Effects of tree size and competition on the growth of Norway spruce and silver birch trees in monocultures and mixed planted stand during mid-rotation

In Northern Europe, growing a mixture of shade-tolerant spruce and light-demanding birch instead of monocultures of either species is a potential way to improve forest resilience and adaptation to climate change. Planting both species at the establishment phase is a rare practice in silviculture, but it can be a viable approach for success in designing the mixture. Currently, there is a lack of knowledge on how these two species respond in size-growth relationships under intra- and interspecific competition when nursery-grown seedlings are used for establishing mixtures of both species. We studied how efficiently different-sized trees utilise resources based on the size-growth relationship and evaluated competition effect on individual trees in managed planted spruce-birch mixture and compared them to monocultures of both species aged 17-20 years since afforestation on former agricultural lands. The studied mixture had an equal initial planting density of both species. Our findings revealed some differences in birch size and growth between stand types during the study period, which may partly reflect differences in their recent management histories. However, there were some indications of enhanced birch height growth in the mixture during earlier stand development. In contrast, individual tree height and basal area were lower for spruce in the mixture than in the monoculture. Although absolute periodic annual increments were also smaller in the mixture, size-growth relationships indicated that these differences were largely associated with the smaller initial size of spruce, suggesting that spruce growth had been suppressed during earlier stand development, but no longer experienced significant suppression from birch. This finding was supported by multiple regression analyses, where no statistically significant effect of interspecific competition on periodic growth during the studied three-year period was detected. Intraspecific competition and initial size were significant growth predictors. We found that the tree height to DBH ratio of spruce differed between stand types. In monoculture, smaller spruce trees were slenderer compared with trees of corresponding DBH in the studied mixed stand, indicating increased competition for light in the monoculture. In conclusion, during the studied three-year period at mid-rotation, no statistically significant influence of interspecific competition on the growth of either species was detected.