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-m(2) 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.