Planting of spruce on the sites of natural beech forests is very common across Europe. Its effect on ground vegetation is assumed to be strong on poorly buffered soils. We investigate associated patterns of herb layer composition and diversity on crystalline and carbonate parent rock in the Western Carpathians. We analysed a series of vegetation plots with respect to the partial relationship of spruce cover with vascular understorey species (ordination), to the affinity of plant species to sample groups with different cover of spruce and to plant diversity and species turnover within these groups. In our data set spruce and beech were the most important tree species with the closest association with understorey composition and their covers explained similar proportions of herb layer variability. Species composition was significantly different under spruce canopies, but species richness was not lower (crystalline region) or even higher (carbonate region), and overall diversity of vascular plants was only slightly affected. Most species negatively or positively associated with spruce differed between bedrock types, but those with the same positive relationship are diagnostic for natural spruce forests (Vaccinio-Piceetea). The species turnover between beech- and spruce-dominated stands was much higher on crystalline bedrock. The higher amounts of light and acid litter in spruce stands, limited growth of spruce on dolomite bedrock and the competitive pressure of beech seem to be the most important drivers of the herb layer changes.