Height-diameter models of regenerated saplings of based on dummy variable and quantile regression.

Based on data collected from 2054 saplings of 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, ., Class Ⅰ (SDI<1863 plants·hm), Class Ⅱ (1863 plants·hm≤SDI<2155 plants·hm), Class Ⅲ (2155 plants·hm≤SDI<2459 plants·hm) and Class Ⅳ (SDI≥2459 plants·hm) by using the quartile method. We constructed a dummy variable model and quantile regression model for the height-breast diameter of saplings of 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 , 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 compared with the base model, while RMSE, MAE, and AIC decreased by 2.1%, 1.5%, and 11.2%, respectively. When the quantile was 0.5, 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 SDI, sapling height in SDI-SDI 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.