Silviculture of birch (Betula pendula Roth and Betula pubescens Ehrh.) in northern Europe

J. HYNYNEN 2 3 P. NIEMIST 1 2 A. VIHER-AARNIO 2 3 A. BRUNNER 0 2 S. HEIN 2 4 P. VELLING 2 3 0 Norwegian University of Life Sciences , s, Norway 1 Finnish Forest Research Institute , Parkano, Finland 2 Institute of Chartered Foresters , 2009. All rights reserved 3 Finnish Forest Research Institute , Vantaa, Finland 4 Forest Research Institute of Baden-Wrttemberg & University of Applied Forest Siences , Freiburg, Germany 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 scientific 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 scientific 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 briefly discussed. Summary Introduction Birch species Birches (Betula L.) are an essential ecological component in northern temperate and boreal forests. Birches are lightdemanding early successional pioneer species, which rapidly occupy open areas after forest fires and clear-cuttings due to their prolific seed production and fast juvenile growth (Fischer et al., 2002). In Europe, two commercially important treelike birch species occur naturally: silver birch (Betula pendula 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 2030 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; Johnsson, 1974; Bhat and Krkkinen, 1980; Jonsell, 2000) (Figure 1ac). In some cases, it is difficult to distinguish between silver birch and downy birch in the field on the grounds of morphological traits. A method for species identification, based on chemical properties of the bark, has been developed in Sweden (Lundgren et al., 1995). Silver birch is a diploid species with 2n = 28 chromosomes in its vegetative cells, whereas downy birch is tetraploid (2n = 56) (Helms and Jrgensen, 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 distribution area on the Eurasian continent, ranging from the Atlantic to eastern Siberia (Hultn and Fries, 1986). Both species grow throughout almost the whole of Europe, but silver birch is absent from Iceland and most of Greece and the Iberian 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) Hmet-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 countries, 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 proportion 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 evenaged artificially 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, including 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 fields (Koivisto, 1959; Fries, 1964; Raulo, 1977; Oikarinen, 1983; Gustavsen and Mielikinen, 1984; Niemist, 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 fine sandy soils. On infertile sites, growth is poor. Clay and silt soils are often too compact for silver birch. Silver birch also suffers from flooding. 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, fine 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 conifer needles (Mikola, 1954, 1985), and birch debris is less Total volume of birch, Mm3 Proportion of the total volume, % 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 Assessmen (...truncated)