Functional traits of young seedlings predict trade-offs in seedling performance in three neotropical forests

Understanding the mechanisms that promote the coexistence of hundreds of
species over small areas in tropical forest remains a challenge. Many
tropical tree species are presumed to be functionally equivalent
shade-tolerant species that differ in performance trade-offs between
survival in shade and the ability to quickly grow in sunlight. Variation
in plant functional traits related to resource acquisition is thought to
predict variation in performance among species, perhaps explaining
community assembly across habitats with gradients in resource
availability. Many studies have found low predictive power, however, when
linking trait measurements to species demographic rates. Seedlings face
different challenges recruiting on the forest floor and may exhibit
different traits and/or performance trade-offs than older individuals face
in the eventual adult niche. Seed mass is the typical proxy for seedling
success, but species also differ in cotyledon strategy (reserve vs
photosynthetic) or other seedling traits. These can cause species with the
same average seed mass to have divergent performance in the same habitat.
We combined long-term studies of seedling dynamics with functional trait
data collected at a standard developmental stage in three diverse
neotropical forests to ask whether variation in coordinated suites of
traits predicts variation among species in demographic performance. Across
hundreds of species in Ecuador, Panama, and Puerto Rico, we found
seedlings displayed correlated suites of leaf, stem, and root traits,
which strongly correlated with seed mass and cotyledon strategy. Variation
among species in seedling functional traits, seed mass, and cotyledon
strategy were strong predictors of trade-offs in seedling growth and
survival. Our findings highlight the importance of cotyledon strategy in
addition to seed mass as a key component of seed and seedling biology.
These results also underscore the importance of matching the ontogenetic
stage of the trait measurement to the stage of demographic dynamics.
Synthesis: With strikingly consistent patterns across three tropical
forests, we find strong evidence for the promise of functional traits to
provide mechanistic links between seedling form and demographic
performance.