Abstract:

Objective: To understand the difference in mechanical properties of juvenile woods among Chinese fir clones, a serial of standard wood samples were prepared by two new-certificated clones in a 10-year-old growth performance demonstration plantation to analyze the microstructure characteristics of juvenile woods. Method: The microstructure and mechanical properties of juvenile woods of Chinese fir clones were analyzed by microscopic image analysis system, X-ray diffraction technique, Fourier transform-infrared microscopic imaging and the national standards of mechanical properties testing. The data of microstructures were observed from 'Yang 020' and 'Yang 061', involving the anatomical structure, microfibril angle, crystallinity and chemical composition lignin, as well as main mechanical properties such as modulus of rupture (MOR), modulus of elasticity (MOE) and compressive strength parallel to grain and hardness, and were processed by one-way variance analysis. Result: The tracheid morphology of earlywood was similar between 'Yang 020' and 'Yang 061', however, there was a significant difference on latewood tracheid morphology. In comparison with colne 'Yang 061', clone 'Yang 020' presented with wider latewood zone, thicker tracheid wall, smaller tracheid lumen and a 25% larger wall-lumen ratio. At the aspect of cell walls, the average microfibril angle for 'Yang 020'and 'Yang 061' was 12.06° and 14.97°, respectively, indicating that a smaller microfibril angle would have more fine mechanical properties for 'Yang 020'. The average crystallinity for 'Yang 061' was 35.88%, about 11% lower than 39.73% for 'Yang 020'. The average lignin content, presented by four groups of characteristic peak ratios on 'Yang 020' was 8% higher than that on 'Yang 061'. The average MOR, MOE, compressive strength parallel to wood grain and wood hardness of 'Yang 020' vs 'Yang 061' was 51.36 MPa vs 42.56 MPa, 10.18 GPa vs 8.98 GPa, 30.27 MPa vs 27.20 MPa and 1 497 N vs 1 391 N, respectively. Each mechanical property indicator, such as MOR, MOE, compressive strength parallel to wood grain, and wood hardness, of 'Yang 020' was higher than that of'Yang 061' by 21%, 13%, 11% and 8% respectively, indicating that clone 'Yang 020' had better mechanical properties, in a wider latewood zones, thicker tracheid wall, larger wall-lumen ratio, smaller microfiber angle and higher lignin content, while 'Yang 061' had lower mechanical properties, in a narrower latewood zones, thinner tracheid wall, smaller wall-lumen ratio, larger microfiber angle and lower lignin content. Conclusion: For juvenile woods properties of 'Yang 061' and 'Yang 020', there were significant differences between the two clones in microstructures, such as anatomical structure parameters, microfibril angle and lignin content, except for the crystallinity. Clone'Yang 020'had higher tested values on juvenile wood mechanical properties over clone 'Yang 061', with higher bending, compression resistance and hardness. The juvenile wood mechanical properties of Chinese fir clones were positively correlated with anatomical structure parameters, such as tracheid wall thickness and wall-lumen ratio and chemical composition lignin content, while a negative correlation was shown between microfibril angle of cell wall cellulose and mechanical property. The juvenile wood mechanical properties of Chinese fir clones were synergically influenced by the above-tested factors, and it was suggested that the anatomical structure parameters and microfibril angle among them might be the most important impact factors on the juvenile wood mechanical properties of Chinese fir clones.

Key words: Chinese fir clones, juvenile woods, anatomical structure, microfibril angle, crystallinity, lignin, mechanical properties