Because of human disturbances and climate warming, tree encroachment has become widespread in ombrotrophic peatlands (bogs). The present study used floristic data collected in 775 survey plots in bogs in 5 bioclimatic domains or subdomains in Québec (Canada), to evaluate the impact of this phenomenon on plant richness and alpha and beta diversity. The results showed that plant richness and diversity generally increased with tree cover, mainly in the southernmost bogs. This was due to the establishment of vascular plants with a stronger affinity for drier habitats and of mosses that grow on woody debris, as well as to a decrease in obligate wetlands species and Sphagnum mosses. By contrast, the effect was less significant in bogs in more northern regions. This was probably due to closer similarities between the local plant species assemblages and the species typically found within bogs. The results also showed that the difference in the floristic community between sites in southern bogs increased with increased tree cover. Based on these results, it is important to consider methods to stop or reduce the rate of tree encroachment to ensure the conservation of the unique flora found within bogs.