Post-fire regeneration of Picea glauca on boreal mixedwood sites appears to be highly variable over time. Our objectives were to determine whether ground-level ring counts underestimate root collar age of understorey P. glauca and whether aging errors increase with stand age. Trees were collected from one to nine stands in Alberta, Canada, in each of three fires occurring in mast years between 1961 and 1991. Trees were cut at ground level (humus soil level), and the belowground stumps were excavated, sectioned, and internally cross-dated with skeleton plots after identifying the root-collar location. Ground-level discs were visually cross-dated with a master chronology, which was constructed using the dendrochronology program COFECHA. Ground-level ring counts underestimated age by a mean of 2.4 years (range 0-6) and 6.4 years (range 0-13) in 20- and 38-year-old stands, respectively. Age underestimation was significantly greater at the root collar than ground level because of missing rings. Cross-dated age structures showed that apparent regeneration lags in 20- and 38-year-old stands were artifacts of ground-level ring counts and that the first year post-fire was the most important establishment year in all mast year burns. We conclude that aging errors have led to inaccurate depictions of regeneration patterns during early mixedwood stand development. Our results portray a different picture of P. glauca succession and have important implications for forest management.