Allometry and UAV-based estimation of tree dimensions, volume and aboveground biomass of Eucalyptus robusta plantations in the central highlands of Madagascar

• Smallholder eucalyptus plantations lack species-specific biomass equations • Species-specific allometric models are calibrated for Eucalyptus robusta • UAV imagery enables efficient estimates of tree height and trunk circumference • UAV models offer rapid alternative for assessing stand structure and biomass In the central highlands of Madagascar, smallholder eucalyptus plantations play a crucial role in meeting the energy needs of households. However, these plantations remain poorly characterized due to a lack of species-specific allometric equations, which hampers the precise assessment of tree volume and aboveground biomass (AGB). Additionally, there is currently no accessible method for estimating biomass at the plot scale. Moreover, volume and biomass measurements at this scale still rely on traditional forest inventories – methods that are both time-consuming and labor-intensive. This study aimed to calibrate species-specific models for estimating tree volume, and stem and total AGB in smallholder Eucalyptus robusta plantations. Allometric models were calibrated using destructive biomass measurements on 40 trees during the first stand rotation (4 to 6 years old), including both fertilized and unfertilized plots. Furthermore, two methods for estimating volume, stem AGB and total AGB using unmanned aerial vehicle (UAV) imagery were developed and compared. Allometric models yielded satisfactory results for predicting volume and biomass, with relative Bias% ranging from 0.9 ± 1.2% to 4.7 ± 6.0% and normalized root mean square error (NRMSE) between 11.2 ± 3.5% and 20.6 ± 14.0%. UAV-based models also produced promising results for estimating tree height and circumference at breast height (CBH), with R² of 0.93 and 0.76, respectively. The most effective approach combined UAV-derived estimates of height and CBH with species-specific allometric models. This study provides robust species-specific models for estimating E. robusta volume and biomass and demonstrates the potential of UAV imagery for characterizing tree plantations in smallholder contexts.