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In this study, the dependence of local conspecific abundance on seed set and predation was examined, and determinates of population density of the temperate tree species Carpinus laxiflora (Betulaceae) in Inagi city, Tokyo, Japan were identified. During a good seed year (2013), seeds were sampled from 27 individuals and categorized as Sound, Empty, Predated, Immature, Decayed, or Broken. Empty seeds were identified as those that failed to fertilize and predation rates were defined as the proportion of seeds that were affected by predators. The proportion of the seed set that was fertilized was significantly positively correlated with local abundance of conspecific trees, while the proportion of seeds that escaped predation was significantly negatively correlated with local abundance of conspecific trees. Thereby, the production of Sound seeds was highest at sites with moderate local conspecific abundance. Although this phenomenon was only observed in a few cases, it clearly showed optimal reproductive success under conditions of moderate density of conspecific reproductive adults, suggesting a mechanism that controls the population density of this species. [ABSTRACT FROM AUTHOR]