Estimating Within Tree Spatial Changes in Acoustic Velocity in Felled Douglas-fir Stems

Abstract

Sorting logs for stiffness in the woods using acoustic devices mounted on harvesters and processors could improve the allocation of logs to their most appropriate processing destinations. Estimating spatial changes in acoustic velocity in felled stems, based on a limited number of measurements, will be essential if they are not to unduly impact harvesting or processing productivity and costs. The effect of five variables (length from butt, bark percentage, moisture content, green density, and oven dry density) and four velocity measurement approaches (time of flight [TOF] across the tree bole, resonance of the full tree length, resonance of the first 3 m butt log segment, and resonance of a log segment between 6 and 9 m above the butt) on the ability to estimate spatial changes in acoustic velocity were evaluated in felled Douglas-fir stems from five stands in southwestern Oregon. Distance from the butt was a significant predictor of acoustic velocity along the stem. TOF acoustic velocity measured across the bole had little value in predicting the longitudinal resonance acoustic velocity of a section of the tree. Full tree length resonance acoustic velocity, or single log acoustic velocity (either 3 m butt log or 3 m log 6 m above the butt) were found to be statistically significant and strongly correlated predictors of acoustic velocity of a section of a tree (R2 ranging from 0.68 to 0.89). The models were found to be stand-dependent, indicating a possible need for calibration for each stand to be harvested.