Nonlinear threshold effects of environmental drivers on vegetation cover in mountain ecosystems: From constraint mechanisms to adaptive management

• Analyzed vegetation patterns in the Qilian Mountains, emphasizing climate and terrain drive ecological adaptation. • Identified critical thresholds (4000 m, 0°C, 300 mm) governing nonlinear vegetation responses to environmental gradients. • Developed a terrain-climate-management framework to shift restoration focus from greening to enhancing ecosystem resilience. Based on a nonlinear theoretical framework, this study systematically reveals the spatiotemporal evolution characteristics of fractional vegetation cover (FVC) in the Qilian Mountains and its environmental constraints, providing scientific support for mountain ecosystem restoration and territorial spatial management. By integrating remote sensing inversion, GIS spatial analysis, and the constraint line model, the following key findings were obtained: (1) Spatiotemporal Evolution Characteristics: From 2000 to 2023, the FVC in the study area exhibited a fluctuating upward trend (annual growth rate of 0.26 %, R 2 = 0.5654, p < 0.001), forming a significant longitudinal gradient pattern. A sharp contrast was observed between the high-coverage areas (FVC > 60 %) in the southeast and the lowcoverage areas in the northwest, where bare land and extremely low to low-coverage regions accounted for 56.25 %. Notably, 54.84 % of the region experienced significant vegetation improvement, with the “greening” effect particularly pronounced in the central and western regions. (2) Topographic Vertical Constraint Mechanism: The altitudinal gradient shaped a four-stage response pattern: in the 2000–2900 m range, vegetation cover increased at a rate of + 1.56 %/100 m, mainly driven by water-heat synergy; in the 2900–4000 m range, accumulated environmental stress reversed the growth trend, leading to a decline of −1.32 %/100 m; in the 4000–4800 m range, thermal constraints intensified sharply, accelerating the decline to −8.01 %/100 m; in the 4800–5800 m range, vegetation approached its survival limit, with the decline rate slowing to −0.57 %/100 m. (3) Climatic Regulation Threshold Effects: Temperature control exhibited a biphasic hump-shaped pattern ( R 2 = 0.9449). In the −15 °C to −7°C range, the FVC gain rate reached 14.22 %/°C; between −7°C and 0 °C, the gain rate decreased to 3.07 %/°C, with 0 °C identified as a critical threshold. Above 0 °C, increasing competition pressure led to a decline in FVC (−0.68 %/°C). Moisture regulation showed a diminishing marginal effect: in the 20–300 mm range, each additional 10 mm of precipitation increased FVC by 2.97 %; in the 300–450 mm range, the marginal gain decreased to 0.88 %; above 450 mm, moisture constraints were lifted, and light-heat conditions became the primary limiting factors. This study identifies critical ecological thresholds of 4000 m altitude, an annual mean temperature of 0 °C, and 300 mm annual precipitation, elucidating the nonlinear response mechanisms of mountain ecosystems. Furthermore, it establishes a multidimensional “topography-climate-management” adaptive regulation framework, providing a scientific paradigm for transitioning ecological restoration from “maximum greenness” to “optimal resilience.”