managing birch woodlands for the production of quality timber · variety of purposes including...

Managing birch woodlands for theproduction of quality timberA. D. CAMERON

Department of Forestry, University of Aberdeen, Aberdeen AB9 2UD, Scotland

Summary

Interest in silver birch (Betula pendula Roth) and downy birch (Betula pubescens Ehrh.) hasgreatly increased in recent years partly as a result of pressures to restore and expand nativewoodlands but also due to renewed interest in birch as a tree capable of producing quality tim-ber. 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 inBritain largely as a result of the poor form of the existing, mainly unmanaged resource. The fol-lowing points need to be considered if stands of quality birch trees are to be produced in an eco-nomical timescale. (1) Sites: silver birch needs good sites that are relatively well drained withlight mineral soils. Downy birch does well on moist to wet sites. (2) Regeneration: natural regen-eration through a shelterwood is the preferred system of regenerating birch as some overheadprotection 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 arerecruited in the first year of the regeneration cycle therefore planting should be considered if theinitial 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 woodsbut these should not be too big, e.g. gaps or strips 20-60 m wide have been suggested in the liter-ature. (3) Maintenance: density of regeneration needs to be reduced to about 2500-3000 stemsha"1 by the time the trees are about 3—6 m tall. Birch seedlings must always be taller than thecompeting vegetation. (4) Thinning: thinning should begin when the mean height of the stand isabout 8—10 m. At this point at least half the number of trees should be removed with the empha-sis on retaining dominants and co-dominants of good form. The aim is to maintain about halfthe height of the tree as living crown to sustain a high rate of growth. Additional thinnings willbe required at intervals of 5 to 7 years and final thinning should leave around 300-500 stemsha"1. (5) Rotation: a rotation of 40-50 years is possible on good sites and perhaps 50-55 years onless favourable sites.

n o u ion colour and bright in appearance. It is one of theBirch is the commonest native woodland tree in strongest timbers commonly grown in BritainBritain and yet until recently its potential for with a basic density of around 530 kg m~3

timber production has been virtually ignored. (Lavers, 1983), and when dried to 12 per centBirch produces a finely textured timber, white in moisture content the total weight per m3 is 630O Institute of Chartered Foratero 1996 Forestry, VoJ. 69, No. 4, 1996

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358 FORESTRY

kg. This is only slightly less than the value forhome-grown beech (690 kg m~3) and greaterthan for sycamore (560 kg m~3). The woodworks and finishes well and can be used for avariety of purposes including fittings, furnitureand paper. Birch, however, has a poor reputa-tion as a timber tree in Britain, probably as aresult of the poor form of the existing stands oftrees. By contrast, birch is an important timberspecies in Finland, for the manufacture of veneerboards, plywood and pulp, although even hereunmanaged birch was long considered a weed(Koski, 1991). Frivold and Mielikainen (1990)state that one of the factors that lead to the neg-ative attitude towards birch throughoutFennoscandia was the habit of foresters com-paring well-managed coniferous stands withunmanaged or poorly managed stands of downybirch. If British birch is to achieve an economicstatus similar to that found in Finland, thenappropriate management techniques must beadopted with the aim of quality timber produc-tion in a financially acceptable timescale.

The aim of this paper is to draw together therelevant literature on birch silviculture, produc-tivity and timber properties from NorthernEurope and North America and to make silvi-cultural recommendations appropriate to Britishconditions based on this review.

Distribution and site requirements

The genus Betula, belonging to the family Betu-laceae, comprises around 40 species distributedthroughout North America, Europe and Asiasouth to the Himalayas (Mitchell, 1976). InBritain there are two arborescent species—silverbirch {Betula pendula Roth) and downy birch{Betula pubescens Ehrh.). The main distinguish-ing morphological characteristics of silver anddowny birch are listed in Table 1.

The birches are pioneer species with life spansof around 50 to 100 years although in Scotlandit can be much older than this (Mitchell, 1976).They show the characteristic early period of fastdevelopment after which growth is slow (Kin-naird, 1968). Silver birch is associated with rel-atively well drained, light soils and downy birchwith moist to wet and frequently peaty soils(Clapham et al., 1981). Two sub-species ofdowny birch are recognized in Britain; ssp.pubescens (Ehrh.) found in the lowlands andglens and ssp. odorata (Bechst.), a shrubbier treeof the moors and mountains (Clapham et al.,1981). In Britain the range of silver and downybirch overlap but with the tendency for silverbirch to be more common in the south and eastof the country and downy birch more frequentin the north and west at higher altitudes onmore exposed sites (Kinnaird, 1968, Forbes andKenworthy, 1973) although distribution maps

Table 1: Main distinguishing morphological characteristics of silver and downy birch (Mitchell, 1976)

Silver birch Downy birch

Height

Bark

Crown

Foliage

up to 30 m

pinkish-white with horizontal greybands; white with big black diamondsin older trees

narrow, pointed and upswept in youngtrees; domed at the top with longpendulous branchlets

shoots purplish-brown, covered withraised white warts; leaves slightlyrounded-triangular tapering to a finepoint with tapering teeth separated by2-3 small triangular teeth; petioleslender with no hairs

up to 20 m

red-brown becoming smooth greyish-white, variable horizontal bands greyor brown often cherry-like inappearance until big

irregular, soon losing pointed top,rather spreading and twisted branches,shoots spreading, crowed and notpendulous

shoots greyish red-purple, covered inshort, soft, shiny white hairs; leavesrounded, often nearly circular, broadrounded teeth; petiole denselypubescent

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MANAGING BIRCHWOODS FOR QUALITY TIMBER 359

show that both species have been recorded inmost 10 kilometre squares of mainland Britain(Perring and Walters, 1976). It often grows inmixtures with other species particularly oak inthe south and west and pine in the north andeast, although pure birch stands are common inthe Highlands.

The birches are more sensitive to soil physicaland chemical properties than most other northtemperate tree species (Helliwell, 1974; Helliwelland Harrison, 1978). They are, however, knownto ameliorate unfavourable soils, their deep rootsystems bringing up and depositing nutrients onthe ground surface through leaf fall (Gardiner,1968), and have been found to reverse theprocess of podzolization leading to the forma-tion of mull type humus (Dimbleby, 1952).Raulo (1978) emphasized that silver birch growswell and produces stems of good quality only onnutrient rich sites. Raulo also noted that silverbirch seedlings grow poorly or may even die ifthe ground water table lies close to the soil sur-face.

Although birches do best on protected siteswith rich, fresh soils, they are noted for theirability to survive and grow on the most forbid-ding landscapes. Many mine spoils, landslidesand drained peatlands have regenerated natu-rally with birch (Game et al., 1982; Kaunistoand Paivanen, 1985; Kulagin, 1985) althoughsoil N must reach at least 700 kg ha"1 throughweathering before silver birch can invade byseeding (Marrs et al., 1981). Silver birch is alsowidely planted on unweathered spoil heapsbecause it tolerates soil pH as low as 3.3 (Bris-sot, 1975; Plass, 1975).

Establishment

Seed production

The birches are monoecious with flowers borneon catkins. Low spring temperature is related topoor pollen production in silver and downybirches and appears to be more important thanrain during pollen shed (Sarvas, 1952, 1956,1967). When the weather is favourable 70-80per cent of silver and downy birch pollen is shedwithin 2—3 days (Sarvas, 1952). The percentageof germinable seed is directly related to theamount of pollen shed; seed with the highest

viability will be produced during years of abun-dant flowering (Sarvas, 1952). Birch seed ripensfrom July to October depending on species withthe rate of ripening dependent on temperaturesums (Sarvas, 1956). With each silver anddowny birch catkin containing about 450 seed,seed falls can be very large with levels of177 000, 157 000 and 258 000 seeds per m2

recorded for paper birch (Betula papyriferaMarsh.), silver and downy birches respectively(Koski and Tallqvist, 1978).

Seed dispersal

Silver and downy birch disperse seed mostly inSeptember arid October (Fries, 1984). The dis-persal range of the seed is limited by its smallsize and by much more than may be expected.Sarvas (1948) observed that silver and downybirch seed dispersal declined exponentially withdistance from seed source, and very few seedstravelled more than 50 m. The theoretical max-imum dispersal distance for 20 m tall trees in a5 m s"1 wind without turbulence is 192 m for sil-ver birch and 145 m for downy birch. Given therelatively limited dispersal of birch seed, theshelterwood system, where seed trees remain ona site, has been suggested for regeneratingstands of yellow birch (Betula alleghaniensisBritton) (Tubbs et al., 1983), paper birch (Per-ala, 1988) and for both silver and downy birch(Sarvas, 1948).

Germination requirements

Seed viability of silver and downy birch on theforest floor declines dramatically after one yearto about 6 per cent (Granstr6m and Fries, 1985).Stratification is not required for birch seedregenerated under light (Brinkman, 1974). Vaar-taja (1959) showed that silver, downy and paperbirch seed germinated well in both darkness andlight at 25—35°C. At temperatures lower than25°C, germination depended on photoperiod,however, no light was needed if the seed wasstratified to break down the seed coat (Blackand Wareing, 1955, 1959). Sarvas (1950) foundthat while silver and downy birch germinatedmuch faster in light than darkness, total germi-nation was unaffected and advised that fieldsown seed should only have a light covering.

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360 FORESTRY

Although silver birch will readily regenerateinto open spaces where ground conditions aresuitable (Kinnaird, 1974), birch seedlings do notneed full sunlight to grow well and an optimumfigure of 43 per cent full light has been recordedfor silver birch (Helliwell and Harrison, 1979)and similar figures have been published for theNorth American yellow and paper birches(Logan, 1965). Older birch trees, however, donot tolerate shade (Lyr et al., 1963) althoughprecise light requirements vary for differentspecies. For example, downy birch toleratesshade better than silver birch (Lappi-SepSla,1947). Figures of 20—40 per cent canopy coverappear to be appropriate to regenerate mostbirch species (Perala and Aim, 1989, 1990).

Brown (1984) emphasized that successfulestablishment of regeneration requires a vegeta-tion-free substrate where the seedling is not sub-jected to frost or to summer drought and,initially at least, freedom from competing vege-tation. The birches appear to regenerate andsurvive better on mineral soil than on litter ormosses (e.g. Linteau, 1948; Sarvas, 1948; Kin-naird, 1974). The extreme surface temperaturesof litter and its limited moisture-holding capac-ity make it a poor germination medium (Lin-teau, 1948). The presence of sufficient moistureand moderate temperatures during the growingseason are known to be important for the sur-vival of birch seedlings. For example, a goodlink has been found between the poor survivalof paper birch seedlings and hot, dry weather(Zasada et al., 1978).

Exposing mineral soils through burninggreatly benefits birch regenerating (e.g. Lutz,1956) due to greater availability of soil moistureand nutrients. Fires, however, must be hotenough to burn most of the surface organiclayer to provide good seedbeds (Mork, 1944;Sarvas, 1948; Tubbs et al, 1983). Mechanicalcultivation either by scarifying (Raulo andMalkonen, 1976; Perala and Aim, 1989) andploughing (Perala and Aim, 1989) may also beused to provide a suitable seedbed. With fewexceptions, carrying out a seeding felling with-out scarification will not be sufficient to regen-erate birches (e.g. Sarvas, 1948; Filip, 1973).Almgren (1990) found that site preparation sub-stantially improved the regeneration of silverbirch. It is not sufficient simply to exclude graz-

ing animals as mere enclosure will allow the re-growth of grazed vegetation thus reducing thesites available for incoming birch seed. Forexample, Kinnaird (1974) found in his study thatwhile first-year seedlings were numerous on arange of sites visited, few survived for 2 yearsand the survivors became fewer with age. Graz-ing, shading and slope did not affect seedlingestablishment. Prepared seedbeds have beenfound to remain receptive to birch germinationand establishment for only 2-3 years (Sarvas,1948; Zasada et al., 1978) and that the vastmajority of seedlings were recruited in the firstyear of the regeneration cycle (Zasada et al.,1978).

Heavy grazing by domestic and wild animalsis known to reduce the number of birch saplings(e.g. Kinnaird, 1974) and that some form of pro-tection such as fencing is required. New growthof previously grazed seedlings, which may havesurvived in this form for as long as 15 years, isoften very rapid when grazing is reduced,although the danger is that trees from such asource will be multi-stemmed (Brown, 1984).

Direct seeding

Direct seeding is widely used in many parts ofthe United States to regenerate clear-cut strips(Bjorkbom, 1969). Downy birch may be sown ineither autumn or spring (Arnborg, 1947)whereas silver and paper birches have beenfound to germinate better when autumn sown(Nash and Duda, 1951). Seedbeds that are toowarm should be avoided. For example, twoyears after seeding silver birch on scarified soils,Luke and McPherson (1983) found 10 times asmany seedlings on cooler, sloping seedbeds witha northerly aspect than on level seedbeds. Strati-fying seed before sowing is not normally done,however, there is evidence that pre-soakingseeds of silver birch improves germination (Kos-nikov, 1982). The germination requirements fordirectly sown seeds are the same as thosedescribed for natural regeneration.

Planting

Birch is commonly planted in Europe and is themost practical method of introducing geneticallyimproved plants. Nurseries often sell birch

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under the name white birch (Betula alba L.)which covers both silver and downy birch. Caremust be taken over choice of provenance asnon-local birch provenances may be susceptibleto late frost (Blackburn and Brown, 1988).

Soil preparation greatly facilitates the estab-lishment of planted birch particularly by reduc-ing competing vegetation (Raulo, 1978),although some recently harvested areas mayneed no further preparation (e.g. Bagaev, 1984).Ploughing has been successfully used for theestablishment of plants of paper birch (Bjork-bom, 1972) and silver and downy birch (Soder-strom, 1975; Pohtilla, 1977; Raulo and Rikala,1981). Kaunisto and Paivanen (1985) noted thatmounding provided good planting sites on peatsoils.

Birch is available from nurseries in a range ofsizes both as bare-root or container plants.Small plants are more adaptable and conse-quently they establish more easily, however,large seedlings (60-80 cm) are sometimes used inFinland where weeds are a problem so thatplanted trees are clear of competing vegetation(Raulo, 1987). Good handling of birch plants isessential as high temperatures and low humiditywill quickly kill the fine roots (Insley, 1979).Planting in spring is best because autumn plant-ing, even if successful, is often associated withdie-back during the winter months (Blackburnand Brown, 1988). A stocking density of a min-imum of 2500 plants/ha (2 X 2 m) is generallyused in Finland on sites with little or no naturalregeneration; the density reduced accordinglydepending on existing regeneration (Raulo,1978).

While fertilizer is appropriate on nutrientdeficient sites, care must be taken to avoidkilling the seedlings by direct root contact withthe fertilizer (Kaunisto and Paivanen, 1985).Birch plants are vulnerable to browsing damage(e.g. Raulo and Lahde, 1977; Hannah andTurner, 1981; Billetoft, 1983; Hoyle, 1984) sofencing may be necessary.

Regeneration systems

Clearcutting is widely used for the regenerationof birch species (e.g. Sarvas, 1948; Nash, 1949;Lehto, 1960; Filip, 1973; Safford, 1983; Kaunistoand Paivanen, 1985) with the coupes large

enough to admit sufficient light but smallenough to accommodate the seed dispersalrange. Size of felling coupes varies greatly withstrip width suggested from 20 m to 60 mdepending on species. For example, Mork (1944)recommended 40 m wide strips to regenerate sil-ver and downy birch whereas Sarvas (1948) sug-gested two tree heights (50-60 m) as themaximum seeding distance. Generally, largercoupes are possible where the climate is cooland moist and good seed crops are frequentwhereas smaller areas are more successful inwarm, dry conditions with small or infrequentseed crops (Perala and Aim, 1990).

On cutting larger coupes than that alreadydescribed, use of the shelterwood system isfavoured. The value of overhead canopy toenhance the germination of birch has alreadybeen described in this review. A two-cut shel-terwood system is recommended for yellowbirch (Tubbs et al., 1983) and shelterwood sys-tems may be suitable for paper birch (Safford,1983; Perala, 1988) and for both silver anddowny birch (Sarvas, 1948). For paper birch,Safford (1983) recommends removing 60 percent of stand basal area leaving at least 10-12seed trees per hectare as a uniform shelterwood.Frivold and Mielikainen (1991) suggest leaving20—40 seed trees of silver or downy birch perhectare. The shelterwood should be removedafter 1 to 2 years to minimize birch suppression(Nash, 1949; Perala, 1988) and to control shadetolerant species. This may also be important asshade may favour downy birch over silver birchas the former tolerates shade better (Lappi-Sepala, 1947). An irregular shelterwood similarto group selection was suggested by Sarvas(1948) to regenerate mixed stands of birch andNorway spruce (Picea abies (L.) Kast.) on min-eral soils and indicated a canopy cover of 20—40per cent.

Although many species of birch can be cop-piced, the practice is generally not recom-mended. Sarvas (1948) noted that silver anddowny birch coppice was of inferior quality toseedlings, a view supported by Kauppi et al.(1988) who found that young downy birch treesthat developed from coppice had more branchesthan seedling trees. Similar findings were indi-cated by Cameron and Sani (1994) with silverbirch. Sarvas (1948) also noted that the capacity

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362 FORESTRY

of downy birch to coppice is strong in compar-ison with silver birch suggesting that downybirch would be favoured in stands regeneratedthis way. Almgren (1990) suggests that coppiceshould only be used to fill in gaps.

Stand management

Weeding and cleaning

If regeneration is good and stocking high, thencleaning will be required. In a review of birchsilviculture in Scandinavia, Frivold andMielikainen (1991) suggest cleaning down toaround 2-m spacing when the trees are 3—6 mtall depending on initial stocking and the likeli-hood of damage through browsing and snow.Cutting the top off the tree is usually sufficientto kill birch (Raulo, 1987). The initial growth ofbirch is fast, however, it is important that thetop of the plant is clear of competing vegetationif it is to be successful (Raulo, 1987). Rauloemphasized that this is particularly importantfor planted trees and good weed control isessential. He also emphasized the importance ofcompleting the weeding by the beginning of thesummer to always ensure good light conditionsfor the plants. Perala (1988) increased the stock-ing but not growth of paper birch seedlings bypre-treating the ground with glyphosate beneatha shelterwood.

Thinning

As light demanders, birch stands need heavythinning to ensure minimal competition fromsurrounding trees. Raulo (1987) suggests firstthinning when dominant height is between 12and 14 m with stocking reduced from 2000-3000stems ha"1 to 700-900 stems ha"1. Frivold andMielikainen (1991) emphasize the importance ofmaintaining a minimum living crown length of40-50 per cent and not to allow the expectedmaincrop trees to be overtopped by competingtrees. Cameron etal. (1995) found no significantdifference in the length of merchantable stembetween heavily thinned stands of birch—wherearound half the height of the tree is maintainedas living crown, and lightly thinned stands—where about one third of the height of the tree

is maintained as living crown. The importanceof maintaining the crowns of shade intolerantspecies is underlined by Dawkins (1963) whofound a strong linear relationship between treecrown diameter and breast height diameter inshade intolerant species; shade tolerant speciesdo not show this clear relationship. Once a treeis constrained, with less than one half its heightas living crown, it responds poorly to furtherthinnings and, once neglected, birch standsrarely recover. Cameron et al. (1995) showedthat relatively high rates of growth of silverbirch were possible through heavy thinningwithout affecting the technical properties of thetimber. They state that logs of veneer dimen-sions and quality could be produced in rotationsof perhaps as little as 40 years if heavy thinningis adopted.

Pruning may be done but it is not essential asthinning operations alone may be enough toremove many of the dead branches (Raulo,1987). Dead branches can be removed at anytime of year, however, green pruning shouldonly be carried out after sapflow in spring orshortly before leaf-fall in autumn, otherwisethere is a considerable risk of discolouration androt (Raulo, 1987).

Protection

Birch is relatively tolerant of exposure althoughin exposed locations it has poor form. In hot drysummers birch is one of the first species to showdamage as premature browning, defoliation anddeath of the bark (Evans, 1984). Birch is suscep-tible to damage by fire, however, young treesusually recover producing vigorous coppiceshoots and the exposed soil surface for 2-3 yearsafter a fire is ideal for natural regeneration.Sheep, cattle, deer and rabbits will all browsebirch seedlings and young trees and ultimatelymay eliminate natural regeneration and stuntthe growth of trees (Evans, 1984).

Birches are sometimes defoliated by thesawfly Croesus septentrionalis L., and the leafrust Melampsoridium betulinum is very com-mon but only causes significant damage toseedlings and young natural regeneration.Witches' brooms, caused by Taphrina betulinaRostrup, are found on downy birch—very rarely

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on silver birch, but these appear to have littleeffect on the growth of trees. Cherry leaf rollvirus has been isolated from silver birch andcauses bushiness and small leaves on affectedbranches. It may also be the cause of the 'flamybirch' figuring in the wood (Cooper, 1979).Birch wood is not durable and the bracket fun-gus Piptoporus betulinus (Bull, ex Fr.) Karst. iscommon on standing trees. Birch is oftenattacked by honey fungus Armillaria spp. Birchthat is not under stress, however, is unlikely tosuccumb to serious pests or diseases.

Wood quality and end use

Birch is a diffuse porous timber, light in colourwith usually no obvious distinction betweensapwood and heartwood. There is apparentlyno appreciable difference in wood propertiesbetween silver and downy birch (Krames andKrenn, 1986) and these compare well with theproperties of the American or Canadian whitebirch. The grain is normally straight in timbercut from trees of good form. Occasionally thewood may exhibit figuring such as 'flamy' or'curly' birch. Birch produces a remarkably denseand strong timber and compares well with beechand oak (Table 2).

Birch timber dries quickly but has a tendency

to distort (Pratt, 1986), however, drying underrestraint would help to reduce this defect. Birchwood can be worked without difficulty withboth hand and power tools, the blunting ofwhich is moderate (Farmer, 1972). The woodglues well and can be stained and polished togood effect (Lincoln, 1986). Birch can be bentsatisfactorily provided knots and cross-grain areabsent (Farmer, 1972). Both sapwood and heart-wood possess no natural durability againstfungal attack; their life in ground contact is lessthan 5 years. Fortunately the sapwood andheartwood can be penetrated completely bypreservatives during pressure treatments(Farmer, 1972).

Birch is commonly used for plywood andblockboard manufacture in Finland and Russia.Straight-gained material with few knots is desir-able for plywood veneers. The uniform textureand density of birch make it well suited for ply-wood commanding high prices even when thecore plies are interleaved with spruce. The tim-ber is equally well suited for particle board pro-duction and Krames and Krenn (1986) found itof intermediate quality between pine and spruceparticle board. The relatively high density ofbirch, however, indicates that ideally it shouldbe used in mixtures with other, less densespecies. Birch produces good quality hardwoodpulp, and it is used for this purpose in northern

Table 2: Density and strength properties of British beech, birch, oak, sycamore and Sitka spruceand 12 per cent moisture content (Lavers, 1983)

Wood property

Density (kg m~3)

Bending strength(modulus of rupture)(N mm2)

Modulus of elasticity(stiffness)(N mm2)

Compressive strengthparallel to grain(N mm2)

Impact strength(toughness) (m)

Beech

690

118

12 600

56

1.14

Birch

670

123

13 3000

60

1.04

Species

Oak

690

97

10 100

52

0.84

Sycamore

560

99

9400

48

0.84

Sitka spruce

384

67

8 100

36

0.51

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364 FORESTRY

Europe and North America. The high volumet- erally better form and rate of growth. In Sweden,ric content of the fibre material, lack of extrac- Finland and Norway, studies have shown thattives, relatively high length/width ratio of the silver birch gives a 15—20 per cent greater yieldfibres and moderate fibre cell wall width all con- by volume than downy birch (Koivisto, 1959;tribute to the suitability of birch for pulping Fries, 1964; Braastad, 1966, 1967, 1977). Knowl-(Koch, 1985). Birch is very suitable for furniture edge of the yield of birch in Britain is sketchymanufacture and high quality joinery (Lincoln, but is generally believed that on reasonable sites1986), and in the USA it is one of the principal it will achieve YC 6-8 although on the best sitesfurniture woods (Panshin and de Zeeuw, 1980). it may achieve YC 10. In Sweden, Finland andTreated birch is suitable for fencing (Farmer, Norway, silver birch will typically yield 8-10 m3

1972), and for boxes, pallets, crates and cooper- ha"1 a"1 on fertile sites (Koivisto, 1959; Fries,age. Where logs produce decorative veneers they 1964; Braastad, 1966; 1967, 1977; Oikarinen,are used in panelling and marquetry (Lincoln, 1983).1986). Based on the heavily thinned stands used in

the study by Cameron et al. (1995), potentialproduction on good sites after 40 years (esti-

n . . . , . , c . . . . mated to be close to the age of maximum meanProductivity and potential profitability a n n u a , i n c r e m e n t M YQ * i s a r Q u n d 1 7 0 _ 2 0 0

Brown (1991) recommends that anyone inter- m3 ha"1. As a comparison, the YC 8 yield modelested in growing birch in Britain with the aim of by Oikarinen (1983) for plantation silver birchobtaining the best possible yield of timber in southern Finland with two thinnings at age 15should only consider silver birch given its gen- 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

Log length

Branch and knot number (per 150 cm

• live branches• dead branches

Maximum branch thickness and knot

• live• dead or decayed

bark

length)

size

3.1 m minimumintervals)

18 cm

to 7.0 m maximum (by 30 cm

no limit5

7 cm4 cm

Stem bend (per log) measured on one side(dependent on top diameter)

• 18-23 cm 2 cm• 24-35 cm 4 cm• 36 cm+ 5 cm

Firm wounds 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

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each time starting from 2300 stems ha"1 yields235 m3 ha"1 at 40, close to the age of maximummean annual increment.

Timber value is critical to the success of birchas a timber producing species. While there is asubstantial industry based on birch in Scandi-navia, particularly Finland, this is not the casein Britain where markets are mainly for fire-wood and pulpwood. Lorrain-Smith (1991)showed that using current prices for these prod-ucts makes birch, even on good sites, a poorinvestment when compared with conifers evenwhen the higher grants for broadleaves aretaken into account. If, however, a modestimprovement in prices were possible based ontypical softwood prices then birch wouldbecome a profitable exercise at YC 6. Lorrain-Smith (1991) pointed out that it only takes asmall increase in the price and productivity ofbirch for it to become more profitable than YC8 Scots pine. The assumption that birch timberfrom British stands on good sites could attractbetter prices than firewood and pulpwood wasrecently demonstrated by Cameron etal. (1995).They showed that much of the timber frommanaged stands in Britain could meet the cur-rent Finnish veneer birch quality and dimensionstandards (Table 3). They found that evenunmanaged stands contain some veneer stan-dard logs. They also showed that heavy thinninggreatly enhances growth on individual trees, sig-nificantly shortening rotation length, withoutcompromising the technical properties of thetimber, thus further improving the economicpotential of birch.

Birch in mixtures

In Scotland, birch often grows in mixtures withother species, particularly oak in the south andwest, and pine in the north and east, althoughpure birch stands are common in the Highlands.In Scandinavia, much of the birch resource isfound in mixture with spruce and/or pine. Friv-old and Mielikainen (1990) found silver birch tobe a stronger competitor than spruce in youngand middle-aged stands and, in the first decadeafter stand establishment, height growth wasgreater than that of spruce. Downy birch wasfound to be an approximately equal competitor

with spruce with similar height growth. Whilethe competition from the birch reduces volumeproduction in the spruce, studies have shownthat total volume yield in mixed silverbirch—spruce stands exceeds that of pure sprucestands if the spruce is felled early (Mielikainen,1985). Total volume production in mixed silverbirch—pine stands can be equal to or exceed thatof pure pine stands, if thinnings are directed atthe birch, while total volume production in mix-tures with downy birch is slightly inferior(Mielikainen, 1980). In Finland, such mixedstands using silver birch are financially superiorto pure stands. In a review of the subject Gar-diner (1968) notes that such mixtures are alsoless prone to insect attack and catastrophicwindthrow. In addition, conifer form is usuallysignificantly improved, as is timber quality,owing primarily to a reduction in early growthproducing a smaller juvenile core.

The main problem with even-agedspruce-birch mixtures is the damage caused tospruce trees by whipping and it is the main rea-son in Britain to remove birch from stands ofSitka spruce. In Scandinavia, whipping damageis severe in about 10 per cent of the spruce trees(Frivold and Mielikainen, 1990).

Conclusion

Birch has many valuable attributes as a timbertree and its commercial potential has been welldemonstrated in Fennoscandia and NorthAmerica. It can produce valuable timber in rel-atively short rotations if grown on good sites.When regularly and heavily thinned, with theaim of maintaining about half of the height ofthe tree as living crown, diameter growth is fastand rotations as short as 40 years may be possi-ble on the best sites. The technical properties ofBritish birch timber have been shown to beequivalent to that from continental stands and iscapable of being used for many purposes—veneers for plywood, sawn timber, turnery, pulpand particle board. In comparison with manybroadleaved species commonly grown inBritain, birch is relatively easy and cheap toregenerate and, providing that young trees areprotected from browsing, there should be noserious pest and disease problems.

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Table 4: Area and standing volume of the birch resource in Britain (Forestry Com-mission, 1984)

Area(X 1000 ha)

Standing volume(X 1 million m3)

High forest Scrub woodland High forest Scrub woodland

ScotlandEnglandWales

Total

16.745.9

5.6

68.2

41.720.4

1.1

63.2

1.454.040.61

6.09

0.0810.1890.102

0.372

There is a substantial area of birch in Britain,much of which is classed as scrub woodlandwith many of the stems below 20 cm averaged.b.h. (Table 4). At the same time high foresttrees are frequently of poor form and this toodetracts from the value of those larger sized logsthat may be available (Brazier, 1990). Thesebirch woodlands must come under appropriatemanagement if the commercial potential ofBritish birch is to improve. The main effortshould concentrate on bringing existing birchwoodlands located on favourable sites back intomanagement. The emphasis should be on nat-ural regeneration using local provenances wherepossible. Expansion of the birch resource shouldtake place on good sites by a combination ofnatural regeneration, planting and/or directseeding.

The following silvicultural recommendationshave been prepared based on this review; theiraim to ensure quality timber production in aneconomical timescale. These recommendationslargely assume pure stands of birch and no dis-tinction has been made between silver anddowny birch unless otherwise stated.

Silvicultural recommendations

Sites

• If birch is to produce quality timber in a rea-sonable time then it must grow on good sitescapable of sustaining a relatively high growthrate. Good silver birch sites are characterizedby having freely drained and light mineral

soils. Downy birch does well on moist to wetsites.

• Silver birch is generally the preferred speciesdue to its good growth and stem formalthough downy birch can develop into goodquality stands particularly where the climateis cooler and wetter.

• Many birchwoods growing on infertile,exposed, upland sites are unlikely to producehigh quality timber even with intensive man-agement and are probably best managed fornon-timber benefits such as wildlife conserva-tion and amenity, although some useful tim-ber such as firewood and pulpwood may beproduced.

The normal silvicultural cycle of birch wood-lands

1. Regeneration felling• Natural regeneration is the preferred method

of regenerating a prolific seed producer suchas birch although planting is quite acceptablewhere seed supply is restricted or where newsites are involved. Birch produces seed everyyear so there is no need to wait for a seed yearto carry out a regeneration felling.

• The shelterwood system is generally favouredas some overhead protection benefits regener-ation success. The first step is to selectively fellthe overstorey leaving about 20-40 trees ha"1

with well-developed crowns and no obviousdefects such as twists and forks.

• Birch will readily regenerate into open areason the edge of woodlands or clear-cut coupesbut these should not be too big as young

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seedlings suffer badly from warm dry condi-tions without the benefit of some shelter. Inaddition, the dispersal range of birch seed ismuch less than that generally perceived. As arule, felling coupes or strips 20-60 m wideappear to be appropriate in most situationswith perhaps larger coupes where the climateis cool and moist and smaller areas in warm,dry conditions.

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 suf-ficient to prepare the ground surface for nat-ural regeneration, however, preparingseedbeds by scarifying or ploughing is usuallyneeded both to control competing vegetationand to provide the optimum medium for seedgermination and seedling growth. Burningmay be an option but the fire must be hotenough to burn most of the litter layer.

• Time of greatest seed-fall is usually betweenSeptember and November so ground prepara-tion should occur just before this.

• Planting or direct seeding may be required ifnatural regeneration fails or is not possiblethrough the lack of seed trees. Containerplants have been found to be successful butcarefully stored, handled and planted -bare-rooted plants will produce good results. Col-lecting seed for direct seeding is readily doneduring the months of August and September.

• There is a general presumption that plantsand seed should be of native origin as theseare better adapted to local climate.

• Control of grazing animals by fencing or othermeans is essential for good survival andgrowth. Browsed plants often become multi-stemmed.

3. Regeneration• It is important that good regeneration is

achieved in the first year of the regenerationcycle as studies have shown that very little isrecruited in subsequent years. If the level ofregeneration in the first year is inadequatethen planting may have to be considered.Maintaining a relatively high stocking densityduring the early years of a birch rotation isessential for the development of stem form

and to reduce the number and size of knots inthe timber.

• When full regeneration has been achieved theseed trees should be removed to prevent sup-pression of the seedlings although leaving afew enhances the landscape.

• Collecting seed and scattering it over bareareas is one method of improving theprospects of regeneration. Direct seeding cantake place either in late autumn or spring.

• If a site is planted then a stocking density ofat least 2500 plants ha"1 should be used.

• Birch seedlings must always be taller thancompeting vegetation to survive and grow.Weed control, therefore, is essential during thefirst few years.

4. Maintenance of regeneration• Density of regeneration needs to be reduced,

in steps if the initial stocking is very high, toabout 2500-3000 stems ha"1 by the time thetrees are perhaps 3-6 m tall. Cutting the topoff the tree is usually sufficient to kill a shadeintolerant species such as birch. The aim dur-ing this period is to maintain about one halfof the height of the tree as living crown.

• Emphasis should be placed on removing poorquality trees (forked, misshapen, poor vigour,etc.) and having the selected dominants andco-dominants evenly spaced.

5. Thinning• Thinning of birch should begin when the

mean height of the stand is around 8-10 mthat may occur at anything from 15 to 25years old depending on growth rate. At thispoint at least half the trees should be removedas birch, particularly silver birch, must havelight to grow well. The emphasis should be onretaining dominants and co-dominants ofgood form. The aim is to maintain about halfthe height of the tree as living crown to main-tain a high rate of growth.

• Further thinnings will be required at intervalsof 5-7 years to maintain crown depth and tofavour the best stems (straight, healthy, fewbranches on the lower half of the tree).

• The final thinning should leave around300-500 stems ha"1.

• Pruning may be done but it is not essential asthinning operations alone may be enough to

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368 FORESTRY

remove many of the dead branches. Greenpruning should only be done after sapflow inspring or shortly before leaf-fall in autumn,otherwise there is a considerable risk of dis-colouration and rot.

6. Felling and regeneration• On good sites (say around YC 8) it is possible

for birch to achieve an average d.b.h. of about30 cm in a rotation of 40-45 years, however,rotation length will need to vary depending onsite quality and markets. On less good sites(around YC 4—6) a rotation of about 50-55years is more appropriate.

• Seed trees should be left at harvesting and thecycle begins again.

• Coppicing birch is generally not recommendedas the quality is inferior to that of seedlings,however, it may be used to fill in gaps.

Bringing neglected birch woods back into man-agement

• While young birch seedlings grow well undera modest canopy, older trees will not tolerateshade and suppression of the crown is theinevitable result if thinning is not done. Thelonger that a birch woodland remains unman-aged the less is its response to thinning and itscapacity to produce quality timber.

• Young woodlands up to the age of around 15to 25 years with at least 2000-3000 trees ha"1

with reasonably well developed crowns andrelatively even height are worth investing timeand money into with the aim of producingsome quality timber. Such stands should bemanaged according to the prescription givenabove.

• Lack of cleaning and respacing may have ledto dense regeneration where the intense com-petition will have reduced the crown size andthus vigour of many of the trees. Thus, clean-ing and thinning should favour dominant andco-dominant trees of good form with aboutone third to one half of the height of the treeas living crown. It is important to avoid leav-ing spindly trees with greatly reduced crowns(less than one third of the height of the tree)in the open as these are vulnerable to windand snow damage. Very suppressed trees willnot respond to thinning.

• Trees in older, neglected woodlands decline invigour over time, as their crowns becomereduced through competition, and will notrespond to thinning. These stands shouldundergo the regeneration cycle describedabove.

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Received 4 October 1995

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