Modeling Diameter Growth and Self-Thinning in Planted Sugi (Cryptomeria japonica) Stands

Abstract: The objectives of this study were to analyze diameter growth in relation to natural thinning in high-density stands in even-aged, pure plantation forests and to develop a growth prediction system based on Japanese permanent plot data. Long-term data recorded in even-aged, pure, unthinned stand plots of Sugi (Cryptomeria japonica) forests were available. Relationships between log-transformed average diameter at breast height (DBH) and stand density were analyzed. In addition, the self-thinning ratio (STR) was analyzed from data gathered from unthinned permanent plots. Relationships between STR and the yield index, Ry (the ratio of actual stand volume to that at full stand density), were also analyzed and modeled. Based on these analyses, the diameter growth rate was formalized as a function of DBH, stand density and stand age, using parameters derived from full density curves. Goodness of fit of predictions of diameter growth in unthinned stands using the estimated parameters were evaluated by comparing predicted stand density and DBH with those observed in the permanent plots. The average error rate, derived by averaging the absolute values of all calculated error rates for the estimated stand density, was 1.8% with a maximum of 6.4%. The average error rate for the DBH was 2.9%, with a maximum of 8.1%. The squared Pearson’s correlation coefficients of predicted and observed average DBH were between 0.97 and 1.0.