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  • Author: R. Washusen x
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Abstract

The determination of microfibril angle (MFA), cellulose crystallite width and microdensity by SilviScan-2 at CSIRO confirmed that several dark bands in the heartwood of an Acacia species were associated with tension wood (TW) fibres. The straight log was taken from a tree of about 30 years of age that had been apparently growing vertically as there were no signs of the usual stimuli for TW formation. The continuous dark bands of about 5 mm width extended radially from the inner heartwood in a spiral pattern and were separated by continuous normal wood bands of similar width. The TW bands continued in the sapwood from the heartwood boundary to the cambium but were not coloured. The dark colour of the TW was probably caused by extractives being polymerised at the heartwood boundary by enzymes in the TW fibres in the sapwood. The MFA and cellulose crystallite width indicated the TW in a band was of similar intensity throughout its length, indicating similar microstructural characteristics at different cambial ages. The initiation of spiral grain formation occurs before that of tension wood in the Acacia samples studied.

Summary

The potential for growth strain measurements for detection of tension wood was assessed in trees from two plantations of 10- to 11-year-old Eucalyptus globulus. Tension wood had commonly developed at or near the stem periphery of straight, vertical and dominant trees. At a localized level growth strain was found to be a good indicator of tension wood. However, in some cases moderate to low growth strain was also detected in some trees where tension wood had been overgrown with small amounts of normal wood. On a whole tree basis the relationship was not as clear. In this case growth strain values determined from multiple measurements appear to be influenced primarily by tissue close to the site of measurements and not by wood at relatively remote locations. In some cases low growth strain values were found in trees with significant tension wood and in others high growth strain values where little tension wood had developed.

Summary

Density and microfibril angle (MFA) of tension wood and normal wood were assessed in the sapwood and heartwood, from three provenanaces of 10-year-old Eucalyptus globulus Labill. Density was measured using a modified saturation method that also enabled the calculation of the extractives lost during saturation. Microdensity and MFA were determined by SilviScan 2, a rapid X-ray densitometry and X-ray diffraction system developed at CSIRO. Significant differences were found in density and extractives between provenances and also density between the sapwood and adjacent heartwood from each provenance. This result may explain some of the drying differences between provenances found in an earlier study (Washusen and Ilic 2000). Sapwood samples with high percentages of tension wood fibres had high density and a significant positive correlation was found between microdensity and tension wood fibre percentage. MFA was found to be very low in normal wood in the sapwood, where most tension wood was found, so tension wood could not be identified by MFA. The positive association between tension wood and wood density suggests that caution should be taken when selecting trees for high wood density in tree improvement programs.