Multi-layered Scots pine forests in boreal Sweden result from mass regeneration and size stratification

•All stands were multi-layered.•Age distributions were dominated by a few 20-year age classes.•The multi-layered structure developed from a more homogeneous structure.•No ingrowth of Scots pine saplings when basal area exceeded 13 m2 ha−1. Understanding historic development of multi-layered Scots pine (Pinus sylvestris L.) stands and how they became multi-layered is essential for assessing the feasibility of using the selection system in these stands. To address this we measured trees (dbh ≥ 4 cm) and saplings (height > 0.5 m dbh