Radial trend in several anatomical characteristics is a well known phenomenon in tree trunks, as the young cambium forms juvenile wood before maturing. This phenomenon is stronger in softwoods than in hardwoods, and thus investigated less in the hardwoods. Another factor that may cause variations in the wood anatomy is growth rate, as it differs between growing sites of different fertility. In this study several anatomical characteristics were compared between silver birch (Betula pendula Roth.) trees grown on different growing sites, of which two were fertile planted sites, and two less fertile naturally regenerated sites. Several of the measured characteristics had a radial trend, commonly increasing with cambial age, but decreasing in the case of number of rays per millimetre. However, radial diameter of earlywood fibres seemed to reach a stable size around 20-30 years of age, and thus it could be suitable for determining the maturation of silver birch wood. The clearest difference in wood anatomy between growing sites was that walls of both vessels and fibres were thicker in trees of natural than planted origin. In addition, the diameter of ray cells was clearly larger in naturally regenerated than in planted birches, although ray width was similar for all origins. The differences observed in cell wall thickness between growing sites most probably affect other properties of birch wood (e.g. density and colour) that are important in the mechanical use of birch timber.

Key words:Birch, diameter, fertility, fibre, growth rate, ray, vessel, wall thickness