Patterns of woody plant species abundance and diversity in the seedling layer of a tropical forest

Questions: 1. How does the composition and diversity of established seedlings compare to that of larger size classes in tropical forests? 2. How do species abundances in the seedling layer vary with adult abundance and life history strategies? Location: Barro Colorado Island (BCI), Panama. Methods: We inventoried woody seedlings ≥ 20 cm tall and < 1 cm DBH in ca. 20000 1-m² quadrats within the BCI 50-ha Forest Dynamics Plot, where all trees and shrubs ≥ 1 cm DBH have been identified. We compared diversity and composition of seedlings to that of larger size classes and tested whether adult abundance, growth form, and shade tolerance contributed to variation in seedling abundance among species. Results: We encountered 60056 seedlings of 332 tree, shrub, and liana species. Diversity of tree seedlings was lower than that of trees ≥ 1 cm DBH. Species abundances in the seedling layer increased non-linearly with reproductive adult abundance, such that per capita seedling abundance declined with adult abundance. Per capita seedling abundance was highest for canopy tree species and lowest for understorey trees. For canopy trees, shade-tolerant species had significantly higher per capita seedling abundance than more light-demanding species. Conclusions: The woody seedling layer on BCI is composed of a subset of the species present in larger size classes. Most species were present in less than 1% of seedling plots, suggesting strong recruitment limitation. Tree species abundance in the seedling layer is largely a function of reproductive adult abundance, but is also influenced by life history strategies and compensatory processes.