Conversion of pure Norway spruce stands is one of the principal silvicultural challenges in Europe. With forest grazing, charcoaling, and clear-cutting for spruce planting, man has severely altered Alpine forests that we often deem to be natural. Natural regeneration of forests replaced by spruce is often rendered difficult and the success of regeneration is the key element of sustainability of the forest and all of its functions. The study was carried out in subalpine fir-beech forests replaced by spruce in the study areas Jelendon and Krašica. Measurements were carried out in 2003 and 2008 on 18 plots in Jelendol, and in 1993, 1998 and 2010 on 32 plots in Krašica. The impact of ecological factors on success of natural regeneration was studied with linear mixed models and generalised linear mixed models. In both study areas we established successional development towards a greater share of broadleaves; spruce and beech are being recruited in Jelendol and spruce and sycamore maple in Krašica. Tree species have different needs with regard to ecological factors and the ecological requirements change depending on the height of regeneration. The presence of seed trees is the key factor for beech regeneration. Establishement of seedlings is best below closed canopies and in small gaps, but survival of sufficient number of seedlings requires sufficient amount of diffuse radiation, which is not available until gaps are medium-sized. In both study areas regeneration required more light for continued growth; in the lower-altitude Krašica, gaps the size of one tree height are sufficient, while in Jelendol gaps must be larger. Development of regeneration is significanty slower in Jelendol. In both sites regeneration is strongly affected by overbowsing.