In Europe, 6–7 million ha even-aged, pure stands of Norway spruce grown outside its natural range are prone to butt rot, windthrow, and bark beetle infestation. Climate change may well accelerate these problems and there is an urgent need to develop methods for converting Norway spruce plantations into more heterogeneous forest with larger resistance and resilience towards natural disturbances. Based on a field experiment located on former heathland on an outwash plain in western Denmark, we examined the mortality and height growth of European beech (Fagus sylvatica L.), European silver fir (Abies alba Mill.), and Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) planted under a shelterwood of 17–19 m tall Norway spruce (Picea abies (L.) Karst.) at shelter densities initially ranging by basal area from 10 to 36 m2 ha−1 (271–1099 stems ha−1). Underplanting included plots of beech (100 per cent) and silver fir (100 per cent) as well as plots with two-species mixtures of silver fir/beech (66 per cent/33 per cent and 33 per cent/66 per cent) and Douglas fir/beech (66 per cent/33 per cent). The experiment also included two soil scarification methods (patch and strip scarification). The experiment was laid out in an unbalanced factorial split-plot design, replicated in time and space, and totaling 144 plots in four blocks. The experiment was followed for 19 years during which a gradual reduction in shelter density took place although in some plots the shelterwood was removed abruptly by windthrow and bark beetle infestation. Shelter densities larger than 10 m2 ha−1 had only a minor effect on the mortality of beech and silver fir, while shelter densities larger than 20 m2 ha−1 resulted in increased mortality of Douglas fir. Subsequent abrupt reduction in shelter density resulted in increased mortality, especially of silver fir. Height growth decreased with increasing shelter density, more for Douglas fir than for beech, and marginally more for beech than for silver fir. For shelter densities <15 m2 ha−1, height growth of silver fir was unaffected. Beech grew faster when mixed with Douglas fir. The soil scarification method did not influence mortality, but strip scarification resulted in marginally larger height growth than patch scarification. We hypothesize that the shelter effect was related mainly to competition for light, while late frost and browsing were important factors in open shelters (<10 m2 ha−1). Based on these results, recommended initial shelter densities are at or <10 m2 ha−1 for the establishment of Douglas fir, at ~15–20 m2 ha−1 for silver fir, and around 15 m2 ha−1 for beech.