•The C and N dynamics are regulated mainly by tree species identity.•Wood density loss and cellulose and lignin degradation are related to the N dynamics.•Bark fragmentation partly controls the C and N amounts dynamics at the log level.•In boreal forests, downed logs contribute significantly to long-term C and N cycles. Quantifying forest carbon (C) and nitrogen (N) turnover rates requires better understanding of the mechanisms of decomposition of coarse woody debris (CWD). We examined the dynamics of bulk density, C, N, cellulose and lignin in downed logs, taking into account initial interspecific trait differences in bark and wood, in a 66-year-long decomposition chronosequence in an old-growth mixed boreal forest. Wood and bark density decreased with increasing decay class and time since tree death. The C concentration per mass did not change or slightly decreased. Nitrogen increased in wood faster than in bark. In aspen wood, it increased two times more intensively than in wood of conifers. The C/N ratio and the cellulose content decreased, whereas the lignin content increased, decreased or remained stable in wood depending on tree species and calculation basis. Spruce wood was characterized by the greatest cellulose degradation rates. The N availability positively influenced wood bulk density loss and cellulose degradation in wood of all tree species, as well as the C loss in conifer bark. The total C amount in bark and wood of individual downed logs decreased. The total N amount decreased in bark and increased in wood with different rates depending on tree species identity. The results indicate the importance of accounting for the CWD tissue tree species specific traits in decomposition studies. In old-growth boreal forests, downed logs contribute significantly to long-term C and N sequestration.