Long-term development of natural regeneration in irregular, mixed stands of silver fir and Norway spruce

•Regeneration development can be modeled with understory and overstory attributes.•High canopy cover and low structural diversity slowed juvenile height development.•Competition from taller juveniles substantially slowed height development.•Silver fir reached a browsing-free height of 2 m faster than Norway spruce.•Fir-dominated cohorts had a stronger impact on spruce density than vice-versa.