Silver Birch, Betula pendulaMany cultivars are available including leaf variants and pendulous forms. See also B. pubescens. Sources of seed and plants Silver birch is found in most parts of Britain but is rarer in the wetter west where it is replaced by B. pubescens. It grows pure and in mixture with Pedunculate and Sessile oak, rowan and Scots pine. Progeny testing shows that careful selection of parent trees followed by cross-breeding leads to production of cultivars with rapid rates of growth, straight stems and good habit of growth. Hybrids between B. pendula and B. pubescens occur. Silviculture A light-demanding pioneer species that colonises fertile heathlands after burning is stopped, and soon appears naturally on felled woodland sites. When grown in mixture it. can damage the leading shoots of nearby trees by the whipping action of its pliant branches. Soil types Prefers brown earths of sandy or loamy texture but tolerates podsols and sands, gravels and steep scree slopes. Is less common on shallow soils over chalk. Rate of growth and yield Young trees grow quickly with single, straight stems and small upward slanting branches to form a slender, conical crown. In pure stands and well-tended mixtures the stem is straight and cylindrical. Grows rapidly up to 20 years, stops after 50 years and rarely lives more than 60 to 70 years. Commonly attains heights of 10 to 12 metres in 20 years, 15 to 18 metres in 30 years and 22 metres in 50 years with stem diameters of 40-50 cms. Protection and resistance The rust fungus Melampsorfdium betulinum is troublesome in the nursery and Polyporus betulinus is associated with the deterioration of old birches. Hardy to late spring and early autumn frosts and winter cold. Silver birch is not often damaged by rabbits or grey squirrels. It is wind firm, resistant to exposure (so useful in shelterbelts) but does not withstand salt-bearing winds. Silver birch is moderately resistant to atmospheric pollution and can be planted in towns and industrial areas. It is semitolerant of pulverised fuel ash and can be used in reclaiming derelict land. Establishing and tending Requires care in transplanting and containerised plants should be used on difficult reclaimed sites. Silver birch fruits freely and coppices strongly. Timber properties A diffuse-porous timber with fine texture, white to light brown in colour and bright. The sapwood is not distinguishable from heartwood. The weight averages 660 kg/m3 making it similar to beech in strength and to ash in toughness. Silver birch kiln dries fairly rapidly with some tendency to distort; it air dries relatively quickly. Clear timber is classified as good for bending and satisfactory for staining and polishing. The heartwood is perishable but is permeable to preservative. Amenity value The smooth silvery-white bark, pendulous secondary branches and pleasant autumn colour make it popular for amenity planting in urban and rural settings. Conservation Silver Birch supports over 200 insect species, so is an important source of food for birds. Timber value Used for furniture, turnery veneers and pulp. Posts are pressure-treated with preservative and used for fencing. It is also good firewood. Economic appraisal An under-rated and neglected species, especially in the uplands. A valuable improver of' infertile sites also a useful nurse for oak and frost tender conifers. Extensive pure stands are not advocated but it is suitable for the edges of plantations and in small groups through them. All sizes of timber can be sold but sawlogs and veneer logs must be free of knots and irregular grain. |
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