###### Abstract  
Understanding vegetation responses to atmospheric drought is critical for arid ecosystem management under climate change. However, the threshold of the response mechanism of grassland in arid regions to atmospheric drought remains unclear. This study investigates how vapor pressure deficit (VPD) regulates grassland gross primary productivity (GPP) in Xinjiang, China, using MODIS and other multi-source remote sensing data (2000-2020). The results show intensified atmospheric drought in central Tianshan Mountains and southern Junggar Basin, with VPD exhibiting a widespread increasing trend (significant increase: 15.75%, extremely significant increase: 4.68%). Intensified atmospheric drought occurred in the central Tianshan Mountains and southern Junggar Basin. Integrated analyses demonstrate that VPD has a dominant negative impact on GPP (path coefficient \(=-0.58\), \(p<0.05\)), primarily driven by atmospheric drought stress. A ridge regression-derived threshold was identified at 0.61 kPa, marking the point where VPD transitions from stimulating to suppressing productivity. Spatially, 58.75% of the total area showed a significant increase in GPP. These findings advance the mechanistic understanding of atmospheric drought impacts on arid ecosystems and inform adaptive grassland management strategies. atmospheric drought; vapor pressure deficit; gross primary productivity; ecological threshold 2025176261703390/su1714626170390/su171462  
benefits resulting from rising atmospheric CO\({}_{2}\) concentrations [4,5], leading to complex effects on surface energy balance. Notably, the coupling between VPD and soil moisture through land-atmosphere feedback mechanisms further amplifies vegetation stress during compound drought-heatwave events [6]. Conversely, hydraulic diversity in plant communities can buffer ecosystem-scale fluctuations in water fluxes under drought conditions [7]. While current academic consensus establishes VPD as a primary regulator of photosynthetic processes through stomatal modulation [8], its ecological impacts exhibit critical biome-specific and context-dependent variations. Seasonal VPD fluctuations in mid-to-high latitudes enhance autumn photosynthetic capacity under low atmospheric aridity [9], yet these benefits are counterbalanced by spatially heterogeneous interactions with temperature (TEM) and soil moisture [10]. Comparative studies in Central Asia suggest that forests exhibit heightened VPD sensitivity compared to grasslands and shrulbands, which are predominantly constrained by soil moisture [11]. This biome-specific response is further complicated by the regulatory effects of VPD on grassland ecosystems: direct stomatal control of transpiration and indirect modulation via soil moisture-radiation feedback [12]. Recent evidence highlights a climatic dichotomy: soil moisture dominates vegetation water constraints in arid/semi-arid zones, whereas VPD-driven limitations prevail in humid regions [13]. Crucially, threshold-dependent responses emerge when VPD exceeds regionally adaptive limits through synergistic interactions with TEM and soil moisture [10], but such thresholds remain undefined for Xinjiang's hyper-arid grasslands. Although grassland ecosystems account for 32% of global natural vegetation and play a crucial role in water vapor exchange and energy balance, systematic studies investigating the relationship between grassland productivity and VPD in arid regions (e.g., Xinjiang) remain limited [14]. This gap is critical given Xinjiang's unique hyper-arid conditions, where vegetation thresholds to drought-causing factors vary markedly by type [11].