Abstract The effect of tree harvesting operations on the loss of species from the understorey of mature deciduous forest in southern Ontario (Canada) was evaluated using a factorial experiment in which 0, 33 or 66% of the basal area of canopy trees was removed from plots 12·5, 25 or 50 m in diameter. The percentage of species lost from plots censused before and immediately after cutting (i.e., the next growing season) increased with cutting intensity but not with plot diameter. Surface disturbance also increased with cutting intensity and this likely contributed to the increase in species loss. The amount of tree slash left behind had less effect on the percentage of species lost. Woody plants in the understorey were more affected than herbaceous species since the meristems of only herbaceous species were protected below ground during the harvesting operations (November–April). Both herbaceous and woody species present initially at a low density (i.e., less than ninetimes per plot) were more likely to be lost than commonly occurring species. To minimise the loss of uncommon species during harvesting operations it would be more effective to minimise surface disturbance than cutting intensity or the area cut.