Abstract The sugar maple decline observed in forests of northeastern North America is one of the disturbances that may have caused changes in the structure and composition of maple-dominated stands. Such changes may alter the sustainability of these uneven-aged ecosystems. The hypothesis that the decline observed in sugar maple populations over recent decades has modified stand structure and composition was tested using data from the Forest Ecosystem Research and Monitoring Network (RESEF) in Québec, Canada. Analysis of the diameter class density distribution, growth rates, and mortality schedule for 12 stands indicated a decline in health and productivity in the sugar maple population. This result suggests that environmental factors have limited the development of this species in all strata. The American beech population responded by doubling its pole cohort density in 10 years. Although increases in American beech stem density tended to counter sugar maple mortality, results suggest that stand structure has been shifted from a sustainable structure to an unsustainable one over a 10-year period. The high mortality of sugar maple and the large increase of pole-size beech trees deeply altered forest structure and composition in the majority of stands studied. These changes question seriously the sustainability of sugar maple in these forest ecosystems.