Seed limitation in a Panamanian forest

1 The role of seed limitation in tropical forests remains uncertain owing to the scarcity of experimental evidence. We performed seed addition experiments to assess seed limitation for 32 shade‐tolerant tropical forest species and monitored the natural seed rain of 25 of these species for 17 years. 2 One, two or five seeds were sown into 0.0079‐m2 plots for large‐ (n = 5 species), medium‐ (n = 5) and small‐seeded species (n = 22), respectively. The experiment was replicated at 69 sites, placed in groups of three at 23 locations. Seedling establishment was evaluated after 1 and 2 years in paired seed addition and control plots. Natural seedling emergence and understorey plant density were also measured. 3 Median natural seed rain was 0.31 seeds m−2 year−1 per focal species. 4 Seed addition enhanced seedling establishment in 31 and 26 of the 32 species after 1 and 2 years, respectively. Mean number of focal species’ seedlings after 2 years was 0.002 seedlings in control plots and 0.12, 0.37 and 0.60 seedlings in seed addition plots for large‐, medium‐ and small‐seeded species, respectively. 5 A 25 seeds added treatment increased seedling establishment by ≥ 2.0‐fold over the five seeds added treatment after 2 years. 6 Community‐wide recruitment and understorey plant density were strongly seed‐limited. The natural density of understorey plants averaged 12 plants m−2 and was significantly less than for seedlings of the single focal species in plots with ≥ 2 seeds added 2 years earlier. 7 The number of established seedlings per seed added was independent of seed size. 8 Treatment (adding zero or five seeds), species identity and location all affected seedling establishment for the 11 small‐seeded species represented at all sites, with treatment and its interactions accounting for 86% of the explained variation. 9 Our results suggest that seed limitation plays a dominant role in seedling recruitment and understorey plant community assembly in tropical forests. Although strong seed limitation may set the stage for species‐neutral community assembly, the species differences in seedling establishment rate and its spatial variation demonstrate an important role for species‐specific processes.

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