Height-DBH Modelling for Young- and Middle-aged Cupressus funebris Plantations in the Central Sichuan Hilly Region

Taking young (12～26 a)- and middle-aged (45～55 a) Cupressus funebris plantations in typical counties of the hilly area of central Sichuan, namely Jintang County, Jianyang City, Jiange County, Yanting County and Fushun County, as research objects, a total of 92 standard plots of 20 m × 20 m were established, and sample data of 1527 trees were obtained (366 young-aged trees: 257 for model calibration and 109 for validation; 1161 middle-aged trees: 703 for model calibration and 458 for validation). Eight commonly-used models including Power, Bates and Watts, Schumacher, Logistic and Chapman-Richards were selected to develop suitable height-diameter at breast height (H-DBH) curves for Cupressus funebris plantations. The Akaike Information Criterion (AIC), root-mean-square error (RMSE), mean absolute error (MAE), relative mean absolute error (RMAE), and adjusted coefficient of determination (R_adj²) were adopted to select the optimal model. The results showed that the Naslund model performed best for young-aged stands, with an adjusted coefficient of determination of 0.9128 and a root-mean-square error of 1.0504 m. The Logistic model was optimal for middle-aged stands, with an adjusted coefficient of determination of 0.9068 and a root-mean-square error of 2.1591 m. For the pooled dataset, the Chapman-Richards model yielded the best fit, with an adjusted coefficient of determination of 0.9524 and a root-mean-square error of 1.9942 m. The findings of this study can provide references for multi-functional management, site quality evaluation and forest carbon stock estimation of Cupressus funebris plantations.