Hierarchical contributions of ontogeny, species identity, stand competition, and local environment to biomass allocation in subtropical coniferous and broadleaved tree species

How ontogeny, species identity, stand competition, and local environment jointly shape biomass allocation in subtropical trees—and differ between conifers and broadleaves — remains unresolved. We destructively sampled 730 trees from five dominant subtropical species in Hunan, China ( Cunninghamia lanceolata , Pinus massoniana , Populus deltoides , Quercus glauca , and Camphora officinarum ), during 2023–2024 across plantations and natural secondary forests. Drivers of root, stem wood, bark, branch, and leaf biomass were partitioned with linear mixed-effects models including plot and species random effects. Individual size was the dominant driver, with standardised effects of 0.49–0.69 exceeding all other predictors by an order of magnitude and explaining 55–99% of variance in organ biomass (marginal R 2 = 0.55–0.99). Species identity contributed a substantial secondary signal, accounting for 15–33% of variance and producing significant random intercepts for all organs and size slopes for all above-ground organs ( P ≤ 0.007); conifer–broadleaf divergence was strongest in bark and branch–leaf scaling, with broadleaves coordinating branch–leaf investment ( r = 0.54) and conifers prioritising stems. After controlling for ontogeny and species, stand density selectively reduced bark allocation ( P = 0.007–0.008) without affecting other organs. Abiotic effects were weak and organ-specific: slope and soil depth marginally influenced leaf and bark allocation, whereas regional climate variables (PET, CWD) at ∼1 km 2 resolution explained no additional variation once size, species, plot and local factors were accounted for, consistent with scale mismatch against plot-level measurements. Allocation in these species therefore reflects mainly individual size and species identity, with stand competition and local habitat contributing organ-specific secondary effects relevant to regional carbon accounting.