Forest disturbance and degradation patterns in China’s Three-North Shelterbelt over the past three decades

The Three-North Shelterbelt Forest Program (TNSFP) is a key ecological initiative in China's arid and semi-arid regions, vital for wind prevention, sand fixation, and ecological restoration. However, long-term drought, extreme climate events, and human disturbances have threatened forest ecosystem stability. A systematic understanding of these disturbance and degradation patterns is lacking, hindering effective restoration strategies. This study aims to identify forest disturbances in the TNSFP from 1991 to 2021 and reveal their spatiotemporal patterns under varying climatic aridity gradients. Using annual Landsat imagery, four indices—NDVI, NBR, SWIR1, and SWIR2—were constructed. The LandTrendr algorithm was applied to extract multi-parameter degradation features, integrated with Hansen Forest Loss Data and the Random Forest model for disturbance classification (OA = 0.96, Kappa = 0.94). Results show that forest degradation peaked in the 1990s, with a decline in degraded areas over time. The high-density degradation zones were mainly in the Greater Khingan Mountains, Changbai Mountains, Loess Plateau, and Altay Mountains. Degradation characteristics such as duration, NDVI decline, and rate varied regionally. Northeastern areas of TNSFP showed slow degradation, while the Loess Plateau and northern Changbai Mountains experienced rapid degradation. Arid regions showed long-duration degradation but weak intensity, while humid regions showed high degradation rates. The findings provide technical support for understanding forest disturbance dynamics and degradation patterns.