Objective Chinese fir (Cunninghamia lanceolata) is one of the most important fast-growing structural timber species in China. Its bending properties are key indicators for evaluating the structural safety of wood materials and are fundamentally determined by the wood microstructure. Clarifying the structure–property relationships between microstructural characteristics and bending properties can provide a scientific basis for rapid assessment, processing modification, and genetic improvement of Chinese fir wood.

Method Chinese fir trees aged over 30 years from a plantation in Nianzhu Forest Farm, Yichun, Jiangxi Province, China, were selected as the study material. Heartwood and sapwood were distinguished along the radial direction. Microstructural parameters, including wall-to-lumen ratio, microfibril angle (MFA), and crystallinity, together with dynamic and static bending properties, were determined using optical microscopy, X-ray diffraction (XRD), stress-wave testing, and a universal testing machine. Ordinary linear regression and multiple linear regression analyses were performed to establish predictive relationships between stress-wave velocity, microstructural factors, and bending properties.

Result Compared with heartwood, sapwood exhibited a wall-to-lumen ratio 27.0% higher, a crystallinity 14.6% higher, and a microfibril angle 21.7% lower. The dynamic modulus of elasticity (MOE_d), modulus of elasticity in bending (MOE), and modulus of rupture (MOR) of sapwood were 5.9%, 7.1%, and 12.4% higher than those of heartwood, respectively. Significant relationships were observed between stress-wave velocity and bending properties, with R² ranging from 0.66 to 0.69. Among the individual microstructural factors, the wall-to-lumen ratio showed the strongest relationship with bending properties (R² = 0.67). The multiple linear regression models yielded R² values ranging from 0.67 to 0.71, indicating that wall-to-lumen ratio, microfibril angle, and crystallinity jointly affected bending properties. The standardized regression coefficients showed that the wall-to-lumen ratio had the greatest relative contribution (β of 0.49–0.59), followed by crystallinity (β of 0.19–0.28) and microfibril angle (β ranged from −0.14 to −0.18).

Conclusion The bending properties of Chinese fir wood are governed by its microstructural factors. A higher wall-to-lumen ratio, lower microfibril angle, and higher crystallinity are associated with superior bending performance. Stress-wave velocity showed relatively strong relationships with bending properties and can therefore serve as an effective indicator for rapid evaluation of bending properties. The multiple linear regression models quantitatively characterized the combined effects and relative contributions of microstructural factors, with wall-to-lumen ratio identified as the dominant factor influencing bending properties.