Abstract Questions: Have the natural plant communities of two mined bogs experienced changes in composition and richness over a three-decade period, and are these changes associated with anthropogenic disturbances? Location: Bas-Saint-Laurent region, southeastern Que´bec, Canada. Methods: We monitored three decades of floristic changes in two disturbed bogs by revisiting 57 plots in 1998, which were previously sampled in 1965 and 1966. Changes in species richness and composition were evaluated using Wilcoxon signed rank tests, principal component analysis and partial redundancy analyses (pRDA). We also used pRDA and an indicator species analysis to determine which species had undergone the greatest changes over time. Variation partitioning was used to evaluate the relative influence of human disturbance in compositional change. Results: The main changes in the vegetation of the two bogs were 1) a decrease of overall species diversity, 2) an increase in the percent cover of trees and of species tolerant of shade or drought, and 3) a decrease in the cover of heliophilous species. Picea mariana, Sphagnum fuscum and Pleurozium schreberi increased in percent cover while Chamaedaphne calyculata and Sphagnum rubellum decreased in percent cover. Variation partitioning suggested that human activities had a significant impact on vegetation composition. Conclusion: The results indicated that substantial changes occurred in the vegetation of the natural fragments of these severely disturbed bogs. Although human activities were partially responsible for the changes, our study suggested that the drying of the peat surface due to drought during the 1960s and 1980s may have contributed to the vegetation changes
