Results from an extensive vegetation survey of 197 boreal forest stands, encompassing a full spectrum of succession and site types in the regions of Ontario and Quebec south of James Bay, are reported. Non-centered principal component analysis plus varimax rotation (nodal component analysis) is applied to overstory and understory data in order to detect vegetational noda. The overstory data are inherently more structured (i.e., contain more distinct subgroups) than those of the understory. An exception is seen with groups in which Abies balsamea plays a prominent role. These are interpreted as stages in various successions of which only one, a Betula papyrifera to A. balsamea sequence, represents the complete transition from one canopy dominant to another. The understory noda are summarized in terms of the compositional and environmental features of stands belonging to each nodum. Relationships among the understory noda are summarized in a multidimensional scaling ordination which is derived directly from similarity values (conjunction coefficients) representing the overlap between pairs of noda. Two major environmental gradients seem to affect understory composition. These are a site moisture-nutrient concentration gradient and a general fertility-productivity gradient. Canopy composition and understory vegetation are compared by nodal conjunction between the two analyses. Groups of noda, representing wet bog forests and also upland mesic forests, show moderate to good coincidence but beyond this little canopy specificity emerges. These results indicate that overstory-understory coincidence is due primarily to similar site requirements. Understory species' respone to an overstory succession is examined. Many abundant herbs seem indifferent to dramatic canopy change. This observation and results from nodal component analysis are discussed with respect to forest fire.© by the Ecological Society of America. Abstract reproduced by permission.