We investigated the response of boreal understory communities to gap formation in order to evaluate the relationship between community stability and changes in community composition. We described the early changes in understory species composition after gap formation and assessed some of the factors responsible for higher resistance to disturbance. At six sites, in the southern boreal forest of north-western Quebec, representing a successional gradient after fire, trees were cut and removed from an area of 400 m2. In each experimental gap, we recorded the cover of the understory species in four 4-m2 permanent quadrats from before to 4 years after gap formation. We estimated resistance to disturbance in terms of changes in species composition and abundance. Younger communities changed less than older ones after disturbance. Sites with more species changed less in composition after gap formation than less rich sites. Richer quadrats contained species that could react quickly to disturbance by increasing their cover and preventing invaders from dominating the understory. Stronger resistance of understory species communities to gap formation can persist in older sites because of the particular disturbance regime. Gaps created by the spruce budworm in older sites dominated by balsam fir increase microsite heterogeneity and diversity by allowing the co-existence of early and late successional species.