Variation of microfibril angle, density and fibre orientation in twenty-nine Eucalyptus nitens trees

Abstract

SilviScan-2 was used to estimate and map microfibril angle (MFA) and density in twenty-nine 15-year-old Eucalyptus nitens trees. Over 4000 MFA measurements were made at about 100/hour. Each measurement represented a weighted average for about 50000 fibres. After an initial decrease near ground level, density increased with height in the stem. MFA decreased with height in the stem, reached a minimum around 30 to 50% of tree height, then increased towards the top. In the radial direction, density first decreased for a few years, then increased towards the bark. MFA was in the range 20 to 30 degrees near the pith at all heights and generally decreased towards the bark over most of the height of the stem. The lowest MFA values (approx. 10 degrees) were found close to the bark at 30 to 50% of stem height. Both density and MFA varied more rapidly near ground level, increasing the uncertainty of correlations between breast-height properties and whole tree properties. On some samples, high-resolution (0.2 mm) MFA scans were performed to confirm an earlier finding that MFA is strongly inversely correlated with density over a few growth rings but not over larger distances. X-ray diffraction was also used for estimating the orientation of the fibres within the samples. The radial variation in fibre orientation generally decreased with distance from the ground. Breast-height sample properties were moderate to good predictors of whole tree properties. Better correlations were obtained using samples from 5.5 m. Whole tree average density and MFA were found to be uncorrelated, indicating that tree improvement strategies could be designed to simultaneously optimise these properties.