Vertical Trait Variation Modulates Forest Biomass Dynamics in Two Japanese Forest Types

Aims While plant functional traits are known to influence biomass dynamics, the nuanced variations across layers, and the underlying ecological mechanisms, especially in temperate zones, are poorly understood. This study aimed to elucidate the influence of key functional traits on demographic processes across different forest strata. Location Our investigation took place between 2015 and 2023 in two distinct temperate forest ecosystems: cool‐temperate old‐growth forests and warm‐temperate secondary forests. Methods We quantified aboveground biomass (AGB) increment, recruitment, and mortality, alongside community‐weighted mean (CWM) values for functional traits, across three distinct layers within each study plot. Generalized linear mixed models were then applied within each layer to examine the relationship between CWM traits and AGB dynamics. Results AGB dynamics exhibited consistent patterns across the three forest layers at both study sites. However, CWM trait patterns varied significantly between study sites across layers, suggesting that successional stage and microenvironmental conditions shape the functional composition and ecological strategies of temperate tree communities. Remarkably, CWM wood density (CWM‐WD) positively correlated with AGB increment in both canopy and sub‐canopy layers, and with AGB recruitment in the canopy. In the understory, CWM‐specific leaf area (CWM‐SLA) showed significant positive effects on both increment and mortality. Conclusions Our study reveals that CWM trait–AGB dynamic relationships vary vertically, underscoring the significance of vertical heterogeneity in trait‐based community assembly processes. This necessitates that future research on forest turnover dynamics accounts for both size‐ and layer‐specific dynamic processes.