Mapping common and glossy buckthorns (Frangula alnus and Rhamnus cathartica) using multi-date satellite imagery WorldView-3, GeoEye-1 and SPOT-7

Buckthorns (Glossy buckthorn, Frangula alnus and common buckthorn, Rhamnus cathartica) represent a threat to biodiversity. Their high competitivity lead to the replacement of native species and the inhibition of forest regeneration. Early detection strategies are therefore necessary to limit invasive alien plant species' impacts, and remote sensing is one of the techniques for early invasion detection. Few studies have used phenological remote sensing approaches to map buckthorn distribution from medium spatial resolution images. Those studies highlighted the difficulty of detecting buckthorns in low densities and in understory using this category of images. The main objective of this study was to develop an approach using multi-date very high spatial resolution satellite imagery to map buckthorns in low densities and in the understory in the Quebec city area. Three machine learning classifiers (Support Vector Machines, Random Forest and Extreme Gradient Boosting) were applied to WorldView-3, GeoEye-1 and SPOT-7 satellite imagery. The Random Forest classifier performed well (Kappa = 0.72). The SVM and XGBoost's coefficient Kappa were 0.69 and 0.66, respectively. However, buckthorn distribution in understory was identified as the main limit to this approach, and LiDAR data could be used to improve buckthorn mapping in similar environments.