Gradient characteristics and the mechanisms that influence vegetation water use efficiency in the Pan-Central Asian Arid Region

Ecosystem water use efficiency (WUE) is a key indicator for assessing the coupling relationship between carbon sequestration and water consumption in ecosystems, and is of significant importance for understanding regional climate adaptability and ecosystem stability. Based on satellite remote sensing data, ERA5 reanalysis data, and land-use classification data, this study analyzed the multi-year mean spatial distribution characteristics of WUE across the Pan-Central Asian arid region from 2001 to 2024, as well as the spatiotemporal variations and altitudinal gradients of WUE among different vegetation types. Mann–Kendall trend tests, correlation analyses, and multiple regression methods were employed to examine long-term trends along altitudinal gradients in WUE, identify dominant driving factors, and quantify their relative contributions to WUE changes. Furthermore, structural equation modeling (SEM) was applied to explore the direct and indirect driving effects of climatic and environmental factors on WUE. The results indicate that: (1) WUE in the Pan-Central Asian arid region exhibits pronounced altitudinal gradients, with high-value zones concentrated in mid-altitude areas (1500–2500 m). Areas where the multi-year mean WUE exceeds 0.6 account for 58.1% of the total area, which is 40.2 percentage points higher than that in areas below 1500 m, and are primarily located across most of the Tianshan Mountains, the central and eastern Qilian Mountains, and the Mongolian Plateau. (2) In terms of median values, Cropland exhibits the highest WUE, which is closely associated with scientific irrigation and fertilization practices; forest exhibits the lowest WUE, while grassland WUE shows higher sensitivity to hydrological, climatic, and environmental factors. Interannually, the WUE of forests exhibits the largest fluctuations and the steepest declining trend, which are larger than those of grassland and cropland. (3) Overall, WUE across the Pan-Central Asian arid region shows a declining trend. Although vegetation growth has improved against the background of regional climate warming and humidification, carbon-water coupling efficiency has not increased. The rate of WUE decline is higher in mid-altitude zones than in low- or high-altitude zones; however, in the high-altitude Qilian Mountain region, both vegetation net primary productivity (NPP) and WUE show simultaneous increases. (4) Precipitation and leaf area index (LAI) act as direct positive drivers of WUE. Multiple regression results reveal that vapor pressure deficit (VPD) is the dominant contributor to WUE variations in low-altitude areas (43.3%), whereas radiation and soil moisture are the dominant contributors in mid-altitude areas (46.7% and 41.6%, respectively), and radiation contributes most in high-altitude areas (35.9%). This altitudinal shift in dominant WUE drivers reflects a transition from water limitation to energy limitation. SEM analysis further indicates that, at low altitudes, moisture-related factors not only significantly enhance WUE through direct pathways but also exert substantial indirect positive effects by increasing soil moisture (SM) and promoting vegetation growth; at mid-altitudes, these moisture factors shift toward indirect regulation mediated by vegetation growth; and at high altitudes, energy-related factors become the critical constraint on ecosystem WUE. Collectively, these findings systematically elucidate the altitudinal gradient characteristics of ecosystem WUE and its underlying mechanisms in the Pan-Central Asian arid region, providing a scientific basis for regional water resource management and ecological adaptation strategies.