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Tree species mixtures - A common feature of southern Swedish forests


Abstract and Figures

The proportions of forests in southern Sweden with the most common single or mixed species compositions were determined using the data provided by the Swedish National Forest Inventory. Forests including a second tree species with a basal area of at least 10 per cent, in addition to the most abundant species, were defined as mixed forests. The most common compositions were spruce, pine/spruce, pine and spruce/birch. Overall, mixed forest was more common than single-species forest, and approximately two-thirds of both spruce-dominated and pine-dominated forests included a sufficient proportion of other tree species to be classified as mixed forests, according to the above definition. Differences in composition related to age, site and ownership classes were analysed. Hypotheses that pine/birch mixtures only occur in early-successional stages and that spruce/birch mixtures occur mainly on wet sites were rejected. In fact, 90 per cent of spruce/birch mixtures were found on mesic and moist-mesic sites. The hypothesis that pine/spruce mixtures are more frequent in private forest was also rejected. In conclusion, the study found high proportions of mixed forest across age and site classes and provides background information for assessing the potential to refine silvicultural methods applicable, e.g., to young spruce/birch or very old pine/spruce stands.
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© Institute of Chartered Foresters, 2010. All rights reserved. Forestry, Vol. 83, No. 4, 2010. doi:10.1093/forestry/cpq025
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In southern Sweden, there is a natural convergence zone
between boreal and nemoral forests (Ahti et al., 1968;
Bohn and Weber, 2000). While nemoral broadleaved spe-
cies are more competitive in the southernmost part, boreal
species have an advantage in the north. Due to differential
growth dynamics, tree species can intermingle in boreal
and nemoral forests following natural or anthropogenic
disturbances, leading to site-specific species mixtures and
compositions (Nilsson et al., 2002; Bolte et al., 2009). Fur-
thermore, climate change and associated shifts in range
boundaries can lead to species compositions changing at
particular sites over extended periods of time (Lindbladh
et al., 2000).
The southern part of Sweden is called Götaland (Figure 1)
that comprises 5 million ha of forest. Here, the current pro-
portion of mixed forest amounts to 20.5 per cent – defined
by the Swedish National Forest Inventory (NFI) as forest
with a minimum of two tree species, the least abundant of
which accounts for 30 per cent of the total basal area.
Other forest types are 39.1 per cent spruce, 24.2 per cent
pine, 4.4 per cent valuable broadleaves, 8.5per cent other
broadleaves and 3.3 per cent bare land (NFI, 2007). Broad-
leaf forests can also contain a mixture of other tree species
in high proportions and here we show that even the pine
or spruce forests may contain substantial proportions of
other tree species.
In general, the percentage of tree species in mixed for-
ests strongly depends on site characteristics and patterns
of disturbance, including those caused by forest manage-
ment. There is clearly potential for pioneer tree species to
establish after final clear felling in Swedish forests. Thus, in
young age classes, there is a high proportion (40 per cent)
of mixed forests, in which a second tree species accounts
for 25 per cent of the total basal area (Skogsdata, 2002).
After long periods without major disturbances, shade-
tolerant species become increasingly dominant, and in the
last ~200 years a major disturbance factor (fire) has been
heavily suppressed in hemiboreal forests, which has com-
plicated attempts to elucidate natural dynamics (Niklasson
and Drakenberg, 2001). Important reference sites in this
respect include pristine boreal forests (Leemans, 1991;
Shorohova et al., 2009), hemiboreal forest reserves such
as Norra Kvill (Niklasson and Drakenberg, 2001) and re-
serves in the transition zone to temperate forests such as
Siggaboda and Biskopstorp (Björkman, 1996; Lindbladh
et al., 2008). All of these forests comprise a mixture of
tree species, although spruce tends to be strongly dominant
in boreal reserves at higher latitudes (Fraver et al., 2008;
Shorohova et al., 2008). In Central Europe, the proportion
of beech generally increases over time in forest reserves
(Meyer et al., 2000). In Götaland (where the three major
types of natural vegetation are hemiboreal mixed forest,
temperate mixed broad-leaved forest and temperate beech/
mixed beech forest; Hickler et al., 2009), the processes
Tree species mixtures – a common feature
of southern Swedish forests
Southern Swedish Forest Research Centre, Swedish Agricultural University, PO Box 49, SE-23053 Alnarp, Sweden
The proportions of forests in southern Sweden with the most common single or mixed species compositions were
determined using the data provided by the Swedish National Forest Inventory. Forests including a second tree species
with a basal area of at least 10 per cent, in addition to the most abundant species, were defined as mixed forests. The
most common compositions were spruce, pine/spruce, pine and spruce/birch. Overall, mixed forest was more common
than single-species forest, and approximately two-thirds of both spruce-dominated and pine-dominated forests
included a sufficient proportion of other tree species to be classified as mixed forests, according to the above definition.
Differences in composition related to age, site and ownership classes were analysed. Hypotheses that pine/birch
mixtures only occur in early-successional stages and that spruce/birch mixtures occur mainly on wet sites were rejected.
In fact, 90 per cent of spruce/birch mixtures were found on mesic and moist-mesic sites. The hypothesis that pine/
spruce mixtures are more frequent in private forest was also rejected. In conclusion, the study found high proportions
of mixed forest across age and site classes and provides background information for assessing the potential to refine
silvicultural methods applicable, e.g., to young spruce/birch or very old pine/spruce stands.
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involved in the natural transition of existing forests into
pure stands of spruce or beech remain speculative since
there is too little knowledge of the effects of disturbance by
wind and fire in the region.
Regardless of this ecological background, mixed species
stands are generally considered to have a lower risk pro-
file than pure stands (Olsthoorn et al., 1999; Röhrig et al.,
2006). However, this general rule has to be scrutinized on
a species-specific basis. The combination of species with
different ecological demands can be an advantage in case
of extreme climatic events. For example, in a drought year,
an admixed drought-tolerant species may be able to com-
pensate for incremental losses suffered by species intolerant
of water stress. Similarly, in the event of an outbreak of
a monophagous insect, which might defoliate one par-
ticular tree species, the presence of a second tree species,
not susceptible to attack, may balance any losses (Scherer-
Lorenzen et al., 2005). Thus, combining tree species with
different tolerances and susceptibilities to ecological and
environmental stresses may be an effective silvicultural
strategy. However, stands of mixed species can have two
important disadvantages: timber productivity can be lower
compared with pure stands of the species with the highest
increment (Agestam, 1985; Kelty, 1992; Pretzsch, 2003;
Vilà et al., 2003) and more effort may be required to pro-
mote the growth of less competitive species (e.g. Nichols
et al., 2006).
The stability of mixed species stands and the flexibility
in relation to the timber market is also considered to be
advantageous by many forest owners. Three-quarters of
forests in southern Sweden are owned by 98000 private
individuals (Statistical Yearbook, 2008). This broad own-
ership is naturally reflected in diverse management objec-
tives and a wide range of specific preferences in relation to
general goals such as sustainable timber production, bio-
logical diversity and aesthetics (Hugosson and Ingemars-
son, 2004). Forest owners usually define their management
goals as promoting stand stability, minimizing investment
costs and maximizing profit. The choice of tree species is
integral to the decision processes involved in attempts to
achieve these goals, except at sites that are already limited
to a single species.
In the present study, a lower threshold for defining
mixed forest was chosen compared with that used in previ-
ous studies (e.g. NFI, 2007), because although the common
definition of mixed forest (forest in which a second tree
species accounts for at least 30 per cent of the basal area)
provides very important information for routine manage-
ment and planning, it may mask options to promote ad-
mixed tree species. Thus, an alternative definition of mixed
forest is also applied here; forest in which the second tree
species accounts for at least 10 per cent of the basal area.
At this level, it is possible to promote the admixture of a
reasonably low proportion of other tree species for ecologi-
cal reasons, to spread risks or to meet criteria of certifica-
tion schemes. Furthermore, at this level we are also better
able to assess the response and resilience of forest produc-
tivity following disturbances to which the different species
have varying susceptibility.
The primary aim of the study presented here was to
determine the proportions of the main single-species and
mixed tree species types of forest in Götaland by age, site
and ownership classes. At the outset, we hypothesized that
(1) forests with mixed tree species are more common than
forests with a single tree species, (2) pine–birch mixtures
are limited to early-successional stages of stand develop-
ment, (3) spruce–birch mixtures mainly grow on wet sites
and (4) the proportion of pine–spruce mixtures is higher in
forests owned by private individuals than in forests with
other owners. The second and third hypotheses were con-
sidered because such assumptions may limit the develop-
ment of appropriate silvicultural strategies for these forest
types. The last hypothesis is based on two ideas: first, that
old pine forests with well-developed spruce undergrowth
are very adequate to form multi-layered stands and sec-
ondly that multiple stand layers are more favoured by pri-
vate forest owners than by forest companies.
Material and methods
Data description
The data analysed were obtained from two inventory
cycles of the Swedish NFI in the period 1998–2007, in
which different sample plots were examined, to increase
the representativeness of minor tree species compositions.
In each case, the plots were temporary, undivided, circular
(154 m2) and located on forest land in the Götaland coun-
ties of Skåne, Blekinge, Halland, Kronoberg, Jönköping,
Kalmar, Östergötland, Västra Götaland and Gotland
(Figure 1). Details of the NFI design and protocols are avail-
able on the Internet ( and have been
Figure 1. Map of southern Sweden showing borders of the
counties of Götaland in light grey.
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described by Ranneby et al. (1987). In total, data from
8200 plots along 2425 tracts (each of which originally con-
sisted of a rectangular trail in the field, along which eight
sample plots were situated) were used here.
Within each plot, tree species and the diameter at breast
height (d.b.h.) of trees 10 cm were recorded in the inven-
tory, and smaller trees with d.b.h. <10 but 4 cm were
callipered on a smaller subplot within a 5-m radius of the
same centre. Still smaller trees with d.b.h. <4 but 0.1 cm
were counted on two 5-m2 plots within the larger plot.
All three size classes were used to calculate stem number
and basal area. Further data relevant to the present study
were also recorded by the NFI, in particular, stand age, soil
moisture class (dry, mesic, moist-mesic, moist or wet) and
ownership (private, forest company, state, church, others).
Three definitions of mixed forests (in the sample plots)
are applied in this paper: (1) forests in which each of the
species account for at least 30 per cent of the basal area
(the common definition, e.g. NFI, 2007); (2) forest in
which each of the species account for at least 10 per cent
of the basal area (the alternative definition and the main
focus of the present paper) and (3) forest in which more
than one tree species were present in any proportions (the
strict definition).
If the mean height of the forest was <7 m, then stem
number percentages were used instead of basal areas in the
definitions of mixed forest.
The trees registered by the NFI in the sample plots are
classified in this study in the 12 groups listed in Table 1
(eight consisting of frequent single or congeneric species,
two consisting of infrequent species with similar growth
patterns and two consisting of other conifers’ and ‘other
Statistical tests
An approximation of Hotelling’s T2 test (Winer, 1962) was
used to determine, for the most frequent species composi-
tions, significant differences in the proportions of old and
young forest, the soil moisture class distribution and the
ownership distribution. Due to the sampling design of the
NFI (which includes several plots per tract, thus the plots
along each tract are not independent), each tract is consid-
ered as an independent sample. For that reason, according
to Wald (1943, cited in Agresti, 1990), the Hotelling test
was applied as generalization of the Student’s t test that al-
lows an observed distribution (e.g. proportions of different
site classes in a particular forest type) to be compared with
an expected distribution (e.g. proportions of different site
classes in the total forest area).
In a first step, the average percentage of a particular
forest type in each site and ownership class was calculated
(equation 1):
p p
= (1)
In the next step, the difference between the average and
expected values (=percentage of the total forest area) was
calculated (equation 2):
j j
z p p
= −
Since the variances and covariances of zj are needed,
these were obtained from the
values using equation (3)
(j = k for calculating variances of zj):
1 1
21 1
1 1
ˆ ˆ ˆ ˆ
ˆ ˆ ˆ ˆ ˆ ˆ
( , ) ( , ) ( , ) ( , )
1ˆˆ ˆ
( , ).
m m
j k j k j r k r
r r
m m
r s
r s
C z z C p p C p p C p p
m m
C p p
= =
= =
= −
∑ ∑
( , )
j k
C z z
for j, k = 1, . . . m 1 yields a matrix of the
order (m 1)and this is inverted. The elements in this
inverted matrix are
. Finally, the T-value was calculated
from equation (4):
1 1 1
1 1
m m
j k jk
j k
T z z s
= ⋅ ⋅
If T exceeded the 95th percentile of the χ2 distribution
with df = m 1, the null hypothesis was rejected.
To compare the observed frequencies of young and old
forest with a hypothetical, even distribution of young and
old forest, the same procedure was applied, but only two
age classes j = 1,2 (1 = young, 2 = old) were used. The test
was defined as
1 2
1 2
ˆ ˆ
( ) ,
ˆˆ ˆ
( )
p p
V p p
1 2 1 2 1 2
ˆ ˆ ˆ
ˆ ˆ ˆ ˆ ˆ ˆ
( ) ( ) ( ) 2 ( , ).
V p p V p V p C p p
= + − ⋅
Usually, different forest types are subjected to specific
rotation periods; in the present study, however, the pro-
portion of forest in the age class 10–49 years was com-
pared with that in the age class 50–89 years, irrespective
of forest type. The youngest age class (from 0 to 9 years)
was excluded from the analysis because the proportion of
Table 1: Classification of tree species recorded by the Swedish
Tree species group Tree species
Pine Pinus sylvestris
Spruce Picea abies
Other conifers Larix spp., Abies spp., Picea spp. except
P. abies, Pinus spp. except P. sylvestris
and other conifers
Birch Betula spp.
Aspen Populus spp.
Oak Quercus petrea, Q. robur
Beech Fagus sylvatica
Noble broadleaves Fraxinus excelsior, Acer spp., Ulmus spp.,
Prunus avium
Lime and hornbeam Tilia spp., Carpinus betulus
Alder Alnus spp.
Rowan Sorbus aucuparia
Other broadleaves Salix spp., Sorbus intermedia and other
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this class could be artificially high in the hypothetical dis-
tribution since the period between harvesting and planting
could exceed 1 year.
Occurrence and proportion of tree species combinations
As stated above, in Sweden, mixed forest is usually defined
by the presence of a second tree species accounting for at
least 30 per cent of the basal area. According to this defini-
tion, 30 per cent of Götaland’s forest could be classified as
mixed. Using the alternative definition of mixed forest, i.e.
forests containing a second species at a minimum 10 per
cent of basal area, the overall percentage of mixed forest
in Götaland increases to 59 per cent, and for forests with
pine and spruce, the proportion increases from 35 to 70
per cent and from 33 to 68 per cent, respectively.
Using the alternative definition for mixed forest, forests
with 193 combinations of tree species were found. Table 2
lists the 28 most frequent combinations, each covering
more than 10000 ha. Together, these 28 combinations
represented 4.4 million ha of the total 5 million ha of for-
est in Götaland. The remaining 165 combinations col-
lectively cover ~320000 ha of the total area. For most of
these combinations, the data were not suitable for deriving
quantitative descriptions, since the standard error (SE) of
the estimated area of the 165 tree species combinations not
shown in Table 2 was >25 per cent. The combination of
spruce/birch/other broadleaves had a SE of 24.9 per cent,
but covered <10000 ha. Bare land covered 245000 ha.
The most common type of forest, covering 20 per cent
of Götaland’s total forest area, was pure spruce. This type,
together with single-species pine and mixed spruce/pine
covered 52 per cent, and birch plus combinations of birch
and conifers covered 26 per cent of the area.
Beech, oak and oak/beech mixtures (excluding other tree
species) covered just 3 per cent of the area. If the third or
strict definition of mixed forest is applied, i.e. forests in
which any other species are present at any proportion, oak
occurred on 14 per cent and beech on 4 per cent of the
sample plots.
When applying the minimum 10 per cent basal area defi-
nition of a mixed forest, pine was found on 46 per cent
of the forest area, spruce on 62 per cent and birch on 33
per cent (Table 3). However, these percentages cannot be
treated as additive since otherwise, due to multiple over-
lapping of tree species, the total forest area sums to 166
per cent. Applying the common definition with a minimum
30 per cent basal area covered by a second species, spruce
(pure and mixed) occurred on 50 per cent of the forest
area, while with the strict definition, that percentage in-
creased to 73 per cent.
According to the strict definition of mixed forest, plots
with more than one tree species represented 78 per cent of
the stocked forest area, leaving only 22 per cent as single-
species stands in the strictest sense.
Table 2: Estimated proportions of tree species or combinations
of species (each accounting for at least 10% of the total basal
area) covering >10 000 ha of forest in Götaland
Tree species
combination Area (ha)
(%) SE (ha)
of plots
Spruce 1012140 20.44 25280 1676
Pine/spruce 895587 18.09 23277 1483
Pine 669728 13.52 23726 1109
Spruce/birch 513317 10.37 18091 850
Pine/spruce/birch 310406 6.27 14317 514
Pine/birch 239749 4.84 12186 397
Birch 207742 4.20 11211 344
Beech 57975 1.17 7447 96
Oak 49520 1.00 5966 82
Spruce/aspen 41669 0.84 5022 69
Spruce/oak 39254 0.79 5019 65
Spruce/birch/alder 35026 0.71 4680 58
Alder 31403 0.63 4652 52
Birch/alder 30195 0.61 4228 50
Spruce/birch/aspen 29591 0.60 4268 49
Spruce/alder 27780 0.56 4397 46
Spruce/birch/oak 21740 0.44 3889 36
Pine/oak 20533 0.41 3605 34
Birch/aspen 18721 0.38 3451 31
Birch/oak 18117 0.37 3294 32
Aspen 16305 0.33 3240 27
Pine/spruce/aspen 15701 0.32 3060 26
15701 0.32 3507 26
Oak/beech 15701 0.32 3804 26
Pine/spruce/oak 15098 0.30 3005 25
Birch/rowan 13890 0.28 2883 23
Spruce/birch/rowan 11474 0.23 2625 19
Pine/birch/oak 10266 0.21 2481 17
Others 3484 6.51 13662 532
Bare land 245184 4.95 12191 406
Total sample 4952000 100 8200
Table 3: Proportions of tree species groups based on the three
definitions of a mixture applied in this study
Tree species group
of sample
plots where
the species
Percentage of
sample plots
where the
accounts for
10% of
basal area
of sample
plots where
the species
accounts for
30% of
basal area
Pine 49.7 45.8 36.5
Spruce 72.7 62.4 50.0
Other conifers 0.4 0.3 0.2
Birch 52.1 32.8 19.0
Aspen 6.7 4.1 2.1
Oak 13.7 5.5 3.3
Beech 4.4 2.5 2.1
Noble broadleaves 2.9 1.1 0.6
Lime and hornbeam 0.9 0.3 0.2
Alder 5.5 3.7 2.0
Rowan 9.1 1.5 0.5
Other broadleaves 5.1 1.9 0.7
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Seven tree species or species combinations represented
78 per cent of Götaland’s forest (Table 2). These forest
types were selected for the analyses of differences in pro-
portions related to age, soil moisture and ownership of for-
ests, as described below.
Age distribution of the seven most frequent tree species
A large proportion of spruce forest was assigned to the age
classes from 20 to 49 years (Figure 2), and spruce forests
were significantly more frequent in the age classes from
10 to 49 years than in the older age classes (50–89 years)
(Table 6). The pine/spruce combination was most frequent
in the age class from 70 to 99 years, but showed a second
peak in the 30- to 39-year age class (Figure 2). Pine/spruce
was significantly more frequent in 50–89 years old forest
than in younger forest (Table 6). Moreover, 6 per cent of
the total forest area in Götaland, equivalent to 300000 ha,
was of pine/spruce forests older than 100 years (Figure 2).
Pine, spruce/birch, pine/birch and birch were species com-
binations that were significantly more abundant in young
forest than in older forest 50–89 years old. No significant
differences were found among the different age classes for
the species combinations of pine/spruce/birch or pine/birch
(Table 6).
Soil moisture of the seven most frequent tree species
Pine forest occurred more frequently on dry sites, com-
pared with other forest types (Table 4). In total, 91.2 per
cent of the spruce/birch mixtures occurred on mesic and
moist-mesic sites. However, proportionally, spruce/birch
was the most frequent type on moist-mesic sites. Pine/birch
was the most equally distributed forest type across all mois-
ture classes. The frequency distributions of all forest types
among moisture classes differed significantly from the dis-
tribution of moisture classes across the total forest area
(Table 6). A fifth moisture class ‘wet’ is not represented in
Table 4 due to its presence in only marginal amounts.
Ownership of the seven most frequent tree species
In general, differences in the distribution of forest types
among ownership categories were low (Table 5). Statisti-
cally significant differences among the distribution of own-
ership across the total forest area were, however, observed
for spruce, pine, pine/birch and birch (Table 6). Forest com-
panies owned a higher relative proportion of pine than other
forest owners (Table 5). The relative proportion of spruce
owned by private individuals was no lower than average.
Mixed forest and monocultures
The analyses based on different definitions highlight the
widespread occurrence of forest composed of various mix-
tures of tree species and show that use of a lower than
conventional threshold in the definition of a mixed species
stand may facilitate forest management when low propor-
tions of additional tree species need to be considered or are
desired, e.g. spruce with a low proportion of birch (Linden,
With respect to percentages of mixed and pure forest in
Götaland, the present study has provided data that differ
from official forestry statistics. The latter indicate that 39
per cent of Götaland’s forest area is covered with ‘spruce’
stands and that spruce accounts for almost 50 per cent of
the timber volume (Statistical Yearbook, 2008). However,
these statistics can be misinterpreted if the stands are con-
sidered to represent pure monocultures. The present study
reveals a considerable proportion of spruce in mixture with
other tree species on the stand level. According to the ap-
plied definition, only 20 per cent of the forest area is cov-
ered with ‘spruce stands. This may be advantageous, since
mixed species stands are generally regarded as being more
stable than pure stands, and spruce generally grows more
rapidly than other native tree species.
Site characteristics
Tree species compositions of forest stands are substantially
influenced by site characteristics (Ellenberg, 1996; Engelmark
and Hytteborn, 1999). The present study found signifi-
cant differences among the distributions of seven impor-
tant forest types (Table 6). However, large proportions of
each forest type were located on the most typical mesic
and moist-mesic sites: e.g. 91 per cent of spruce/birch mix-
tures occurred on these sites (Table 4). The hypothesis that
spruce/birch mainly grows on wet sites was therefore re-
jected since this forest type was not found on any wet site.
Figure 2. Age class distribution of the seven most frequent
tree species or species combinations (in which each species ac-
counts for at least 10% of the total basal area). Species/groups
of species with proportions <0.2% are not shown.
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Stand age and silvicultural implications
The hypothesis that pine/birch mixtures are limited to early-
successional stages was also rejected, since this forest type
was found among stands in a broad range of age classes
(albeit at higher proportions in young stands than in older
stands). A similar pattern was generally observed for spe-
cies mixtures containing birch and conifers. With respect to
the proportion of mixed stands that occur in Götaland, our
results complement the findings of Fahlvik (2005) that pre-
commercial thinning has great potential to promote mixed
stands with birch and conifers. While the regeneration of
Table 4: Percentages of total forest area and the seven most frequent forest types (each species in mixtures accounting for 10% of
basal area) in each of four soil moisture classes
Moisture Total forest area Spruce Pine/spruce Pine Spruce/birch Pine/spruce/birch Pine/birch Birch
Dry 5.7 2.3 6.1 15.4 2.0 5.3 9.1 2.6
Mesic 60.1 69.3 67.8 60.2 48.6 48.7 39.8 39.5
Moist-mesic 29.0 26.6 23.3 20.7 42.6 37.8 38.3 46.5
Moist 5.1 1.8 2.7 3.6 6.8 8.2 12.8 11.4
Total 100 100 100 100 100 100 100 100
Table 5: Percentages of total forest area and the seven most frequent forest types (each species in mixtures accounting for 10% of
basal area) in the indicated ownership classes (not shown: numbers representing <10 000 ha of forest area)
Ownership Total forest area Spruce Pine/spruce Pine Spruce/birch Pine/spruce/birch Pine/birch Birch
Private 78.1 79.1 77.3 68.9 80.7 81.1 72.0 82.3
11.8 11.1 12.8 18.3 11.4 12.5 14.4 7.6
State 1.9 1.1 2.4 2.5 –
Church 2.8 3.8 2.9 2.4 2.2
Others 5.4 4.9 4.6 7.8 4.0 0.8 3.3 4.9
Total 100 100 100 100 100 100 100 100
Table 6: T-values of comparisons: (1) between the proportions
of young and old forest for each forest type, (2) between the
observed and expected site class distribution of the total forest
area for each forest type and (3) between the observed and
expected ownership class distribution of the total forest area, for
each forest type
Forest type
of the
of 10- to
vs 50- to
of the
site class
of total forest
area vs forest
of the
of total forest
area vs forest
Spruce 27.9* 193.9* 8.2*
Pine/spruce 213.7* 62.7* 2.7
Pine 50.2* 87.7* 24.8*
Spruce/birch 136.9* 107.5* 2.3
Pine/spruce/birch 1.1 28.4* 2.3
Pine/birch 9.7 72.9* 3.9*
Birch 38.9* 82.2* 9.8*
Statistically significant differences are indicated by an asterisk (*).
mixed tree species is easily achievable on many sites (Agestam
et al., 2005), silvicultural measures to maintain spruce/birch
mixtures may reduce timber production and net value dur-
ing the whole rotation (Fahlvik, 2005). However, in young
stages of development, Linden (2003) found no significant
differences in timber productivity between spruce mixed with
20 per cent birch and pure spruce stands. Since both species
differ in height development and growth (Mård, 1996), case
studies covering various sites are needed to assess the poten-
tial height and crown development of species in relation to
age and silvicultural treatment. In the later stages of growth,
the model by Agestam (1985) provides sufficient guidance
for mixed forest.
A third hypothesis, that pine/spruce mixtures are more fre-
quently owned by private individuals, was also rejected.
Our results indicate that, on the contrary, forest companies
hold a higher relative proportion of pine forest than other
forest owners and that pine mixtures with other species did
not seem to be disfavoured (Table 5).
The relatively high proportion of spruce in private for-
ests might reflect the preference of simple management
practices or the advanced natural ingrowth that occurs in
less intensively managed forests. In either case, spruce is an
easy and feasible option for many forest owners, especially
with regard to its establishment. However, there is a broad
range of types of forest owners (Törnqvist, 1995), who are
not distinguished in Table 5.
The total number of tree species combinations was very
high. For most mixtures (e.g. beech mixed with other
at Sveriges Lantbruksuniversitet on December 16, 2010forestry.oxfordjournals.orgDownloaded from
noble broadleaves), the available data were insufficient to
allow quantitative descriptions. However, the proportions
in which they currently occur should not be the only cri-
terion for identifying important forest types, especially if
predicted changes in proportions of different tree species,
based on future climate change scenarios, are taken into
account. Qualitative information can be used too: e.g. for-
est communities can be classified according to phytosociol-
ogy (Diekmann, 1999; Engelmark and Hytteborn, 1999)
or tree species mixtures documented in literature (Linden,
2003; Bolte et al., 2009). In addition, forest development
types applied in German or Danish management planning
(Otto, 1994; LÖWE, 2004; Larsen, 2005) may be useful,
provided that it is applicable to Swedish conditions. An-
other important input is the expertise, which can only be
provided by local silviculturalists, required to fill gaps in
the scientific knowledge concerning the specific develop-
ment of tree species in mixed stands. A key task for forest
scientists is to process all the available information, includ-
ing any uncertainties, and present it in an accessible, read-
ily understandable way to owners and operators who need
to make site-specific and ownership-dependent stand-level
management decisions.
In conclusion, there is a high proportion of mixed for-
ests, but silvicultural guidance for these types of forest is
limited. Even though mixed stands can require more effort
to promote particular tree species, and can yield lower vol-
umes, they can be advantageous for other reasons. Ulti-
mately, the owner must decide whether the advantages of
mixed stands outweigh the disadvantages, but to take such
decisions rationally, information on the associated mea-
sures, costs and future developments is required.
In the context of climate change, Millar et al. (2007)
suggest that no single solution can meet all potential fu-
ture challenges; hence the best strategy is to mix different
approaches in order to meet a range of possible situations.
Especially in Swedish forestry, the climate scenarios for
this century do not limit the silvicultural options that were
given in the past (KSLA, 2004; Eriksson, 2007; SCCV,
2007). However, there is an increasing need to balance
aspects of stand stability and carbon sequestration. Site
characteristics affecting stability, e.g. wind exposure or
water supply, as well as the perception of forest owners,
are becoming increasingly important. These, and other as-
pects of management, can only be evaluated at the level
of individual stands. For that reason, information gained
from analyses at other levels must be provided in a neutral,
accessible form appropriate for stand-level considerations,
in order to allow the most feasible options to be selected.
Methodological limitations
The main goal of the present study was to determine the
proportions of different tree species mixtures present in
Götaland. For that purpose, the NFI provided the most
comprehensive data, although they differed from those
given in the Statistical Yearbook (2008), which were
based on different data sets, mainly due to the exclusion of
permanent plots and partial sample plots (i.e. those divided
by borders of forest land or borders between stands).
For the analysis, the tree species were aggregated into
groups with similar growth patterns (Table 1). Hence, lime
and hornbeam were combined since both of these species
are shade-tolerant and might require similar silvicultural
treatments in mixed stands with oak or pine. Ash, elm,
sycamore, Norway maple and cherry are all relatively fast-
growing noble broadleaves which would have similar silvi-
cultural requirements (although cherry would need special
attention). From a similar perspective, Scots pine and
lodgepole pine could also be combined in a single group.
However, an objective of the study was to obtain clear in-
dications of the proportions and distributions of pure and
mixed Scots pine stands; hence, Scots pine was considered
separately from conifer species that occurred in very low
proportions (and were merged).
The proportions of mixed forests defined according
to the third, strictest definition could have been inflated
by the presence of trees that were nearly dead. However,
their proportional occurrence can be assumed to have been
much lower than the percentage of dead trees recorded by
the NFI survey, which was just 0.85 per cent of the total
number of recorded trees in the data set. Thus, any possible
bias they caused is likely to have been very weak.
The ratio of mixed species recorded in a plot may not
reflect the ratio that occur in the whole forest stand since
a plot recorded as having a sufficiently high proportion of
different species for classification as a mixed stand might
actually have been located at a specific microsite in a stand
that would otherwise have been classified as less mixed or
even pure (e.g. a spruce/birch plot in a pure spruce stand).
However, any potential distortion in the data due to such
sampling errors should be compensated by similar errors
with an opposite bias: e.g. samples from plots comprising
pure spruce in similar stands with scattered birches. This
effect has been demonstrated in an unpublished stand-
based forest survey (översiktigt skogsinventeringen) con-
ducted 15 years ago, which reported similar proportions of
noble broadleaves to those found in the NFI survey.
Theoretically, samples almost always provide under-
estimates of the incidence of rare events (Zöhrer, 1980).
Thus, rare tree species are likely to occur somewhat
more frequently than the data acquired in the present
study suggest.
Conflict of Interest Statement
None declared.
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... Thus, continuous cover and disturbance-based forest management regimes are being promoted to restore multilayered, heterogeneous stands (Axelsson 2008;Pukkala and von Gadow 2012;Kuuluvainen and Russel 2012). However, as heterogeneity increases, forest management becomes more complex as more diverse practices are needed to tend and harvest stands with multiple combinations of tree species and structures when compared with management of even-aged, single-species stands (Drössler 2010;Messier et al. 2014). This article presents a feasible strategy for harvesting heterogeneous forest stands to optimize benefits of integrating landscape-level commodity production and the maintenance of other ecosystem services. ...
... Considering the limited area of protected forest and the prevalent attitudes of particular forest owners in favor of more wildlife or recreational values (Hugosson and Ingemarsson 2004), certain biodiversity values (e.g., tree species admixtures, dead trees, or multiple stand layers) could be promoted through forest management (Messier et al. 2014). Some valuable biodiversity is already present in managed forests and also merits conservation (Rydberg and Falck 2000;Drössler 2010). ...
... Liebl.) in lower layers. The mixture of pine and spruce with evenly scattered oak trees in conifer stands are features commonly found in multilayered forests in southern Sweden (Drössler 2010;Drössler et al. 2012a), although multilayered stands are rare (Axelsson 2008). ...
Full-text available
Forest management in Sweden can be characterized by even-aged silviculture heavily relying on three established harvest regimes: clearcutting, the seed-tree method, and the shelterwood system. Less intense, small-scale retention harvest systems such as single tree and group selection harvest are rarely used. In addition, natural regeneration dynamics without enrichment planting have barely been studied. Consequently, this study examined natural regeneration establishment in a multi-layered Pinus sylvestris-Picea abies forest stand in southwest Sweden after target diameter harvesting and soil scarification. The creation of forest canopy gaps had a positive effect on total seedling density five years after harvest, mainly due to a significantly higher number of Betula pendula individuals. Seedling density of more desirable tree species suitable for continuous cover forestry such as Fagus sylvatica, Quercus petraea and Picea abies also increased substantially in gaps when compared to pre-harvest conditions or the unharvested plots. In contrast, soil scarification did not increase the number of seedlings of desired tree species due to a significant decrease in Picea abies abundance. Soil moisture and gap size significantly improved Betula pendula seedling establishment while a larger number of Quercus petraea seedlings were observed in Vaccinium myrtillus patches. We conclude that canopy gaps are beneficial under the encountered stand conditions to initiate forest regeneration, and that soil scarification without the timely occurrence of a mast year of desired tree species is not effective in the type of forest studied.
... The second problem is that there is no consensus on the cutoff levels of specific indicators in defining the mixed forests. For example, some studies have tried to differentiate mixed forests from pure forests using a threshold of 95% of volume for component species, while others have used a cutoff level 75% of volume to identify mixed forests (Bravo-Oviedo et al., 2014;Drössler, 2010). Due to the lack of standard of a definition for mixed forests, different authors have suggested markedly inconsistent distributions of mixed forests. ...
... definitions and quantitative indicators for different mixed forest types, including mixed evergreen and deciduous broad-leaved forests, remain largely unclear(Bravo-Oviedo et al., 2014;Drössler, 2010). Previous studies have tried to use a set of qualitative and quantitative vegetation characteristics, such as the number of stems, basal area, and/or biomass or canopy cover of component species, to describe and define mixed forests (for mixed forests has never been proposed, and studies have been inconsistent in choosing quantitative indicators.The first obstacle in using quantitative indicators in defining mixed forests is that there is an extensive and variable selection of proposed quantitative indicators, most of which have been developed for forest management, thus providing limited applicability to vegetation classification. ...
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Understanding climatic influences on the proportion of evergreen versus deciduous broad-leaved tree species in forests is of crucial importance when predicting the impact of climate change on broad-leaved forests. Here, we quantified the geographical distribution of evergreen versus deciduous broad-leaved tree species in subtropical China. The Relative Importance Value index (RIV) was used to examine regional patterns in tree species dominance and was related to three key climatic variables: mean annual temperature (MAT), minimum temperature of the coldest month (MinT), and mean annual precipitation (MAP). We found the RIV of evergreen species to decrease with latitude at a lapse rate of 10% per degree between 23.5 and 25°N, 1% per degree at 25–29.1°N, and 15% per degree at 29.1–34°N. The RIV of evergreen species increased with: MinT at a lapse rate of 10% per °C between −4.5 and 2.5°C and 2% per °C at 2.5–10.5°C; MAP at a lapse rate of 10% per 100 mm between 900 and 1,600 mm and 4% per 100 mm between 1,600 and 2,250 mm. All selected climatic variables cumulatively explained 71% of the geographical variation in dominance of evergreen and deciduous broad-leaved tree species and the climatic variables, ranked in order of decreasing effects were as follows: MinT > MAP > MAT. We further proposed that the latitudinal limit of evergreen and deciduous broad-leaved mixed forests was 29.1–32°N, corresponding with MAT of 11–18.1°C, MinT of −2.5 to 2.51°C, and MAP of 1,000–1,630 mm. This study is the first quantitative assessment of climatic correlates with the evergreenness and deciduousness of broad-leaved forests in subtropical China and underscores that extreme cold temperature is the most important climatic determinant of evergreen and deciduous broad-leaved tree species’ distributions, a finding that confirms earlier qualitative studies. Our findings also offer new insight into the definition and distribution of the mixed forest and an accurate assessment of vulnerability of mixed forests to future climate change.
... Because of their fast growth, broadleaves could overtop recently-planted conifers, resulting in reduced growth and mortality of crop trees. Although the regeneration goal is usually a Norway spruce or Scots pine forest, the combination of mortality of coniferous seedlings and B A natural regeneration of broadleaves often results in tree-species mixtures in the young forest (Drössler, 2010;SFA, 2015). Moreover, ungulate browsing irreversibly changes the tree structure, biomass production, and can even cause death of the tree (Bergqvist et al., 2003;Persson et al., 2000). ...
... The definition of what counts as a mixture versus a monoculture varies across studies (Bravo-Oviedo et al. 2014). For example, the Swedish national forest inventory (NFI) sets the limits as a tree species composition for which no more than 65% of the basal area is dominated by one species (Drössler 2010;Nilsson 2013), whereas other studies use a threshold of 70% (Felton et al. 2016). The retention of at least some broadleaf trees throughout a stand's rotation (5-10% of basal area) is also a requirement of some certification standards (FSC 2010). ...
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Admixtures of birch in Norway spruce plantations are being promoted as a means to increase habitat and species diversity. The implications of this mixture were analysed with regional survey data from southern Sweden. Permanent sample plots from the Swedish National Forest Inventory (NFI), with Norway spruce and admixture of birch, were used to describe the temporal trends in the admixture, regarding species composition and competitive strength. Observations from thinned plots show a higher harvest removal in birch (35%) than for Norway spruce (19%). Observations without thinnings in the period before measurement showed that individual birch tree growth was lower compared to Norway spruce and it decreased even more with increasing stand age and competition. In addition, a complementary field survey, with multiple distributed sample plots in each stand, was used to detect within-stand variation of species composition and density. Although within-stand heterogeneity was larger in mixed stands in terms of species composition, it was not different from Norway spruce monocultures in terms of stand density. These two surveys show that the admixture of birch, for several reasons, decreases over stand age and although birch increases tree species diversity, it does not necessary imply a change in density.
... These species cover approx. 30,000 ha today (Drössler 2010). Also, birch occurs naturally in young conifer plantations and provides opportunities for the forest sector. ...
Technical Report
Full-text available
Acknowledgements The report benefited from the helpful comments from external reviewers, Arttu Malkamäki from the University of Helsinki and Margarida Tomé from the University of Lisbon. We wish to express our thanks for their insights and comments that helped to improve the report, and acknowledge that they are in no way responsible for any remaining errors.
... In general, this results from the small number of long-term experiments in mixed stands and the lack of data standardization (Bravo-Oviedo et al., 2014;Pretzsch et al., 2010). To fill such gaps, the use of large-scale forest inventory data has been proposed as an alternative approach allowing the analysis of forest composition in relation to site and management characteristics on large spatial and temporal scales (Corona et al., 2011;Drössler, 2010). However, this requires methodological standardization to avoid bias caused by different sampling areas and dbh thresholds (McRoberts et al., 2009). ...
Knowledge regarding tree species dynamics is essential to understand forest responses to the environment, and to evaluate management options in adapting forest ecosystems to future climates. As maintaining tree species diversity and promoting structural stand heterogeneity are among the strategic elements in adapting forest management to climate change, the monitoring tree diversity is an ongoing challenge. Large-scale forest inventories have been proposed as a suitable basis for forest diversity analysis on large spatial and temporal scales. We used Swiss forest inventory data (NFI) to analyse temporal changes in tree species richness on small plots from 1983 to 2006. For two size groups of trees (‘small’ trees with dbh from 12 to 35 cm from plots with 200m2 area, and ‘large’ trees with dbh≥36 cm from plots with 500m2 area), we identified the number and the tree species appearing (‘gains’) or disappearing (‘losses’) from each plot during the study period, and related these changes to site, stand and management characteristics. We found that species richness change was size-dependent and varied largely due to regional differences in the past land-use history of the Swiss forests. ‘Gains’ of ‘small’ trees were higher in stands with diverse vertical structure, with less competitive pressure as well as in warm environments, whereas ‘gains’ of ‘large’ trees were mostly related to climate and were highest in warm and moderately moist habitats. ‘Losses’ in both tree-size groups were mainly promoted by management. Our analysis suggests high vulnerability of Picea abies and high competitiveness of Fagus sylvatica, and underlines the potential of Abies alba in forming future Swiss forests. Despite of the silvicultural paradigm to create more species rich forests, most silvicultural interventions decreased small-scale species richness. This calls for further studies on the effect of management on tree species diversity.
... As such, stand registries have been lacking information on these areas and thus inefficient for identification of birch shelterwood stands. As more than 500,000 hectares in Southern Sweden comprise mixed spruce and birch stands (Drössler 2010), the difficulty of finding birch shelterwood areas in stand registers was surprising. This is a main contributor to this study being carried out over several years, it was in fact hard to find stands suitable for the study. ...
Full-text available
To improve the micro climate for Norway spruce (Picea abies (L.) Karst.) regeneration and achieve higher growth, a system of birch shelterwoods with naturally regenerated birch (Betula spp.) creating an overstory sheltering planted spruce is implemented in southern Sweden. Even though the primary objective is to establish a new spruce stand, the economic viability depends on efficient birch overstory harvest with little damage to spruce regeneration. This study aimed to analyze time consumption and net revenues for harvester and forwarder work when removing the birch overstory, and to describe the frequency of logging damage in the residual spruce stand. Time consumption data was collected through time studies of harvesting and forwarding of 10 study plots. Sample plots were inventoried after harvesting and forwarding operations to identify damage on the residual spruce. Average harvester productivity was 2.8 oven dry ton per efficient work hour. The variation in time consumption was up to 94%, explained by a positive correlation with the number of trees harvested per hectare and a negative relationship with removed volume per hectare. Forwarder loading time correlated with forwarded volume along the strip road and the number of birch trees per ha prior to logging. Approximately 7–17% of the residual trees were damaged, and the harvester caused 83% of the damage. Due to high harvesting costs and low revenues, only plots with large removals provided positive net revenues. Birch shelterwoods can therefore not be expected to increase net revenues but are best seen as a regeneration method for addressing stand re-establishment challenges.
... Generally speaking, species composition in wood-pastures was broadly similar to expected species composition in the surrounding forest in a given region, apart from a higher occurrence of fruit trees in some regions (Drössler, 2010;Bölöni et al., 2011;Garbarino et al., 2013;Hartel et al., 2013;Roellig et al., 2016). Only in plots from central Portugal was there was only one species present. ...
Full-text available
Europe's woodland and savanna rangelands, often part of silvopastoral systems known as wood-pastures, are deteriorating because of abandonment that leads to return to a forested state or lack of tree regeneration from overgrazing or tree and shrub removal. Despite numerous local studies, there has been no broader survey of the stand structure of European wood-pastures showing which systems are at risk of losing their semiopen character. This overview aims to 1) show some of the differences and similarities in wood-pastures from landscapes across Europe and 2) identify which of these wood-pastures are at risk of losing their semiopen character. We collated a dataset of 13 693 trees from 390 plots in wood-pastures from eight different European regions (western Estonia, eastern Greece, northern Germany, Hungary, northern Italy, southern Portugal, central Romania, and southern Sweden), including tree diameters at breast height, tree density, management type, and tree species composition. On the basis of their structural characteristics, we classified wood-pastures using principal component analysis (PCA) and cluster analysis. The PCA showed a gradient from dense wood-pastures with high levels of regeneration (e.g., in Estonia) to sparse wood-pastures with large trees but a lack of regeneration (e.g., in Romania). Along this gradient, we identified three main groups of wood-pastures: 1) sparse wood-pastures with mostly big trees; 2) dense wood-pastures composed of small trees, and 3) wood-pastures containing a wide range of tree ages. Our results show a large structural gradient in European wood-pastures, as well as regeneration problems varying in their severity, highlighting the importance of social-ecological context for wood-pasture conditions. To maintain the ecological and cultural integrity of European wood-pastures, we suggest 1) more comprehensively considering them in European policies such as the Common Agricultural Policy and EU Habitats Directive, while 2) taking into account their structural characteristics and social-ecological backgrounds.
... CWS ov and CWS bk for the inner and outer area of the forest, were simulated making use of the simulated canopy structures for the different tree species and age classes contained in that pixel. To this aim a simulation over southern Scandinavia was set up by using the CRU-NCEP reanalysis for 2010 making use of the parameter values for G adj and R f , Norway spruce (Picea abies (L.) Karst) and Scots pine (Pinus sylvestris L.); however, a species mixture of coniferous tree and broadleaved species such as birch (Betula pendula Roth and B. pubescens Ehrh.) can be found in this region (Drössler, 2010). The simulated spatial distribution of critical wind speeds for each tree species thus reflects the effects on critical wind speeds of age class structure, forest management, and, to a lesser extent, local climate conditions. ...
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Earth system models (ESMs) are currently the most advanced tools with which to study the interactions among humans, ecosystem productivity, and the climate. The inclusion of storm damage in ESMs has long been hampered by their big-leaf approach, which ignores the canopy structure information that is required for process-based wind-throw modelling. Recently the big-leaf assumptions in the large-scale land surface model ORCHIDEE-CAN were replaced by a three-dimensional description of the canopy structure. This opened the way to the integration of the processes from the small-scale wind damage risk model ForestGALES into ORCHIDEE-CAN. The integration of ForestGALES into ORCHIDEE-CAN required, however, developing numerically efficient solutions to deal with (1) landscape heterogeneity, i.e. account for newly established forest edges for the parameterization of gusts; (2) downscaling spatially and temporally aggregated wind fields to obtain more realistic wind speeds that would represents gusts; and (3) downscaling storm damage within the 2500 km² pixels of ORCHIDEE-CAN. This new version of ORCHIDEE-CAN was parameterized over Sweden. Subsequently, the performance of the model was tested against data for historical storms in southern Sweden between 1951 and 2010 and south-western France in 2009. In years without big storms, here defined as a storm damaging less than 15 × 10⁶ m³ of wood in Sweden, the model error is 1.62 × 10⁶ m³, which is about 100 % of the observed damage. For years with big storms, such as Gudrun in 2005, the model error increased to 5.05 × 10⁶ m³, which is between 10 and 50 % of the observed damage. When the same model parameters were used over France, the model reproduced a decrease in leaf area index and an increase in albedo, in accordance with SPOT-VGT and MODIS records following the passing of Cyclone Klaus in 2009. The current version of ORCHIDEE-CAN (revision 4262) is therefore expected to have the capability to capture the dynamics of forest structure due to storm disturbance on both regional and global scales, although the empirical parameters calculating gustiness from the gridded wind fields and storm damage from critical wind speeds may benefit from regional fitting.
... Over the last hundred years, clear-cutting and other anthropogenic forms of land-use change have dramatically altered tree species composition in Sweden, largely to the detriment of broadleaf and mixed forest cover (Edenius et al. 2002;Lindbladh et al. 2014). Norway spruce (Picea abies) has benefited most from such changes, to the extent that in southern Sweden, spruce-dominated production stands now comprise 40 % of forest area (Drössler 2010). This form of forest management has direct implications for ungulate food resources (Kuijper 2011). ...
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People provide wild ungulates with large quantities of supplementary feed to improve their health and survival and reduce forest damage. Whereas supplementary feeding can positively affect the winter survival of ungulates and short-term hunting success, some of the feeds provided may actually reduce ungulate health and increase forest damage. Here, we highlight how recent advances in ungulate nutritional ecology can help explain why supplementary feeding can lead to undesirable outcomes. Using Europe’s largest cervid, the moose (Alces alces), as a model species, and Sweden, as the socio-ecological context, we explain the concept of nutritional balancing and its relevance to supplementary feeding. Nutritional balancing refers to how animals alter their food intake to achieve a specific nutritional target balance in their diet, by selecting balanced food items or by combining items with nutritional compositions that are complimentary. As the most common supplementary feeds used contain higher concentrations of non-structural carbohydrates than the ungulates’ normal winter diet, the consumption of such feeds may cause animals to increase their intake of woody browse, and thereby exacerbate forest damage. We also explain how animal health may be negatively affected by large intakes of such feed if complementary browse items are not available. We therefore suggest that the use of inappropriate feed is an additional means by which supplementary feeding may result in negative outcomes for hunters, forest owners, and wild animals.
Das von Alfred Dengler 1930 begründete Buch wurde für die 8. Auflage überarbeitet, aktualisiert, deutlich erweitert und gestalterisch modernisiert. Nach einer umfassenden Einführung in die Ziele des Waldmanagements im Kontext der Geschichte des Waldbaus und den aktuellen Anforderungen liefert das moderne Lehr- und Handbuch Studierenden der Forstwirtschaft das komplette Handwerkszeug des Waldbaus: Baumartenwahl, Begründung, Verjüngung und Pflege von Waldbeständen sowie die Gestaltung der Wälder in Betriebsarten und Verjüngungsformen.
High resolution pollen analysis was carried out on five peat profiles from small forest hollows at four sites in southern Sweden. The sites (Bocksten in Halland; Flahult, Mattarp, and Siggaboda in Smaland) were all within the area where the present distribution limits of Fagus and Picea overlap. All forest hollows used to reconstruct the past vegetation have relatively small pollen source areas. The results from the different sites are presented in separate papers. These results are compared and discussed in more detail in this synthesis. Viewed on a continental scale the migration pattern of Fagus can be correlated with climate and its change over the millennia, but at finer scales there are factors other than climate that are crucial for establishment (e.g. disturbance, seed dispersal, human activities). The establishment of Fagus does not show a regional coherence in southern Sweden. This may imply that climate was not the limiting factor for its establishment. The present day distribution of Fagus in southern Sweden suggests a migration with a discontinuous front with outlying populations, and this model probably applies to its past distribution. Picea invaded southern Sweden from the north diring a period when the cultural landscape had already been evolving for some time. Picea is a dominant tree with an effective seed dispersal, and the relatively open and probably grazed forests in the area were not particular resistant to Picea invasion. The timing of local Picea establishment seems to be mostly controlled by its migration, ie, it became established when its front reached the studied sites.
This contribution focuses on the question how tree species richness is related to forest productivity. Based on long-term experimental plots the effects of the transition from mono-cultures to mixed stands are analysed. Important commercial tree species from temperate and boreal climate zones are considered. Depending on the type of mixture and site conditions the effects from species mixtures on productivity may vary considerably (Fig. 2). Combinations of early and late successional tree species, ontogenetically early and late culminating species, shade-intolerant and shade-tolerant species may raise the efficiency of resource utilisation by up to 30% compared with that in pure stands. By contrast, where ecological amplitudes and functional characteristics are similar, species will compete for the same resources in crown and root system. Antagonistic effects from species interaction and productivity reductions by up to 30% may ensue (Fig. 6). With the occurrence of risk the productivity relationships between pure and mixed stands, related to medium stand densities, may experience a shift in favour of mixed stands (Fig. 7 and 8). The reason being that the combination of several species is synonymous with risk distribution. As a rule, mixed stands are more flexible in the face of changes in site conditions and more resilient to natural disturbances or perturbations on account of silvicultural treatment (Fig. 9). In mixed stands silvicultural removals or severe loss of one species may be compensated for by accelerated growth of the remaining species.
Ecological theory suggests that there is a potential productivity advantage to be gained by designing managed forest stands to contain more than one tree species. The basis for this advantage, as noted by Ewel (1986) and Vandermeer (1989) is rooted in fundamental niche theory—two or more species must use resources differently if they are to coexist on a site. Differential resource use among species suggests that the species in a mixture may utilize the resources of a site more completely than any single species would be able to do, leading to greater overall productivity. However, the link between differential resource use among species and greater total resource use does not necessarily exist in all cases. For example, it is possible that a mixture of species may simply subdivide the total resource base that one highly efficient species may completely use on its own. In addition to niche separation, which potentially applies to all species in mixture, certain specific combinations of species may exist in which one species may directly benefit from the presence of another.