Species-rich communities are composed of several locally rare and a few abundant species. A trait-based approach can derive general explanations for such species abundance distribution. However, for tree species in forests, the intraspecific variation of traits associated with the ontogeny and understory/canopy conditions must be considered. Here, we quantified intraspecific variation of leaf traits across saplings and adults in trees and tested if leaf and/or regenerative traits mediate the species abundance distribution of saplings and adults, and if saplings abundance also determines the abundance of adults in tree communities of Araucaria forests in Southern Brazil. We found intraspecific variation for specific leaf area (SLA) and leaf dry matter content (LDMC), but not for leaf area (LA). Adults have higher LDMC and lower SLA than saplings, suggesting a shift in investment strategies. Species abundant in saplings were associated with larger LA, whereas abundant species in the adults had lower SLA and LA and a larger number of seeds per fruit. The abundance of saplings also contributed to explaining the abundance of species as adults. Saplings experience low light availability in the understory, larger leaves are essential to maximising light capture. On the other hand, adult trees face high irradiation, windy and frost exposure conditions; species with lower SLA reach higher abundance. Regenerative traits were only relevant for the model of adults. Species abundance distribution in forests is related to functional traits, but the relationship is distinct for saplings and adults as leaf traits vary according to the ontogenetic stage and/or the environmental conditions imposed to individuals. In addition, we suggest the assembly of trees in the upper forest stratum is not only mediated by abiotic and biotic filters that favour some leaf and regenerative attributes, but also by lasting effects of variables that filtered out individuals in former development stages.