The effects of forest floor disturbance by six mechanical site preparation (MSP) treatments on structural and compositional diversity of early successional communities were examined in a clearcut in central Ontario. Periodic assessment of floristic diversity and abundance was performed to quantify treatment effects on species richness and abundance of three discrete plant life forms (cryptogam, herbaceous and woody) in the first five growing seasons after MSP. Botanical surveys of the surrounding uncut forest provided an estimate of pre-harvest forest species composition of the clearcut study site. Intensity of forest floor disturbance differed significantly among MSP treatments, creating a variety of microsites. The percentage of mineral soil exposure and abundance of downed woody material ranged from 7% to 52% and 18% to 1%, respectively, from the least to most severe treatments. Five years after MSP, 134 species (50 cryptogams, 47 herbaceous, 37 woody) were present in the clearcut study site, compared with 102 species (54 cryptogams, 25 herbaceous, 23 woody) in the uncut forest. Species richness and abundance of cryptogam, herbaceous and woody plant species increased with time after MSP, except for herbaceous species richness. MSP treatment significantly influenced species richness of cryptogams, herbaceous and woody plants, and abundance (percent cover) of woody plants. Treatment differences in richness and abundance were strongly correlated with intensity of forest floor disturbance. Scalping treatments that created the most mineral soil exposure and removed the most downed woody material typically supported the lowest species diversity and lacked several native forest species. Cryptogams were most affected by MSP, with higher richness and abundance where forest floor disturbance was relatively low. MSP did not affect the incidence of alien plant species: only two alien species were observed on the study site, representing less than 1% of total cover. Evaluation of MSP effects on early successional plant communities based on beta diversity in an ordination analysis revealed several distinct species groups associated with specific treatments. The relative amount of forest floor disturbance and downed woody material among treatments were strong predictors of species groupings within the ordination. Plant communities in slash piles were distinct from those resulting from MSP and contained several remnant forest species. Examination of species’ autecology provided supporting evidence that the displacement of downed woody material and forest floor organic layers was the leading cause of changes in species richness and abundance on this site.