Abstract: Seedling establishment is a critical demographic bottleneck for species regeneration and diversity maintenance in forests, yet the temporal contributions of biotic and abiotic factors remain poorly understood. This is because most previous studies focused disproportionately on biotic factors or limited abiotic factors and used observational data lacking standardized baselines for seedling age. We conducted a two-year transplant experiment in the 25-ha Baishanzu subtropical forest dynamics plot, monitoring 2,886 seedlings from six species spanning a range of abundance across conspecific adult density gradients. We analyzed the temporal effects of neighborhood conspecifics, heterospecifics, light, soil nutrients, and topography on seedling establishment both in aggregate and for individual species. Our results revealed that negative effects from conspecific adults intensified as seedlings grow. Conversely, heterospecific neighbors consistently exerted positive effects throughout the establishment. Environmental factors exhibited temporal variation in their effects. While light availability negatively impacted early establishment, soil nutrients emerged as strong predictors for older seedlings. For individual species, responses were highly species-specific. Negative coefficients for conspecific adult density were observed across species over time, yet statistical significance was detected only in species Daphniphyllum macropodum and Photinia davidiana. Heterospecific effects remained neutral or positive across all six species. Notably, abiotic factors showed species-specific directional effects, benefiting certain species while inhibiting others, suggesting that niche differentiation across heterogeneous microhabitats drives regeneration dynamics. Our results highlight that both abiotic and biotic factors are indispensable drivers during seedling establishment, and their shifting temporal dynamics combined with species- specific responses fundamentally shape seedling dynamics in heterogeneous forests.

Key words: seedling establishment, recruitment limitation, neighborhood effects, environmental filtering, conspecific negative density dependence, subtropical forest dynamics plot