Drought and soil phosphorus explain variation in functional trait-vital rate relationships in tropical tree seedlings

Background/Question/Methods

A fundamental goal of ecology is to understand how phenotype relates to performance. Functional traits (i.e., phenotypic traits that impact fitness) are commonly used to infer 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 uncertainty is that the adaptive value of a trait and thus its relationship with vital rates (e.g., growth and mortality) is dependent on local environmental conditions, which would lead to variation in trait-rate relationships across gradients. For example, in high resource environments, traits that maximize carbon gain and competitive ability are advantageous, whereas the same strategy can be disadvantageous in low resource environments. In this study, we tested the prediction that traits related to resource acquisitive/conservative life history strategies would show stronger relationships with mortality and weaker relationships with growth in low compared to high resource environments. We examined trait-rate relationships for six functional traits (seed mass, wood density, maximum height, and three leaf traits), using long-term data on seedling growth and survival of diverse woody plant species from eight forest sites spanning a pronounced precipitation and soil phosphorous gradient in central Panama.

Results/Conclusions

For all sites combined, seedling relative growth rates decreased with increasing wood density, increasing seed mass, and increasingly conservative leaf traits, while seedling mortality rates decreased with increasing wood density, decreasing maximum height, and increasingly conservative leaf traits. However, the strength of most trait-rate relationships varied among sites. In some cases, trait-rate relationships were explained by local dry season severity or soil phosphorus availability. For example, the relationship between growth and wood density was stronger at sites with lower soil phosphorus; wood density-mortality relationships were stronger at sites with more severe dry seasons and higher soil phosphorus; and maximum height-mortality relationships were stronger at sites with higher soil phosphorus. Our results demonstrate that the strength of trait-vital rate relationships can vary among sites. Supporting our prediction, in our system, dry season severity and soil phosphorus predicted how functional traits are related to growth and mortality rates, though much variation remains unexplained. Accounting for site-to-site variation in resource availability may help resolve a fundamental issue in trait-based ecology and improve the utility of functional traits for explaining key ecological and evolutionary processes.