Background Air and satellite remote sensing can provide detailed data about forests. However, cooperation between satellite service providers and smaller research institutions is often difficult to orchestrate. Stratospheric long endurance (SLE) unmanned air vehicles (UAVs) could provide small R&D organisations with the means to improve and harmonise their international research activities. SLE UAVs use solar photovoltaic cells to provide energy for propulsion and can, therefore, stay in the air for extremely long periods without the need for refuelling. Indeed, they can fly almost permanently, as long as there is no need to change mission or technical specifications. They can follow any monitoring or measurement path, or circle over a specified area at an altitude of up to 15-20 kilometres. Forests in Poland are threatened by multiple land use pressures, as well as invasive phytopathogens such as those belonging to the genera Phytophthora, which have reduced the density and strength of trees. Treatments to tackle invasive Phytophthora pathogens are costly and not so effective, mainly because of the challenges of covering such large areas, and the difficulty in observing and monitoring the problem from the ground. Objectives The HESOFF project aims to integrate innovative SLE UAV technologies with innovative methods for forest cultivation. It seeks to demonstrate how SLE UAVs can be used to effectively evaluate the health of forests according to different characteristics, allowing for more targeted interventions. The project will test the effectiveness of phosphites in improving the resistance of trees to the invasive Phytophthora pathogens. It will use experimental techniques to evaluate the health of oak trees, and their environmental conditions, before and after applying the phosphite treatment. HESOFF will also introduce new methods for assessing the state of forests and the effectiveness of cultivation through images produced by SLE UAVs. A forest is typically defined by properties such as the age and distribution of trees, the species, the density, the yield, the necessity of intervention measures, the site quality, etc. These properties can be assessed through visual interpretation of SLE UAV aerial images. Expected results The most important result of the HESOFF project will be to verify the use of SLE UAV technology as a more environmentally and economically advantageous means of evaluating the health of forests. This will include optimising the use of multispectral cameras on planes. The project also expects to demonstrate the effectiveness of phosphite treatments in tackling Phytophthora pathogens and improving the health of forests. Specific targets for treated areas include: A 50% improvement in the health of oak stands; A 70% decrease in the DNA of invasive Phytophthora pathogens in soil and plant tissue; An improvement in forest tree root systems, with an increase in the amount of fine root from 30 to 50%; 30% of tree crowns rebuilt, with decreased transparency and defoliation.