Impact of tree age on values of selected parameters and structural indices of wood fibres of silver birch (Betula pendula Roth.) in Poland

Silver birch is one of the most important forest-forming tree species in Poland and north-eastern Europe. Birch fibres are a major source of paper and fibre-based products. Tree age impacts the fibre structure, affecting the pulp quality produced from them. Understanding this effect is key to ensuring the sustainable use of wood. This paper presents the results of the most extensive study in Poland concerning the variability of selected parameters and structural indices of silver birch fibre structure depending on tree age. The study covered 23 locations in 3 age groups of 30, 50, and 70 years, where 414 trees were felled. A total of 12,420 fibres were measured. Tree age has a statistically significant effect on selected parameters and structural indices of silver birch fibres, except for the solids index. Mean fibre length, width, wall thickness, and slenderness ratio increased with tree age. Lumen width, Runkel ratio, rigidity index, Mühlsteph’s index, and flexibility coefficient values of 50-year-old trees differed from those of other age classes. Tree age has a relatively great and statistically significant effect on fibre diameter, wall thickness, and slenderness index. The study of the characteristics of birch wood aged 30, 50, and 70 years shows its high potential value in the pulp and paper industry. Due to the higher value and size of older wood, and for economic reasons, the paper industry uses wood from younger trees or older trees that do not qualify for industrial standards.