DIVERSITY AND STABILITY OF UNDERSTOREY COMMUNITIES FOLLOWING DISTURBANCE IN THE SOUTHERN BOREAL FOREST

1 We investigated the response of boreal understorey communities to ga p formation in order to evaluate the relationship between community st ability and changes in community composition. We described the early c hanges in understorey species composition after gap formation and asse ssed some of the factors responsible for higher resistance to disturba nce. 2 At six sites, in the southern boreal forest of north-western Qu ebec, representing a successional gradient after fire, trees were cut and removed from an area of 400 m(2). In each experimental gap, we rec orded the cover of the understorey species in four 4 m(2) permanent qu adrats from before to 4 years after gap formation. We estimated resist ance to disturbance in terms of changes in species composition and abu ndance. 3 Younger communities changed less than older ones after distu rbance, 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 prev enting invaders from dominating the understorey. 4 Stronger resistance of understorey species communities to gap formation can persist in ol der sites because of the particular disturbance regime. Gaps created b y the spruce budworm in older sites dominated by balsam fir increase m icrosite heterogeneity and diversity by allowing the co-existence of e arly and late successional species.