Accuracy and precision of measuring cross-sectional area in stem disks of Douglas-fir infected by Armillaria root disease

Abstract

Stem cross-sectional areas were checked for accuracy and precision of area measurements in healthy and Armillaria ostoyae (Romagn.) Herink infected 18-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). Ten trees were randomly selected in each combination of two size classes and four infection classes, and stem disks were taken at the soil line (0 m) and at 1.3 m. Disks were marked at the longest radius, 90° to the longest radius, the shortest radius, and at radii determined by the sum of the largest diameter and the diameter at 90° divided by four. These radii were used to calculate cross-sectional area, then these calculated areas were compared with the corresponding digitized areas. Cross-sectional areas calculated from radial measurements were generally not within 5% of digitized areas. Radii were also drawn on the disks corresponding to the positions at which healthy and infected roots arose from the root collar below. For 0-m disks, the stem radii over healthy roots averaged 7 mm longer than over infected roots. At 1.3 m, the stem radii over healthy roots were 4 mm longer, but this was reduced with increasing infection of the root system. Offset piths and irregular shapes formed because of radial reduction over infected roots, and corresponding radial expansion over healthy roots affected the accuracy of disk area estimation.