Nondestructive Evaluation of the Heartwood Color of Standing Sugi Trees by Using the Lateral Impact Vibration Method

Abstract

The heartwood color of standing sugi (Cryptomeria japonica) trees was evaluated using the lateral impact vibration method. Evaluations were obtained using the diagnostic index (DF_r), which is the product of the diameter D of a stem and the resonance frequency F_r obtained when the stem is laterally impacted. The heartwood color was evaluated on the basis of the following assumptions. A lower DF_r implies a higher moisture content (M_c) in the heartwood, and a higher M_c implies lower brightness (L^*) of the heartwood color. Therefore, a low DF_r might imply a low L^*. Seven hundred trees were measured using this method. Seventy of the 700 trees were cut to check the color of the heartwood. The increase or decrease rate R in the DF_r of each tree against the regression line of D and DF_r for all the cut trees was calculated as the index for evaluating L^*. The calculations revealed a negative correlation between R and M_c (r = 0.71) and between M_c and L^* (r = 0.70). Further, a positive correlation (r = 0.53) was observed between R and L^*. The correlation coefficient between R and M_c was not high, but the brightness of the heartwood color (L^*) of the standing sugi trees could be classified into three groups by using the index R. The use of heartwood color evaluation by this method in thinning is also discussed.