The increased frequency and severity of heat and drought events in the coming years will challenge locally adapted temperate tree species and increase their vulnerability to disturbances. Traditional forests assessments performed every few years in the frame of national forest inventories might no longer be sufficient to capture the impact of extreme events on our forests. In this project, we investigate the potential of spectral proxies, obtained from unmanned aerial vehicles (UAV) high-resolution multispectral and thermal imaging, to detect trees pre-visual drought stress responses. Center of our investigation is the Pfynwald long-term irrigation experiment, a forest in one of the driest inner alpine valleys of the European Alps. UAV-based monitoring of tree physiological responses opens the way for high-throughput phenotyping of thousands of individuals and tree species in parallel, which will help us define the forest of the future.