Silviculture of birch ( Betula pendula Roth and Betula ... · Birch seeds are small light-winged...

© Institute of Chartered Foresters, 2009. All rights reserved. Forestry, Vol. 83, No. 1, 2010. doi:10.1093/forestry/cpp035For Permissions, please email: [email protected] Advance Access publication date 28 December 2009

Introduction

Birch species

Birches ( Betula L.) are an essential ecological component in northern temperate and boreal forests. Birches are light-demanding early successional pioneer species, which rapidly occupy open areas after forest fi res and clear-cuttings due to their prolifi c seed production and fast juvenile growth ( Fischer et al. , 2002 ). In Europe, two commercially important treelike birch species occur naturally: silver birch ( Betula pen-dula Roth) and downy birch ( Betula pubescens Ehrh.).

Silver birch and downy birch resemble each other in their general appearance, i.e. they are both white-stemmed and usually monocormic trees, reaching a height of 20 – 30 m. However, they differ regarding the morphology of the

leaves, twigs, branches, bark, seeds and catkin scales, as well as cell size and wood anatomy ( Kujala, 1946 ; Johns-son, 1974 ; Bhat and Kärkkäinen, 1980 ; Jonsell, 2000 ) ( Figure 1a – c ). In some cases, it is diffi cult to distinguish between silver birch and downy birch in the fi eld on the grounds of morphological traits. A method for species identifi cation, based on chemical properties of the bark, has been developed in Sweden ( Lundgren et al. , 1995 ). Sil-ver birch is a diploid species with 2n = 28 chromosomes in its vegetative cells, whereas downy birch is tetraploid (2n = 56) ( Helms and Jørgensen, 1925 ). Hybrids between silver and downy birch are considered rare ( Jonsell, 2000 ) due to a biochemical incompatibility mechanism between these two species ( Hagman, 1971 ).

Both silver birch and downy birch have a wide natural dis-tribution area on the Eurasian continent, ranging from the

Silviculture of birch ( Betula pendula Roth and Betula pubescens Ehrh.) in northern Europe J. HYNYNEN 1 * , P. NIEMISTÖ 2 , A. VIHERÄ-AARNIO 1 , A. BRUNNER 3 , S. HEIN 4 and P. VELLING 1

1 Finnish Forest Research Institute, Vantaa, Finland 2 Finnish Forest Research Institute, Parkano, Finland 3 Norwegian University of Life Sciences, Ås, Norway 4 Forest Research Institute of Baden-Württemberg & University of Applied Forest Siences, Freiburg, Germany * Corresponding author. E-mail: [email protected]

Summary

In Europe, two commercially important treelike birch species occur naturally: silver birch ( Betula pendula Roth) and downy birch ( Betula pubescens Ehrh.). Both species have a wide natural distribution area on the Eurasian continent, ranging from the Atlantic to eastern Siberia. Although birches occur throughout almost the whole of Europe, the most abundant birch resources are in the temperate and boreal forests of Northern Europe. In the Baltic and Nordic countries, the proportion of birch out of the total volume of the growing stock varies between 11 and 28 per cent. In Northern Europe, birch is commercially the most important broadleaved tree species.

Birches are light-demanding early successional pioneer species, which grow both in mixed stands and in pure stands. This article provides an overview of the most important ecological characteristics and typical growth and yield patterns of birch, based on European scientifi c literature. Growth and yield research on birch has been relatively active in Northern Europe, where numerous growth and yield models have been developed during the last decades. In this paper, a list of published scientifi c articles on growth modelling is provided and is grouped according to the different types of model.

When growing in forest stands, birches have a relatively straight slender stem form. The current practices and silvicultural recommendations, based on research directed at high-quality timber production in silver birch stands, are reviewed. Although the emphasis is on even-aged pure silver birch stands, the management of mixed stands as well as the silviculture of downy birch and curly birch are also briefl y discussed.

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Figure 1. (a) The leaves of silver birch (on the left) are gla-brous and broadly triangular to almost rhombic in shape with acute apex double serrate margin and a slender petiole. The leaves of downy birch (on the right) are rounded triangular to ovate or suborbicular in shape with less acute apex and thick petiole. The margin is serrate but not double serrate. Photo: Erkki Oksanen. (b) Twigs of the current year of silver birch (on the left) are glabrous and covered with resinous warts and usually numerous lenticels, whereas twigs of downy birch (on the right) are pubescent without resinous warts and with few or no lenticels. Photo: Erkki Oksanen. (c) Bark of silver birch (on the left) is usually towards the base of the trunk deeply fi ssured, coarse and dark, whereas bark of downy birch (on the right) is rather smooth and not coarse and fi ssured. Photo: Erkki Oksanen.

Atlantic to eastern Siberia ( Hultén and Fries, 1986 ). Both spe-cies grow throughout almost the whole of Europe, but silver birch is absent from Iceland and most of Greece and the Ibe-rian Peninsula. Downy birch is found also in south-western Greenland and Iceland, but it is absent from most of the Mediterranean area ( Jonsell, 2000 ). In general, downy birch occurs slightly more frequently in more northern, cool and humid areas than silver birch. Mountain birch ( B. pubescens Ehrh. subsp. czerepanovii (Orlova) Hämet-Ahti), a subspecies of downy birch, forms the subarctic and subalpine timber line and a wide birch forest zone between the boreal coniferous forest and treeless tundra in Fennoscandia and north-western Russia ( Jonsell, 2000 ; Wielgolaski, 2001 ).

Birch resources

For forestry, birch is the most important broadleaved tree species in Northern and Eastern Europe. In the Nordic coun-tries, the proportion of birch out of the total volume of the growing stock varies between 11 and 16 per cent and in the Baltic countries, 17 and 28 per cent ( Table 1 ). Birch is also a very important commercial tree species in Russia and in Belarus. In Central and Southern Europe, where the propor-tion of birch out of the growing stock is only a few per cent, birch has only a marginal role in forestry. Most of the birch resources occur in mixed stands dominated by conifers but, in Northern Europe, silver birch is also grown in pure even-aged artifi cially regenerated stands.

As the most common broadleaved species in northern Europe, birches are very important for the biodiversity of coniferous forests. In different phases of succession, a large number of species feed on or live together with birch, includ-ing mycorrhiza- forming fungi, herbivores, wood- decaying fungi and saproxylic insects.

Species characteristics

Site requirements

Silver birch occurs most frequently on fertile forest site types and on afforested abandoned fi elds ( Koivisto, 1959 ; Fries, 1964 ; Raulo, 1977 ; Oikarinen, 1983 ; Gustavsen and Mielikäinen, 1984 ; Niemistö, 1995b ). The most important site characteristics for the vigorous growth of silver birch are adequate moisture and air content. The best forest sites for silver birch are sandy and silty till soils and fi ne sandy soils. On infertile sites, growth is poor. Clay and silt soils are often too compact for silver birch. Silver birch also suffers from fl ooding. The site requirements of downy birch are not as strict as those of silver birch. Downy birch can survive also on compact soils and on wet peatlands. In northernmost Europe, silver birch prefers similar sites to Scots pine, i.e. dry soils with low solute concentration, whereas downy birch dominates wet, cool, fi ne textured and poorly aerated soils ( Sutinen et al. , 2002 ).

The effect of birch on site properties differs from that of conifers. Birch leaves decompose more quickly than coni-fer needles ( Mikola, 1954 , 1985 ), and birch debris is less

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SILVICULTURE OF BIRCH IN NORTHERN EUROPE 105

cross-pollination due to a biochemical self-incompatibil-ity mechanism ( Hagman, 1971 ). Birches fl ower simultane-ously with leafi ng in the spring and the seeds ripen in July to August in Northern Europe ( Sarvas, 1952 ). Birches are prolifi c seed producers, but there is wide annual variation in the quantity and quality of the seed crop. Abundant seed crops are repeated at 2 – 3 years intervals in North-ern Europe ( Sarvas, 1948 ; Koski and Tallqvist, 1978 ). In Central Europe, birch produces seed every year ( Cameron, 1996 ). Birch seeds are small light-winged nut-lets with a good dispersal ability ( Sarvas, 1948 ; Jonsell, 2000 ; Wagner et al. , 2004 ). Their germination is regu-lated by the interaction of photoperiod and temperature ( Vaartaja, 1952 ; Black and Wareing, 1954 , 1955 ; Vanhatalo et al. , 1996 ). The wide and continuous area of distribu-tion, outcrossing breeding system, long-distance dispersal of pollen by wind, abundant seed production and good dis-persal of seeds all maintain gene fl ow from one population to another, wide genetic variation within populations and continuous variation among birch populations ( Hamrick et al. , 1992 ; Eriksson et al. , 2003 ).

Birches are also able to regenerate vegetatively by sprouting from dormant basal buds, when the apical dominance of the leader shoot is weakened or removed due to, for example felling or browsing damage ( Kauppi et al. , 1987 ; Perala and Alm, 1990 ). The number of basal buds and ability to produce coppice varies greatly from tree to tree and are affected by the age of the mother tree, site quality, stump height and felling time ( Ferm and Kauppi, 1990 ). Both silver and downy birch are able to coppice, but vigorous coppicing is regarded as a typical feature of downy birch. The growth of coppice shoots is much faster than that of planted seedlings at an early age, but they are reached and overgrown by the planted seedlings at the age of 4 – 5 years. Coppice shoots have bigger leaves, higher leaf area and higher chlorophyll con-tent, as well as a denser crown than the seed-born plants ( Kauppi et al. , 1988 ; Ferm and Kauppi, 1990 ). Coppicing of downy birch can be used as a regeneration method in short-rotation intensive management, but this option may not be profi table on the basis of biomass production and economics ( Ferm, 1993 ). More often, the vigorous coppicing of downy birch is regarded as a silvicultural problem because the sprouts are strong competitors to conifer seedlings.

Shade tolerance

Being pioneer tree species, birches are shade intolerant. They maintain their vitality and vigorous growth only when growing as dominant trees in a stand with a rela-tively wide spacing and low degree of within-stand com-petition. Silver birch is even more shade intolerant than downy birch. According to the self-thinning models for even-aged stands developed by Hynynen (1993) , the max-imum stem number of a silver birch stand with a mean diameter of 25 cm is ca. 600 stems ha � 1 , whereas in a Norway spruce stand, the corresponding stem number can be ca. 1400 ( Figure 2 ).

acidic. In birch stands, nutrient cycling can be faster than that in pure conifer stands ( Mälkönen, 1977 ; Priha, 1999 ). In birch stands, more light reaches the forest fl oor thus fa-vouring the development of ground vegetation. The root systems of birch trees are often deep and intensive. How-ever, they are very adaptive to changes in their growth en-vironment ( Laitakari, 1935 ; Köstler et al. , 1968 ; Ostonen et al. , 2007 ). The decomposition of dead roots is fast, which is favourable for soil porosity.

Regeneration ecology

Birches are monoecious wind- and cross-pollinating spe-cies with small simple fl owers situated in separate male and female catkins ( deJong, 1993 ; Jonsell, 2000 ). Their light small pollen grains ( Johnsson, 1974 ) are produced in huge quantities and carried long distances by the wind ( Hjelmroos, 1991 ). Seeds usually develop as a result of

Table 1 : The birch growing stock in different countries

CountryTotal volume of

birch, Mm 3 Proportion of the total volume, %

Countries where birch is listed among the 10 most common species Russia 11 023 14 Finland 357 16 Sweden 334 11 Belarus 233 24 Latvia 154 28 Norway 126 16 Estonia 101.6 22 Poland 76.7 4.3 Ukraine 75 4 Lithuania 69.4 17 France 38.0 1.8 Czech Republic 16.2 1.8 UK 11 3.6 Belgium 5 3 The Netherlands 3.2 5

Countries where birch is not listed among the 10 most common species Germany * <179 <5 Spain † <33 <4 Hungary † <9 <2.5 Italy † <9 <0.5 Austria † <8 <1 Slovakia † <4 <1 Croatia † <4 <1 Slovenia † <4 <1 Switzerland † <3 <1 Denmark † <0.6 2

Source: Global Forest Resource Assessment 2010. Country Reports. Forestry Department of Food and Agriculture Organization. United Nations ( http :// www . fao . org / forestry / site / fra / en / ). * Includes all the species with a low age expectance: Betula spp., Sorbus torminalis , Alnus spp., Populus spp., Prunus serotina , Prunus avium L., other broadleaved if not mentioned above. † Numbers refer to the growing stock of the 10th most common tree species.

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0

500

1000

1500

2000

2500

3000

0 5 10 15 20 25 30 35 40

Scots pine

Norway spruce

Birch

stem number, trees/ha

Mean diameter, cm

Figure 2. Relationship between stand mean diameter and maximum stem number according to the self-thinning models by Hynynen (1993) .

Growth and yield

Growth characteristics

Birch species have a sympodial height growth pat-tern, which is typical of broadleaved tree species. De-spite this pattern, the stem form of silver birch is often relatively straight and slender ( Heiskanen, 1957 ; Heräjärvi, 2001 ). Birch is a typical pioneer tree spe-cies with rapid early growth. On the best sites, birch can reach a height of up to 24 – 25 m within 30 years ( Oikarinen, 1983 ; Eriksson et al. , 1997 ). On the poor-est sites, the height increment remains relatively modest, reaching only 6 m in 30 years according to Eriksson et al. (1997) . According to Finnish studies, pure and managed silver birch stands reach the culmination of height growth at stand ages of 10 – 20 years and of volume growth 5 years later ( Raulo, 1977 ; Oikarinen, 1983 ). Growth is vigor-ous until the stand age of 40 – 50 years ( Koivisto, 1959 ; Fries, 1964 ; Oikarinen, 1983 ). Dominant height in a birch stand at the age of 50 years can be up to 30 m ( Oikarinen, 1983 ). After this, growth starts to decline and, before the age of 100 years, the vitality of birch trees decreases and they become more susceptible to decay and other defects.

According to the height curves developed in Central Eu-rope ( Lockow, 1997 ; Hein et al. , 2009 ), fl attening of the height growth in naturally regenerated birch stands occurs at somewhat earlier ages compared with the models developed in Norway ( Strand and Braastad, 1967 ). The height curves for birch plantations are only applicable in stands under 60 years old ( Oikarinen, 1983 ). They show a relatively steep height increment up until the age of 60 years ( Figure 3 ).

Annual diameter growth begins in spring after the leaves have fl ushed. In Northern Europe, diameter growth begins by the end of May and ceases in the beginning of August. In favourable growing conditions, the annual ring width is 3 – 4 mm.

In average growing conditions, ca. 70 per cent of the total biomass of a mature birch tree is allocated to the stem, ca.

10 per cent to the crown (branches and leaves) and ca. 20 per cent to the stump and coarse roots ( Repola et al. , 2007 ). However, the development of both the crown and the stem are very sensitive to stand density. Birch crowns in dense stands are much shorter than those in open stands ( Figure 4 ). With increas ing stand density, diameter growth decreases before height growth, resulting in a very slender stem form.

Volume yield

The most extensive growth and yield research on birch has been carried out in the Nordic countries, although the old-est yield tables were developed by Schwappach (1903) in Germany. According to the Finnish yield tables ( Koivisto, 1959 ), the cumulative volume yield in naturally regener-ated pure unmanaged birch stands up to the age of 80 years varies between 320 and 540 m 3 ha � 1 depending on the site productivity, which corresponds the mean annual incre-ment (MAI) between 4 and 6.75 m 3 ha � 1 . Correspondingly, total volume yield in managed silver birch plantations up to the age of 60 years varies between 360 and 560 m 3 ha � 1 (MAI 6 and 9.3 m 3 ha � 1 , respectively) ( Oikarinen, 1983 ). In Sweden, MAI of silver birch on good sites is ca. 10 m 3 ha � 1 for rotations between 30 and 60 years ( Dahlberg et al. , 2006 ). According to the yield tables of Schwappach (1903) , silver birch reaches only a cumulative volume pro-duction of 389 m 3 per hectare at the age of 80 on the best sites in Central Europe (MAI = 4.9 m 3 ha � 1 ).

In Nordic countries, birch is the most productive species of all the commercially important native broadleaved tree species. The high productivity, combined with straight and slender stems, is the main reasons why birch is the most important broadleaved tree species in forestry. How-ever, in most of European countries, the volume yield of birch is lower than that of other fast-growing broadleaves, like Fraxinus excelsior L. or Acer pseudoplatanus L. This suggests that there a completely different silvicultural strategy is required, and the growing of silver birch is a

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viable management option only if the establishment costs for alternative species are high or if the return from the production of high-quality birch timber is considerable.

Growth and yield research on birch

Growth and yield research has been the most active in the Nordic countries, where the importance of birch for for-estry is considerably greater than in Central or in Southern Europe ( Table 2 ). For this region, both growth and yield tables and statistical growth and yield models have been developed. For Sweden and Finland, models have been de-veloped that predict the risk of snow and wind damage for

birch stands ( Päätalo et al. , 1999 ; Valinger and Fridman, 1999 ). Effects of climate change on growth of birch have been predicted for Finland ( Briceno-Elizondo et al. , 2006 ; Garcia-Gonzalo et al. , 2007 ).

Management of silver birch

Goals

Silver birch can be grown either in pure stands or in mixed stands with other tree species. Most of the birch resources of Europe occur in mixed stands dominated by coniferous

Figure 3. A selection of height growth models for silver birch in Europe. (A) Site classes I – V from Strand and Braastad (1967) /Norway – naturally regenerated, (B) Site indices 30, 28, 26, 24 m from Oikarinen (1983) /Finland – cultivated, (C) Yield classes I, II from Schwappach (1903) /Germany – naturally regenerated and (D) Site indices 32, 28, . . . 16 m (base age = 100 years) from Lockow (1997) /Germany – naturally regenerated.

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Figure 4. The infl uence of stand density on the living crown ratio in silver birch stands (according to Niemistö, 1995a ).

species. Naturally regenerated birch in mixed stands is mainly harvested for pulpwood. However, silver birch stems can develop into valuable high-quality saw timber in mixed stands, if birch is favoured by management practices. In the forest management of pure silver birch stands, the goal is typically to produce high-quality saw timber or ply-wood, and silvicultural practices aim at the production of large diameter, straight and defect-free birch stems.

Regeneration

Birch as a pioneer species regenerates abundantly natu-rally if seed sources are available. Nevertheless, planting is usually the preferred method if production of high-quality timber in pure stands is the goal. Also afforestation with birch often requires planting. Birch is a frequently used pioneer in this context, especially on poor and degraded soils ( Frivold and Borchgrevink, 1981 ; Karlsson, 2002 ; Jogiste et al. , 2003 ; Liepins, 2007 ; Renou et al. , 2007 ; Uri et al. , 2007 ). Stem sprouts can be used as a regeneration method in short- rotation intensive management. Seeding is frequently mentioned as a viable option for regeneration of birch in textbooks but so far has not been commonly prac-ticed. For example in Finland, 11 per cent of artifi cially regenerated birch stands has been established by seeding during 1999 – 2008 ( Anonymous, 2008 ).

Birch is most often regenerated after clear-cutting. For natural regeneration, coupe size and shape are a compro-mise between light requirements and seed supply. Seed trees might be used on larger coupes.

Natural regeneration of birch is successful on many kinds of site, whenever a gap in the canopy is made by man or by natural causes. Natural regeneration of birch species is abundant also in cultivated young pine- and spruce-dominated stands (e.g. Götmark et al. , 2005 ; Eerikäinen et al. , 2007 ). Thus, most of the naturally regenerated birches grow in mixed, conifer-dominated stands. In Central Europe, birch can also regenerate in

oak ( Quercus robur L. or Quercus petraea (Mattuschka) Liebl.), Douglas-fi r ( Pseudotsuga menziesii (Mirbel) Franco) and even in beech ( Fagus sylvatica L.) stands ( Oosterbaan, 2000 ).

Natural regeneration is the most common and preferred regeneration method for birch in many European coun-tries. In Britain, Cameron (1996) recommends a shelter-wood system to provide some overhead protection, which is benefi cial for regeneration. The recommended number of overstorey trees with well-developed crowns is ~ 20 to 40 trees per hectare. The results of Karlsson (2001) from Sweden also suggest that a sparse shelterwood of birch can be used in the regeneration of birch. However, a high shel-terwood density decreases the survival of birch seedlings ( Nilsson et al. , 2002 ). Site preparation promotes natural regeneration (e.g. Sarvas, 1948 ; Kinnaird, 1974 ; Karlsson, 1996 ). Light soil scarifi cation is a recommended practice in the natural regeneration of birch (e.g. Karlsson, 2001 ).

There are several risks of failure in natural regeneration. Competition from ground vegetation and the coppicing of other deciduous tree species are the most important factors preventing the successful natural regeneration of birch. Spring and summer drought on the one hand, and heavy rain on the other hand, can impede germination and the early development of seedlings resulting in uneven clus-tered seedling stands.

Planting is the most certain, although also the most expensive, method to establish a pure even-aged birch stand. Planted seedlings are more competitive against the ground vegetation and coppice than naturally regener-ated seedlings. On the most fertile forest sites or on for-mer agricultural land, planting is the only regeneration alternative owing to vigorous development of the com-peting ground vegetation. After planting, weed control is found to be a necessary treatment. In Finland, planting is the main artifi cial regeneration method for silver birch.

The fi rst initiatives for plus tree selection and breeding of birch were made in Finland and Sweden in the 1940s ( Johnsson, 1974 ; Viherä-Aarnio, 1994 ). In order to improve

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Table 2 : Growth and yield tables and models for birch grouped into the following categories: Y = yield table (only in table format, model not presented), SI = height growth model (stand level), site index model, G = other stand-level growth model (volume, basal area and mean diameter), M = mortality model and self-thinning model, damages, R = recruitment model, g t = individual-tree growth model (height, diameter and volume), v t = model for stem volume, stem form, tapering, etc. and b = model for biomass, crown, branches, roots and bark thickness

Area/country Authors Type of model

Nordic countries Y SI G M R g t v t b

Finland Gustavsen and Mielikäinen (1984) x Hynynen (1993) x Hynynen et al. (2002) x x x x x Koivisto (1959) x Lehtonen et al. (2004) x Mäkinen et al. (2003) x Mielikäinen (1980) x x Mielikäinen (1985) x x x x Mielikäinen and Valkonen (1995) x Niemistö (1991) Oikarinen (1983) x x x x Repola et al. (2007) x Saramäki (1977) x x x

Norway Andreassen and Tomter (2003) x Bollandsås et al. (2008) x Braastad (1967) x x Braastad (1977) x x Eid and Oyen (2003) x Eid and Tuhus (2001) x Gobakken (2000) x Lexerød and Eid (2005) x Lexerød (2005) x Strand and Braastad (1967) x

Sweden Agestam (1985) x x Claesson et al. (2001) x x Ekö (1985) x Eriksson et al. (1997) x Fahlvik and Nyström (2006) x Fries (1964) x x Johansson (1999) x Johansson (2004) x Johansson (2006) x Johansson (2007) x Karlsson et al. (1997) x Marklund (1988) x Petersson and Stahl (2006) x Söderberg (1986) x x Tham (1988) x x Valinger and Fridman (1999) x

Other countries Austria Frauendorfer (1954) x x Estonia Jogiste (1998) x x Germany Altherr et al. (1976) x

Hein et al. (2009) x Lockow (1996 , 1997 , 2000) x x Schölch (1998) x Schwappach (1903) x x

France Prevosto et al. (1999) x x Ireland Nieuwenhuis and Barrett (2002) x The Netherlands Jansen et al. (1996) based upon Braastad (1967) x x Slovakia Halaj et al. (1987) x x Great Britain Hamilton and Christie (1971 , 1973) x x

x refers to the type of model that is presented in the cited article.

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the genetic quality of planted silver birch, an extensive breeding programme including plus tree selection, cross-ing and progeny testing was started in the 1960s in Finland ( Raulo and Koski, 1977 ; Viherä-Aarnio, 1994 ; Koski and Rousi, 2005 ). Similar breeding activities have since been initiated in other European countries as well ( Kleinschmit and Otto, 1980 ; Malcolm and Worrell, 2001 ; Stener and Jansson, 2005 ). The yield and stem quality of silver birch have been signifi cantly improved by breeding ( Hagqvist and Hahl, 1998 ). Large-scale production of genetically improved birch seed can be arranged effectively in polythene green-house seed orchards, and this has become a well-established profi table practice ( Lepistö, 1973 ; Hagqvist, 1991 ; Stener and Werner, 1997 ; Ahtikoski, 2000 ). Successful methods for the vegetative propagation of birch by tissue culture are also available ( Simola, 1985 ; Ryynänen and Ryynänen, 1986 ; Viherä-Aarnio and Velling, 2001 ).

In Nordic countries, 1-year container grown seedlings (50 – 100 cm) are often used (seedling density 500 m � 2 ; pot volume 75 cm 3 ) in silver birch plantations. Pure silver birch planta-tions are usually established with planting densities between 1600 and 2500 seedlings ha � 1 . When planting on abandoned agricultural land, herbicides may be used before planting, but in forested areas, planting is usually performed within 1 – 2 years after clear-cutting without any ground preparation.

Severe damage is often caused in young birch planta-tions by mammalian herbivores, i.e. voles, hare, moose and deer. When vole populations are high, birch planta-tions can be totally destroyed ( Koski and Rousi, 2005 ). On former agricultural land, especially, voles have a sig-nifi cant infl uence on the survival of planted birch ( Rousi et al. , 1990 ; Brunvatne, 1997 ; Hytönen and Jylhä, 2005 ). Small seedlings may be killed by voles, but larger saplings are able to heal over the wounds, although discoloration and decay often follow as a result of minor damage. Dam-age by the fi eld vole ( Microtus agrestis L.) is the most com-mon and economically the most serious, but other species of vole can also cause damage ( Rousi et al. , 1990 ). Because dense ground vegetation offers nutrition and shelter to voles, intensive site preparation and weed control are the most effective means to reduce vole populations and pre-vent vole damage. Individual tree shelters are also used, but this is a rather costly method and is mainly employed for the most valuable birch seedlings, e.g. curly birch seedlings. Mountain hare ( Lepus timidus L.) may also harm young seedlings by cutting off the top of the shoot.

Moose ( Alces alces L.) cause signifi cant damage in young birch plantations in the Nordic countries ( Heik-kilä and Raulo, 1987 ; Persson et al. , 2005 ; Viherä-Aarnio and Heikkilä, 2006 ), and it is often impossible to regener-ate birch by planting because of the high risk for moose damage. Moose browse leaves and young twigs of birch, and when striving for these, may break stems of even tall saplings ( Löyttyniemi and Lääperi, 1988 ). The damage, depending on the severity, may reduce growth or lower the stem and timber quality ( Lavsund, 1987 ; Heikkilä et al. , 1993 ; Lilja and Heikkilä, 1995 ). Stem breakage usually leads to crookedness, wood discoloration and decay of the main stem ( Heikkilä et al. , 1993 ; Lilja and Heikkilä,

1995 ). As regards height growth, the ability of birch to re-cover from moose browsing is considered good ( Heikkilä and Raulo, 1987 ; Heikkilä et al. , 1993 ). Preventing moose damage is diffi cult because the damage may occur at any time of the year over a period lasting for up to a decade. Fencing is effective, but, for example in Finland, it is only used when planting highly valuable seedlings like curly birch. Roe deer ( Capreolus capreolus L.) are also a com-mon reason for failure in birch regeneration ( Van Hees et al. , 1996 ). Local damage can be caused by white-tailed deer ( Odocoileus virginianus Zimm.). Reindeer ( Rangifer taran-dus L.) has a signifi cant effect on birch regeneration in the northern reindeer management area ( Helle, 2001 ).

In the nurseries and young plantations, birches are at-tacked by fungal and bacterial pathogens that cause black lesions and dieback of small seedlings ( Lilja et al. , 1997 ). The most common of these are Phytophtora cactorum (Leb. & Cohn) Schr. and Godronia multispora J.W. Growes. Birch-leaf rust is caused by Melampsoridium betulinum (Fr.) Kleb. which occurs in birch stands especially during rainy wet summers. In nurseries, it weakens the seedlings and increases mortality after planting ( Lilja et al. , 1997 ).

Management of established stands

Being a light-demanding tree species, the crown development and stem growth of silver birch are retarded when grow-ing at high densities. Therefore, intensive pre-commercial and commercial thinnings are required for the profi t-able production of sawn timber ( Cameron et al. , 1995 ; Niemistö, 1995a , 1995b ). In the management of birch stands, the rule of thumb is that the proportion of living tree crown should be at least 50 per cent of the tree height in order to ensure vigorous growth ( Niemistö, 1991 ). Thinning speeds up stem diameter growth and sawn timber yield, shortens the rotation and thus increases the cutting revenues ( Oikarinen, 1983 ). In silver birch stands, fast di-ameter increment does not impair wood quality ( Heräjärvi, 2001 , 2004a , 2004b ), which is often the case in conifer stands. Furthermore, a shortened rotation decreases the risk of decay, which is a typical defect in old birch stands ( Hallaksela and Niemistö, 1998 ).

Young stand management In even-aged birch stands, up to 1600 trees can reach the merchantable stem size. Therefore, precommercial thin-ning is recommended in young birch stands with high den-sities. In Northern Europe, the recommended spacing for a birch stand after precommercial thinning varies between countries from 1600 to 2500 stems per hectare ( Braastad et al. , 1993 ; Niemistö, 1995a , b ; Cameron, 1996 ; Braastad, 1998 ; Zalitis and Zalitis, 2007 ; Rytter et al. , 2008 ). In seeded and naturally regenerated young stands, more intensive silvi-cultural management practices are required than in planted stands due to higher initial density. In seeded and naturally regenerated birch stands, precommercial thinnings should be carried out two or three times, whereas in planted birch stands, no precommercial thinnings are usually needed.

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The silviculture of birch aims at homogeneous even-sized stocking. Due to competition for light within the stand, the smallest trees tend to have suppressed height develop-ment, are susceptible to snow damage and will not reach the size of merchantable stems. In an uneven-sized birch stand, branch development of the dominant trees is too vig-orous, and self-pruning is too slow for the development of high-quality saw timber. In young and dense birch stands, precommercial thinning is needed before the mean height of 7 m to ensure vigorous crown development and diameter growth of dominant trees ( Rytter and Werner, 2007 ).

Top cutting and breaking of secondary stems as an alterna-tive to traditional brush saws for precommercial thinning in birch was found to be a viable method according to Swedish re-search results ( Karlsson and Albrektson, 2000 , 2001 ; Fällman et al. , 2003 ; Ligne et al. , 2005 ). In these studies, top cutting of secondary stems up to 40 – 70 per cent of the mean height of main stems prevented the main stems becoming over-topped by secondary stems and improved their stem quality compared with traditional precommercial thinning.

Commercial thinnings The typical management schedule of a planted silver birch stand includes two commercial thinnings during the rota-tion. Relatively high thinning intensities with removal per-centages from 30 to 40 per cent are applied in order to ensure a high yield of good quality timber in fi nal fellings and adequate removals of merchantable wood in thinnings (e.g. Oikarinen, 1983 ; Rytter et al. , 2008 ).

In silver birch stands, it is recommended that the fi rst commercial thinning is carried out before the crown ratio of the dominant trees falls below 50 per cent, which is re-garded as an indicator of good vitality and vigorous growth ( Almgren, 1990 ; Niemistö, 1995a ; Cameron, 1996 ; Rytter and Werner, 2000 ; Juodvalkis et al. , 2005 ). By that time, the stems of dominant silver birch are free of living branches along the length of the butt log. In Finnish silver birch plan-tations with planting density of 1600 trees per hectare, fi rst commercial thinning is usually carried out at the dominant height of 13 – 15 m to a density of 700 – 800 trees ha � 1 . The thinning removal is mainly pulp or energy wood.

The second thinning of silver birch stands is advisable ca. 15 years after an intensive fi rst thinning ( Oikarinen, 1983 ). The stem number after thinning is ca. 350 – 400 trees per hectare. In the second thinning, the removal includes some saw logs, in addition to pulp wood. If three commercial thinnings are required, as is often case in irregular or ini-tially very dense stands, the density of the fi nal crop is the same, but the thinnings are lighter.

For naturally regenerated birch stands, the widely used silvicultural management regime for the production of high-quality timber includes intensive spacing control with the help of pre-commercial and commercial thinnings ( Braastad et al. , 1993 ; Cameron, 1996 ; Braastad , 1998 ). Because of the high initial density, pre-commercial thin-ning is needed before the trees reach 5 m dominant height in order to decrease competition and to select the best-quality trees for the future crop trees. Later, heavy commer-cial thinnings are recommended after the branch-free trunk

has been formed in order to promote diameter growth and to ensure vital crowns (e.g. Cameron, 1996 ). Throughout the rotation of silver birch stands in Nordic countries, thin-ning should not be delayed because of the high risk of snow and ice damage after heavy thinning of a dense birch stand (e.g. Fries, 1964 ). The recommended density in naturally regenerated birch stand at the time of fi nal felling varies between 300 and 600 stems ha � 1 (e.g. Braastad et al. , 1993 ; Cameron, 1996 ; Braastad, 1998 ).

In Finland, where the management of silver birch is the most widely practiced in Europe, the typical rotation in silver birch plantations varies between 40 and 60 years depending on the site productivity and the quality of the growing stock ( Oikarinen, 1983 ) ( Table 3 ). In stands where high-quality sawn timber can be produced, rotations tend to be longer than those in birch stand of poorer timber quality.

Pruning Due to the self-pruning of branches, silver birch stems are usually free of living branches up to 5 – 7 m height, i.e. along the length of the butt log, by the time of fi rst commercial thin-ning. Thus, high-quality saw timber can be grown without ar-tifi cial pruning in silver birch stands with densities over 1500 trees ha � 1 until the fi rst commercial thinning. However, high pruning of birch has been practised on a small scale in order to produce top quality timber. Documented research results are restricted to local reports based on quite limited material ( Heiskanen, 1958 ; Zumer, 1967 ). In Finland, pruning is usually carried out in two phases. Birch trees are pruned for the fi rst time at the height of 6 – 7 m up to a pruning height of 2.5 – 3 m. The number of pruned trees is 600 – 700 stems ha � 1 . The second pruning takes place when the stand height exceeds 10 m. Then 400 – 500 birch trees ha � 1 are pruned, with a pruning height of 5 – 6 m. The fi nal number of pruned trees at the time of fi nal felling is ca. 350 trees ha � 1 . Longer rotations are applied in pruned stands containing top quality trees than in stands of average timber quality.

In pruning birch, clippers are recommended instead of pruning saws in order to avoid bark and stem damage. The risk of damage is low if the diameter of the pruned branches is below 2 cm and the work carried out carefully. Birch trees should be pruned during the growing season in July or well before the start of the growing season during late winter or early spring.

Crop tree management

Crop tree management is a frequently employed silvicul-tural concept in western and central Europe. For birch, there exist many local recommendations (e.g. Bigot, 2000 ; De Grandmaison and Sevrin, 2000 ; Lemaire, 2000 , 2004 ). Some of them are based on local experience, and transfer-ring them to other conditions needs further research.

Recently, a model framework within crop tree silvi-culture for silver birch has been developed by Hein et al. (2009) . One potential outcome of crop tree-oriented silvi-culture is based on the relationship between age, diameter at breast height and crown width development. The den-sity of the stand is expected to be 95 – 120 mature dominant

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Table 3 : Growth and yield table for planted silver birch stands growing on a fertile forest site type in Southern Finland (Site index H50 = 26 m )

The Development of growing stock

Stand age (years)

Dominant height (m)

Stem number (n ha � 1 )

Basal area

(m 2 ha � 1 )Volume

(m 3 ha � 1 )

Mean diameter

(cm)

Total yield

(m 3 ha � 1 )

IV (m 3 ha � 1 year � 1 )

MAI (m 3 ha � 1 year � 1 )

10 6.9 2000 3 9 5 9 0.9 0 15 11 2000 10.2 49 8.7 49 8 3.3 20 14.4 2000 15.8 99 10.7 99 10 4.9

20 14.4 816 9.4 59 12.8 99 4.9 25 17.3 816 13.5 102 15.3 142 8.6 5.7 30 19.7 816 17 146 17 186 8.8 6.2 35 21.7 816 20.1 190 18.5 230 8.8 6.6

35 21.7 389 12.1 114 20.7 230 6.6 40 23.4 389 14.9 152 22.9 268 7.6 6.7 45 24.8 389 17.6 190 24.8 306 7.6 6.8 50 26 389 20.1 228 26.5 344 7.6 6.9 55 27.1 389 22.5 266 28 382 7.6 6.9 60 28 389 24.8 303 29.4 419 7.4 7

Removals

CuttingStand age

(years)Dominant height (m)

Stem number (n ha � 1 )

Basal area (m 2 ha � 1 )

Volume (m 3 ha � 1 )

First thinning

20 14.4 1184 6.4 40

Second thinning

35 21.7 427 8 76

Final felling 60 28 389 24.8 303

Management regime includes two commercial thinning and fi nal felling at the age of 60 years.

crop trees of 45 – 50 cm in diameter ha � 1 by the end of a rotation of 50 – 55 years. Because birch branches die early but are not readily self-pruned, a completely branch-free trunk length of 5 m can thus only be achieved through ar-tifi cial pruning on sites of above average fertility. Longer rotation times should be avoided because birch seems to develop brown heart rot at ages greater than 50 – – 55 years ( Hein et al. , 2009 ).

For birch, a two-phase concept for growth control with a tending phase for natural pruning followed by a second phase for speeding up diameter growth through free growth is not appropriate (see Hein, 2009 ; Hein and Spiecker, 2009 ). Instead, heavy thinnings until full and permanent crown release are needed to reach large diameters, which is essential for growing high valuable timber ( Hein et al. , 2009 ). These recommendations are in line with earlier fi nd-ings from Belgium (e.g. Lemaire, 2000 , 2004 ) and for other species with fast early height growth (e.g. Hein, 2009 ; Hein and Spiecker, 2009 ; Hein et al. , 2008 ).

Fertilization Fertilization is not a common practice in the management of birch stands. A few empirical fertilization trials in Finland have shown only a weak growth response to fertilization ( Oikarinen and Pyykkönen, 1981 ).

Management of mixed stands

Most of the birch resources are growing in mixed stands ( Johansson, 2003 ). In Northern Europe, the typical species composition is a birch admixture in stands dominated by Scots pine or Norway spruce. In such stands, birch will not become suppressed as easily as many other broadleaved pio-neer species. Dominant birch trees are capable of maintain-ing their vitality throughout the commercial rotation period, and their stem quality is often better than if growing in pure birch stands. Even if the total yield of mixed stands with birch does not exceed the sum of the components, some additional yield can be realized by favouring birch early in the rotation and spruce and/or pine later ( Frivold and Groven, 1996 ; Frivold and Frank, 2002 ). On fertile mineral soil sites in Northern Europe, a mixture of silver birch and Norway spruce is the most common species mixture in commercial stands. Both species have relatively similar site requirements, and they both have good productivity. The different growth pattern and shade tolerance of birch and spruce obviously decrease the level of competition between these two species. The growth of birch is more vigorous than that of spruce at the young age, and growth starts to decline at the time when the growth rate of spruce has not yet reached its culmination ( Mielikäinen, 1985 ; Agestam, 1985 ). As a shade-tolerant

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tree species, Norway spruce does not suffer too much from shading by the birch admixture and can even survive when growing in the understorey ( Mielikäinen and Valkonen, 1995 ). The growth of Norway spruce and birch in young stands of different mixtures has been described for a large number of experimental plots ( Braathe, 1988 ; Mård, 1996 ; Brække and Granhus, 2004 ; Repola et al. , 2006 ). Gobakken and Naesset (2002) developed a diameter growth model for young spruce growing in mixture with birch in Norway. Fahlvik et al. (2005) demonstrated the effect of different mixtures on total production using a Swedish growth model. Jogiste (1998) analysed the growth of mixed stands with a growth model for Estonia and demonstrated the dynamics of the growth relationship between the species as stands age. Valkonen and Valsta (2001) presented an analysis on the economics of the two-storied birch-spruce mixture in Fin-land. Their results indicated that growing birch overstorey in a spruce plantation up to commercial volume is profi table in Finnish economic and technical conditions.

In Nordic countries, typical sites for Scots pine are too poor for silver birch. However, mixtures of silver birch and Scots pine can be found on the best site types for pine. Because both species are shade intolerant, the competition between pine and birch is stronger than that between birch and spruce. How-ever, silver birch and Scots pine can be grown successfully in a mixed stand through intensive silviculture ( Mielikäinen, 1980 ). Valkonen and Ruuska (2003) established a model de-scribing the effects of birch admixtures on the growth and quality of Scots pine in Finland and demonstrated that Scots pine can compete with birch and that a birch admixture can reduce the branch sizes of pines. Early thinnings in mixed pine stands should leave enough birches and other broadleaved species as moose forage ( Härkönen, 1998 ).

A small admixture of birch in a spruce- dominated mixed stand can even slightly increase the total yield compared with that of a pure spruce stand ( Mielikäinen, 1985 ; Tham, 1988 ). Conversely, Agestam (1985) reported greater yields in pure spruce stands than in mixed stands. A small birch admixture in a Scots pine-dominated stand has a negligible effect on the yield according to Mielikäinen (1980) . Agestam (1985) found ca. 2 per cent greater yield in mixed stand of Scots pine and birch on sites with Scots pine site index of 24 m. On sites with lower site indexes yield of pine stands were found out to be higher than in mixed stands. A high proportion of birch in a mixed stand has been found to result in decreased wood pro-duction compared with that of pure conifer stands ( Heräjärvi, 2001 ). Silver birch requires wide spacing and heavy thinnings in order to maintain its vitality. Heavy thinnings, in turn, re-sult in lower wood production during the rotation.

Successful management of an even-aged mixed birch-conifer stand requires an intensive silvicultural regime be-cause of the different growth patterns of the two tree species. Being fast-growing pioneer species, birch trees readily start to suppress conifers in a sapling stand. Therefore, the co-nifer seedlings should be advanced in their early growth compared with the birches. In practice, the difference in tree ages between conifers and birches should be 5 – 10 years in naturally regenerated stands. In spruce plantations, where site preparation has been carried out, naturally regenerated

birches emerging on the site after planting cannot suppress planted spruce trees and can later be grown in the same tree storey together with spruce ( Valkonen, 2000 ).

Management of curly birch

Curly birch ( B. pendula var. carelica (Mercklin) Hämet-Ahti) is a variety of the silver birch, but curly grain is also occasionally observed in other tree species (such as downy birch, black and grey alder, mountain ash and aspen; pine and spruce) ( Heikinheimo, 1951 ; Saarnijoki, 1961 ; Velling et al. , 2000 ; Hagqvist and Mikkola, 2008 ). It grows natu-rally mainly in the Nordic countries, western Russia, the Baltic countries, Belarus and the Ukraine (e.g. Pagan and Paganová, 1994 ). There are also scattered occurrences in the eastern parts of Central Europe. In Finland, curly birches can be found in the southern third of the country, but the tree can be cultivated as far north as Lapland.

A number of forms of curly birch can be distinguished on the basis of the external appearance of the trunk: curly grain with protuberances, necks, rings and stripes ( Saarnio, 1976 ). Mixed types, usually consisting of necks and protu-berances, also occur. Typical features of the curly-grained wood include faulty cell orientation, abnormally wide wood rays and ingrown bark, giving a brown fl amy pattern.

There are a number of theories about the formation of curly (or wavy) grain, which is caused by a disturbance in the cambium ( Hintikka, 1922 ; Ruden, 1954 ; Johnsson, 1974 ; Velling et al. , 2000 ; Hagqvist and Mikkola, 2008 ). Curly grain formation is a hereditary trait: a maximum of only 50 per cent of the plants grown from the seed of an open-pollinated curly birch tree exhibit curly grain for-mation. On average, 60 – 70 per cent of the seed orchard material is curly grained, and micropropagated plants all have curly grain formation. The curly grain property fi rst becomes visible when the trees are ~ 5 to 6 years old.

The establishment procedure for a curly birch stand is similar to that for normal silver birch. Sites suitable for curly birch are light fertile soils: heavy clay or peaty soil should be avoided. Some 1600 – 2000 seedlings ha � 1 are planted. However, if there is risk of pest damage, then it is recommended to plant up to 2500 seedlings ha � 1 and/or fence the plantation. If micropropagated plantlets are used, then a planting density of as low as 800 – 1000 plant-lets ha � 1 can be employed, thereby considerably lowering costs. A mixture of micropropagated (e.g. 400) and seed-borne (1200) plants is also a good option.

Cleaning is needed to remove other species, especially broadleaved sprouts. Normal silver birch trees without any visible signs of curly-grained formation are removed at the age of ~ 10 to 13 years, i.e. when the dominant height is 7 – 9 m ( Heikinheimo, 1951 ; Velling et al. , 2000 ). Repeated thinning of the remaining curly birch individuals is needed to ensure enough light (more than for normal birch). The fi nal felling takes place at the age of 40 – 50 years.

Artifi cial pruning of the stems must preferably be started 2 – 3 years after establishment and then continued in stages ( Heikinheimo, 1951 ; Raulo et al. , 1978 ; Hagqvist and Mikkola, 2008 ). July is the best time to carry out pruning.

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Curly-grained wood is especially valuable (see Kosonen, 2004 ; Hagqvist and Mikkola, 2008 ) and unlike other wood, it is priced according to weight . With a green wood density of more than 900 kg m � 3 and a current (2008) price of curly-grained stem wood, suitable for turning, of 3 – 5 € k � 1 green wood, a price of more than 3000 € m � 3 can easily be achieved. Prices can vary considerably, de-pending on the quality of the curly grain, log length and diameter and consignment size. There is even a demand for good quality curly-grained branch wood. Small or bushy forms of curly birch not suitable for wood production can be used as decorative garden and park trees.

Management of downy birch

The main aim of growing downy birch is to produce pulp wood and fuel wood with low costs. The size or quality of the stems is often too poor for veneer and saw logs, especially when growing on peatlands and other wet sites ( Verkasalo, 1997 ). Heavy precommercial and commercial thinnings are not recommended in young downy birches ( Niemistö, 1991 ; Ferm, 1993 ). When managed for pulp wood production, young downy birch stands should be thinned at 4 – 6 m height to a density of 2000 – 2500 stems ha � 1 . It is recommended to carry out only one commercial thinning at a stand domi-nant height of 13 – 14 m to a density of 1000 stems ha � 1 . In this management regime, fi nal cutting takes place at the age of 50 – 60 years ( Niemistö et al. , 2008 ). If the management goal is the production of fuel wood, then precommercial thinning can be delayed up to a stand dominant height of 10 – 11 m. Downy birch has also been frequently used in trials with short-rotation energy crops ( Ferm, 1993 ; Paukkonen and Kauppi, 1998 ; Hytönen and Kaunisto, 1999 ; Telenius, 1999 ; Rydberg, 2000 ; Luostarinen and Kauppi, 2005 ; Walle et al. , 2007 ). According to Finnish studies, however, fuel wood production through short rotation and vegetative regeneration by coppicing is not feasible ( Hytönen and Issakainen, 2001 ).

In downy birch stands on mineral soils or the most fertile peatland sites, it is possible to grow veneer or saw logs as well as pulp wood. In this case, the second com-mercial thinning to a density of 400 – 500 stems ha � 1 in-stead of fi nal cutting is recommended at the age of 50 years. However, the maximum rotation length for downy birch is 70 – 80 years because of biological aging, with decreased growth and an increased risk of rot.

Confl ict of Interest Statement

None declared .

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Received 7 October 2008

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Silviculture of birch ( Betula pendula Roth and Betula ... · Birch seeds are small light-winged...

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