We examined the response of tree seedling emergence and survival to the dieback of Sasa and canopy gap formation in an old-growth forest near Lake Towada, northern Japan. Synchronous death of Sasa occurred in 1995. We established four types of sampling sites differing in forest canopy conditions (Closed or Gap) and Sasa status (Dead or Live). Gap-Dead sites had the highest light levels and the greatest fluctuation in soil temperatures. The death of Sasa alone facilitated the emergence (Acer japonicum, Fagus crenata, Fraxinus lanuginosa, and Tilia japonica) and survival (Acanthopanax sciadophylloides, F. crenata, F. lanuginosa, Kalopanax pictus, and Sorbus commixta) of species with a seedling bank strategy. Cercidiphyllum japonicum grew at all sites at a higher density than other species, but survived well only in Gap-Dead sites. This behaviour was associated with a seed rain strategy. The additive effects of Sasa death and canopy gap formation promoted seedling emergence of pioneer tree species (Betula maximowicziana, Lindera umbellata, and Magnolia obovata), probably through break of dormancy by the large temperature fluctuation. In addition, the scarcity of advance regeneration in canopy gaps due to Sasa cover facilitates the regeneration of pioneer species. The dominance and dieback cycle of Sasa contributes to species diversity in this forest.Nomenclatur: Satake et al. (1989).Abbreviations: RPPFD = Relative Photosynthetic Photon Flux Density; ANOVA = Analysis of Variance; MANOVA = Multivariate Analysis of Variance.