Determinants of seedling survivorship in tropical rainforest on the island of Dominica, Lesser Antilles

Background/Question/Methods

Many of the mechanisms proposed to maintain woody plant species diversity in forested ecosystems involve processes affecting seedlings. To determine the importance of light and competition in structuring hurricane-prone tropical wet forests of Dominica, an island in the Lesser Antilles, we examined the relative effects of biotic and abiotic factors on survivorship of  seedlings. We followed 6568 individuals of 90 tree, liana, and shrub species from 425 1 x 1 m seedling plots in 17 permanent vegetation plots over one year in Dominica. Individual seedling probabilities of survival were modeled as a function of initial height, light levels in the understory, and conspecific and heterospecific seedling density. We conducted these analyses for all individuals and then separately for different growth forms (trees, lianas, and shrubs) and the eight most common seedling species. For trees, we also included the effects of conspecific and heterospecific basal area of individuals greater than 1 cm in diameter. The eight most common species in order of decreasing abundance were the following: Amanoa caribaea (Phyllanthaceae), Licania ternatensis (Chrysobalanaceae), Psychotria urbaniana (Rubiaceae), Faramea occidentalis (Rubiaceae), Tapura latifolia (Dichapetalaceae), Dacryodes excelsa (Burseraceae), Paullinia vespertillio (Sapindaceae), and Rudgea citrifolia (Rubiaceae).

Results/Conclusions

Of the seedlings encountered in the initial survey, 83% of individuals were trees, 10% shrubs, and 5.8% lianas. Survivorship in each group was 85%, 90%, and 94%, respectively. Regardless of growth form, seedling survivorship was positively related to initial seedling height, light, and heterospecific seedling density. This overall pattern was driven by tree species. Tree survivorship was also positively associated with heterospecific basal area and negatively associated with conspecific basal area. For shrubs, initially larger seedlings had higher survivorship, but no other variables were significantly associated with their survival. Lianas survivorship was strongly positively affected by light but not by initial height or seedling density. Of the eight most common species, seedlings of the canopy tree D. excelsa were much less likely to survive than those of the understory tree F. occidentalis and the liana P. vespertillio. Higher light levels were significantly associated with greater survivorship only for the canopy trees A. caribaea and L. ternatensis, while heterospecific seedling density was positively associated with survivorship for A. caribaea and T. latifolia. In Dominican rain forests, both biotic and abiotic factors significantly affected seedling survival, with light levels having the greatest effects on trees and lianas.