Ecologists usually find that plant demography (e.g., survival and growth) changes along with plant size and environmental gradients, which suggests the effects of ontogeny-related processes and abiotic filtering. However, the role of functional traits underlying the plant size- and environment-demography relationships is usually overlooked. By measuring the individual-level leaf traits of more than 2700 seedlings in a temperate forest, we evaluated how seedling functional traits mediated the size- and environment-demography relationships. First, we found leaves were larger for taller seedlings, leaf economics traits were more conservative in taller seedlings and under high light and low elevation conditions. Then, structural equation modeling showed that a higher survival probability for taller seedlings was indirectly driven by their larger leaf area. Although taller seedlings had lower growth rates, larger and more resource-conservative leaves could promote the growth rate of these tall seedlings. Environmental variables did not appear to influence seedling survival and growth directly but did influence growth indirectly by mediating trait variation. Finally, species-specific variation in traits along with size and environments was associated with the species-specific variation in seedling survival and growth. Our study suggests that, not only plant ontogeny- and environment-related ecological processes, functional traits are also important intermediary agents underlying the usually observed plant size- and environment-demography relationships.