A remote sensing method for mapping alpine grasslines based on graph-cut

Climate change has induced substantial shifts in vegetation boundaries such as alpine treelines and shrublines, with widespread ecological and climatic influences. However, spatial and temporal changes in the upper elevational limit of alpine grasslands ("alpine grasslines") are still poorly understood due to lack of field observations and remote sensing estimates. In this study, taking the Tibetan Plateau as an example, we propose a novel method for automatically identifying alpine grasslines from multi-source remote sensing data and determining their positions at 30-m spatial resolution. We first identified 2895 mountains potentially having alpine grasslines. On each mountain, we identified a narrow area around the upper elevational limit of alpine grasslands where the alpine grassline was potentially located. Then, we used linear discriminant analysis to adaptively generate from Landsat reflectance features a synthetic feature that maximized the difference between vegetated and unvegetated pixels in each of these areas. After that, we designed a graph-cut algorithm to integrate the advantages of the Otsu and Canny approaches, which was used to determine the precise position of the alpine grassline from the synthetic feature image. Validation against alpine grasslines visually interpreted from a large number of high-spatial-resolution images showed a high level of accuracy (R2, .99 and .98; mean absolute error, 22.6 and 36.2 m, vs. drone and PlanetScope images, respectively). Across the Tibetan Plateau, the alpine grassline elevation ranged from 4038 to 5380 m (5th-95th percentile), lower in the northeast and southeast and higher in the southwest. This study provides a method for remotely sensing alpine grasslines for the first-time at large scale and lays a foundation for investigating their responses to climate change. Alpine grasslands are widely distributed in alpine regions of the world, and their distribution is highly sensitive to climate change, but spatial and temporal changes in the upper elevational range of alpine grasslands (hereafter "alpine grasslines") remain poorly understood because of lack of observations. We propose a graph-cut based method, combining the Otsu binarization and modified Canny gradient, for automatically identifying alpine grasslines from multi-source remote sensing data and determining their positions at 30-m spatial resolution. Validation showed a high level of accuracy (R2, .99 and .98; mean absolute error, 22.6 and 36.2 m, vs. drone and PlanetScope images, respectively).image