This study examined the importance of substrate, stand age, and environmental factors on diversity and abundance of nonvascular species in aspen mixedwood forests. Sampling was carried out on downed woody material (DWM) in young (23-26 years), mature (51-63 years), and old (122-146 years) aspen mixedwood stands. DWM was categorized into one of seven decay stages. Environmental variables including light, pH, and wood type were measured for a subset of DWM. Diversity and abundance of nonvascular species were related to decay stage and stand age. Old stands had the greatest number of species and the greatest diversity of woody substrates in each of the decay stages. Decay stage was the main determinant of variance in the species distributions, followed by stand age, pH, and wood type. These results suggest that assemblages of nonvascular species may be altered if old aspen stands are removed from the forest landscape, and that wood in different stages of decay is important to maintaining such assemblages.