Managing birch woodlands for the production of quality timber

Transcription

Managing birch woodlands for the
production of quality timber
A. D. CAMERON
Summary
Interest in silver birch (Betula pendula Roth) and downy birch (Betula pubescens Ehrh.) has
greatly increased in recent years partly as a result of pressures to restore and expand native
woodlands but also due to renewed interest in birch as a tree capable of producing quality timber. Despite the many advantages of birch as a commercial timber tree—ease of establishment,
fast growth on good sites, high value timber and a short rotation, it has a poor reputation in
Britain largely as a result of the poor form of the existing, mainly unmanaged resource. The following points need to be considered if stands of quality birch trees are to be produced in an economical timescale. (1) Sites: silver birch needs good sites that are relatively well drained with
light mineral soils. Downy birch does well on moist to wet sites. (2) Regeneration: natural regeneration through a shelterwood is the preferred system of regenerating birch as some overhead
protection is beneficial to germination success. About 20-40 seed trees should be left per hectare.
Good ground preparation and control of grazing are essential. The vast majority of seedlings are
recruited in the first year of the regeneration cycle therefore planting should be considered if the
initial regeneration success is poor. Direct seeding is also a successful method of regeneration.
Birch readily regenerates naturally into suitably prepared open areas next to existing birch woods
but these should not be too big, e.g. gaps or strips 20-60 m wide have been suggested in the literature. (3) Maintenance: density of regeneration needs to be reduced to about 2500-3000 stems
ha"1 by the time the trees are about 3—6 m tall. Birch seedlings must always be taller than the
competing vegetation. (4) Thinning: thinning should begin when the mean height of the stand is
about 8—10 m. At this point at least half the number of trees should be removed with the emphasis on retaining dominants and co-dominants of good form. The aim is to maintain about half
the height of the tree as living crown to sustain a high rate of growth. Additional thinnings will
be required at intervals of 5 to 7 years and final thinning should leave around 300-500 stems
ha"1. (5) Rotation: a rotation of 40-50 years is possible on good sites and perhaps 50-55 years on
less favourable sites.
n o u ion
Birch is the commonest native woodland tree in
Britain and yet until recently its potential for
timber production has been virtually ignored.
Birch produces a finely textured timber, white in
O Institute of Chartered Foratero 1996
colour and bright in appearance. It is one of the
strongest timbers commonly grown in Britain
with a basic density of around 530 kg m~3
(Lavers, 1983), and when dried to 12 per cent
moisture content the total weight per m3 is 630
Forestry, VoJ. 69, No. 4, 1996
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Department of Forestry, University of Aberdeen, Aberdeen AB9 2UD, Scotland
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FORESTRY
Distribution and site requirements
The genus Betula, belonging to the family Betulaceae, comprises around 40 species distributed
throughout North America, Europe and Asia
south to the Himalayas (Mitchell, 1976). In
Britain there are two arborescent species—silver
birch {Betula pendula Roth) and downy birch
{Betula pubescens Ehrh.). The main distinguishing morphological characteristics of silver and
downy birch are listed in Table 1.
The birches are pioneer species with life spans
of around 50 to 100 years although in Scotland
it can be much older than this (Mitchell, 1976).
They show the characteristic early period of fast
development after which growth is slow (Kinnaird, 1968). Silver birch is associated with relatively well drained, light soils and downy birch
with moist to wet and frequently peaty soils
(Clapham et al., 1981). Two sub-species of
downy birch are recognized in Britain; ssp.
pubescens (Ehrh.) found in the lowlands and
glens and ssp. odorata (Bechst.), a shrubbier tree
of the moors and mountains (Clapham et al.,
1981). In Britain the range of silver and downy
birch overlap but with the tendency for silver
birch to be more common in the south and east
of the country and downy birch more frequent
in the north and west at higher altitudes on
more exposed sites (Kinnaird, 1968, Forbes and
Kenworthy, 1973) although distribution maps
Table 1: Main distinguishing morphological characteristics of silver and downy birch (Mitchell, 1976)
Silver birch
Downy birch
Height
up to 30 m
up to 20 m
Bark
pinkish-white with horizontal grey
bands; white with big black diamonds
in older trees
red-brown becoming smooth greyishwhite, variable horizontal bands grey
or brown often cherry-like in
appearance until big
Crown
narrow, pointed and upswept in young
trees; domed at the top with long
pendulous branchlets
irregular, soon losing pointed top,
rather spreading and twisted branches,
shoots spreading, crowed and not
pendulous
Foliage
shoots purplish-brown, covered with
raised white warts; leaves slightly
rounded-triangular tapering to a fine
point with tapering teeth separated by
2-3 small triangular teeth; petiole
slender with no hairs
shoots greyish red-purple, covered in
short, soft, shiny white hairs; leaves
rounded, often nearly circular, broad
rounded teeth; petiole densely
pubescent
kg. This is only slightly less than the value for
home-grown beech (690 kg m~3) and greater
than for sycamore (560 kg m~3). The wood
works and finishes well and can be used for a
variety of purposes including fittings, furniture
and paper. Birch, however, has a poor reputation as a timber tree in Britain, probably as a
result of the poor form of the existing stands of
trees. By contrast, birch is an important timber
species in Finland, for the manufacture of veneer
boards, plywood and pulp, although even here
unmanaged birch was long considered a weed
(Koski, 1991). Frivold and Mielikainen (1990)
state that one of the factors that lead to the negative attitude towards birch throughout
Fennoscandia was the habit of foresters comparing well-managed coniferous stands with
unmanaged or poorly managed stands of downy
birch. If British birch is to achieve an economic
status similar to that found in Finland, then
appropriate management techniques must be
adopted with the aim of quality timber production in a financially acceptable timescale.
The aim of this paper is to draw together the
relevant literature on birch silviculture, productivity and timber properties from Northern
Europe and North America and to make silvicultural recommendations appropriate to British
conditions based on this review.
MANAGING BIRCHWOODS FOR QUALITY TIMBER
Establishment
Seed production
The birches are monoecious with flowers borne
on catkins. Low spring temperature is related to
poor pollen production in silver and downy
birches and appears to be more important than
rain during pollen shed (Sarvas, 1952, 1956,
1967). When the weather is favourable 70-80
per cent of silver and downy birch pollen is shed
within 2—3 days (Sarvas, 1952). The percentage
of germinable seed is directly related to the
amount of pollen shed; seed with the highest
viability will be produced during years of abundant flowering (Sarvas, 1952). Birch seed ripens
from July to October depending on species with
the rate of ripening dependent on temperature
sums (Sarvas, 1956). With each silver and
downy birch catkin containing about 450 seed,
seed falls can be very large with levels of
177 000, 157 000 and 258 000 seeds per m2
recorded for paper birch (Betula papyrifera
Marsh.), silver and downy birches respectively
(Koski and Tallqvist, 1978).
Seed dispersal
Silver and downy birch disperse seed mostly in
September arid October (Fries, 1984). The dispersal range of the seed is limited by its small
size and by much more than may be expected.
Sarvas (1948) observed that silver and downy
birch seed dispersal declined exponentially with
distance from seed source, and very few seeds
travelled more than 50 m. The theoretical maximum dispersal distance for 20 m tall trees in a
5 m s"1 wind without turbulence is 192 m for silver birch and 145 m for downy birch. Given the
relatively limited dispersal of birch seed, the
shelterwood system, where seed trees remain on
a site, has been suggested for regenerating
stands of yellow birch (Betula alleghaniensis
Britton) (Tubbs et al., 1983), paper birch (Perala, 1988) and for both silver and downy birch
(Sarvas, 1948).
Germination requirements
Seed viability of silver and downy birch on the
forest floor declines dramatically after one year
to about 6 per cent (Granstr6m and Fries, 1985).
Stratification is not required for birch seed
regenerated under light (Brinkman, 1974). Vaartaja (1959) showed that silver, downy and paper
birch seed germinated well in both darkness and
light at 25—35°C. At temperatures lower than
25°C, germination depended on photoperiod,
however, no light was needed if the seed was
stratified to break down the seed coat (Black
and Wareing, 1955, 1959). Sarvas (1950) found
that while silver and downy birch germinated
much faster in light than darkness, total germination was unaffected and advised that field
sown seed should only have a light covering.
show that both species have been recorded in
most 10 kilometre squares of mainland Britain
(Perring and Walters, 1976). It often grows in
mixtures with other species particularly oak in
the south and west and pine in the north and
east, although pure birch stands are common in
the Highlands.
The birches are more sensitive to soil physical
and chemical properties than most other north
temperate tree species (Helliwell, 1974; Helliwell
and Harrison, 1978). They are, however, known
to ameliorate unfavourable soils, their deep root
systems bringing up and depositing nutrients on
the ground surface through leaf fall (Gardiner,
1968), and have been found to reverse the
process of podzolization leading to the formation of mull type humus (Dimbleby, 1952).
Raulo (1978) emphasized that silver birch grows
well and produces stems of good quality only on
nutrient rich sites. Raulo also noted that silver
birch seedlings grow poorly or may even die if
the ground water table lies close to the soil surface.
Although birches do best on protected sites
with rich, fresh soils, they are noted for their
ability to survive and grow on the most forbidding landscapes. Many mine spoils, landslides
and drained peatlands have regenerated naturally with birch (Game et al., 1982; Kaunisto
and Paivanen, 1985; Kulagin, 1985) although
soil N must reach at least 700 kg ha"1 through
weathering before silver birch can invade by
seeding (Marrs et al., 1981). Silver birch is also
widely planted on unweathered spoil heaps
because it tolerates soil pH as low as 3.3 (Brissot, 1975; Plass, 1975).
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FORESTRY
ing animals as mere enclosure will allow the regrowth of grazed vegetation thus reducing the
sites available for incoming birch seed. For
example, Kinnaird (1974) found in his study that
while first-year seedlings were numerous on a
range of sites visited, few survived for 2 years
and the survivors became fewer with age. Grazing, shading and slope did not affect seedling
establishment. Prepared seedbeds have been
found to remain receptive to birch germination
and establishment for only 2-3 years (Sarvas,
1948; Zasada et al., 1978) and that the vast
majority of seedlings were recruited in the first
year of the regeneration cycle (Zasada et al.,
1978).
Heavy grazing by domestic and wild animals
is known to reduce the number of birch saplings
(e.g. Kinnaird, 1974) and that some form of protection such as fencing is required. New growth
of previously grazed seedlings, which may have
survived in this form for as long as 15 years, is
often very rapid when grazing is reduced,
although the danger is that trees from such a
source will be multi-stemmed (Brown, 1984).
Direct seeding
Direct seeding is widely used in many parts of
the United States to regenerate clear-cut strips
(Bjorkbom, 1969). Downy birch may be sown in
either autumn or spring (Arnborg, 1947)
whereas silver and paper birches have been
found to germinate better when autumn sown
(Nash and Duda, 1951). Seedbeds that are too
warm should be avoided. For example, two
years after seeding silver birch on scarified soils,
Luke and McPherson (1983) found 10 times as
many seedlings on cooler, sloping seedbeds with
a northerly aspect than on level seedbeds. Stratifying seed before sowing is not normally done,
however, there is evidence that pre-soaking
seeds of silver birch improves germination (Kosnikov, 1982). The germination requirements for
directly sown seeds are the same as those
described for natural regeneration.
Planting
Birch is commonly planted in Europe and is the
most practical method of introducing genetically
improved plants. Nurseries often sell birch
Although silver birch will readily regenerate
into open spaces where ground conditions are
suitable (Kinnaird, 1974), birch seedlings do not
need full sunlight to grow well and an optimum
figure of 43 per cent full light has been recorded
for silver birch (Helliwell and Harrison, 1979)
and similar figures have been published for the
North American yellow and paper birches
(Logan, 1965). Older birch trees, however, do
not tolerate shade (Lyr et al., 1963) although
precise light requirements vary for different
species. For example, downy birch tolerates
shade better than silver birch (Lappi-SepSla,
1947). Figures of 20—40 per cent canopy cover
appear to be appropriate to regenerate most
birch species (Perala and Aim, 1989, 1990).
Brown (1984) emphasized that successful
establishment of regeneration requires a vegetation-free substrate where the seedling is not subjected to frost or to summer drought and,
initially at least, freedom from competing vegetation. The birches appear to regenerate and
survive better on mineral soil than on litter or
mosses (e.g. Linteau, 1948; Sarvas, 1948; Kinnaird, 1974). The extreme surface temperatures
of litter and its limited moisture-holding capacity make it a poor germination medium (Linteau, 1948). The presence of sufficient moisture
and moderate temperatures during the growing
season are known to be important for the survival of birch seedlings. For example, a good
link has been found between the poor survival
of paper birch seedlings and hot, dry weather
(Zasada et al., 1978).
Exposing mineral soils through burning
greatly benefits birch regenerating (e.g. Lutz,
1956) due to greater availability of soil moisture
and nutrients. Fires, however, must be hot
enough to burn most of the surface organic
layer to provide good seedbeds (Mork, 1944;
Sarvas, 1948; Tubbs et al, 1983). Mechanical
cultivation either by scarifying (Raulo and
Malkonen, 1976; Perala and Aim, 1989) and
ploughing (Perala and Aim, 1989) may also be
used to provide a suitable seedbed. With few
exceptions, carrying out a seeding felling without scarification will not be sufficient to regenerate birches (e.g. Sarvas, 1948; Filip, 1973).
Almgren (1990) found that site preparation substantially improved the regeneration of silver
birch. It is not sufficient simply to exclude graz-
Regeneration systems
Clearcutting is widely used for the regeneration
of birch species (e.g. Sarvas, 1948; Nash, 1949;
Lehto, 1960; Filip, 1973; Safford, 1983; Kaunisto
and Paivanen, 1985) with the coupes large
enough to admit sufficient light but small
enough to accommodate the seed dispersal
range. Size of felling coupes varies greatly with
strip width suggested from 20 m to 60 m
depending on species. For example, Mork (1944)
recommended 40 m wide strips to regenerate silver and downy birch whereas Sarvas (1948) suggested two tree heights (50-60 m) as the
maximum seeding distance. Generally, larger
coupes are possible where the climate is cool
and moist and good seed crops are frequent
whereas smaller areas are more successful in
warm, dry conditions with small or infrequent
seed crops (Perala and Aim, 1990).
On cutting larger coupes than that already
described, use of the shelterwood system is
favoured. The value of overhead canopy to
enhance the germination of birch has already
been described in this review. A two-cut shelterwood system is recommended for yellow
birch (Tubbs et al., 1983) and shelterwood systems may be suitable for paper birch (Safford,
1983; Perala, 1988) and for both silver and
downy birch (Sarvas, 1948). For paper birch,
Safford (1983) recommends removing 60 per
cent of stand basal area leaving at least 10-12
seed trees per hectare as a uniform shelterwood.
Frivold and Mielikainen (1991) suggest leaving
20—40 seed trees of silver or downy birch per
hectare. The shelterwood should be removed
after 1 to 2 years to minimize birch suppression
(Nash, 1949; Perala, 1988) and to control shade
tolerant species. This may also be important as
shade may favour downy birch over silver birch
as the former tolerates shade better (LappiSepala, 1947). An irregular shelterwood similar
to group selection was suggested by Sarvas
(1948) to regenerate mixed stands of birch and
Norway spruce (Picea abies (L.) Kast.) on mineral soils and indicated a canopy cover of 20—40
per cent.
Although many species of birch can be coppiced, the practice is generally not recommended. Sarvas (1948) noted that silver and
downy birch coppice was of inferior quality to
seedlings, a view supported by Kauppi et al.
(1988) who found that young downy birch trees
that developed from coppice had more branches
than seedling trees. Similar findings were indicated by Cameron and Sani (1994) with silver
birch. Sarvas (1948) also noted that the capacity
under the name white birch (Betula alba L.)
which covers both silver and downy birch. Care
must be taken over choice of provenance as
non-local birch provenances may be susceptible
to late frost (Blackburn and Brown, 1988).
Soil preparation greatly facilitates the establishment of planted birch particularly by reducing competing vegetation (Raulo, 1978),
although some recently harvested areas may
need no further preparation (e.g. Bagaev, 1984).
Ploughing has been successfully used for the
establishment of plants of paper birch (Bjorkbom, 1972) and silver and downy birch (Soderstrom, 1975; Pohtilla, 1977; Raulo and Rikala,
1981). Kaunisto and Paivanen (1985) noted that
mounding provided good planting sites on peat
soils.
Birch is available from nurseries in a range of
sizes both as bare-root or container plants.
Small plants are more adaptable and consequently they establish more easily, however,
large seedlings (60-80 cm) are sometimes used in
Finland where weeds are a problem so that
planted trees are clear of competing vegetation
(Raulo, 1987). Good handling of birch plants is
essential as high temperatures and low humidity
will quickly kill the fine roots (Insley, 1979).
Planting in spring is best because autumn planting, even if successful, is often associated with
die-back during the winter months (Blackburn
and Brown, 1988). A stocking density of a minimum of 2500 plants/ha (2 X 2 m) is generally
used in Finland on sites with little or no natural
regeneration; the density reduced accordingly
depending on existing regeneration (Raulo,
1978).
While fertilizer is appropriate on nutrient
deficient sites, care must be taken to avoid
killing the seedlings by direct root contact with
the fertilizer (Kaunisto and Paivanen, 1985).
Birch plants are vulnerable to browsing damage
(e.g. Raulo and Lahde, 1977; Hannah and
Turner, 1981; Billetoft, 1983; Hoyle, 1984) so
fencing may be necessary.
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FORESTRY
of downy birch to coppice is strong in comparison with silver birch suggesting that downy
birch would be favoured in stands regenerated
this way. Almgren (1990) suggests that coppice
should only be used to fill in gaps.
Stand management
If regeneration is good and stocking high, then
cleaning will be required. In a review of birch
silviculture in Scandinavia, Frivold and
Mielikainen (1991) suggest cleaning down to
around 2-m spacing when the trees are 3—6 m
tall depending on initial stocking and the likelihood of damage through browsing and snow.
Cutting the top off the tree is usually sufficient
to kill birch (Raulo, 1987). The initial growth of
birch is fast, however, it is important that the
top of the plant is clear of competing vegetation
if it is to be successful (Raulo, 1987). Raulo
emphasized that this is particularly important
for planted trees and good weed control is
essential. He also emphasized the importance of
completing the weeding by the beginning of the
summer to always ensure good light conditions
for the plants. Perala (1988) increased the stocking but not growth of paper birch seedlings by
pre-treating the ground with glyphosate beneath
a shelterwood.
Thinning
As light demanders, birch stands need heavy
thinning to ensure minimal competition from
surrounding trees. Raulo (1987) suggests first
thinning when dominant height is between 12
and 14 m with stocking reduced from 2000-3000
stems ha"1 to 700-900 stems ha"1. Frivold and
Mielikainen (1991) emphasize the importance of
maintaining a minimum living crown length of
40-50 per cent and not to allow the expected
maincrop trees to be overtopped by competing
trees. Cameron etal. (1995) found no significant
difference in the length of merchantable stem
between heavily thinned stands of birch—where
around half the height of the tree is maintained
as living crown, and lightly thinned stands—
where about one third of the height of the tree
Protection
Birch is relatively tolerant of exposure although
in exposed locations it has poor form. In hot dry
summers birch is one of the first species to show
damage as premature browning, defoliation and
death of the bark (Evans, 1984). Birch is susceptible to damage by fire, however, young trees
usually recover producing vigorous coppice
shoots and the exposed soil surface for 2-3 years
after a fire is ideal for natural regeneration.
Sheep, cattle, deer and rabbits will all browse
birch seedlings and young trees and ultimately
may eliminate natural regeneration and stunt
the growth of trees (Evans, 1984).
Birches are sometimes defoliated by the
sawfly Croesus septentrionalis L., and the leaf
rust Melampsoridium betulinum is very common but only causes significant damage to
seedlings and young natural regeneration.
Witches' brooms, caused by Taphrina betulina
Rostrup, are found on downy birch—very rarely
Weeding and cleaning
is maintained as living crown. The importance
of maintaining the crowns of shade intolerant
species is underlined by Dawkins (1963) who
found a strong linear relationship between tree
crown diameter and breast height diameter in
shade intolerant species; shade tolerant species
do not show this clear relationship. Once a tree
is constrained, with less than one half its height
as living crown, it responds poorly to further
thinnings and, once neglected, birch stands
rarely recover. Cameron et al. (1995) showed
that relatively high rates of growth of silver
birch were possible through heavy thinning
without affecting the technical properties of the
timber. They state that logs of veneer dimensions and quality could be produced in rotations
of perhaps as little as 40 years if heavy thinning
is adopted.
Pruning may be done but it is not essential as
thinning operations alone may be enough to
remove many of the dead branches (Raulo,
1987). Dead branches can be removed at any
time of year, however, green pruning should
only be carried out after sapflow in spring or
shortly before leaf-fall in autumn, otherwise
there is a considerable risk of discolouration and
rot (Raulo, 1987).
Wood quality and end use
Birch is a diffuse porous timber, light in colour
with usually no obvious distinction between
sapwood and heartwood. There is apparently
no appreciable difference in wood properties
between silver and downy birch (Krames and
Krenn, 1986) and these compare well with the
properties of the American or Canadian white
birch. The grain is normally straight in timber
cut from trees of good form. Occasionally the
wood may exhibit figuring such as 'flamy' or
'curly' birch. Birch produces a remarkably dense
and strong timber and compares well with beech
and oak (Table 2).
Birch timber dries quickly but has a tendency
to distort (Pratt, 1986), however, drying under
restraint would help to reduce this defect. Birch
wood can be worked without difficulty with
both hand and power tools, the blunting of
which is moderate (Farmer, 1972). The wood
glues well and can be stained and polished to
good effect (Lincoln, 1986). Birch can be bent
satisfactorily provided knots and cross-grain are
absent (Farmer, 1972). Both sapwood and heartwood possess no natural durability against
fungal attack; their life in ground contact is less
than 5 years. Fortunately the sapwood and
heartwood can be penetrated completely by
preservatives during pressure
treatments
(Farmer, 1972).
Birch is commonly used for plywood and
blockboard manufacture in Finland and Russia.
Straight-gained material with few knots is desirable for plywood veneers. The uniform texture
and density of birch make it well suited for plywood commanding high prices even when the
core plies are interleaved with spruce. The timber is equally well suited for particle board production and Krames and Krenn (1986) found it
of intermediate quality between pine and spruce
particle board. The relatively high density of
birch, however, indicates that ideally it should
be used in mixtures with other, less dense
species. Birch produces good quality hardwood
pulp, and it is used for this purpose in northern
Table 2: Density and strength properties of British beech, birch, oak, sycamore and Sitka spruce
and 12 per cent moisture content (Lavers, 1983)
Species
Wood property
Density (kg m~3)
Beech
Birch
Oak
Sycamore Sitka spruce
690
670
690
560
384
Bending strength
(modulus of rupture)
(N mm2)
118
123
97
99
67
Modulus of elasticity
(stiffness)
(N mm2)
12 600
13 3000
10 100
9400
8 100
Compressive strength
parallel to grain
(N mm2)
56
60
52
48
36
1.14
1.04
0.84
0.84
0.51
Impact strength
(toughness) (m)
on silver birch, but these appear to have little
effect on the growth of trees. Cherry leaf roll
virus has been isolated from silver birch and
causes bushiness and small leaves on affected
branches. It may also be the cause of the 'flamy
birch' figuring in the wood (Cooper, 1979).
Birch wood is not durable and the bracket fungus Piptoporus betulinus (Bull, ex Fr.) Karst. is
common on standing trees. Birch is often
attacked by honey fungus Armillaria spp. Birch
that is not under stress, however, is unlikely to
succumb to serious pests or diseases.
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FORESTRY
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. .
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Productivity and potential profitability
Brown (1991) recommends that anyone interested in growing birch in Britain with the aim of
obtaining the best possible yield of timber
should only consider silver birch given its genn
erally better form and rate of growth. In Sweden,
Finland and Norway, studies have shown that
silver birch gives a 15—20 per cent greater yield
by volume than downy birch (Koivisto, 1959;
Fries, 1964; Braastad, 1966, 1967, 1977). Knowledge of the yield of birch in Britain is sketchy
but is generally believed that on reasonable sites
it will achieve YC 6-8 although on the best sites
it may achieve YC 10. In Sweden, Finland and
Norway, silver birch will typically yield 8-10 m 3
ha" 1 a"1 on fertile sites (Koivisto, 1959; Fries,
1964; Braastad, 1966; 1967, 1977; Oikarinen,
1983).
Based on the heavily thinned stands used in
the study by Cameron et al. (1995), potential
production on good sites after 40 years (estimated to be close to the age of maximum mean
a n n u a , i n c r e m e n t M YQ * i s a r Q u n d
170_200
m 3 ha" 1 . As a comparison, the YC 8 yield model
by Oikarinen (1983) for plantation silver birch
in southern Finland with two thinnings at age 15
and 30 reducing stem number by about one half
Table 3: Current Finnish veneer birch quality and dimensions standards (KeskusmetsOlautakunta Tapio,
1991)
Log minimum diameter top end over bark
Log length
18 cm
3.1 m minimum to 7.0 m maximum (by 30 cm
intervals)
Branch and knot number (per 150 cm length)
• live branches
• dead branches
no limit
5
Maximum branch thickness and knot size
• live
• dead or decayed
7 cm
4 cm
Stem bend (per log) measured on one side
(dependent on top diameter)
• 18-23 cm
• 24-35 cm
• 36 cm+
Firm wounds
2 cm
4 cm
5 cm
On one side up to 90 cm in length, and depending
on top diameter, 2—4 cm deep or 10% in diameter.
Decayed wounds are not permitted
If there are more than two quality defects then that log is not acceptable
Europe and North America. T h e high volumetric content of the fibre material, lack of extractives, relatively high length/width ratio of the
fibres and moderate fibre cell wall width all contribute to the suitability of birch for pulping
(Koch, 1985). Birch is very suitable for furniture
manufacture and high quality joinery (Lincoln,
1986), and in the USA it is one of the principal
furniture woods (Panshin and de Zeeuw, 1980).
Treated birch is suitable for fencing (Farmer,
1972), and for boxes, pallets, crates and cooperage. Where logs produce decorative veneers they
are used in panelling and marquetry (Lincoln,
1986).
Birch in mixtures
In Scotland, birch often grows in mixtures with
other species, particularly oak in the south and
west, and pine in the north and east, although
pure birch stands are common in the Highlands.
In Scandinavia, much of the birch resource is
found in mixture with spruce and/or pine. Frivold and Mielikainen (1990) found silver birch to
be a stronger competitor than spruce in young
and middle-aged stands and, in the first decade
after stand establishment, height growth was
greater than that of spruce. Downy birch was
found to be an approximately equal competitor
with spruce with similar height growth. While
the competition from the birch reduces volume
production in the spruce, studies have shown
that total volume yield in mixed silver
birch—spruce stands exceeds that of pure spruce
stands if the spruce is felled early (Mielikainen,
1985). Total volume production in mixed silver
birch—pine stands can be equal to or exceed that
of pure pine stands, if thinnings are directed at
the birch, while total volume production in mixtures with downy birch is slightly inferior
(Mielikainen, 1980). In Finland, such mixed
stands using silver birch are financially superior
to pure stands. In a review of the subject Gardiner (1968) notes that such mixtures are also
less prone to insect attack and catastrophic
windthrow. In addition, conifer form is usually
significantly improved, as is timber quality,
owing primarily to a reduction in early growth
producing a smaller juvenile core.
The
main
problem
with
even-aged
spruce-birch mixtures is the damage caused to
spruce trees by whipping and it is the main reason in Britain to remove birch from stands of
Sitka spruce. In Scandinavia, whipping damage
is severe in about 10 per cent of the spruce trees
(Frivold and Mielikainen, 1990).
Conclusion
Birch has many valuable attributes as a timber
tree and its commercial potential has been well
demonstrated in Fennoscandia and North
America. It can produce valuable timber in relatively short rotations if grown on good sites.
When regularly and heavily thinned, with the
aim of maintaining about half of the height of
the tree as living crown, diameter growth is fast
and rotations as short as 40 years may be possible on the best sites. The technical properties of
British birch timber have been shown to be
equivalent to that from continental stands and is
capable of being used for many purposes—
veneers for plywood, sawn timber, turnery, pulp
and particle board. In comparison with many
broadleaved species commonly grown in
Britain, birch is relatively easy and cheap to
regenerate and, providing that young trees are
protected from browsing, there should be no
serious pest and disease problems.
each time starting from 2300 stems ha"1 yields
235 m3 ha"1 at 40, close to the age of maximum
mean annual increment.
Timber value is critical to the success of birch
as a timber producing species. While there is a
substantial industry based on birch in Scandinavia, particularly Finland, this is not the case
in Britain where markets are mainly for firewood and pulpwood. Lorrain-Smith (1991)
showed that using current prices for these products makes birch, even on good sites, a poor
investment when compared with conifers even
when the higher grants for broadleaves are
taken into account. If, however, a modest
improvement in prices were possible based on
typical softwood prices then birch would
become a profitable exercise at YC 6. LorrainSmith (1991) pointed out that it only takes a
small increase in the price and productivity of
birch for it to become more profitable than YC
8 Scots pine. The assumption that birch timber
from British stands on good sites could attract
better prices than firewood and pulpwood was
recently demonstrated by Cameron etal. (1995).
They showed that much of the timber from
managed stands in Britain could meet the current Finnish veneer birch quality and dimension
standards (Table 3). They found that even
unmanaged stands contain some veneer standard logs. They also showed that heavy thinning
greatly enhances growth on individual trees, significantly shortening rotation length, without
compromising the technical properties of the
timber, thus further improving the economic
potential of birch.
365
366
FORESTRY
Table 4: Area and standing volume of the birch resource in Britain (Forestry Commission, 1984)
Area
(X 1000 ha)
High forest
Standing volume
(X 1 million m3)
Scrub woodland
High forest
Scrub woodland
16.7
45.9
5.6
41.7
20.4
1.1
1.45
4.04
0.61
0.081
0.189
0.102
Total
68.2
63.2
6.09
0.372
There is a substantial area of birch in Britain,
much of which is classed as scrub woodland
with many of the stems below 20 cm average
d.b.h. (Table 4). At the same time high forest
trees are frequently of poor form and this too
detracts from the value of those larger sized logs
that may be available (Brazier, 1990). These
birch woodlands must come under appropriate
management if the commercial potential of
British birch is to improve. The main effort
should concentrate on bringing existing birch
woodlands located on favourable sites back into
management. The emphasis should be on natural regeneration using local provenances where
possible. Expansion of the birch resource should
take place on good sites by a combination of
natural regeneration, planting and/or direct
seeding.
The following silvicultural recommendations
have been prepared based on this review; their
aim to ensure quality timber production in an
economical timescale. These recommendations
largely assume pure stands of birch and no distinction has been made between silver and
downy birch unless otherwise stated.
Silvicultural recommendations
Sites
• If birch is to produce quality timber in a reasonable time then it must grow on good sites
capable of sustaining a relatively high growth
rate. Good silver birch sites are characterized
by having freely drained and light mineral
soils. Downy birch does well on moist to wet
sites.
• Silver birch is generally the preferred species
due to its good growth and stem form
although downy birch can develop into good
quality stands particularly where the climate
is cooler and wetter.
• Many birchwoods growing on infertile,
exposed, upland sites are unlikely to produce
high quality timber even with intensive management and are probably best managed for
non-timber benefits such as wildlife conservation and amenity, although some useful timber such as firewood and pulpwood may be
produced.
The normal silvicultural cycle of birch woodlands
1. Regeneration felling
• Natural regeneration is the preferred method
of regenerating a prolific seed producer such
as birch although planting is quite acceptable
where seed supply is restricted or where new
sites are involved. Birch produces seed every
year so there is no need to wait for a seed year
to carry out a regeneration felling.
• The shelterwood system is generally favoured
as some overhead protection benefits regeneration success. The first step is to selectively fell
the overstorey leaving about 20-40 trees ha"1
with well-developed crowns and no obvious
defects such as twists and forks.
• Birch will readily regenerate into open areas
on the edge of woodlands or clear-cut coupes
but these should not be too big as young
Scotland
England
Wales
seedlings suffer badly from warm dry conditions without the benefit of some shelter. In
addition, the dispersal range of birch seed is
much less than that generally perceived. As a
rule, felling coupes or strips 20-60 m wide
appear to be appropriate in most situations
with perhaps larger coupes where the climate
is cool and moist and smaller areas in warm,
dry conditions.
3. Regeneration
• It is important that good regeneration is
achieved in the first year of the regeneration
cycle as studies have shown that very little is
recruited in subsequent years. If the level of
regeneration in the first year is inadequate
then planting may have to be considered.
Maintaining a relatively high stocking density
during the early years of a birch rotation is
essential for the development of stem form
•
•
•
and to reduce the number and size of knots in
the timber.
When full regeneration has been achieved the
seed trees should be removed to prevent suppression of the seedlings although leaving a
few enhances the landscape.
Collecting seed and scattering it over bare
areas is one method of improving the
prospects of regeneration. Direct seeding can
take place either in late autumn or spring.
If a site is planted then a stocking density of
at least 2500 plants ha"1 should be used.
Birch seedlings must always be taller than
competing vegetation to survive and grow.
Weed control, therefore, is essential during the
first few years.
4. Maintenance of regeneration
• Density of regeneration needs to be reduced,
in steps if the initial stocking is very high, to
about 2500-3000 stems ha"1 by the time the
trees are perhaps 3-6 m tall. Cutting the top
off the tree is usually sufficient to kill a shade
intolerant species such as birch. The aim during this period is to maintain about one half
of the height of the tree as living crown.
• Emphasis should be placed on removing poor
quality trees (forked, misshapen, poor vigour,
etc.) and having the selected dominants and
co-dominants evenly spaced.
5. Thinning
• Thinning of birch should begin when the
mean height of the stand is around 8-10 m
that may occur at anything from 15 to 25
years old depending on growth rate. At this
point at least half the trees should be removed
as birch, particularly silver birch, must have
light to grow well. The emphasis should be on
retaining dominants and co-dominants of
good form. The aim is to maintain about half
the height of the tree as living crown to maintain a high rate of growth.
• Further thinnings will be required at intervals
of 5-7 years to maintain crown depth and to
favour the best stems (straight, healthy, few
branches on the lower half of the tree).
• The final thinning should leave around
300-500 stems ha"1.
• Pruning may be done but it is not essential as
thinning operations alone may be enough to
2. Ground preparation
• Birch needs a weed-free site with a minimal
litter layer to ensure successful regeneration.
Felling and removal of the timber may be sufficient to prepare the ground surface for natural
regeneration,
however,
preparing
seedbeds by scarifying or ploughing is usually
needed both to control competing vegetation
and to provide the optimum medium for seed
germination and seedling growth. Burning
may be an option but the fire must be hot
enough to burn most of the litter layer.
• Time of greatest seed-fall is usually between
September and November so ground preparation should occur just before this.
• Planting or direct seeding may be required if
natural regeneration fails or is not possible
through the lack of seed trees. Container
plants have been found to be successful but
carefully stored, handled and planted -barerooted plants will produce good results. Collecting seed for direct seeding is readily done
during the months of August and September.
• There is a general presumption that plants
and seed should be of native origin as these
are better adapted to local climate.
• Control of grazing animals by fencing or other
means is essential for good survival and
growth. Browsed plants often become multistemmed.
•
367
368
FORESTRY
remove many of the dead branches. Green
pruning should only be done after sapflow in
spring or shortly before leaf-fall in autumn,
otherwise there is a considerable risk of discolouration and rot.
Bringing neglected birch woods back into management
• While young birch seedlings grow well under
a modest canopy, older trees will not tolerate
shade and suppression of the crown is the
inevitable result if thinning is not done. The
longer that a birch woodland remains unmanaged the less is its response to thinning and its
capacity to produce quality timber.
• Young woodlands up to the age of around 15
to 25 years with at least 2000-3000 trees ha"1
with reasonably well developed crowns and
relatively even height are worth investing time
and money into with the aim of producing
some quality timber. Such stands should be
managed according to the prescription given
above.
• Lack of cleaning and respacing may have led
to dense regeneration where the intense competition will have reduced the crown size and
thus vigour of many of the trees. Thus, cleaning and thinning should favour dominant and
co-dominant trees of good form with about
one third to one half of the height of the tree
as living crown. It is important to avoid leaving spindly trees with greatly reduced crowns
(less than one third of the height of the tree)
in the open as these are vulnerable to wind
and snow damage. Very suppressed trees will
not respond to thinning.
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Managing birch woodlands for the production of quality timber

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