Dacrydium pectinatum de Laubenfels is a flagship and constructive species in the tropical montane rainforest of Hainan, China. The pattern of its community functional traits and the mechanisms sustaining diversity are key to understanding the adaptive strategies of this ecosystem. However, research on the variation of functional traits within D. pectinatum communities and their coupling with environmental drivers remains limited. Based on 30 fixed plots of D. pectinatum communities in Jianfengling, this study examined eight leaf functional traits: LA, SLA, LDMC, LT, CC, LNC, LPC, and LCC. We used the coefficient of variation to analyze interspecific variability in leaf functional traits and explore their ecological strategies, Spearman correlation to assess the relationships between functional traits and species/functional diversity, and structural equation modeling to uncover the direct and indirect effects of altitude and soil factors on trait-diversity relationships with environmental dependency. The results showed that: (1) tree species in D. pectinatum communities exhibited significant interspecific divergence in physiological traits (e.g., LDMC, SLA) but convergence in nutrient-related traits (e.g., LNC, LPC), indicating a conservative resource allocation strategy adopted by plants in response to environmental filtering pressures; (2) there was a strong correlation between species diversity and functional diversity, while only certain traits (e.g., LDMC, CC, LPC) were significantly correlated with plant diversity, suggesting that the distribution of community diversity is influenced not only by species composition but also by environmental filtering and interspecific competition; (3) altitude and soil factors drove trait-diversity relationships through both direct and indirect pathways—altitude indirectly enhanced functional diversity by selecting stress-tolerant species, while nutrient-poor soil conditions reduced functional diversity by constraining the differentiation of physiological traits. These findings support the diversity-function hypothesis and demonstrate how environmental filtering mediates community structure and ecosystem function through leaf functional traits, providing important theoretical insights into the relationship between biodiversity and ecosystem functioning in tropical forest ecosystems.