Soil mediates the impact of functional traits on tree growth and survival across life stages in a temperate forest

Trait‐based approaches are widely used to understand variation in plant demographic performance, yet their predictive power is often limited by context dependence. In particular, trait–demography relationships may shift across life stages and be modulated by local abiotic and biotic conditions. However, few studies have simultaneously examined how ontogeny, soil fertility, and neighborhood trait composition jointly shape the outcomes of trait–performance relationships. Here, we integrate long‐term demographic data, functional traits, and soil variables from a 20‐ha warm temperate forest dynamics plot to evaluate how three key traits, specific leaf area (SLA), leaf dry matter content (LDMC), and wood density (WD), affect tree growth and survival across seedling, sapling, and adult stages. We also examine how these effects, including neighborhood interactions for trait dissimilarity, are modified by soil fertility. Our study showed that soil fertility modulated trait effects on tree performance, which varied across life stages. Direct trait effects were detected primarily at the adult stage, where adult survival increased with WD but decreased with LDMC. In contrast, at the seedling and sapling stages, trait effects on survival were largely contingent on soil nutrients. Increasing soil organic matter, moisture, and available potassium intensified the negative effects of conservative traits (LDMC and WD) on seedling and sapling survival, while alleviating the negative effects of acquisitive traits (SLA). Seedlings exhibited lower survival when surrounded by heterospecific neighbors with dissimilar SLA. The negative effect of SLA dissimilarity among neighboring seedlings on seedling survival was amplified in soils rich in organic matter, moisture, and available potassium, whereas between seedlings and neighboring trees it was alleviated under higher nitrogen and phosphorus availability. Our results demonstrate the importance of explicitly considering life stage, local abiotic conditions, and biotic neighborhood context. This multidimensional approach offers new insights into trait‐based mechanisms underlying forest community dynamics across life stages.