A study of Eucalyptus grandis and Eucalyptus globulus branch wood microstructure

Experimental measurements of cellulose crystallite width and microfibril angle (MFA) by X-ray diffractometry on SilviScan-2 and by conventional microtechniques revealed that the branch wood of the two species exhibited very similar trends in cellulose crystallite width and MFA. Cellulose crystallite width was greater on the upper side of the branches. Tension wood, as defined by the occurrence of gelatinous fibres, was found where cellulose crystallite width was greater than 3.0 nm and 3.1 nm in Eucalyptus grandis and E. globulus respectively. In the tension wood zones, MFA was lower than in the rest of the samples and so could be used to differentiate tension wood. On the lower side of the branches MFA determined from X-ray diffractometry unexpectedly exceeded 40 degrees and fibres were often buckled in both the tangential and radial directions in both species. This local variation in the direction of the fibre axes contributed only slightly to the magnitude of the MFA determined by SilviScan-2. Even given this misalignment, the additional evidence gained from pit angles and cracks in fibre walls suggested that the MFA was indeed around 40 degrees in the lower radius of the branches. This MFA is considerably larger than would be expected for eucalypt stem wood and it is suggested that opposite wood in eucalypt branches may provide a complimentary structural role to that of the tension wood. Experimental measurements of crystallite width produced by SilviScan-2 may be used to accurately locate tension wood zones in both species.