Legacy effects of initial density on Scots pine (Pinus sylvestris L.) stand structure: evidence from three decades of undisturbed development

Initial planting density and spacing are key silvicultural decisions that influence forest stand development over long-time horizons, yet their long-term effects under non-managed conditions remain insufficiently documented. This study analyses a unique long-term experiment examining the undisturbed development of artificially regenerated Scots pine ( Pinus sylvestris L.) stands established under seven initial spacing variants in western Poland. Since planting in 1974, all stands have been left unmanaged, allowing natural processes such as competition and self-thinning to shape stand structure over more than three decades. Tree biometric characteristics, spatial and non-spatial structural indices, and biosocial structure were assessed at different stages of stand development. Results show that initial spacing strongly influenced tree diameter, height, crown length, mortality rates, spatial patterns, and social differentiation during early stand development. However, density-dependent self-thinning progressively reduced differences among spacing variants, leading to convergence in stand density and tree-size diversity by age 35 years. Non-spatial indices of size diversity remained generally low and were not enhanced by non-management. In contrast, spatial structure and social stability showed more persistent responses, particularly in stands established at wide, regular spacing. The findings indicate that spontaneous natural processes in even-aged Scots pine stands do not necessarily increase size diversity, highlighting that non-management alone is unlikely to generate structurally diverse stands during early stand development. Initial spacing decisions, however, can exert long-lasting effects on individual tree growth and spatial organisation, providing important guidance for adaptive silvicultural strategies under changing environmental conditions.