We examined the effects of woody substrate and stand age on diversity and relative abundance of bryophytes, lichens, and fungi in trembling aspen (Populus tremuloides Michx.) mixedwood boreal forests in Alberta. Point sampling was carried out on surfaces of downed woody material in young (23-26 years), mature (51-63 years), and old (122-146 years) aspen mixedwood stands. Downed woody material was categorized into one of seven decay stages. Diversity and relative abundance of bryophytes, lichens, and fungi were related to decay stage of the log and to stand age. Old aspen mixedwood stands had the highest nonvascular species richness and the greatest diversity of woody substrates in each of the decay stages. The seven decay stages shared many species. Species of bryophytes, lichens, and fungi on particular decay stages were different across the age-classes, indicating that time, as well as structural attributes, were important in determining species assemblages. These results suggest that both the presence of old aspen mixedwood stands and downed wood in different stages of decay are important to maintaining assemblages of bryophytes, lichens, and fungi.