THE EFFECTS OF HEAVY THINNING ON STEM QUALITY AND TIMBER PROPERTIES OF SILVER BIRCH (BETULA-PENDULA ROTH)

A study was carried out into the effects of heavy thinning on stem qua lity and timber properties of silver birch (Betula pendula Roth). Comp arisons were made with lightly thinned and unthinned birch stands. Bir ch stands from the UK were also compared with similarly managed stands from Germany and Denmark. In general, trees from lightly thinned and unthinned stands had similar characteristics. Heavy thinning enhanced diameter increment through greater ring width in comparison with the o ther treatments. The timber properties of basic density, grain angle a nd shrinkage coefficients in the radial, tangential and longitudinal d irections, however, were largely unaffected. More and bigger branches and greater stem taper were found on trees from heavily thinned stands , but these stems still met the current Finnish veneer birch quality a nd dimension standards. Comparisons between birch stands, managed by h eavy and light thinning, from the UK and Germany and Denmark revealed no meaningful differences in timber properties. The results suggest th at veneer dimension and quality logs could be produced in the UK in ro tations as short as 40 years on good sites if heavy thinning is adopte d.