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The entry of trees into a community is an irregular process often constrained by species interactions and regeneration niche. We experimentally examined the effect of light intensity (high, medium and low), substrate type (mineral and cover with litter) and presence of Chusquea culeou (Poaceae), on the performance of regeneration of Nothofagus dombeyi, N. obliqua and N. alpina (Nothofagaceae). These species constitute the natural forest within the lake Lácar watershed (40° 08´ W, 71° 30´ S). Populations of C. culeou, an abundant understorey component that restrict tree regeneration, experimented in 2001 a gregarious flowering and subsequent mortality comprising 200,000 ha and the development of an abundant seedlings cohort. During tests, Nothofagus species exhibited a very low germination (< 2.2%) positively correlated with seed weight. Plant emergence and size was larger and mortality smaller under high irradiation, probably as an adaptation to colonize open areas. Reduced germination of seeds on organic substrate was ascribed to the role of litter as a physical barrier. Nothofagus exhibited 76% of increased mortality under the presence of C. culeou, whose germination was very abundant (93% in laboratory) and occurred preferably with high irradiance and litter. However, C. culeou also showed a good response with lower light levels. After senescence, C. culeou would form an abundant and fast-growing cohort that probably limits Nothofagus ability to establish, given increased mortality and litter accumulation. These results contribute to a better understanding of succession, which is crucial for implementing a sound management of this valuable ecosystem.