Ontogenetic shifts in trait-mediated mechanisms of community assembly

Background/Question/Methods

Identifying the processes that maintain highly diverse communities remains a central goal in ecology. Species variation across ontogenetic stages (e.g. tree size) in growth and survival rates and variation in the strength of niche partitioning are potentially important mechanisms for promoting diversity. Ontogenetic tradeoffs in performance may equalize the performance of functionally diverse species, promoting coexistence. However, the roles of size in mediating competitive dynamics and in promoting diversity are not well understood. We studied whether functional trait (seed size, maximum height, SLA, leaf N and wood density) associations with niche and average growth and survival rates change as trees transition from seedlings to four size classes of trees in a subtropical rainforest in Puerto Rico. We used a hierarchical Bayes model of growth and survival to infer trait relationships with ontogenetic change in competitive dynamics.

Results/Conclusions

Average growth and survival were the strongest trait-mediated assembly processes and were highly consistent across tree ontogeny for most traits. Adult trait associations with seedling performance were generally weaker than associations with performance of larger tree size classes. The association between species trait values and their response to crowding by neighbors were weaker and showed inconsistent shifts as trees grew. Trait associations with growth that suggested niche differences were most important for the largest tree size class. For survival, however, trait associations that suggested niche differences were most important for intermediate tree size classes, the stage at which stand thinning occurs. Our results identify potential ontogenetic axes of niche partitioning and performance-equalizing functional tradeoffs that promote species coexistence in this diverse tropical forest community.