Growth and quality of silver birch with different improvement levels

Sammanfattning: Silver birch is the most important broadleaf tree species in the context of wood production in Fennoscandia stating 12.9% of the total volume in Sweden (Nilsson et al., 2021). The issue is that birch wood currently available from Swedish forests is usually of low-quality classes and thereby is used mainly for pulp and fuelwood therefore its economic potential is not fully used and the willingness to invest in planting birch is low. Luckily tree breeding leads to the creation of fast-growing genotypes with higher quality. To increase interest in planting genetically improved silver birch (Betula pendula Roth.) further research is needed to assess the economic profit and popularize the knowledge about benefits coming from using genetically improved material. In this study, data from two field trials (Remningstorp and Tagel) have been used where the same 10 material groups were planted. The material groups included 22 genotypes created in the elite populations (full-sib crossings), progenies of 14 selected +Trees, 50 comparison trees selected in proximity to +Trees, and 7 reference sorts. The planting design was randomized with a 2 x 2 m spacing in each trial. The measurements and assessment conducted in the field covered diameter at breast height (DBH), stem (SQ), and branches quality (BQ). The presence of damage meaning the occurrence of spike knots, double stems, and double tops were registered. The objective of this study was to examine the differences between different planting materials and to select the best individuals for future breeding. The results showed that genetic improvement in silver birch brought improvement in diameter growth and quality characteristics compared to comparison trees. The offspring from the greenhouse plantation Ekebo 1 exhibited the largest diameter, showing a 22% increase. Improved material from Ekebo 4 seed orchard had the highest stem and branches quality with the lowest frequency of spike knots and double tops occurrence. Furthermore, the offspring of +Trees, selected based on phenotypic traits, outperformed comparison trees in diameter (4% increase), stem quality, and branches quality. The occurrence of damages was varied as the occurrence of spike knots and double tops was higher in +Trees but double stems were less frequent in this group. There was also some variability observed in the comparison on the stand level, where the advantage of +Trees was less pronounced. Due to the fact that phenotypic differences might be affected by many factors including the site, further research is needed on this topic. Lastly, the elite populations exhibited an average of 9% better diameter growth than comparison trees. The results showed high variability within Elite populations both in diameter and occurrence of damages. These findings highlight the positive impact of genetic improvement in silver birch, emphasizing the potential for enhancing diameter growth and quality traits through breeding programs.