Regeneration of Norway spruce in conversion forest management : analysing natural regeneration in the conversion phase from a single layered to a multi-layered forest structure

Sammanfattning

Swedish forestry is dominated by even-aged, single-layered stands managed through the clear-cut system, a practice that has increased timber yields but also led to the decline of complex forest structures and associated biodiversity. Alternative management methods are being promoted which implies conversion of a large proportion even-aged Norway spruce (Picea abies) stands to multi-layered structures. However, practical knowledge about effective conversion methods is scarce. This thesis investigates how different thinning strategies, site preparation and fertilization affect natural regeneration during the early phase of converting even-aged Norway spruce stands to multi-layered forests. The study is based on a unique long-term field experiment established at four sites in Sweden, with two sites in the north (Jämtland) and two in the south (Småland). Regeneration was assessed in relation to treatment and site conditions, with focus on seedling abundance, species composition, and ground vegetation. Results show that heavy thinning was the only treatment that consistently stimulated regeneration, significantly increasing seedling numbers compared to untreated controls. Other treatments, such as different thinning strategies, fertilization, and site preparation, had little or no effect on overall seedling establishment. Regeneration was significantly higher at the northern sites than at the southern sites, a difference that is more likely influenced by site specific factors such as stand age, soil moisture, soil fertility and seed production dynamics. One of the sites showed little or no regeneration regardless of treatment. The findings highlight the complexity of regeneration dynamics in conversion management and suggest that successful transformation to multi-layered Norway spruce stands depends not so much on silvicultural treatments but rather on a combination of site-specific factors. More long term field experiments are needed to learn about these factors.