niemistö 
1998, Ylioja et al. 1998; Verkasalo 1997).
 Regarding the stem dimensions and quality 
of birch from thinning stands, the results were 
generally in line with the few previous studies 
available from Cameron et al. (1995), Niemistö 
(1995a, 1998), Dunham (1996), Niemistö et al. 
(1997) and Lindblad et al. (2003). It should be 
noted that mixed stands of birch and spruce are 
the dominant types of birch forests in most other 
countries in Europe. Practical experiences have 
shown there generally severe crookedness and 
knottiness, for example, steep knot angle, obvi-
 ously traceable to bad management history (e.g., 
Walfridsson 1976, Langhammer 1982, Agestam 
1985, Ekström 1987, see also Cameron et al. 
1995).
 Niemistö et al. (1997) published an extensive 
study where 30-year old planted silver birch was 
examined so that the 600 largest trees per hec-
 tare (potential plywood log stock for final felling 
stage) were taken into account. Compared with 
that study, sample trees of the current study were 
2–4 cm smaller in diameter and 1.5–3.5 metres 
shorter in height, on average. The lowest dead 
branch was 0.3–1.7 metres lower but the lowest 
living branch was higher in naturally regenerated 
pure stands and lower in mixed stands, indicating 
a more delayed thinning except for the planted 
stands. Compared with the birch trees harvested 
from thinning stands in the report by Lehtimäki et 
al. (2002), the branchiness was at about a similar 
level in the current study. 
At a log level, the distribution of knots depends 
on the log’s vertical location in a tree. In the 
study of Lindblad et al. (2003), the 1.1-metre 
sections of birch logs from thinning operations 
were made up of 48% of knot-free logs and 
17% of fully or partly sound-knotted logs. Cut 
from the tree base of three metre length, knot-
 free sections accounted for 75%, but only 35% 
from 3–6 m height and 15% from 6–9 m height. 
Respectively, sections with fully or partly sound 
knots accounted for 2%, 15% and 35%. However, 
birch is a challenging species to determine the 
internal knottiness from the external knottiness 
(e.g., Heräjärvi 2002, Lindblad et al. 2003). Only 
6% of the sample trees were defect-free in this 
study from the viewpoint of sawmilling, whereas 
the percentage was 36 in the study of Niemistö et 
al. (1997) from the viewpoint of plywood manu-
 facturing. Surface defects were less frequent in 
this study, indicating also less stem defects from 
previous tending and harvesting operations.
 Multiple crooks and crooks in the middle sec-
 tions of the stem were prevalent in this study, 
more frequent than in the material of Niemistö 
et al. (1997), and clearly more common than in 
the material of Lehtimäki et al. (2002). These 
form defects are typical for birch logs, and much 
more common than, e.g., uniform sweepness. 
The practical importance for saw milling lies in 
the fact that irregular deviations from straightness 
are very difficult to manage in the optimisation 
of sawing to maximise the recovery of sawn 
timber (Heräjärvi 2002, Lehtimäki et al. 2002, 
Lindblad et al. 2003). This can be seen in the 
high log consumption ratio when sawing small-
 diameter logs (Lehtimäki et al. 2002, Lindblad 
2003) and conventional large logs (Luostarinen 
and Verkasalo 2000, Heräjärvi 2002) with 2.8–4.4 
m3 and 2.6–3.2 m3 of logs per one cubic metre of 
sawn timber, respectively. It is also reflected to 
the lowering sawn timber yield when full-square 
timber is aimed for, and in the ineffective appli-
 cation of curve sawing. In the sawing simulation 
by Lindblad et al. (2003), the yield of full-square 
sawn timber decreased only by 5–10 percentage 
units, while the sweepness of birch logs grew 
from less than 5 mm/m up to more than 15 mm/m, 
or the crookedness grew from less than 10 mm/m 
up to more than 15 mm/m.
 In the sites with a high fertility such as in this 
study, silver birch in particular grows rapidly 
both in the diameter of the bole and branches, as 
shown by the before-mentioned researchers. This 
279
 Kilpeläinen et al. Saw Log Recovery and Stem Quality of Birch from Thinnings in Southern Finland
 causes also a risk of too heavy branchiness for 
the log quality. However, tree spacing and thin-
 ning rate affect largely, heavy thinning resulting 
in enhanced diameter development in the bole 
through greater ring width and more and thicker 
branches in comparison with light thinning and 
no thinning (Cameron et al. 1995). Nevertheless, 
the physical properties of wood remain largely 
unaffected. In a planting design of silver birch 
high initial density resulted in thin branches, 
early branch death and early self-pruning, the 
adequate density being 1600 trees per hectare, 
except for the most fertile sites (Niemistö 1995a). 
The diameter of the thickest branch in the butt 
log was 23 mm at the density of 5000 trees per 
hectare, growing to 30 mm at the density of 800 
trees per hectare.
 The results on harvesting recovery of saw logs 
of this study are unique in the scientific forum, 
hence, in this respect there are no reference results 
available. However, there is no reason to expect 
unrealistic outcomes from the simulated harvest-
 ing recoveries. Two provincial exploratory stud-
 ies in Finland showed results differing to some 
extent from this study. Rantanen et al. (2000) 
observed the recovery and percentage of birch 
saw logs being 2–27 m3 per hectare and 3–23% 
in individual marked second-thinning stands of 
birch in a more or less random sampling, the 
results being somewhat lower than reported in this 
study. Lehtimäki et al. (2002) noted the respective 
results of 4–22 m3 per hectare and 14–30% in a 
more critically selected stand data, the results 
being therefore at a similar level to this study. 
In mixed stands dominated by conifers, the pro-
 portion of saw log from the entire harvesting 
recovery of birch was relatively high: depending 
on the bucking option, from 4–10% and 10–16% 
in the first and second commercial thinnings, 
respectively (Table 6). This can be explained by 
the fact that in mixed stands the birch trees have 
more severe competition for light, which may 
lead to straighter stem form and more efficient 
dying and self-pruning of lower branches. Many 
authors have also reported that the birch trees that 
have been saved growing in mixed stands until the 
time of final felling actually represent the highest 
possible quality and saw or plywood log recovery 
in Finland (e.g., Heiskanen 1957, 1966, Verkasalo 
1997, Heräjärvi 2001, Kaurala et al. 2004).
 Depending on the stratum (stand type), the pro-
 portion of logs suitable for sawing varied roughly 
between 10 and 20 per cent. The recovery of logs 
was the highest in planted birch stands and the 
lowest in naturally regenerated pure birch stands. 
However, the results of this study were based on 
rather strict quality requirements. In practical 
timber procurement, the saw log recoveries might 
be somewhat higher. Especially, the defect class 
2 (defects decreasing the quality of sawn timber) 
might have been treated in this study too strictly. 
The conclusion is supported by the results of 
Lindblad et al. (2003) with approximately one 
third of saw logs in the thinning recovery in 
practical cutting of birch stands with a single-grip 
harvester compared with one quarter according 
to the measurement and evaluation of standing 
tree stocks. In the study of Arponen et al. (2008), 
planted silver birch trees from four selected later 
thinning stands provided even a higher saw log 
recovery of 43–57 per cent, when the minimum 
log diameter of 12 cm was applied in the evalua-
 tion of standing stocks.
 The availability of diameters and lengths of saw 
logs and pulpwood had an effect of 5–10 percent-
 age units on the recovery of saw logs. Short stem 
parts with adequate quality for saw milling are 
typical in thinning stands, thus, short log lengths 
are needed. In this study, decreasing the number 
of log lengths from two to one reduced the saw 
log percentage as much as by one third. The 
availability of different pulpwood lengths was 
almost as important: both the approximate length 
(3 m ± 10%) and free length (2.7–5.0 m) indirectly 
added to the saw log percentage.
 Small diameter of the trees in the removal sets 
also limits bucking saw logs from thinning stands 
of birch. In the study of Lindblad et al. (2003), 
saw log percentage increased from dbh class 13 
cm to dbh class 17 cm, and remained thereafter at 
an approximately constant level. Lindblad et al. 
(2003) applied similar minimum saw log diameter 
than we did in this study. Accordingly, larger tree 
dimensions along with less defects in the potential 
log section mainly explain the higher saw log 
recovery in the second thinnings compared with 
the first thinnings. The higher technical quality of 
the timber recovery in the second thinnings can 
be expected due to the elimination of the trees 
with the lowest quality in the first thinnings (see: 
280
 Silva Fennica 45(2), 2011 research articles
 Rantanen et al. 2000, Lindblad et al. 2003). This 
is also obvious judged from the results of Arponen 
et al. (2008).
 The advanced wood processing technology, for 
example, cross-cutting, finger jointing, laminat-
 ing and surface treatment supported by machine 
vision, nevertheless, enables economic processing 
of birch lumber with variable lengths and some 
defects, as well. Apparently, in the bucking simu-
 lations even relatively small defects caused timber 
assortment downgrade from saw log to pulpwood, 
especially in case of smaller-diameter logs. This 
would not be the case in real life. It must also be 
kept in mind that in commercial thinning the best 
trees should remain standing. Thus, it is unac-
 ceptable to increase the saw log recovery at the 
expense of silviculture.
 Based on this study, harvesting recoveries of 
end-use based timber assortments can be esti-
 mated in different kinds of thinning birch stands. 
In addition, the results provide birch users with 
information concerning timber quality attributes 
in various thinning stand types. Judged from the 
tree and log dimensions and stem quality, silver 
birch firstly from plantations and secondly from 
mixed stands, particularly from the second and 
later thinnings should be in the largest interest 
for the saw milling, furniture and interior product 
sector, and, thus, for saw log production in the 
forest management. Accordingly, a careful selec-
 tion of birch thinning stands for timber procure-
 ment may provide a considerable raw material 
potential for mechanical wood processing.
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