Process-dependent shifts in conspecific negative density dependence during boreal forest succession

Abstract Conspecific negative density dependence (CNDD)—the stronger negative effects of conspecific than heterospecific neighbours on plant demographic performance—is a key process proposed to stabilize forest diversity, yet how it changes during succession remains poorly understood. Using a 25-year spatially explicit dataset from 148 post-fire plots in boreal forests, we show that species richness increases with stand age and that CNDD varies in process- and abundance-dependent ways across recruitment, survival, and growth. Plot-level CNDD shows no significant change with stand age in recruitment or survival, but mean relative effects are generally stronger for recruitment, and CNDD shifts from destabilizing to stabilizing for growth as stands mature. Species-level CNDD is strongly associated with species abundance, with rare species consistently experiencing stronger CNDD in recruitment and growth, whereas common species tend toward neutral or destabilizing effects. For survival, by contrast, CNDD is stronger in common than in rare species. These abundance-related differences remain broadly consistent across stand development, except in recruitment, where CNDD intensifies in rare species but remains consistently weak or near-neutral in common species. This shifting balance of density-dependent pressures highlights succession-driven trade-offs as central to species coexistence and increases in diversity in disturbance-driven forests.