From Data to Discovery: Characterizing the Scope and Limitations of Seedling Trait Studies in the U.S. With the EPiC Database.

The seedling stage is a uniquely vulnerable and influential period of the plant life cycle with outsized impacts on plant recruitment, and population and community dynamics. Despite this, plant morphology, structure, and function during the seedling stage often differ markedly from later developmental stages, complicating generalizations about ecological strategy across ontogeny. Trait-based ecology offers a valuable framework to understand how functional variation shapes plant performance at early-life stages and its ecological consequences. Yet, trait-based approaches applied at the seedling stage remain limited and inconsistent across studies. To address these limitations, our systematic review of 181 published studies investigated seedling traits of native grass, forb, and shrub species across diverse environmental conditions in the continental United States. Following extraction of covariates, we identified emergent patterns, limitations, and opportunities to expand seedling functional ecology, particularly in relation to regeneration strategies. We found that most studies focused on individuals younger than 90 days old, with only 24% of studies measuring seedling traits beyond this window. Most research emphasized morphological traits (36%), followed by performance/growth (34%), physiological (15%), and phenological traits (13%), with aboveground biomass as the most studied trait. We identified nearly 300 traits but suggest future research efforts focus on key traits for better comparability of seedling responses. Most taxa were in Poaceae (24%) and Asteraceae (25%) families, and nearly 60% of field studies were conducted in dryland ecoregions. Water availability was the most frequently manipulated abiotic factor (39%), while physical disturbance-despite its central role in ecosystem dynamics-was rarely examined (5%). To advance seedling functional ecology and its relevance in applied and community assembly contexts, greater representation of phenological and belowground traits is needed alongside evaluations in diverse environments in the US. Measuring traits that span distinct ontogenetic stages, ecoregions, and taxonomic groups will advance understanding of plant regeneration under accelerating change.