Enhancing stem volume estimation for savanna species using variable-exponent taper equation and close-range photogrammetry

Stem volume estimation is crucial in forest ecology and management, particularly for timber harvesting strategies and carbon stock assessments. This study aimed to develop a variable-exponent taper equation specifically tailored to savanna tree species using close-range photogrammetry (CRP) data and to evaluate its performance against conventional volume equations for stem volume estimation. A dataset of 30 trees across five dominant savanna species was used to fit the taper model, which was validated using a separate dataset of 322 trees from 14 species. The results demonstrated significant improvements in volume estimation accuracy when using the taper equation. At the tree level, the root mean square error (RMSE) decreased by 47%, from 598 to 319 ​dm³, and the mean absolute bias (MAB) by 48%, from 328 to 172 ​dm³, compared to volume equations. Similarly, at the plot level, RMSE was reduced by 42% and MAB by 40%. The model performed well for species with regular forms. However, species with irregular tapers exhibited higher errors, reflecting the challenges of modeling stem forms of mixed species. The use of CRP proved valuable, providing high-resolution diameter measurements that improved model parameterization. This study underscores the importance of advanced data collection methods for enhancing taper model accuracy and suggests that further species-specific adjustments are needed to improve performance for species with irregular forms. The findings support the broader application of taper equations for improving stem volume estimates in savanna ecosystems, contributing to better forest management and resource monitoring practices.