Forest expansion (afforestation) is one of the main vegetation changes currently observed in peatlands worldwide in response to natural and anthropogenic disturbances. We examined the relationships between plant and bird richness and frequency and extent of the forest cover in bogs of southeastern Québec to evaluate the impact of forest expansion on typical bog species. A total of 154 plant species and 36 bird species were recorded in the 16 peatlands studied (2-189 ha). Richness and frequency of ombrotrophic plant species were negatively associated with an increase in forest cover. Yet, minerotrophic and minerotrophic-ombrotrophic species did not appear to take advantage of this decline. Afforestation influenced bird species composition by altering the vegetation structure in all strata of the bogs (fewer mosses and shrubs, more trees) and by homogenizing the spatial distribution of plant communities (open patches progressively replaced by forested patches). Peatlands of southeastern Québec remain islands of boreal vegetation in a matrix of deciduous forests and agricultural fields, but afforestation appears to progressively reduce the diversity of peatland habitats. Afforestation does not imperil plant or bird species across their entire range, but it contributes to impoverishing regional biodiversity.