Modeling tree stem volume for hill Shorea robusta Gaertn. forests in Karnali Province, Nepal

Sal (Shorea robusta Gaertn.) is a major tree species of Nepal, which plays a vital role in the socio-economic development of livelihoods through multi-purpose uses. Developing a tree stem volume model provides a fundamental tool for estimating forest biomass, carbon stock, and economic value of timber and is useful for modeling growth and yield and analysis of forest ecosystems. This study developed tree stem volume models using measurements from 503 S. robusta trees of different community-managed forests in both the Siwalik Hill and non-Siwalik hilly regions of Nepal. As significant differentiation of the stem volume was observed by region in the analysis, a common tree stem volume model applicable to S. robusta forests in both regions was developed by applying the dummy variable modeling approach. Among some versatile growth functions (power, fractional and exponential functions) considered for fitting data with diameter at breast height, total tree height and crown width used as predictors, the power function provided the best fits (R²_adj = 0.9730; RMSE = 0.1427) with no systematic residual trends observed. The model simulation exhibited an increased volume with increasing tree height but decreasing crown width. The presented model was proved to be statistically flexible and biologically plausible and thus can be applied for a precise volume prediction of the species of interest. Model accuracy can be increased with the model recalibrated using additional predictor variables (e.g., site and climate variables) and more data collected in wider geographical ranges of the Siwalik and non-Siwalik hills of the Karnali province and beyond.