[Nonlinear Impacts and Threshold Effects of Land Use Change on Ecosystem Service Value in Northwest China]

As a crucial ecological barrier in China， the fragile ecosystems in Northwest China exhibit high sensitivity to land use changes. Therefore， investigating the relationship between land use/cover change （LUCC） and ecosystem service value （ESV） is of great significance for regional ecological protection and sustainable development. Using Northwest China as the study area， this study systematically analyzed the spatiotemporal evolution patterns of land use based on land use data from 2008 to 2023 using spatial statistical methods. The ESV was assessed using a modified equivalent factor method and geodetector to reveal its spatiotemporal differentiation characteristics and driving factors. A boosted regression tree （BRT） model was employed to quantitatively identify the nonlinear impact mechanisms of proportional changes in different land use types on the ESV and determine their critical threshold effects. The results indicated the following： ① During the study period， the overall land use structure in Northwest China remained relatively stable. Construction land showed the most significant increase， expanding by 9 941.85 km2， and was primarily converted from cultivated land and grassland. ② ESV exhibited an initial decline followed by a rising trend over time. Spatially， it displayed a pattern of "higher values in the southeast and lower values in the northwest，" closely correlated with land use types， with different ESV grades showing clustered distribution. ③ The spatial heterogeneity of ESV was jointly driven by natural and socio-economic factors. Land use intensity contributed the most among the drivers. All factor interactions exhibited enhancing effects， with the interaction between land use intensity and other factors being particularly prominent in explanatory power. ④ Forest land， grassland， and unused land were identified as the key land use types influencing ESV in the mountain system. Unused land， water bodies， and cultivated land were the key factors affecting the ESV in the oasis and desert systems. ⑤ The proportion of key land use types exhibited a significant threshold effect on ecosystem service value （ESV）. To maintain ESV at a medium or higher level， the land use proportions in each system must meet the following conditions： For the mountain system， forest land should account for 21.56%-98.99%， grassland for 61.07%-98.99%， and unused land for 3.03%-11.39%； for the oasis system， water area should account for 3.75%-98.99%， unused land for 1.01%-28.01%， and cultivated land for 3.03%-14.22%； and for the desert system， water area should account for 0.97%-98.99%， unused land for 1.01%-74.39%， and cultivated land for 0.00%-4.03%.