Widespread variation in functional trait-vital rate relationships in tropical tree seedlings across a precipitation and soil phosphorus gradient

A fundamental assumption of functional ecology is that functional traits
are related to interspecific variation in performance. However, the
relationship between functional traits and performance is often weak or
uncertain, especially for plants. A potential explanation for this
inconsistency is that the relationship between functional traits and vital
rates (e.g., growth and mortality) is dependent on local environmental
conditions, which would lead to variation in trait-rate relationships
across environmental gradients. In this study, we examined trait-rate
relationships for six functional traits (seed mass, wood density, maximum
height, leaf mass per area, leaf area, and leaf dry matter content) using
long-term data on seedling growth and survival of woody plant species from
eight forest sites spanning a pronounced precipitation and soil phosphorus
gradient in central Panama. For all traits considered except for leaf mass
per area-mortality, leaf mass per area-growth, and leaf area-mortality
relationships, we found widespread variation in the strength of trait-rate
relationships across sites. For some traits, trait-rate relationships
showed no overall trend but displayed wide site-to-site variation. In a
small subset of cases, variation in trait-rate relationships was explained
by soil phosphorus availability. Our results demonstrate that
environmental gradients have the potential to influence how functional
traits are related to growth and mortality rates, though much variation
remains to be explained. Accounting for site-to-site variation may help
resolve a fundamental issue in trait-based ecology – that traits are often
weakly related to performance – and improve the utility of functional
traits for explaining key ecological and evolutionary processes.