Abstract: Based on data collected from 2054 saplings of Larix gmelinii forest in 55 fixed plots in 2018-2019 in Cuigang Forestry Station, Daxing’anling area, we classified the stand density index (SDI) into four classes, i.e., Class Ⅰ (SDI₁<1863 plants·hm^-2), Class Ⅱ (1863 plants·hm^-2≤SDI₂<2155 plants·hm^-2), Class Ⅲ (2155 plants·hm^-2≤SDI₃<2459 plants·hm^-2) and Class Ⅳ (SDI₄≥2459 plants·hm^-2) by using the quartile method. We constructed a dummy variable model and quantile regression model for the height-breast diameter of saplings of L. gmelinii with dummy variable method introduced SDI. The results showed that among the five selected representative non-linear tree height curve models, the Richards model fitted the best, with R_a², RMSE and MAE of 0.7637, 0.8250 m and 0.5696 m. The dummy variable model including the SDI constructed based on the Richards model showed a 1.3% increase in R_a² compared with the base model, while RMSE, MAE, and AIC decreased by 2.1%, 1.5%, and 11.2%, respectively. When the quantile τ was 0.5, R_a² of quantile regression model was the maximum, and RMSE, MAE, AIC was the minimum, being 0.7612, 0.8294 m, 0.5657 m, and -767.19, respectively. Compared with SDI₁, sapling height in SDI₂-SDI₄ was increased by 5.6%, 5.6%, and 11.3%, suggesting reasonable that regulation of stand density was conducive to increase the height growth of saplings in regeneration.

Key words: dummy variable, quantile regression, tree height, diameter at breast height, stand density index