Abstract: Shrubs and tree saplings are important components of understory in natural forests, possessing rich species diversity and broad ecological adaptability. They play a key role in soil moisture retention, water conservation, and wind and sand prevention. However, current research has mostly focused on the interactions between the tree layer and understory, with less attention on the growth dynamics, biomass renewal, and environmental adaptability of understory. Based on data of 18 shrub species and sapling of six tree species from the understory of Fenglin County in Heilongjiang Province, along with climate data from the sampling sites, we introduced two dummy variables, stand density and age class, to construct a biomass model for understory shrubs and tree saplings. The results showed that incorporating climate factors (mean annual temperature and mean annual precipitation) into the base model to construct a climate-sensitive biomass model significantly improved the model's fitting accuracy. The coefficient of determination (R²) increased from 0.732 in the base model to 0.741, with an improvement of 6.8%. Based on the climate-sensitive biomass model, single dummy variables for age class and a dual dummy variable model combining stand density and age class were introduced. The dual dummy variable model showed the best fit, with an R² of 0.840, being significantly higher than the single dummy variable model (R²=0.787) and the climate-sensitive biomass model (R²=0.741). The model constructed based on the climate-sensitive biomass model and incorporating dual dummy variables for stand density and age class, could effectively reflect the biomass variations of shrubs and tree saplings under different age classes and stand densities. It would provide a basis for forest ecological management and the scientific estimation of biomass, with significant practical value.

Key words: shrub, young tree, biomass, climate factor, single dummy variable model, double dummy variable model