Development and application of a vegetation degradation classification approach for the temperate grasslands of northern China

•Grassland degradation classification based on the annual maximum leaf area index.•Determined degradation thresholds are temporally stable and spatially universal.•Degradation thresholds are specific for six temperate grasslands in northern China.•Degraded grasslands in northern China accounted for 8.2 % from 1981 to 2017.•Our results can accurately assess degradation status for different grassland types.