Effect of radial increment core diameter on tracheid length measurement in Norway spruce

The effect of radial increment core diameter (comparing the effects of 12, 8 and 4 mm diameters, with sampling from chips used as a control) on the measurement of tracheid mean length and the percentage of broken tracheids was examined for mature wood of Norway spruce. Two methods of measuring tracheid length were used: image analysis, in which arithmetic mean length (AML) was calculated for unbroken tracheids, and the Kajaani FS-200, in which AML was calculated for all tracheids, regardless of damage. In addition to AML, length weighted mean length (LWML) was calculated for the Kajaani. Almost 70 % of all tracheids remained undamaged when the 12 mm increment core was used. This was 18 % units less than for chips. Proportion of broken tracheids increased as core diameter decreased, and for the 4 mm increment core 30 per cent of all tracheids remained undamaged. All increment core diameters tested gave LWML values that were significantly lower than the control. For AML by Kajaani, all mean values were about 50 per cent lower than corresponding LWML or image analysis values, thus showing the direct influence of broken tracheids. However, only the 4 and 8 mm increment cores differed significantly from the control. For unbroken tracheids measured by image analysis, the 4 mm increment core gave a significantly lower value than the control, thus showing the indirect influence of broken tracheids. Values obtained from the 8 and 12 mm increment cores did not differ significantly from the control. It was concluded that in species with an average tracheid length of approximately 3 mm (which includes mature Norway spruce wood), AML can be correctly measured if increment cores of 12 mm are used. When unbroken tracheids are chosen for measurement the same is true for an 8 mm increment core. However, if measurements are to be made on tracheids regardless of damage, length weighting is necessary to obtain a reliable value. In that case, none of the increment core diameters tested in this investigation provided accurate measurements.