Roundwood pre-grading with longitudinal acoustic waves for production of structural boards

The assessment of timber quality at any stage of the production chain promises to be an advantage for the forest and timber industry. This paper presents results of wood pre-grading by means of the dynamic modulus of elasticity, which was performed along the complete processing chain—from standing trees to sawn timber. The measurements were conducted on 154 forty-year-old trees of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) from two forest experimental stands in Southern Germany. Long logs of about 13 m were bucked into short logs of 4.1 m; the dataset contained 1,820 boards cut out of 332 short logs. Pre-grading was performed at different levels: standing tree, long log, and short log. The boards were machine strength graded by a ViSCAN-COMPACT grading machine. The investigation showed high correlations between different stages in the production chain of timber. For strength classes C24 and C30 an increase in yield of about 10 percentage points by rejecting the 25 % worst logs (short or long) was observed. It did not matter if pre-grading was realized at the stage of long logs or short logs. However, methods of pre-grading at standing tree must be improved to obtain reliable prediction of dry sawn timber quality of managed forest stands (r² = 0.17).