Many studies have shown that certain species of bryophytes and lichens require old-growth forests for their survival. The objective of this study is to evaluate the composition and diversity of epiphytic lichen and bryophyte communities on trembling aspen (Populus tremuloides), as a function of the time elapsed since stand initiation. The study was carried out in the forests surrounding Lake Duparquet, in the southwestern part of Québec's boreal forest. Stands representing different post-fire successional stages, corresponding to 278, 125, 79, and 51 yr since the last fire were selected. For each stand age, 10 trees from four different sites were sampled. A total of 75 species of non-vascular plants were found, including 34 species of mosses, seven species of liverworts, and 34 species of lichens. More species were observed in the 278-yr old stand, which also contained a greater number of exclusive species. In a correspondence analysis performed on species presence/absence data, axes one and two were correlated with distance from the closest unburnt area and time since fire. For each tree, species richness, diversity, and cover indices were calculated and compared for the different stand ages. These indices tended to be higher in the 79-yr old stand and lower in the 51-yr old stand. By using mixed log-linear models, we found that the frequency of occurrence of some species was linked to the time since fire, and others species to tree age. Old-growth forests are important to bryophytes and lichens since they have a greater number of species and some species are found exclusively or more frequently in old-growth forests. Furthermore, each stand age has some species associated with it. Therefore, forest management practices should be adopted to maintain all successional stages present in the natural forest landscape in order to preserve the diversity of non-vascular plants.