Timber volume estimation is fundamental for sustainable forest management. This study focused on estimating the form factor of Shorea robusta in various physiographic regions of Nepal, with a specific aim to develop a diameter-height model for the Terai region. Data collection involved the destructive sampling of 109 randomly selected S. robusta trees, with recorded information including diameter at breast height (DBH), tree height, and crown parameters. After destructive felling, overbark and underbark stem diameters, along with section lengths, were measured at intervals of 1 m up to the tree tip.

The analysis revealed average overbark form factors of 0.42, 0.41, and 0.4, and underbark form factors of 0.41, 0.4, and 0.38 in the Terai, Siwalik, and Middle Mountain regions, respectively. Form factor significantly varied with diameter class in Terai (P = 0.02 < 0.05) and (P = 0.04 < 0.05), Siwalik (P = 0.05 ≤ 0.05) and (P = 0.05 ≤ 0.05), and the Middle Mountain region (P = 0.006 < 0.05) and (P = 0.01 < 0.05) for overbark and underbark, respectively. Correlation analysis indicated a significant relationship, with DBH increasing with height (0.86, 0.80, and 0.83) while decreasing with both DBH (-0.47, -0.42, -0.64) and form factor (-0.41, -0.34, -0.40), respectively, in all regions. The Pearson correlation test further confirmed these relationships at the 5% level of significance.

For estimating tree height in the Terai region, a power form of the model H = 1.3 + 4.194 * DBH^0.464 demonstrated the best fit, with an adjusted R-squared of 0.79, RMSE of 3.47, AIC of 267.22, and a significant p-value (P < 0.0001). This model contributes valuable insights for height estimation in Shorea robusta stands in the Terai region.

This research provides species-specific form factors that help improve the quantification of volume and other forest products, contributing to sustainable forest management in Nepal.