Estimation of aboveground biomass and carbon stocks of Quercus ilex L. saplings using UAV derived RGB imagery

Abstract

Abstract Key message Crown area, sapling height, and biovolume extracted from UAV-acquired RGB images provided accurate estimates of aboveground biomass and carbon stocks in a 5-year-old holm oak (Quercus ilex L.) plantation. Our models regressing UAV-derived sapling variables against ground-based measurements exhibited high R2 values (0.78–0.89), thereby refecting that RGB data can be used as an efective tool for measuring young individuals. Context The monitoring of tree sapling performance from the early stages of reforestation is of particular importance in the context of the global eforts to restore forests. Yet, most models to estimate carbon sequestration are devel‑ oped for adult trees. Thus, the few models specifcally developed for young trees rely on ground-based feld sampling of tree growth parameters, which is time-consuming and difcult to implement at large spatial scales. Aims Our objectives were as follows: (1) to study the potential of UAV-based RGB imagery to detect and extract sap‑ ling variables (e.g., crown area, height, and biovolume) by comparing ground-based sapling measurements with UAVderived data and (2) to compare the accuracy of the data estimated from RGB imagery with existing traditional feld-based allometric equations. Methods We used a 5-year-old holm oak (Quercus ilex L. subsp. ballota (Desf.) Samp.) plantation (N = 617 plants), and their crown area, height, and biovolume were estimated from RGB imagery. Subsequently, the plants were harvested and the UAV-derived data were compared with feld-measured sapling height and aboveground biomass values. Carbon content in leaves and stems was measured in a subsample of the saplings to estimate carbon stocks. Results The models ftted with UAV-derived variables displayed high performance, with R2 values from 0.78 to 0.89 for height, leaf and stem biomass, total aboveground biomass, and carbon stocks. Moreover, aboveground biomass out‑ puts calculated with feld height and UAV-derived height using allometric equations exhibited R2 values from 0.65 to 0.68. Conclusions Given the afordable cost of RGB cameras and the versatility of drones, we suggest that UAV-based models may be a cost-efective method to estimate the biomass and carbon stocks of young plantations. However, further studies conducting drone fights in diferent conditions are needed to make this approach more scalable.