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Keywords:

  • community ecology;
  • dispersal assembly;
  • Panama;
  • seed addition experiment;
  • seed limitation;
  • seed size;
  • seedling ecology;
  • seedling recruitment;
  • tropical ecology;
  • tropical forest

Summary

  • 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.