The early growth stage of plants is vital to community diversity and community regeneration. Understandably, it is critical to explore the mechanisms underpinning the spatial and temporal dynamic patterns of seedling survival and growth. The Janzen--Connell hypothesis predicts that conspecific density dependence lowers the survival of conspecific seedlings by attracting specialist natural enemies, promoting the recruitment and performance of heterospecific neighbors. Recent work has underscored how this conspecific negative density dependence may be mediated by mutualists -- such as how arbuscular mycorrhizal fungi may mediate the accrual of host-specific pathogens beneath the crown of conspecific. Aboveground mutualist and enemy interactions exist as well, however, and may provide useful insight into density dependence that are as of yet unexplored. Using a long-term seedling demographic dataset in a subtropical forest plot in central China, we found that a mutualist association with ants had significant positive effects on seedling survival, but a negative effect on seedling growth. We also confirmed that conspecific neighborhoods had a significant negative effect on seedling survival, confirming that the Janzen-Connell hypothesis may influence community composition in the subtropical forest. Overall, our findings suggested that ants and conspecific neighborhoods played important but inverse roles on seedling survival and growth, which indicated that ants may mediate conspecific negative density dependence at community level to some extent.