Forestry
An International Journal of Forest Research
Forestry 2014; 87, 437 – 447, doi:10.1093/forestry/cpu004
Advance Access publication 3 March 2014
Wood density and tracheid properties of Scots pine: responses to
repeated fertilization and timing of the first commercial thinning
Harri Mäkinen* and Jari Hynynen
Finnish Forest Research Institute, Vantaa Unit, PO Box 18, FI-01301 Vantaa, Finland
*Corresponding author. Tel.: +358 408015265; Fax: +358 295322103; E-mail: [email protected]
Received 8 October 2013
The aim of this study was to compare the effects of fertilization and thinning on the growth rate and wood and
tracheid properties of Scots pine (Pinus sylvestris L.) in southern Finland. The study was based on two long-term
experiments with two fertilization treatments (unfertilized and 150 kg N ha21 every 5 years) and two thinning
levels (delayed and intensive first commercial thinning) in a randomized block design. A total of 80 trees were
sampled 30 years after the onset of the treatments. Intensive thinning enhanced the radial growth of the remaining trees by 52 per cent compared with the delayed first thinning. Accordingly, the fertilization increased the radial
growth by 37 per cent compared with the unfertilized trees. However, only small differences were found between
the treatments in the earlywood/latewood ratio (3 –9 per cent), wood density (2 – 8 per cent), tracheid diameter
(2 – 5 per cent), cell wall thickness (0 –10 per cent) and tracheid length (4 –5 per cent). The results demonstrated
that the prevailing fertilization and thinning treatments of Scots pine stands considerably enhance the growth
rate but do not cause major detrimental changes in the wood and tracheid properties.
Introduction
Wood formation is controlled both by environmental and genetic
factors. Because wood properties result from the relative
amounts of different cell types, as well as their properties, silvicultural practises promoting tree growth may cause changes in wood
properties (Lundgren, 2004a,b; Beets et al., 2007). Wood properties
determine the usability of wood as end products, in pulp and paper
industry, the wood panel industry and in solid wood manufacturing. Thus, the integrated examination of the wood production
and conversion chain would allow further gain in wood manufacturing industry (e.g. Houllier et al., 1995; Schneider et al., 2008; Gardiner et al., 2011).
In conifers, increased tree growth via improved resource availability usually decreases wood density (Jozsa and Brix, 1989;
Pape, 1999a,b; Saranpää, 2003), a decline that is due to the accelerated production of earlywood relative to latewood, and the
density of earlywood and latewood may also decrease, as reported
for black spruce (Picea mariana (Mill.) B.S.P.) (Zhang et al., 1996;
Alteyrac et al., 2005) and Norway spruce (Picea abies (L.) Karst.)
(Mäkinen et al., 2002b). A faster growth rate caused by more intensive treatments has also resulted in wider and shorter tracheids
with thinner cell walls (Herman et al., 1998; Mäkinen et al.,
2002a; Lundgren, 2004a; Jaakkola et al., 2005a). However, some
studies have suggested that growth rate has no effect on wood
density of conifers (Megraw, 1985; Watson et al., 2003; Kärenlampi
and Riekkinen, 2004).
Forest management has changed in the Nordic countries in
recent decades. In particular, neglected or delayed pre-commercial
thinning and first commercial thinning are a problem in young
stands. According to the Finland’s National Forest Programme
2015 (Anonymous, 2011), the annual area requiring first commercial thinning is estimated to be 250 000 ha. However, on average,
only 185 000 ha of the first commercial thinning was performed
annually in the twenty-first century (Finnish Statistical Yearbook
of Forestry, 2011). The reason for the neglected or delayed first
thinning is the low profitability due to the small stem size and
high harvesting costs. However, the first commercial thinning can
be delayed in managed young stands in which pre-commercial
thinnings have been performed, resulting in increased thinning
removal and revenue (Huuskonen and Ahtikoski, 2005; Huuskonen
and Hynynen, 2006). Additionally, there is a general trend in silviculture toward lower stand densities, reduced number of thinnings
and shorter rotations (Anonymous, 2011).
In Nordic countries, nitrogen is usually regarded as the nutrient
that most limits tree growth (e.g. Tamm, 1991; Jarvis and Linder,
2000; Ingerslev et al., 2001). Indeed, nitrogen fertilization either
alone or together with phosphorus has proven to have a great
impact on tree growth (Kukkola and Saramäki, 1983; Bergh et al.,
1999; Saarsalmi and Mälkönen, 2001; Saarsalmi et al., 2006). In
2010, the area of forest fertilized for growth was 34 000 ha, which
is clearly less than the maximum record of over 240 000 ha during
the peak years in the mid-1970s (Finnish Statistical Yearbook of Forestry, 2011). According to the Finland’s National Forest Programme
2015 (Anonymous, 2011), the goal is to increase the annual area of
forest fertilization up to 80 000 ha year21.
In Finland, Scots pine (Pinus sylvestris L.) is an ecologically
and economically important tree species that comprises
# Institute of Chartered Foresters, 2014. All rights reserved. For Permissions, please e-mail: [email protected].
437
Forestry
49 per cent of the growing stock volume (Finnish Statistical Yearbook of Forestry, 2011). For Scots pine, most of the previous studies
on the effects of radial growth on wood and tracheid properties
have been based on short-term sample plots, or information on
the previous management history has been lacking. A deeper
understanding of the variation in wood and tracheid properties
could lead to improved silvicultural regimes with respect to both
timber and paper quality. In particular, more information is
needed to understand the long-term effects of the timing and intensity of the first commercial thinning and fertilization on wood
and tracheid properties.
The aim of this study was to investigate the long-term effects of
repeated fertilization and thinning of various timings and intensities on the growth rate and wood and tracheid properties of
Scots pine. Based on the samples taken at breast height (1.3 m),
wood density and tracheid dimensions (width, length and cell
wall thickness) were examined separately in earlywood and latewood, from pith to bark. The study was based on long-term
fertilization-thinning experiments in southern Finland. At the
onset of the treatments in the late 1970s, the stands were at the
phase of their first commercial thinning. Thus, the stand development in the experiments has been recorded for 30 years, and the
stands are approaching maturity. This provides an opportunity to
investigate wood and tracheid properties during the whole stand
rotation.
This study is a sequel to the reports of Hynynen and Saramäki
(1995) and Hynynen and Arola (1999) who reported the areabased growth and yield results (m3 ha21) for the first 15 and 20
years of the experiments. This study is a parallel study to those
by Jaakkola et al. (2006, 2007) describing the effects of fertilization
and thinning on wood and tracheid properties of Norway spruce,
based on the experiments belonging to the same experimental
series.
Materials and methods
The experiments
The material was collected from two fertilization-thinning experiments in
Juva and Iitti, southern Finland (Table 1). The experiment in Juva was on
a fertile site for Scots pine, with a site index (H100) of 31.4 m. The other experiment in Iitti was on a more typical Scots pine site classified as the
Vaccinium forest site type (Cajander, 1949), with a site index of 23.9 m.
The experiments were initiated in homogenous even-aged stands with an
age of 29 and 48 years, initial density of 3360 and 2677 stems ha21,
basal area of 25.3 and 19.1 m2 ha21, and basal area weighted mean
height of 10.7 and 10.6 m in Juva and Iitti, respectively.
The treatments, which began in 1977 and 1978 in Juva and Iitti, respectively, included three thinning and three fertilization treatments in a
randomized block design (Figure. 1). The fertilization treatments included
in this study were applied as follows: (F0) – unfertilized and (F1) – 150 kg
nitrogen (N), 75 kg phosphorus oxide (P2O5) and 75 kg potassium oxide
(K2O) ha21. NPK fertilizer was applied at 5-year intervals. The thinning
treatments included in this study were applied as follows: (T0) – delayed
first thinning, i.e. 60 per cent of the original number of stems was
removed 15 years after the onset of the experiment, and (T2) – intensive
first thinning with 60 per cent of the original number of stems removed
at the onset of the experiment. The removals correspond to 35 and 43
per cent of the basal area in the delayed and intensive first thinnings,
Figure 1 Number of stems (% of the original number of stems) and
fertilization treatments in Juva and Iitti. The experiment in Juva was
started in 1977. The parallel experiment in Iitti was started in 1978, and
all of the treatments were performed 1 year later. Thinning treatments:
T0 – delayed first thinning, i.e. 60% of the original number of stems was
removed 15 years after the onset of the experiment, and T2 – intensive
first thinning, i.e. 60% of the original number of stems was removed at
the onset of the experiment. The second commercial thinning was
performed on the plots of intensive thinning (T2) by again removing 60%
of the stem number. The fertilized (F1T2) plots were thinned 20 years
after the onset of the experiment, and the unfertilized plots (F0T2) were
thinned 25 years after the onset of the experiment. Fertilizer was applied
every 5 years to the fertilized plots (F1).
Table 1 Characteristics of the experiments at the time of most recent re-measurement in 2008
Experiment:
location
(number)
No. of
sample trees
Latitude
Longitude
Altitude
a.s.l. (m)
Temperature
sum (d.d.)1
Stand
age2
Hdom
(m)3
H100
(m)4
d.b.h.
(cm)5
Juva (LH509)
Iitti (LH512)
40
40
61847′
60850′
24815′
24811′
107
100
1229
1259
60
78
25.0
21.3
31.4
23.9
26.8
23.8
Sum of degrees by which the daily average temperature exceeds +58C (Ojansuu and Henttonen 1983).
Total age (years).
3
Dominant height, height of 100 thickest trees per hectare.
4
Site index, dominant height at age 100 years, calculated using the functions of Gustavsen (1980) and Vuokila and Väliaho (1980) for naturally regenerated and for planted and sowed stands, respectively.
5
Arithmetic mean diameter of the sample trees at breast height.
1
2
438
Wood density and tracheid properties of Scots pine
respectively. The second commercial thinning was performed on the plots
of intensive thinning (T2) by again removing 60 per cent of the stem
number. The fertilized (F1T2) plots were thinned 20 years after the onset
of the experiment, and the unfertilized plots (F0T2) were thinned 25 years
after the onset of the experiment. The treatments were applied in a factorial design, i.e. plots F0T0, F1T0, F0T2 and F1T2 were included.
The thinning type employed was thinning from below, which directed
the removal to the smallest trees. However, a regular spatial distribution
of the trees throughout the plot was maintained, i.e. smaller trees were
left standing if their removal would have resulted in large openings. In
addition, all of the clearly damaged trees and trees of poor quality due
to thick branches or crooked stems were removed, irrespective of their
size.
At the time of the most recent re-measurement in 2008, sample trees
were felled for analysing the wood properties. For the sample tree selection,
the cumulative basal area distribution of the trees on each plot was divided
into five classes. Two sample trees were randomly selected from each class,
i.e. the total number of trees sampled was 40 in both experiments. Sample
discs were collected at breast height (1.3 m).
Measurements
For all of the discs, the earlywood and latewood width, wood density, tracheid width and lumen diameter were measured using a SilviScan instrument (Evans et al., 1995). The measurements were performed from the
pith to bark along the southern radius of each disc, with averages for the
radial intervals of 25 mm. If necessary, the measurement direction was
rotated to avoid any cracks, knots or compression wood.
Several approaches have been used for demarcating between earlywood and latewood (Koubaa et al., 2002; Antony et al., 2012). The
minimum and maximum density method uses the minimum and
maximum density of the densitometry profiles of individual annual rings
(Polge, 1978). For each annual ring, a transition point (WDtp) between the
early- and latewood was separately defined as follows:
WDtp = (WDmax − WDmin ) × 0.8
(1)
where WDmax and WDmin are the maximum and minimum wood density
of an annual ring, respectively. The factor 0.8 was found to have the best fit
to the visually defined early – latewood boundary. In the visual comparison, earlywood was defined as the lighter-colored component of the
annual ring, characterized by an abrupt transition from earlywood to latewood.
Half of the trees sampled were selected for tracheid length measurements (one sample tree per size class from each plot). Seven annual rings
were selected at regular intervals from pith to bark (in Juva, the rings
formed in years 1965, 1975, 1980, 1985, 1990, 1995 and 2006; in Iitti,
the rings formed in 1966, 1976, 1981, 1986, 1991, 1996 and 2007). The
selected rings were split into small sticks and macerated in a solution of
glacial acetic acid and hydrogen peroxide (1 : 1, v/v) at 608C for 48 h (Franklin, 1945). The tracheid length distributions were measured using an automated fibre analyser – Kajaani FS-200 instrument (Kajaani Electronics Ltd.,
Kajaani, Finland). FS-200 measures tracheid lengths using an indirect
optical method which is based on the ability of the fibres to change the direction of polarized light, according to the TAPPI standard method T271. The
initial tracheid length distribution is affected by the processing, i.e. tracheids
are broken and other shorter wood elements such as ray and parenchyma
cells are liberated. Therefore, the mean, length-weighted tracheid length
was calculated from the tracheid length distribution.
Statistical analyses
In the analysis, the weighted means for the wood and tracheid properties
were calculated over the entire study period after the treatment onset.
The weighting factor was the relative basal area of each annual ring,
i.e. the values shown in the tables refer to the basal area weighted
means of each property. The statistical significance of the fertilization
and thinning effects on the measured wood and tracheid properties
was tested using the mixed model analysis for nested data structure.
The MIXED procedure of the SAS statistical software (SAS Institute Inc.,
2009), including both fixed and random effects, was used with the
following model:
Y ji = E + F + T + F × T + aC + Yj + 1 ji
(2)
where Yji is the dependent variable, E is the fixed effect of the experiment,
F is the fixed effect of fertilization, T is the fixed effect of thinning, F ×T is
their interaction, a is a fixed regression coefficient for covariate C, g j is the
random effect for plot j and 1ji is the uncorrelated random error for tree i
on plot j. The random plot effect was included to account for the unexplained variation between the plots which is not included in the fixed
treatment effects. The restricted maximum likelihood (REML) method
was used to estimate the parameters.
The initial differences between the plots and trees were removed using a
covariate (C) for each tree. The covariate was calculated as an arithmetic
mean of the property in question in the annual rings formed 5 years prior
to the treatment onset. The differences between the treatments were considered statistically significant when the P-value was ,0.05.
Results
On the intensively thinned plots (T2), both the first and second thinning clearly increased the radial increment of the remaining trees
(Figure. 2). The radial increment was the highest in the third year
after the intensive thinning, thereafter approaching the increment
of the T0 plots. In Juva, the delayed first thinning 15 years after the
onset of the experiment did not markedly enhance the increment
of the sampled trees. Accordingly, in Iitti, the radial increment was
only moderately enhanced after the delayed first thinning on the
unfertilized plot. In contrast, the radial increment was the
highest among all of the plots some years after the delayed first
thinning on the fertilized plot (F1T0) in Iitti. During the entire
study period, the intensive thinning treatment increased the
radial increment by 52 per cent compared with the delayed first
thinning (Tables 2 and 3). On the fertilized plots, the radial increment was, on average, 37 per cent higher than that on the unfertilized plots, but the difference was only significant at P ¼ 0.06 level.
The fertilization treatment slightly increased the latewood proportion (0.30 versus 0.33 on the unfertilized and fertilized plots,
respectively), but there was no significant difference between the
thinning treatments (Figure. 2, Tables 2 and 3).
The wood density increased toward the bark, and it had large
annual variations (Figure. 3). In Juva, the fertilization treatment
slightly decreased the wood density, whereas the differences
between the treatments were small in Iitti. During the entire
study period, fertilization and thinning, on average, decreased
the wood density by 4 and 2 per cent, respectively (Tables 2 and
3). The fertilization treatment decreased density of latewood relatively more than earlywood. In contrast, the difference in the latewood density between the thinning treatments (T0 versus T2) was
not statistically significant.
The tracheid diameter had marked synchronous annual variations (Figure. 4). Fertilization decreased and intensive early thinning
increased the average overall tracheid diameter and earlywood
tracheid diameter (Tables 2 and 3). In contrast, the difference in
439
Forestry
Figure 2 Average annual radial increment and latewood proportion in different fertilization (F) and thinning treatments (T) in Juva (A) and Iitti (B). The
vertical continuous line indicates the delayed first thinning on the T0 plots; the vertical dashed lines indicate the second commercial thinning 20 and
25 years after the onset of the experiment on plots F1T2 and F0T2, respectively, and the triangles indicate the fertilization applications on the F1 plots.
See the Materials and methods for an explanation of the treatments.
the tracheid diameter of latewood between the fertilized and unfertilized plots was not significant. Although most of the differences were significant, the average differences between the
treatments were small, at 1 mm or less.
The cell wall thickness also displayed synchronous annual variations (Figure. 5). In Juva, the fertilized plots had clearly lower
average cell wall thickness, and thinner earlywood and latewood
cell walls, compared with the unfertilized plots, but the differences
between the thinning treatments were small. In contrast, no major
differences were found between the treatments in Iitti. On average,
fertilization decreased the cell wall thickness by 7 –10 per cent, but
the differences between the thinning treatments were not significant (Tables 2 and 3).
Near the pith, the tracheid length rapidly increased when
moving outward, but the increase levelled off in the outer parts
of the stem (Figure. 6). On average, the tracheids were 5 per cent
shorter on the fertilized plots compared with the unfertilized
plots during the treatment period and 4 per cent shorter on the
plots with intensive thinning compared with the plots with
delayed first thinning (Table 3). However, the differences between
the treatments were not significant.
For almost all variables studied, the interaction term between
the fertilization and thinning treatments (F × T) was not significant
(Table 2). Thus, the effects of fertilization and thinning treatments
on the radial growth rate and wood and tracheid properties were
additive.
440
Discussion
Radial growth and latewood proportion
This case study compared the long-term effects of fertilization and
thinning on the growth and wood and tracheid properties of Scots
pine and identified the impacts of various timings and intensities of
the treatments. At the time of the onset of the treatment in the late
1970s, the intensive first thinning (T2) was considered to be very intensive. After the thinning, the number of stems per hectare was
900– 1000. According to the current recommendations for forestry practice, the recommended density for Scots pine stands
after the first commercial thinning is 900–1200 stems ha21
(Finnish Forestry, 2011). On the intensively thinned plots, 50 per
cent of the stand basal area was removed at the second commercial thinning. The intensity of the second thinning on treatments
T2F0 and T2F1 can be considered intensive when compared with
current thinning guidelines for mature Scots pine stands (Finnish
Forestry, 2011).
The intensive thinning increased the basal area increment of the
remaining trees by 50 per cent, an increase that corresponds with
the responses to thinning reported previously for Scots pine (e.g.
Mäkinen and Isomäki, 2004a,b; Mäkinen et al., 2006). Although
the increment of the remaining trees increased, Hynynen and Saramäki (1995) showed that during the first 15 years of the experiments the annual volume increment per hectare on the
Wood density and tracheid properties of Scots pine
Table 2 Mixed model analysis (equation 2) on the basal area increment,
latewood proportion and wood and tracheid properties showing the
F- and P-values for the effects of the experiment (E), fertilization (F),
thinning (T), their interaction (F×T) and covariate (the 5-year mean
before the onset of the experiment) (C)
Parameter
DF
Basal area increment
E
3
F
71
T
71
F×T
71
C
71
Latewood proportion
E
3
F
71
T
71
F×T
71
C
71
Wood density
E
3
F
71
T
71
F×T
71
C
71
Wood density, earlywood
E
3
F
71
T
71
F×T
71
C
71
Wood density, latewood
E
3
F
71
T
71
F×T
71
C
71
Tracheid diameter
E
3
F
71
T
71
F×T
71
C
71
Tracheid diameter, earlywood
E
3
F
71
T
71
F×T
71
C
71
Tracheid diameter, latewood
E
3
F
71
T
71
F×T
71
C
71
Cell wall thickness
F-value
P-value
0.67
3.67
9.38
0.80
29.27
0.47
0.06
0.00
0.37
0.00
15.26
4.79
0.02
0.24
33.12
0.03
0.03
0.89
0.63
0.00
27.81
20.08
7.00
0.29
139.02
0.01
0.00
0.01
0.59
0.00
5.30
32.66
7.40
0.02
89.73
0.10
0.00
0.01
0.90
0.00
0.41
27.28
1.47
0.25
128.73
0.57
0.00
0.23
0.62
0.00
6.31
10.41
7.33
0.75
103.46
0.09
0.00
0.01
0.39
0.00
1.70
22.19
11.47
0.56
114.73
0.28
0.00
0.00
0.46
0.00
11.04
1.84
13.13
5.26
10.85
0.05
0.20
0.00
0.02
0.00
Continued
Table 2 Continued
Parameter
DF
E
3
F
71
T
71
F×T
71
C
71
Cell wall thickness, earlywood
E
3
F
71
T
71
F×T
71
C
71
Cell wall thickness, latewood
E
3
F
71
T
71
F×T
71
C
71
Tracheid length
E
3
F
31
T
31
F×T
31
C
31
F-value
P-value
16.26
64.6
1.84
0.87
155.76
0.03
0.00
0.20
0.35
0.00
3.29
75.67
3.41
0.09
102.94
0.17
0.00
0.07
0.77
0.00
0.24
27.46
0.00
0.01
145.06
0.66
0.00
0.99
0.94
0.00
2.28
3.99
0.13
0.04
13.75
0.23
0.05
0.72
0.83
0.00
The significant (P , 0.05) effects are shown in bold type.
Table 3 Mean basal area increment and wood and tracheid properties in
different fertilization (F) and thinning treatments (T)
Basal area increment (cm2)
Latewood proportion
Wood density (kg m23)
Wood density, earlywood (kg m23)
Wood density, latewood (kg m23)
Tracheid diameter (mm)
Tracheid diameter, earlywood (mm)
Tracheid diameter, latewood (mm)
Cell wall thickness (mm)
Cell wall thickness, earlywood (mm)
Cell wall thickness, latewood (mm)
Tracheid length (mm)
F0
F1
T0
T2
234.5
0.30
595.5
444.5
951.4
32.0
35.7
24.2
3.39
2.61
5.23
2.68
320.3
0.33
574.1
424.4
891.3
31.0
34.7
23.9
3.16
2.43
4.73
2.54
220.0
0.31
590.6
440.7
931.7
31.1
34.8
23.6
3.28
2.54
5.00
2.66
334.7
0.32
579.0
428.3
911.1
31.9
35.5
24.6
3.27
2.50
4.97
2.56
The values shown are the predicted means for the main effects of
fertilization and thinning, using the fixed part of equation 2. For
explanation of the treatments, see the Materials and methods.
intensively thinned plots decreased by 1 m3 ha21 compared with
the plots with the delayed first thinning.
The positive growth response to the fertilization treatment corresponds to the relative growth increases obtained in coniferous
441
Forestry
Figure 3 Average wood density in each annual ring and the densities of earlywood and latewood in different fertilization (F) and thinning treatments (T) in
Juva (A) and Iitti (B). The vertical continuous line indicates the delayed first thinning on the T0 plots; the vertical dashed lines indicate the second
commercial thinning 20 and 25 years after the onset of the experiment on plots F1T2 and F0T2, respectively, and the triangles indicate the fertilization
applications on the F1 plots. See the Materials and methods for an explanation of the treatments.
stands after a single N application (e.g. Kukkola and Saramäki,
1983; Saarsalmi et al., 2006). According to earlier fertilization
studies (e.g. Viro, 1972; Kukkola and Saramäki, 1983), the duration
of the N fertilization effect in mature pine stands in southern
Finland varies between 6 to 9 years. In our study, the positive
growth response comprised the entire study period due to the successive N applications. Therefore, the absolute growth increase as a
result of N application surpassed the conventional silvicultural fertilizations (Finnish Forestry, 2011) and, therefore, represents the
impacts of a high N dose on tree growth.
The proportion of earlywood in conifers has been suggested to
increase with increasing radial growth (e.g. Smith, 1980; Pape,
1999a,b). In Scots pine, Peltola et al. (2007) also found that the
442
earlywood percentage increased up to 10 and 5 per cent over a
12-year study period on heavily and moderately thinned plots, respectively. In our study, the differences in the proportion of latewood between the different fertilization and thinning treatments
were small (0 –3 per cent) (cf., Zhang et al., 1996). Accordingly, in
loblolly pine (Pinus taeda L.), Tasissa and Burkhart (1998) found
that thinning had no effect on the proportion of latewood. Based
on the experiments similar to those used in the present study, Jaakkola et al. (2006) also reported for Norway spruce that fertilization
and thinning treatments resulted in a relatively small decrease in
the proportion of latewood (0 –9 per cent). In fact, the change in
latewood proportion with tree size is a confounding factor: the latewood proportion generally increases with increasing stem
Wood density and tracheid properties of Scots pine
Figure 4 Average tracheid diameter in each annual ring and tracheid diameters in earlywood and latewood in different fertilization (F) and thinning
treatments (T) in Juva (A) and Iitti (B). The vertical continuous line indicates the delayed first thinning on the T0 plots, the vertical dashed lines indicate
the second commercial thinning 20 and 25 years after the onset of the experiment on plots F1T2 and F0T2, respectively, and the triangles indicate the
fertilization applications on the F1 plots. See the Materials and methods for an explanation of the treatments.
diameter (e.g. Jyske et al., 2008; Mäkinen and Hynynen, 2012). The
fertilization and intensive thinning enhanced radial growth, and we
also weighted the measured latewood proportions by the basal
area of each annual ring when calculating the averages for the
entire study period. Thus, together with the long study period,
these effects most likely negated the minor differences in the relative latewood width.
Wood density and tracheid dimensions
The fertilization and intensive thinning resulted in a reduction of
2 –8 per cent in the wood density compared with that of the
unfertilized plots and the plots with delayed first thinning. Our
results agree well with the previous findings. In a 56-year-old
Scots pine stand in northern Sweden, Mörling (2002) found an increase of 14 per cent in the ring width following N addition and
by 40 per cent after thinning (40 per cent basal area removal) but
a decrease by only 0 –4 per cent in the wood density compared
with that of unthinned and unfertilized plots. Similar results were
also found by Peltola et al. (2007), reporting an average reduction
of 2 per cent in the wood density of Scots pine after a range of thinning intensities, despite a considerable increase in the radial increment. The effect of thinning on the predicted wood density of Scots
pine was also found to be small by Auty (2011). In red pine (Pinus
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Forestry
Figure 5 Average cell wall thickness in each annual ring and the wall thicknesses in earlywood and latewood in different fertilization (F) and thinning
treatments (T) in Juva (A) and Iitti (B). The vertical continuous line indicates the delayed first thinning on the T0 plots; the vertical dashed lines indicate
the second commercial thinning 20 and 25 years after the onset of the experiment on plots F1T2 and F0T2, respectively, and the triangles indicate the
fertilization applications on the F1 plots. See the Materials and methods for an explanation of the treatments.
resinosa Ait.) and loblolly pine, Larocque and Marshall (1995) and
Tasissa and Burkhart (1998) also found little or no relationship
between the wood density and growth rate. In jack pine (Pinus
banksiana Lamb.), the effect of thinning on the wood density was
also small (Schneider et al., 2008). Similarly, Jaakkola et al.
(2005b) found that both thinning and fertilization significantly
increased the increment of individual Norway spruce trees, but
the treatments had a negligible effect on wood density. In a nutrient optimization experiment in a Norway spruce stand in Sweden,
Mäkinen et al. (2002b) and Lundgren (2004b) demonstrated that
the optimal nutrient addition resulted in a higher increase (100 –
200 per cent) in the radial increment and a concurrent reduction
of 20 per cent in the wood density. In the experiment, the yield
444
of stemwood in the optimized treatments surpassed the best
yields obtained by conventional silvicultural means (Bergh et al.,
1999), and therefore represents the extreme impact of fertilization
on tree growth. Obviously, higher nutrient additions and subsequent
larger increases in tree growth, higher than those in this study, are
required for considerable reductions in wood density.
The effects of fertilization and thinning on the tracheid diameter
and cell wall thickness were small and variable among the treatment combinations. In previous studies, the rapid growth rate of
conifers has resulted in the formation of larger tracheids but
thinner cell walls (e.g. Denne, 1973; Mäkinen et al., 2002a; Lundgren, 2004a). However, the opposite results for the relationship
between ring width and tracheid dimensions have also been
Wood density and tracheid properties of Scots pine
Figure 6 Average tracheid length in the selected annual rings in different fertilization (F) and thinning treatments (T) in Juva (A) and Iitti (B). The vertical
continuous line indicates the delayed first thinning on the T0 plots; the vertical dashed lines indicate the second commercial thinning 20 and 25 years after
the onset of the experiment on plots F1T2 and F0T2, respectively, and the triangles indicate the fertilization applications on the F1 plots. See the Materials and
methods for an explanation of the treatments.
reported (Denne, 1973; Brix and Mitchell, 1980; Bergqvist et al.,
2000). In Norway spruce, Jaakkola et al. (2005a) found that an increasing growth rate due to thinning had no profound effect on the
lumen diameter and cell wall thickness of the tracheids. Based on
similar fertilization-thinning experiments in Norway spruce, Jaakkola et al. (2007) also found only small differences in the lumen
diameter and cell wall thickness for the different treatments. Furthermore, Jyske et al. (2010) reported for Norway spruce that
drought stress and fertilization had only minor effects on the tracheid diameter and cell wall thickness.
The mean tracheid length found in this study is in accordance
with earlier studies on Scots pine in Finland (Varhimo et al., 2003;
Havimo et al., 2009; Rautiainen and Alén, 2009). The average tracheid length showed a rather regular increase from the pith to
the outer part of the stem (cf., Molteberg and Høibø, 2006; Rautiainen and Alén, 2009).This trend is due to the increasing cambial maturity (e.g. Olesen, 1978,1982; Saranpää et al., 2000). In this study,
a faster growth rate due to intensive thinning and/or fertilization
tended to slightly decrease the tracheid length (4 –5 per cent),
but statistically significant differences were not found. Differences
of similar magnitude (0– 6 per cent) in the tracheid length were
found after similar treatments in Norway spruce (Jaakkola et al.,
2007). In previous studies, a faster growth rate due to fertilization
and thinning has also resulted in somewhat shorter tracheids
(Erickson and Harrison, 1974; Herman et al., 1998; Mäkinen et al.,
2002a; Jaakkola et al., 2005a). According to Sirviö and Kärenlampi
(2001),the tracheid length is mostly determined by the maturation
of the cambium, with the growth rate being a less important factor.
This is in agreement with the observations of Varhimo et al. (2003)
and Rautiainen and Alén (2009), who reported that the variation in
the tracheid length is much larger within a single Scots pine tree
than between trees. However, Helander (1933) and Mäkinen
et al. (2002a) reported that the tracheid length decreased 17 –20
per cent when the ring width was doubled or tripled.
treatments did not change the investigated wood and tracheid
properties, with the relative changes in the wood and tracheid
properties being negligible in relation to the changes in the radial
growth rate. The effects of fertilization and thinning treatments
on the radial growth rate and wood and tracheid properties were
additive, i.e. an increasing growth rate resulted in a similar kind of
changes irrespective of the factor promoting enhanced growth.
The material used in this study was small, with only 80 stems
sampled from two stands in southern Finland. Therefore, although
the results are consistent with the previous Finnish, Nordic and
European studies, the results are only indicative due to the small
dataset. The results also support the recent Finnish results on
Norway spruce, allowing at least preliminary conclusions. It
seems that the prevailing fertilization and thinning treatments of
Scots pine stands in the Nordic countries do not cause detrimental
changes in the wood and tracheid properties.
Acknowledgements
We are greatly indebted to the following people: Tapio Järvinen, Jukka
Lehtimäki, Irmeli Luovula, Tapio Nevalainen, Lars Olsson, Timo Siitonen
and Pia Vento for their skilful assistance in obtaining the study material,
and Dr Sven-Olof Lundqvist and Prof. Kari Edelmann for allowing us to
make use of the measurement facilities in their institutions.
Funding
The study was conducted in the Finnish Forest Research Institute. The work
was supported by Tekes (the Finnish Funding Agency for Technology and
Innovation) as part of the EffFibre (‘Value through Intensive and Efficient
Fibre Supply’) Research Program of the Finnish Forestcluster Ltd.
Conclusions
The results demonstrated that intensive thinnings and fertilization
considerably enhance the growth rate of the remaining trees. The
Conflict of interest statement
None declared.
445
Forestry
References
the wood yield and economic yield of a pine stand on a fairly dry forest
site type.] Metsät. Aikakausk 2005, 99–115. (in Finnish).
Alteyrac, J., Zhang, S.Y., Cloutier, A. and Ruel, J.-C. 2005 Influence of stand
density on ring width and wood density at different sampling heights in
black spruce (Picea mariana (MILL.) B.S.P.). Wood Fiber Sci. 37, 83– 94.
Huuskonen, S. and Hynynen, J. 2006 Timing and intensity of precommercial
thinning and their effects on the first commercial thinning in Scots pine
stands. Silva Fenn. 40, 645– 662.
Anonymous. 2011 Finland’s National Forest Programme 2015 Finnish
Ministry of Agriculture and Forestry, Juvenes Print, 52 p.
Hynynen, J. and Saramäki, J. 1995 Ensiharvennuksen viivästymisen ja
harvennusvoimakkuuden vaikutus nuoren männikön kehitykseen. Folia
For. 1995, 99– 113.
Antony, F., Schimleck, L.R. and Daniels, R.F. 2012 A comparison of
earlywood – latewood demarcation methods – a case study in loblolly
pine. IAWA J. 33, 187–195.
Auty, D. 2011 Modelling the effects of forest management on the wood
properties and branch characteristics of UK-grown Scots pine. PhD thesis,
University of Aberdeen, 158 p.
Hynynen, J. and Arola, M. 1999 Ensiharvennusajankohdan vaikutus
hoidetun männikön kehitykseen ja harvennuksen kannattavuuteen.
Metsätieteen Aikakauskirja 1999, 5– 23.
Beets, P., Kimberley, M. and McKinley, R. 2007 Predicting wood density of
Pinus radiata annual growth increments. N. Z. J. For. Sci. 37, 241– 266.
Ingerslev, M., Mälkönen, E., Nilsen, P., Nohrstedt, H.-Ö., Óskarsson, H. and
Raulund-Rasmussen, K. 2001 Main findings and future challenges in
forest nutritional research and management in the Nordic countries.
Scand. J. For. Res. 16, 488–501.
Bergh, J., Linder, S., Lundmark, T. and Elfving, B. 1999 The effect of water and
nutrient availability on the productivity of Norway spruce in northern and
southern Sweden. For. Ecol. Manage. 119, 51– 62.
Jaakkola, T., Mäkinen, H., Sarén, M.-P. and Saranpää, P. 2005a Does thinning
intensity affect the tracheid dimensions of Norway spruce? Can. J. For. Res.
35, 2685 –2697.
Bergqvist, G., Bergsten, U. and Ahlqvist, B. 2000 Fibre properties of Norway
spruce of different growth rates grown under birch shelterwoods of two
densities. Can. J. For. Res. 30, 487– 494.
Jaakkola, T., Mäkinen, H. and Saranpää, P. 2005b Wood density in Norway
spruce: changes with thinning intensity and tree age. Can. J. For. Res. 35,
1767– 1778.
Brix, H. and Mitchell, A.K. 1980 Effects of thinning and nitrogen fertilization
on xylem development in Douglas-fir. Can. J. For. Res. 10, 121–128.
Jaakkola, T., Mäkinen, H. and Saranpää, P. 2006 Wood density of Norway
spruce: responses to timing and intensity of first commercial thinning and
fertilization. For. Ecol. Manage. 237, 513–521.
Cajander, A.K. 1949 Forest types and their significance. Acta For. Fenn. 56,
1 –69.
Denne, M.P. 1973 Tracheid dimensions in relation to shoot vigour in Picea.
Forestry 46, 117–124.
Erickson, H.D. and Harrison, T. 1974 Douglas-fir wood quality studies. Part I:
effects of age and stimulated growth on wood density and anatomy. Wood
Sci. Technol. 8, 207– 226.
Jaakkola, T., Mäkinen, H. and Saranpää, P. 2007 Effects of thinning and
fertilisation on tracheid dimensions and lignin content of Norway spruce.
Holzforschung 61, 301–310.
Jarvis, P. and Linder, S. 2000 Constraints to growth of boreal forests. Nature
405, 904– 905.
Jozsa, L.A. and Brix, H. 1989 The effects of fertilization and thinning on wood
quality of a 24-year-old Douglas-fir stand. Can. J. For. Res. 19, 1137– 1145.
Evans, R., Downes, G.M., Menz, D.N.J. and Stringer, S.L. 1995 Rapid
measurement of variation in tracheid transverse dimensions in a radiata
pine tree. Appita J. 48, 134– 138.
Jyske, T., Mäkinen, H. and Saranpää, P. 2008 Wood density within Norway
spruce stems. Silva Fenn. 42, 439–455.
Finnish Forestry – practice and management. 2011 Metsäkustannus,
Helsinki. 271 p. http://www.kauppakv.fi/sis/Metsakustannus/tuote/
9789525694628.
Jyske, T., Hölttä, T., Mäkinen, H., Nöjd, P., Lumme, I. and Spiecher, H. 2010 The
effect of artificially induced drought on radial increment and wood
properties of Norway spruce. Tree Physiol. 30, 103–115.
Franklin, G.L. 1945 Preparation of thin sections of synthetic resins and
wood–resin composites, and a new macerating method for wood. Nature
155, 51.
Kärenlampi, P.P. and Riekkinen, M. 2004 Maturity and growth rate effects on
Scots pine basic density. Wood Sci. Technol. 38, 465– 473.
Gardiner, B., Leban, J.-M., Auty, D. and Simpson, H. 2011 Models for
predicting wood density of British-grown Sitka spruce. Forestry 84,
119–132.
Koubaa, A., Zhang, S.Y. and Makni, S. 2002 Defining the transition from
earlywood to latewood in black spruce based on intra-ring wood density
profiles from X-ray densitometry. Ann. For. Sci. 59, 511– 518.
Gustavsen, H. 1980 Site index curves for conifer stands in Finland. Folia For.
454, 1 –31. (in Finnish with English summary).
Kukkola, M. and Saramäki, J. 1983 Growth response in repeatedly
fertilized pine and spruce stands on mineral soils. Commun. Inst. For.
Fenn. 114, 1 – 55.
Havimo, M., Rikala, J., Sirviö, J. and Sipi, M. 2009 Tracheid cross-sectional
dimensions in Scots pine (Pinus sylvestris) – distributions and comparison
with Norway spruce (Picea abies). Silva Fenn. 43, 681–688.
Larocque, G.R. and Marshall, P.L. 1995 Wood relative density development in
red pine (Pinus resinosa Ait.) stands as affected by different initial spacings.
For. Sci. 41, 709–728.
Helander, A.B. 1933 Variations in tracheid length of pine and spruce. Found.
For. Res. Finland. 14, 1 –75. (in Finnish with English summary).
Lundgren, C. 2004a Cell wall thickness and tangential and radial cell
diameter of fertilized and irrigated Norway spruce. Silva Fenn. 38, 95–106.
Herman, M., Dutilleul, P. and Avella-Shaw, T. 1998 Growth rate effects on
temporal trajectories of ring width, wood density, and mean tracheid
length in Norway spruce (Picea abies (L.) Karst.). Wood Fiber Sci. 30, 6– 17.
Lundgren, C. 2004b Microfibril angle and density patterns of fertilized and
irrigated Norway spruce. Silva Fenn. 38, 107–117.
Houllier, F., Leban, J.-M. and Colin, F. 1995 Linking growth modeling to
timber quality assessment for Norway spruce. For. Ecol. Manage. 74,
91– 102.
Huuskonen, S. and Ahtikoski, A. 2005 Ensiharvennuksen ajoituksen ja
voimakkuuden vaikutus kuivahkon kankaan männiköiden tuotokseen ja
tuottoon. [The effect of the timing and intensity of the first thinning on
446
Mäkinen, H. and Isomäki, A. 2004a Thinning intensity and growth of Norway
spruce stands in Finland. Forestry 77, 349– 364.
Mäkinen, H. and Isomäki, A. 2004b Thinning intensity and long-term
changes in increment and stem form of Norway spruce trees. For. Ecol.
Manage. 201, 295–309.
Mäkinen, H. and Hynynen, J. 2012 Predicting wood and tracheid properties
of Scots pine. For. Ecol. Manage. 279, 11– 20.
Wood density and tracheid properties of Scots pine
Mäkinen, H., Saranpää, P. and Linder, S. 2002a Effect of growth rate on fibre
characteristics in Norway spruce (Picea abies (L.) Karst.). Holzforschung
56, 449– 460.
Saarsalmi, A., Kukkola, M., Moilanen, M. and Arola, M. 2006 Long-term
effects of ash and N fertilization on stand growth, tree nutrient status
and soil chemistry in a Scots pine stand. For. Ecol. Manage. 235, 116– 128.
Mäkinen, H., Saranpää, P. and Linder, S. 2002b Wood-density variation of
Norway spruce in relation to nutrient optimization and fibre dimensions.
Can. J. For. Res. 32, 185–194.
Saranpää, P. 2003 Wood density and growth. In Wood quality and its
biological basis, Barnett, J.R. and Jeronimidis, G. (eds). Blackwell
Publishing Ltd. pp. 87–117.
Mäkinen, H., Isomäki, A. and Hongisto, T. 2006 Effect of half-systematic and
systematic thinning on the increment of Scots pine and Norway spruce in
Finland. Forestry 79, 103–121.
Saranpää, P., Pesonen, E., Sarén, M., Anderson, S., Siiriä, S., Serimaa, R. and
Paakkari, T. 2000 Variation of the properties of tracheids in Norway spruce
(Picea abies (L.) Karst.). In Cell and molecular biology of wood formation.
Savidge, R.A., Barnett, J.R. and Napier, R. (eds). BIOS Scientific Publishers
Limited. pp. 337– 345.
Megraw, R.A. 1985 Wood quality factors in loblolly pine: the influence of tree
age, position in tree, and cultural practices on wood specific gravity, fiber
length, and fibril angle. TAPPI Press.
Molteberg, D. and Høibø, O. 2006 Development and variation of wood
density, kraft pulp yield and fibre dimensions in young Norway spruce
(Picea abies). Wood Sci. Technol. 40, 173–189.
Mörling, T. 2002 Evaluation of annual ring width and ring density
development following fertilisation and thinning of Scots pine. Ann. For.
Sci. 59, 29– 40.
Ojansuu, R. and Henttonen, H. 1983 Estimation of local values of monthly
temperature, effective temperature sum and precipitation sum from the
measurements made by the Finnish Meteorological Office. Silva Fenn. 17,
143–160. (in Finnish with English summary).
Olesen, P.O. 1978 On cyclophysis and topophysis. Silvae Genet. 27,
173–178.
Olesen, P.O. 1982 The effect of cyclophysis on tracheid width and basic
density in Norway spruce. For. Tree Improv. 15, 1– 80.
Pape, R. 1999a Effects of thinning regime on the wood properties and stem
quality of Picea abies. Scand. J. For. Res. 14, 38–50.
Pape, R. 1999b Influence of thinning and tree diameter class on the
development of basic density and annual ring width in Picea abies.
Scand. J. For. Res. 14, 27– 37.
Peltola, H., Kilpeläinen, A., Sauvala, K., Räisänen, T. and Ikonen, V.-P. 2007
Effects of early thinning regime and tree status on the radial growth and
wood density of Scots pine. Silva Fenn. 41, 489–505.
Polge, H. 1978 Fifteen years of wood radiation densitometry. Wood Sci.
Technol. 12, 187–196.
Rautiainen, R. and Alén, R. 2009 Variations in fibre length within a
first-thinning Scots pine (Pinus sylvestris) stems. Cellulose 16,
349 – 355.
Saarsalmi, A. and Mälkönen, E. 2001 Forest fertilization research in Finland: a
literature review. Scand. J. For. Res. 16, 514–535.
SAS Institute Inc. 2009 SAS/STAT 9.2 User’s Guide, Second Edition. SAS
Institute Inc.
Schneider, R., Zhang, S.Y., Swift, D.E., Bégin, J. and Lussier, J.-M., 2008
Predicting selected wood properties of jack pine following commercial
thinning. Can. J. For. Res. 38, 2030– 2043.
Sirviö, J. and Kärenlampi, P. 2001 The effects of maturity and growth rate on
the properties of spruce wood tracheids. Wood Sci. Technol. 35, 541– 554.
Smith, J.H.G. 1980 Influences of spacing on radial growth and percentage
latewood of Douglas-fir, western hemlock, and western red cedar.
Can. J. For. Res. 10, 169–175.
Statistical Yearbook of Forestry. 2010 Finnish Forest Research Institute. 470
pp. (in Finnish with English Summary).
Tamm, C.O. 1991 Nitrogen in terrestrial ecosystems: questions of
productivity, vegetational changes, and ecosystem stability. Ecol Stud 81,
Springer, 115 pp.
Tasissa, G. and Burkhart, H.E. 1998 Modeling thinning effects on ring specific
gravity of loblolly pine (Pinus taeda L.). For. Sci. 44, 212– 223.
Varhimo, A., Kojola, S., Penttilä, T. and Laiho, R. 2003 Quality and yield of
pulpwood in drained peatland forests: pulpwood properties of Scots pine
in stands of first commercial thinning. Silva Fenn. 37, 343– 357.
Viro, P.J. 1972 Die Walddüngung auf finnischen Mineralböden. Folia For. 138,
1 –19. (in German).
Vuokila, Y. and Väliaho, H. 1980 Growth and yield models for conifer cultures
in Finland. Commun. Inst. For. Fenn. 99(2), 1 – 271. (in Finnish with English
summary).
Watson, R., Garner, C., Robertson, R., Reath, S., Gee, W. and Hunt, K. 2003 The
effects of initial tree spacing on the fibre properties of plantation-grown
coastal western hemlock. Can. J. For. Res. 33, 2460 –2468.
Zhang, S.Y., Simpson, D. and Morgenstern, E.K. 1996 Variation in the
relationship of wood density with growth in 40 black spruce (Picea
mariana) families grown in New Brunswick. Wood Fiber Sci. 28, 91 –99.
447