“…In general, machine learning approaches have high potential to capture the non-linear relationship between remote sensing data and vegetation parameters and have the capability of integrating multisource information at different levels (Yao, Qin, and Chen, 2019). In addition, the results acquired confirmed the importance of using machine learning algorithms in remote sensing vegetation classification, due to their powerful adaptation, self-learning, and parallel processing capabilities (Navin and Agilandeeswari, 2020;Meng et al, 2021;. Therefore, some challenges need to be emphasized and both the hardware and software of UAS remote sensing technology require improvements: [1] The endurance of UAS is relatively limited, the flight stability is not strong enough in areas with large terrain fluctuation and the lack of flight altitude limits the image size; [2] Although more lightweight and smaller sensor systems have become available, such as hyperspectral and LiDAR sensors, but they are still expensive; [3] The integration between UAS platforms and sensors requires improvement, e.g., most of the multispectral, hyperspectral, and thermal sensors are built independent of the UAV platform, so, need an extra GPS module and, also, UAS are often equipped with a single sensor, multisensor integration is beneficial to improve monitoring accuracy and efficiency; [4] The mosaic workload is significantly higher than satellite imagery, which takes up more time for image processing, resulting in the need to develop more robust algorithms, like deep learning techniques, in addition, the technology of mass data processing needs to be improved due to the richness and variety of data obtained; [5] The most vegetation classifications via UAS require human participation and interpretation, indicating that the combination between UAS Frontiers in Environmental Science frontiersin.org remote sensing with ground data and satellite data needs to be strengthened; if the dataset used for training is extent, computer learning techniques would generate a satisfactory classification outcome; [6] The use of UAS images to monitor tropical savannas leaf phenology is a challenge due to difficult methods to extract accurate quantitative phenology estimates under variable lighting and viewing conditions; [7] The application scenarios of UAS remote sensing in grassland ecosystem monitoring need to be expanded and deepened, mainly in animal investigation and soil physical and chemical monitoring; and also, the correlation between the scientific research of UAS remote sensing monitoring and practical decision making of grassland management is still insufficient (Neumann et al, 2019;Park et al, 2019;Lyu et al, 2020;Sun et al, 2021).…”