Non-Destructive Methods and Process Capability Analysis to Assess Conformance of Douglas-fir Stands to Customer Quality Specifications
David G. Briggs, Eric C. Turnblom and B. Bruce Bare
Largest branch diameter in the breast-height region (LLBH) and acoustic velocity on lower bole were measured on trees in a 20-year old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) experiment composed of seven density management/fertilization regimes. The less dense regimes tended to have larger mean branch diameter at breast height, with fertilizer increasing mean even further. However, the except for the densest regimes, the difference between a density regime and its counterpart with fertilizer was not statistically significant. The densest regime had significantly higher mean acoustic velocity than the other regimes, which were all the same except for one with very low velocity attributed to abnormal wood formed after damage by black bears. Although statistical significance may be lacking with respect to mean properties, subtle differences in their distributions may be important to timber sellers where purchasers often pay premiums for stands with higher percentages of trees that meet their process and customer needs. A statistical quality control procedure, process capability analysis, was used to assess the conformance of each regime to specifications for largest branch diameter at breast height and acoustic velocity. Conformance of largest branch diameter at breast height to a 35-mm maximum ranged from 84% to 100%, with fertilization reducing conformance by 10 to 15%. Conformance of acoustic velocity to a 3.5 km/sec minimum ranged from 15 to 85% with negligible difference between a thinned regime and its counterpart with fertilizer. Joint conformance ranged from 10 to 85%, with generally lower conformance associated with fertilizer. There is potential for using statistical quality control techniques to assist with timber marketing, harvest planning, and monitoring stand development.
Keywords: statistical quality control, process capability analysis, fertilizer, thinning, knot diameter, wood stiffness, acoustic velocity, nondestructive testing, wood quality.