Topographical Gradient Characteristics of Land-Use Changes in the Agro-Pastoral Ecotone of Northern China

The agro-pastoral ecotone of northern China (AENC) is a significant ecological barrier, where the topographical features play basic roles in land-use change. In order to reveal the influence of topographical factors on land-use changes in the AENC, we used land-use transfer matrix, geo-information graphics, terrain niche, distribution index and geographical detector to explore the topographic gradient effect of land-use changes during 2000–2020 in the AENC based on remote-sensing image data from 2000, 2010, and 2020. The findings indicate that: (1) The total areas of land-use changes were 121,744 km², accounting for 17.41% between 2000–2020. This was characterized by increasing amounts of land-use changes in the AENC. The changes in land-use were dominated by the conversions among farmland, forestland, and grassland, which were distributed widely in the mountainous regions of northern, western, and eastern margins. The expansion of construction land was derived mainly from farmland and grassland occurred in river valleys. (2) The pattern of land-use changes was divided into five types including stable type, prophase change, anaphase change, continuous change, and repeated change. Stable type accounted for 559,868.86 km² and 80.09% of the total area. It was dominant in high altitude and complex terrain areas with terrain niches of more than 1.61. Prophase and anaphase changes accounted for 3.95% and 13.03%, respectively, which occupied to dominant positions in the 0.69–1.17 and 0.04–0.69, 1.17–1.61 terrain niches topographic gradient, respectively. Continuous and repeated changes occupied dominant positions in low altitude and flat complex areas with terrain niches of 0.04–1.17. (3) The topographic gradient effect of land-use changes in the AENC was influenced comprehensively by natural, geographical location, socioeconomic, and policy factors. Natural environmental factors and geographical location determined the topographic gradient pattern of land-use structure, while the direction of the topographical gradient pattern of land-use changes in the AENC is influenced by socioeconomic and policy factors. This research can provide a scientific reference for the development and protection of territorial space and optimal allocation of land resources in the AENC.