Thinning silver birch stands in Finland: impacts on growth, yield and carbon dynamics

Abstract

There has been a notable increase in interest in deciduous trees, particularly the silver birch (Betula pendula Roth.), in Finland and other Nordic countries. However, there is a need for updated knowledge and tools on the growth and management of silver birch. The objective of this study was to evaluate the growth and yield of silver birch stands subjected to varying intensity of thinning. The extensive set of long-term experiments with a monitoring period of 30 years on average, provides updated information on the growth and yield of mature silver birch in Finland. The experimental design comprised seven thinning treatments from below: two heavy thinnings, one heavy thinning, light and heavy thinning, two moderate thinnings, moderate and light thinning, one or two light thinnings, and an unthinned control, applied in the first commercial thinning (FCT) following a second thinning mostly after ten years. In certain instances, second one was a cautious thinning from above. Additionally, a heavy thinning was done in some plots after 20‒25 years of FCT. The findings indicated that more intensive thinning procedures resulted in a reduction in the overall yield of silver birch stands. Nevertheless, intensive thinnings exhibited a higher diameter development. The commercial wood production maintained an average level following moderate thinnings or after one heavy thinning. However, it was reduced following two heavy thinnings and increased following less intensive treatments. In particular, the removal of a significant proportion of the trees through heavy thinning led to an increase in the yield of sawlogs. Former agricultural sites exhibited higher growth rates and yields in comparison to forest sites with similar site indices. However, as the site index increased, the discrepancy in growth and yield between the site classes diminished. Conversely, former agricultural sites exhibited a markedly higher mortality rate in mature stands. Therefore, it would be prudent to consider shorter rotations in former agricultural sites in comparison to forest sites. Further research is required to investigate the impact of juvenile stand management on both yield and profitability, with a particular focus on wood quality.