1. Density-dependent recruitment is fundamental to understanding species diversity and community dynamics in plants. Although there is compelling evidence that seeds and seedlings die from conspecific negative density dependence (CNDD) as predicted by the Janzen–Connell hypothesis, characterising adult recruitment remains a challenge for long-living trees. Previous studies have used the decrease of fine-scale spatial genetic structure (FSGS) across life stages to indicate CNDD; however, this has not been tested rigorously.

2. We addressed these challenges by integrating dispersal kernels and FSGS. To establish links between density dependence and FSGS, we simulated seedlings based on the estimated dispersal kernels from parentage analyses, and further simulated adults under various seedling-to-adult recruitment scenarios, using an individual-based spatially explicit model. We tested this method in an isolated Cyclobalanopsis glauca population on China’s Dajinshan Island.

3. We detected significant FSGS in the seedlings and weaker, though also significant, FSGS in the adults. As expected, the observed FSGS of seedlings was well predicted by the simulated seedlings, with observations falling inside the 95% confidence envelopes over all distance classes. However, the simulations showed that CNDD enhanced the FSGS, while positive density dependence dampened it during the seedling-to-adult transition. The adult FSGS of our population was therefore explained by positive rather than negative density-dependent adult recruitment.

4. Synthesis. Our study demonstrates that the change of FSGS in conjunction with dispersal-based model tests can offer a valuable insight into density-dependent adult recruitment. The results indicate that the transitions from seedlings to adults in C. glauca are dominantly regulated not by Janzen–Connell effects, but by processes of positive density dependence. More broadly, the findings may provide a caution against extrapolations of widespread Janzen–Connell effects in seeds and seedlings to adult recruits, underscoring a critical gap between mechanisms at early stages and long-term population and community dynamics.