Effects of overstory tree species and seed trap design on seed rain in a tropical rainforest

Kirk, Bridgette
Major Professor
Ann E. Russell
Brian Wilsey
Committee Member
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Natural Resource Ecology and Management

Tropical rainforests maintain a high diversity of plant species, but the community assembly mechanisms by which this occurs are unclear. We hypothesized that individual overstory tree species could filter seed rain differentially, owing to differences in species-specific traits that affect forest physiognomy and resources for animals, which in turn influence likelihood of arrival of wind- and animal-dispersed seeds. Quantifying the effects of an individual tree species on seed rain in a highly diverse natural forest is challenging, and thus understudied. Seed rain of very small seeds (<1 mm long) is also relatively uncharacterized because seed trap mesh size is usually ≥1 mm. In a unique experimental setting containing replicated, monodominant, 26-yr-old plantations of four native tree species at La Selva Biological Station (Costa Rica), we studied the effects of four native tree species on seed rain. We collected seed rain of woody species for 14 months in 15 50-à  -50-m plots from 75 traps. In each plot three traps were lined with Teryleneà ® fabric of 1-à µm (fine) mesh and two with 1-mm (regular) mesh.

Across all traps, the total annual seed rain was 133,355 seeds from 121 woody species representing 41 families. Seed rain differed significantly among the four overstory tree species with respect to species richness and composition. Species richness was lowest in Pentaclethra macroloba, the only species in our experiment that was not animal-dispersed or predated. Total abundance, species richness, diversity and composition of seed rain varied significantly between fine- and regular-mesh traps. Regular-mesh traps underestimated species richness by 50% and missed 90% of the very-small-seeded species in seed rain. This indicated that the seed trap mesh size could significantly affect interpretations about life form and successional proclivity of species arriving in seed rain, with consequences for evaluation of restoration strategies. These results suggest that individual tree species influence potential disperser behavior, and therefore seed dispersal patterns, by virtue of differences in food resources provided by tree species. Individual tree species can filter seed dispersal, thereby influencing its own local neighborhood at the onset of plant community assembly, seed arrival.