Woody plant species abundance, distribution, and survival in the seedling bank of a neotropical forest
Comita, Liza Sheera
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Tropical forests possess the most diverse plant communities on earth. Most hypotheses proposed to explain how this diversity is maintained focus on processes occurring during early life stages. However, most field surveys include only large saplings and adult trees, making it difficult to evaluate these hypotheses. I conducted a community-wide survey of seedlings and small saplings in a 50-ha Forest Dynamics Plot on Barro Colorado Island (BCI), Panama. Using these data, along with preexisting data on trees in the BCI plot, I described patterns of species abundance, distribution, diversity and survival in the seedling layer and examined evidence for several hypotheses proposed to explain the maintenance of diversity in tropical forests. Between 2001 and 2004, annual censuses of woody seedlings were conducted in ~20,000 21-m quadrats located throughout the BCI plot. In the initial census, 60,056 seedlings of 332 tree, shrub, and liana species were encountered. The seedling layer was predominantly composed of a subset of the species present in larger size classes. Most species were present in <1% of the seedling quadrats, suggesting strong recruitment limitation. Species abundances in the seedling layer were largely determined by reproductive adult abundance, but were also influenced by life history strategies. Using data on the spatial locations of individuals in the BCI plot, I examined the distribution of 80 species with respect to five habitat types at the seedling and tree stages. Most species were significantly associated with a habitat type at one of the two life stages. Results indicate that pathways leading to species’ habitat associations are complex and vary among species, with differences in seed dispersal and seedling ecology resulting in differences in seedling versus tree habitat associations. Lastly, I examined survival of seedlings from 2001 to 2004 and tested whether conspecific neighbor density reduces the probability of survival. Overall, I found a negative effect of local conspecific density on established seedling survival. In species-level analyses, local conspecific density had a negative effect on survival for 26 out of 59 species. In contrast, at the community level, seedling survival was positively correlated with species abundance in the 50-ha study area.