Factors affecting the occurrence of biomass-dense forests in Sweden

Energy production from wood-based biomass is recognized as a sustainable energy source under current EU regulations. This approach could significantly contribute to Sweden's energy strategy, given the country's vast forestlands, which cover 27.9 million ha. This study explores factors influencing the occurrence of biomass-dense forests' - stem-dense forests with small diameter trees (BDFs) - across Sweden, considering large-scale variables, site characteristics, and management settings. Using Swedish National Forest Inventory data, we found that stand age and site productivity were the strongest predictors of BDF occurrence nationally and regionally, with older, low-productivity stands accumulating high biomass due to prolonged periods without thinning. BDFs were most common in northern regions (Northern and Southern Norrland) where long rotation cycles and seasonal thinning constrains, and high-elevation sites favored BDFs formation. Additionally, soil conditions played a critical role, with BDFs more frequent on wet, and peat soils, which are less productive and are less frequently thinned. Our results also demonstrated high complexity of site conditions and their interactive effects on the occurrence of BDFs. Nevertheless, we suggest that stand age, site productivity, tree species composition, and soil moisture can guide biomass management decisions, including thinning practices, in the context of increasing demand for renewable energy.