Lower‐intensity restoration interventions drive greater seedling establishment for later‐successional tree species

Recovery of tree community composition in restored tropical forests relies on successful recruitment of later‐successional species. However, the long‐term effects of different restoration interventions on establishment success of arriving seeds are poorly understood. We evaluated the effects of three restoration treatments on the seed‐to‐seedling transition for later‐successional tree species in a fragmented agricultural landscape in southern Costa Rica. Restoration plots (0.25 ha) were established in a block design nearly two decades prior and spanned a gradient of intervention intensity: natural regeneration (not planted), applied nucleation (planted tree clusters) and plantation (fully planted). We conducted seed addition experiments from 2021 to 2023 using eight species at seven replicate restoration sites and in four nearby remnant forests. We defined the seed‐to‐seedling transition as two stages: seedling emergence and post‐emergence survival. Seedling emergence was highest in natural regeneration (48%), intermediate in applied nucleation (37%) and lowest in plantation (25%) and remnant forests (27%), with high variability across species. First‐year survival of emerged seedlings was greater in applied nucleation than in plantation and was lower in remnant forest than in all restoration treatments. On average, seedlings grew slightly taller in natural regeneration compared to the other treatments. Despite seed mass spanning four orders of magnitude among the eight species, this trait did not explain variation in seedling emergence or survival. Synthesis and applications : Taken together, results suggest that after ~15 years, applied nucleation and natural regeneration plots may provide more favourable conditions for establishment of later‐successional trees than do plantation plots. When the seed‐to‐seedling transition is not a major barrier for the recruitment of later‐successional tree species in restoration plots, practitioners should focus on increasing seed arrival for these species. Seed addition could be a viable strategy to introduce later‐successional species that fail to colonize restoration sites, particularly in restoration sites with low intensity interventions (i.e. natural regeneration or applied nucleation).