Clonal and within-tree variation in microfibril angle in poplar clones

The microfibril angle (MFA) of seven poplar clones was determined using X-ray diffraction technique. MFA was measured for every growth ring at breast height (1.3 m) for all sample trees, and at 0 (butt), 5.6, 9.6, 13.6, 17.6, 19.6 and 21.6 m for clone I-69, Nanlin-95 and Nanlin-895. A total of 900 samples (chips) were assessed. Significant variation was observed among poplar clones and growth rings. Mean clonal MFAs at the breast height ranged from 15.2 degrees to 21.0 degrees. MFA at breast height varied from 7.8 degrees to 28 degrees between growth rings and showed a consistent pith-to-bark trend of declining angles. Disc-average microfibril angle in poplar clones declines rapidly from 0 to 5.6 m height, after which, it is near constant within tree height. Significant variation was also observed in disc-average MFA among tree heights. Correlation analysis showed that the correlation between MFAs from rings 5 to 11 at breast height was significant, while whole-tree values of each growth ring were significantly correlated with the corresponding MFA at breast height. The results of this study provide some basis for genetic selection of poplar trees based on wood-quality criteria and for devising appropriate sampling strategies for MFA measurement.