Dune bistability identified by remote sensing in a semi-arid dune field of northern China

Sand dunes could have bistable states based on the observation that bare dunes and vegetation-stabilized dunes can coexist in the same area under the same environmental conditions. So far, more quantitative evidence on such an interesting phenomenon has remained elusive. In this study, the vegetation coverage and spatial distribution of dunes in the semi-arid Mu Us dune field, north-central China, were investigated by different remote sensing indices derived from the Landsat images using the Google Earth Engine platform, and the results were verified using the aerial images. Frequency distribution of vegetation coverage in many sub-regions (5 km × 5 km) across the dune field clearly shows two dominant peaks, one around 5% and the other around 40%, representing active and stabilized dune states, respectively. The boundaries between the patches of these two states are sharp, and have hardly shifted during the last twenty years. Such coexistence of bistable states is mostly distributed across a precipitation gradient from 200 to 400 mm in the study area. The relative portion of active dunes in total is reduced with increasing precipitation, while the dominant peaks of vegetation coverage for two dune states remain largely unchanged. All these lines of evidence are in accordance with the theory of alternative stable states and model predictions. Because the reversal would be difficult once the dunes shifted into an undesired state from the standpoint of environmental management, detecting and monitoring these transitions, which are often abrupt, is important for better process-based understanding of the mechanisms involved and anticipating future transitions.