Managing boreal forests while maintaining biodiversity is challenging due to climate change and increasing resource demands. Retention forestry, in which some trees are deliberately left during harvesting, mitigates the negative impacts of clearcutting, but it remains unclear whether regeneration can be ensured as tree retention levels increase. This study assessed Scots pine (Pinus sylvestris L.) regeneration over 4.5 years in the southern boreal zone of Sweden (Effaråsen) under five treatments: four tree retention levels (3%, 10%, 30%, 50%) and a 50% retention treatment with prescribed burning. Mechanical site preparation (MSP) and regeneration methods were key drivers of success in Scots pine forest regeneration. MSP consistently and positively influenced all regeneration variables (height, growth, survival, germination, and recruitment) across planting, seeding, and natural regeneration. Direct seeding produced the highest number of seedlings per hectare, while planting yielded the tallest seedlings with high survival. Natural regeneration produced fewer and smaller seedlings and was insufficient to ensure stand establishment. Stand-scale retention levels generally did not affect regeneration, but retained trees within 20 meters negatively affected the height, growth, and survival of planted and seed-germinated seedlings, likely due to competition, indicating a proximity effect. Burned areas showed greater height and survival, suggesting that prescribed burning enhances regeneration by reducing competition while potentially creating habitat relevant for conservation and specialized species. Overall, the results highlight that retention trees intended for biodiversity have a limited role as seed trees for regeneration, and careful planning is needed to use them for biodiversity purposes without negatively impacting regeneration.