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Growth effects after mountain forest
selective cutting in southeast
Norway
BERNT-HÅVARD ØYEN1 AND PETTER NILSEN2
1
2
Norwegian Forest Research Institute, Fanaflaten 4, N-5244 Fana, Bergen, Norway
Norwegian Forest Research Institute, Høgskolev. 12, N-1432 Ås, Norway
Summary
Growth effects in a sub-alpine, low-yield Norway spruce forest in southeast Norway are reported.
Sixteen sample plots of 400 m2, established 8–9 years after a mountain forest selective (MFS) cutting
in the mid-1970s, were re-investigated in 2000. The selective cutting was heavy, with a mean felling
volume of 72 per cent of the standing volume. Most trees in the remaining stands responded
positively with increased growth after the cutting, and this was most pronounced in small and
medium sized trees. A weak relationship between standing volume before and after felling, and the
actual stand volume increment in the 25-year period was revealed. The felling has stimulated natural
regeneration and increased the proportion of birch. The results indicate that not more than ~65 per
cent of the standing volume should be cut in a single intervention if cutting cycle is less than
50 years.
Introduction
About one-fifth of the productive forest area in
Norway (~1.5 million ha.), is classified as mountain forest, of which half could be defined as protection forests according to the Forestry Act
(Nilsen and Solberg, 1998). Generally, mountain
forests are characterized by difficult conditions
for seeding, regeneration and growth due to harsh
climatic conditions (Kielland-Lund, 1981a). In
central southeast Norway the border between
mountain forest and lowland forest is situated
between 500 and 650 m above sea level. The tree
line of mountain birch (Betula pubescens subsp.
tortuósa Lebed. Nym.) is presently located
between 900 and 1150 m above sea level. Beneath
this birch zone, which is generally about 100 m
© Institute of Chartered Foresters, 2002
in vertical extent, the mountain spruce and pine
forests are located. There has been a great deal of
debate concerning the management of these
mountain forests (Børset, 1994; Bergan and
Skoklefald, 1996; Ohlson and Tryterud, 1999).
One method of management is the mountain
forest selective (MFS) cutting system, where scattered single trees or small groups of trees are
selected and harvested (Figure 1). This follows a
pattern of harvesting of the largest and most marketable trees, which has been a common practice
in the Nordic countries (Nilsen, 1984; Valtanen,1988; Lundqvist, 1993; Suadicani and Fjeld,
2000; Lädhe et al., 2001). Similar silvicultural
systems to MFS cutting have been investigated in
other countries (e.g. Wing, 1977; Burschel et al.,
1992; Schütz 1997; Groot, 1997). The practice of
Forestry, Vol. 75, No. 4, 2002
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F O R E S T RY
(a)
(b)
Figure 1. A mountain spruce forest stand before (a) and after (b) a selective cutting (after Nilsen, 1988).
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GROWTH AFTER SELECTIVE CUTTING
MFS cutting has increased during the last
25 years; however, there have been few investigations of the long-term effects on productivity
and forest structure. As a result of this, proposed
growth and yield models for this type of silviculture are inadequate (Nilsen, 2001). This paper
summarizes some results from a retrospective
study in mountain forest and discusses aspects of
stand growth 25 years after MFS cutting in a subalpine Norway spruce forest in southeast
Norway.
Methods
Site description
Sixteen long-term sample plots in Mannstadlia,
Gausdal municipality (61° N, 10° E), southeast
Norway, about 800 m above sea level, were reinvestigated in September 2000, 25 years after the
MFS cutting. The forest area was harvested in
autumn 1974 and 1975, and the first investigations were performed on the plots in 1983 and
1984 (Nilsen, 1988). The plots were selected
to cover different harvesting intensities and
stand structures. The plots are located on a
northeast-facing terraced slope, on sedimentary
rocks with a thick cover of moraine. In general
the cutting was quite heavy, with a mean felling
volume of 72 per cent of standing volume. The
main vegetation types, according to KiellandLund (1981b), were tall herb type (Melico–Piceetum aconitetosum, four plots), small fern type
(Eu–Piceetum dryopteridetosum, five plots) and
bilberry type (Eu–Piceetum myrtilletosum, seven
plots). A more detailed description of the sites,
stand history and harvesting of the plots is given
by Nilsen (1988).
Assessments
Each plot was 20 20 m (400 m2). All trees above
5 cm diameter at breast height (d.b.h.) (defined as
‘large trees’ for the study) were assessed for d.b.h.,
height and crown length. Trees smaller than this
(defined as potential ‘recruits’) were counted and
mapped. For recruits, the leader length was
measured, but where trees were tall (>5 m) this
403
was estimated using binoculars. Tree volumes of
Norway spruce and birch were calculated according to the methods of Vestjordet (1967) and
Braastad (1966), respectively. Site index estimates
before cutting from Nilsen (1988) have been used
in this study. All large trees were examined for butt
rot, recent snow damage to the top of the tree, and
other types of stem damage including double
stems.
All spruce trees above 5 cm in diameter were
cored using standard procedures. The core
samples were marked and stored in plastic tubes
at –20°C until they were measured in the spring
of 2001. Annual growth rings were measured by
means of WinDendro linked to a scanner, and
supported with visual control (Horntvedt et al.,
2000).
Calculations and statistical analysis
Effects on growth and yield associated with the
MFS cutting were examined using a historical
stand reconstruction method. Diameter for the
harvested trees was estimated using the relationship between stump diameter and d.b.h. (Nilsen,
1988):
Diameter at breast height (mm) = 5.6 + 0.73 stump diameter (mm)
R2 0.91, CV 9.4 per cent
A height curve based on tree diameter (cm) was
calculated for estimating tree height:
Tree height (m) exp[–0.17896 + 0.8165ln (d)]
R2 0.84, CV 18.6 per cent
Tree volume of the harvested trees was calculated
according to the method of Vestjordet (1967).
Linear equations were fitted to data using a
regression procedure with stepwise selection (SAS
Institute, 1990) for indicating parameters
influencing growth 25 years after the MFS
cutting. The function was of the form:
Iv = f (stand attributes, felling volume, site
index, altitude, vegetation type).
Stand attributes included number of large trees
(ha–1), basal area (m2 ha–1) and standing volume
(m3 ha–1).
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F O R E S T RY
Results
Stand structure
Twenty-five years after the MFS cutting, the
number of large trees varied from 280 to 830,
with a mean of 520 per hectare, of which 370
were Norway spruce (Table 1). The number of
large trees was 79 per cent of the level prior to
cutting. The mean basal area has increased from
31 per cent post-MFS-cutting, to 54 per cent of
the pre-harvest level in 2000. Similarly, the standing volume has increased from 28 per cent of
post-MFS-cutting, to 52 per cent of the pre-harvesting level in 2000 (Table 1).
The coefficient of variation for the standing
volume was 24 per cent prior to cutting, 60 per
cent after the cutting, and in 2000 it had
decreased to 39 per cent. Observations of the
leader growth of the remaining trees indicated
that vigour was satisfactory. Only 1 per cent of
the large spruce trees had butt rot at breast height.
Visible snow breakage occurred in 7.1 per cent of
the spruce trees as top damage (double leader or
top breakage) and 7.2 per cent had old stem
damage (double stems or stem breakage).
The site index, basal area and the standing
volume were generally low, reflecting the harsh
climatic conditions and earlier human influence
(Table 2).
The highest standing volume was 136.6 and
the lowest 34.9 m3 ha–1. In 2000 the birch component (percentage of basal area) varied from 1
to 44 per cent with a mean of 18 per cent. The
pre-harvesting proportion of birch was ~6 per
cent (Table 1).
Diameter and age distribution
The distribution of the tree sizes 25 years after the
MFS cutting showed a maximum value at ~18 cm
d.b.h. (Figure 2). The mean tree diameter of
spruce varied between 15.3 and 27.5 cm for the
16 stands and the number of trees above 30 cm
d.b.h. was on average 38 per hectare (Figure 2).
The maximum diameter of spruce in any of the
stands was 45 cm.
The distribution of age-classes for corable
spruce trees above 5 cm d.b.h. was relatively
even. The ages present varied from 10 to
197 years, with the majority (71 per cent) in the
Table 1: Changes in stand parameters 1974/75 to 2000
Stand parameters
No. of trees (ha–1)
Basal area (m2 ha–1)
Standing volume (m3 ha–1)
Proportion of birch (% of BA)
Recruits of spruce (trees ha–1)
Before MFS
After MFS
2000
670 (18)
22.3 (22)
140.2 (24)
6.0 (66)
450 (95)
375 (37)
7.0 (97)
38.9 (60)
12.5 (53)
675 (84)
520 (28)
12.1 ( 35)
72.4 (39)
18.1 (80)
710 (77)
Mean values (n = 16 plots), coefficient of variation (SD as percentage of mean) in brackets. The MFS cutting
was performed in 1974 and 1975. Number of spruce recruits before and after MFS cutting refers to counting
and mapping in 1983 and 1984.
Table 2: Stand parameters and their range, 25 or 26 years after the MFS cutting
Stand properties
Min.
Max.
Mean
SD
Site index, H40 (m)
No. of trees (ha–1)
Basal area (m2 ha–1)
Standing volume (m3 ha–1)
Proportion of birch (% of BA)
Recruits of spruce (trees ha–1)
5.5
280
6.5
34.9
1.2
200
8.0
830
19.7
136.6
44.1
1530
6.7
520
12.1
72.4
18.1
710
0.6
146
4.2
33.2
14.5
547
n = 16 plots.
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GROWTH AFTER SELECTIVE CUTTING
405
No. ha –1
150
100
89
81
59
58
50
48
38
0
7.5
12.5
17.5
22.5
27.5
>30
Midpoint diameter (cm)
Figure 2. Distribution of large spruce trees in different size-classes (5 cm) in 2000, ~25 years after the MFS
cutting. Mean values are given with standard deviation bars (n 16).
Age (years)
range 41–140 years. About 8 per cent of trees
were older than 140 years and there were no
exceptionally old trees (>200 years) present
(Table 3). In a study from the same area, Nilsen
(1988) found that recruits took about 50 years
from seed to breast height. This means that total
age could be calculated as breast height age + ~50
years, and that most of the trees harvested in
1974/75 were 200–250 years old.
Analysis of increment cores and tree data
showed that the relationship between tree diameter and age in breast height measurement was
highly significant (P < 0.01) but only explained 33
per cent of the variation (Figure 3). Hence, the
predictive value of the equation is limited by the
large residual variation, and as such cannot be
relied upon as a method of estimating the age of
standing trees in similar areas.
200
180
160
140
120
100
80
60
40
20
0
y = 4.87x
R2 = 0.33
0
10
20
30
40
50
Diameter at breast height (cm)
Figure 3. Plot of diameter at breast height (d.b.h.)
versus tree age. The regression line is indicated.
0.32 m3 ha–1 a–1, ranging from 0.43 to
3.24 m3 ha–1 a–1 (see Table 4).
Scatter diagrams (Figure 4) show the volume
increment plotted against standing volume after
cutting (V3), felling volume (V2), site index (H40)
and basal area (BA). Only site index showed a
significant and positive relationship with volume
increment (r 0.518, P < 0.05). To explore this
further, a multiple regression analysis of Ivol on
Basal area- and volume-increment
From the MFS cutting up to the year 2000, the
mean annual basal area increment was
0.2 m2 ha–1 a–1,
ranging
from
0.06
to
0.4 m2 ha–1 a–1. Correspondingly, the mean
annual
volume
increment
(Iv)
was
Table 3: Age distribution for large Norway spruce trees
Age class
(years)
%
n = 209 trees.
0–20
21–40
41–60
61–80
14
7
9
12
81–100 101–120 121–140 141–160 161–180
18
18
14
4
3
181+
<1
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F O R E S T RY
Table 4: Basal area and volume increment during the period from the MFS cutting in 1974/75 up to 2000
Increment
Min.
Max.
Mean
SD
Basal area (m2 ha–1 a–1)
Volume (m3 ha–1 a–1)
0.05
0.43
0.40
3.24
0.20
1.32
0.10
0.91
n = 16 plots.
4.0
IV (m 3 ha–1 a–1)
–1
IV (m 3 ha–1 a )
4.0
3.0
2.0
1.0
0.0
0
50
100
3
3.0
2.0
1.0
0.0
0
150
5
IV (m 3 ha–1 a–1)
4.0
3.0
2.0
1.0
0.0
5
6
7
10
15
20
150
200
BA (m 2 ha–1 )
–1
V3 (m ha )
IV (m3 ha–1 a–1)
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8
9
4.0
3.0
2.0
1.0
0.0
0
50
H 40 (m)
100
V2 (m3 ha–1 )
Figure 4. Scatter plots of mean annual volume increment versus standing volume after cutting (V3), basal
area (BA) after cutting, site index (H40) and harvested volume (V2).
combinations of stand variables would describe
the relationship more precisely (Table 5). Of
several candidate variables selected to explain
volume increment, only a model of mean annual
increment with site index and felling volume was
significant at the 5 per cent level. The intercept
and V2 were significant at the 10 per cent level,
and H40 at the 5 per cent level. The coefficient of
variation for the model was 59 per cent, with a
large residual variation present in the data.
Volume increment estimated with the model
(Table 5) considering different harvesting
strategies indicates that very heavy interventions
cause a decline in stocking, while low and intermediate interventions cause its accumulation over
time (Figure 5). However, such a simple model
assumes a stocking before MFS cutting of
100–200 m3 ha–1.
Table 5: Multiple regression of the dependent variable IV on combinations of the independent variables site
index (H40), standing volume before cutting (V1), felling volume (V2)
Independent variables
(x1, x2)
H40; V2
H40; V1; V2
Equation
Sample F values
R2
CV (%)
y = –3.64 + 0.88 1 –0.01 2
3.88
2.54
0.28
59.0
Only significant equations are given.
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Recruitment
3
The recruitment situation 8–9 years after the MFS
cutting in the mid 1970s was satisfactory concerning the advance growth of spruce and despite
irregular spacing (Nilsen, 1988). Before MFS
cutting, the mean number of recruits was 450
trees ha–1 (CV 24 per cent), and this then
increased by 225 trees ha–1 (CV 49 per cent) in
1983/84. The mean number of recruits has now
increased to 710 ha–1, only 5 per cent higher than
in 1983/84 (Table 1). There is a tendency towards
a slight increase of seedlings in the blueberry
vegetation type, whereas only small changes have
taken place in the other ground vegetation types
(Table 6).
The occurrence of seedlings, saplings and
suckers is essential to maintain yield in the long
term. Cutting strength plotted against number of
small spruces and birches is shown in Figure 7
(but this does not distinguish between birch
regenerated from seed and coppice shoots). For
spruce, about 2 per cent of the recruits had rooted
from low branches. The effect of cutting strength
on birch regeneration was significant (r2 0.57,
P < 0.05) and suggests that higher cutting
2
1
0
50
100
150
3
200
–1
Cutting volume (m ha )
Figure 5. The effect of various MFS cutting volumes
on the mean volume increment for maximum
(H40 8.0, filled symbols) and minimum
(H40 5.5, open symbols) site index in this study.
The response after MFS cutting for different
size-classes is indicated (Figure 6). The small and
medium sized trees had the largest proportional
increase in diameter, and diameter increment is
still above the pre-MFS-cutting level. The larger
trees responded to the cutting, but diameter increment in 2000 is now similar to the pre-MFScutting level.
D.Cl. 12.5
4
D.Cl. 22.5
3
ID (mm)
407
4
D.Cl. 32.5
2
1
0
No. of recruits (ha –1)
I Vol (m3 ha–1 a–1)
GROWTH AFTER SELECTIVE CUTTING
6000
5000
4000
3000
2000
1000
0
40
50
1965 1970 1975 1980 1985 1990 1995 2000
60
70
80
90
100
Cutting strength (%)
Year
Figure 6. Development of diameter increment (ID)
versus time for three different size classes (5 cm). The
MFS cuttings were performed in 1974 and 1975.
Figure 7. Cutting strength (percentage of volume)
versus number of spruce (filled symbols) and birch
(open symbols) recruits. The regression line for birch
is indicated.
Table 6: Development of Norway spruce recruitment (no. ha–1) on different vegetation types
Vegetation type
Tall herb type (n = 4)
Small fern type (n = 5)
Bilberry type (n = 7)
1983/84 (no. ha–1)
1110 (55)
720 (104)
400 (38)
2000 (no. ha–1)
Differences (%)
1130 (85)
680 (53)
490 (35)
+2
–6
+23
Coefficient of variation (%) in parentheses. Values from 1983/84 refer to Nilsen (1988).
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F O R E S T RY
strengths are related to a larger number of birch
recruits; however, for spruce, the relationship is
less clear. A large proportion of the birch was
browsed by elk but for many reasons birch is not
suitable for timber production in these areas.
Discussion
The stocking density of the remaining trees is
generally low and irregular, which is likely to be
a result of past exploitation and growth conditions. The spatial distribution of trees now
comprises groups of juvenile spruce trees in gaps
or thickets of birch, and dense groups of mediumsized trees interspersed with larger individuals.
Varied growth characteristics and competition
between trees have resulted in a slightly weak
age-diameter relationship. So far, Norwegian
stand growth models designed for mountain
spruce forest seem to underestimate the potential
growth reaction after MFS cutting (Nilsen, 2001).
The present study supports such a result.
However, the conclusion is based on stands with
a favourable situation for natural regeneration.
The growth reaction 25 years after MFS cutting
has been substantial, in spite of a felling volume
of 72 per cent of the standing volume. The
average volume increment was 1.32 m3 ha–1 a–1
(CV 69 per cent) or 3.5 per cent (pV) of the
standing volume immediately after the MFS
cutting. The corresponding mean yield class
(even-aged system) is ~1.5 m3 ha–1 a–1 with a rotation period of 140 years (Tveite and Braastad,
1981). The lowest volume increment occurred in
dense stands with small felling volumes and low
site indices. Only a weak relationship could be
found between the actual stand volume increment, cutting strength and standing volume. Plot
sizes of 0.04 ha are small for growth studies and
this is probably responsible for a considerable
amount of the unexplained variation. Generally,
on low site indices, large harvesting volumes will
reduce the mean annual increment. The regression model (see Table 2) indicated that a mean
cutting strength on an average site index of 6.7 m
and similar stocking densities should be
~70 m3 ha–1 (~50 per cent of standing volume)
with a target of 1.5 m3 ha–1 a–1. If the harvesting
volume increases to 120 m3 ha–1 the mean annual
increment will decrease to ~1.1 m3 ha–1 a–1. In
old-growth, high-altitude irregular spruce stands
in northern Sweden, Näslund (1942) reported the
mean loss of increment to be ~25 per cent in a
cutting strength of 66 per cent of volume. The
highest increment loss was observed on the
richest sites.
Stem volume increment of even-aged forest
stands has for a long time been considered more
or less independent of differences in stocking
density (Langsæter, 1941; Møller, 1954;
Assmann, 1961). This study supports the theory
that the stem volume increment tends to remain
constant within a wide range of densities. The
original ‘plateau-theory’ of Møller proposes that
the stand volume increment is constant down to
~50 per cent of the maximum basal area. A
relative basal area of 30 per cent corresponds to
~80 per cent of maximum volume increment.
According to growth models for semi-irregular
stands on similar site indices, the maximum basal
area is 20–30 m2 ha–1 (Braastad, 1983). In our
study the mean stocking level was reduced to
7–10 m2 ha–1 and still allowed a volume increment of 1–3 m3 ha–1 a–1 (see Figure 3). However,
heavy cuttings can lead to grass invasion and
problems for seedling establishment, especially in
the blueberry-type vegetation. In high-elevation
sites with cold, wet soils and restricted root
development, endemic windthrow after heavy
thinning in dense stands might occur (Näslund,
1942; Brantseg, 1962). The risk of severe crown
deterioration, particularly through wind and
snow damage, should also be addressed.
The length of the cutting cycle also has economic implications. Calculations indicate that to
avoid a felling cycle >50 years, the cutting
strength should not be extended to more than ~65
per cent of standing volume in a single intervention. These figures assume a target standing
volume equal to the pre-harvesting volume,
similar rate of increment in the next growth
period, and that the favourable conditions for
natural regeneration of spruce are prolonged.
There are few other comparable Norwegian
studies covering growth effects of a selection
forest system in mountain areas. Böhmer (1922,
1957) reported growth and structural changes in
five sites situated above 650 m in southeast
Norway. The mean standing volume of these
plots, which were intensively managed over many
decades, was ~100 m3 ha–1 with an increment rate
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GROWTH AFTER SELECTIVE CUTTING
(pV) of 2.8 per cent. Andreassen (1994) reported
growth and stand structure in two selection
forests in mountain areas with standing volumes
of 119 and 170 m3 ha–1 and increment rates from
2.8 to 1.9 per cent. Both stands have been regularly cut, with short felling cycles (10–20 years).
The growth effect of such low felling volumes are
not really comparable with the harvesting in the
present study.
Providing there is no deterioration in the
condition of the trees due to climatic events or as
a result of mismanagement, it is likely that the
pre-alpine irregular spruce forest will respond
positively after mountain selective cutting. The
irregular stands are robust and the ingrowth of
spruce and birch show promising development.
However, there is great uncertainty about recruitment and ingrowth to large diameter classes and
hence the possibilities for future production and
repeated selective cuttings. Supplementary studies
in a broader range of areas are recommended,
together with the development of flexible single
tree and stand models for selective cutting
regimes.
Acknowledgements
The authors would like to thank Roald Brean and
Sigbjørn Øen for their assistance in carrying out this
work. We also acknowledge the assistance from Tor
Myking and two anonymous referees. The project has
been funded by the Norwegian Research Council (No.
124100/140).
References
Andreassen, K. 1994 Development and yield in selection forest. Communications of Norwegian Forest
Research Institute 47(5), 1–37.
Assmann, E. 1961 Waldertragskunde. Bayerisher Landwirtschaftsverlag, München, 490 pp. [In German]
Bergan, J. and Skoklefald, S. 1996 Rydding av ikke drivverdig virke før tilplanting med norsk gran (Picea
abies) etter foryngelseshogster i høyereliggende skog.
Aktuelt Skogforsk 5(96), 1–10. [In Norwegian]
Braastad, H. 1966 Volume tables for birch. Commun.
Norw. For. Res. Inst. 21, 23–78.
Braastad, H. 1983 Yield level in Picea abies stands with
low initial density and irregular spacing. Norw. For.
Res. Inst. 7(83), 1–42. [English summary]
Brantseg, A. 1962 Skogbestandets pleie. In:
409
Skogskjøtsel. O. Børset(ed.). Skogbruksboka, Bind II.
Skogforlaget A/S, Oslo, pp. 355–384. [In Norwegian]
Burschel, P., El Kateb, H. and Mosandl, R. 1992 Experiments in mixed mountain forests in Bavaria. In: The
Ecology and Silviculture of Mixed Species Forest.
M.J. Kelty, B.C. Larson and C.D. Oliver (eds).
Kluwer Academic, Dordrecht, pp. 183–215.
Böhmer, J.G. 1922 Bledningsskog. Tidsskr. Skogbruk
30, 117–163, 196–269. [In Norwegian, French
summary]
Böhmer, J.G. 1957 Bledningsskog II. Tidsskr. Skogbruk
65, 204–247. [In Norwegian, French summary]
Børset, O. 1994 Norsk fjellskogskjøtsel – et tilbakeblikk og en oppsummering. Aktuelt Skogforsk 1(94),
1–12. [In Norwegian]
Groot, A. 1997 Uneven-aged silviculture for peatland
Black spruce. In: The IUFRO Interdisciplinary
Uneven-aged Management Symposium. W.H.
Emmingham (ed.). Corvallis, OR, pp. 107–120.
Horntvedt, R., Rolstad, J., Storaunet, K.O. and Tveite,
B. 2000 Årringanalyser – anvendelser og metoder.
Annual Report from the Norwegian Forest Research
Institute, pp. 24–27. [In Norwegian, English
summary]
Kielland-Lund, J. 1981a Hva er fjellskog? Tidssk. Skogbruk 88, 1–16. [In Norwegian]
Kielland-Lund, J. 1981b Die Waldgesellschaften SONorwegens. Phytocoenologia 9, 53–250.
Langsæter, A. 1941 Om tynning i enaldret gran- og
furuskog. Commun. Norw. For. Res. Inst. 13,
131–216. [In Norwegian, German summary]
Lädhe, E., Laiho, O. and Norokorpi, Y. 2001 Structure
transformation and volume increment in Norway
spruce dominated forests following contrasting silvicultural treatments. For. Ecol. Manage. 151,
133–138.
Lundqvist, L. 1993 Changes in the stand structure on
permanent Picea abies plots managed with single tree
selection. Scand. J. For. Res. 8, 510–517.
Møller, C.M. 1954 The influence of thinning on volume
increment. Thinning problems and practices in
Denmark. Stat. Univ. N.Y. Coll. For. Tech. Publ. 76,
5–44.
Nilsen, P. 1984 Fjellskoghogst – en vurdering av hogstmåten i granskog. Tidssk. Skogbruk 91, 22–29. [In
Norwegian]
Nilsen, P. 1988 Selective cutting in mountain spruce
forest – regeneration and production after earlier cuttings. Norw. For. Res. Inst. 2(88), 1–26. [English
summary]
Nilsen, P. 2001 Modellering av fjellskoghogst. Aktuelt
Skogforskningen 3/01, 4–5. [In Norwegian]
Nilsen, P. and Solberg, B. 1998 Mountainous forests in
Norway. Ecological, economic and policy aspects.
Univ. Bodenkultur Wien Publ. Ser. 35, 223–228.
10 Øyen (jr/d)
25/10/02
410
9:13 am
Page 410
F O R E S T RY
Näslund, M. 1942 Den gamla granskogens reaktionsförmåga etter genomhuggning. Medd. Statens Skogsförsöksanstalt 33, 1–212. [In Swedish, German
summary]
Ohlson, M. and Tryterud, E. 1999 Long term spruce
continuity – a challenge for a sustainable Scandinavian forestry. For. Ecol. Manage. 124, 27–34.
SAS Institute 1990 SAS. Procedures Guide. Version 6,
3rd edn. Cary, NC, USA, 705 pp.
Schütz, J.-P. 1997 The Swiss experience: more than one
hundred years of experience with single-tree selection
management system in mountainous mixed forests of
spruce, fir and beech. In: The IUFRO Interdisciplinary Uneven-aged Management Symposium.
W.H. Emmingham (ed.). Corvallis, Oregon, pp.
21–34.
Suadicani, K. and Fjeld, D. 2001 Single-tree and group
selection in montane Norway spruce stands: factors
influencing operational efficiency. Scand. J. For. Res.
16, 79–87.
Tveite, B. and Braastad, H. 1981 Bonitering av gran,
furu og bjørk. Norsk Skogbruk 27(4), 17–22. [In
Norwegian]
Valtanen, J. 1988 Stand reforestation at elevated sites in
Northern Finland. Folia For. 718, 1–41.
Vestjordet, E. 1967 Functions and tables for volume of
standing trees. Commun. Norw. For. Res. Inst. 22,
543–574.
Wing, M.R. 1977 Silvicultural systems – uneven-aged
management. USDA, Forest Service. General Technical Report NE 29, 67–72.