The present study aimed to reconstruct the floristic evolution of the bog at the Base de plein air de Sainte-Foy (Québec City, Québec, Canada) between 1978 and 2015, particularly considering the tree encroachment apparently occurring at the site. The specific objectives were to identify which species had shown the greatest change in frequency and abundance, and to assess how such modifications had altered the overall floristic composition of the bog. Thirty-five plots sampled in 1978 were re-sampled in 2015. There was a 152% increase in plant richness between the two sample dates, and the plant assemblages were markedly different (Sørensen dissimilarity index of 0.62). There was a notable decrease in the frequency and abundance of typical wetland associated species over time. By contrast, forest-associated species increased significantly. Biotic differentiation (an increase in beta diversity) was also observed, indicating that plant assemblages differed more between sampling plots in 2015 than they did in 1978. Management approaches for the protection of the bog are discussed.