## 4 Discussion  
### Overall Increasing Trend in GPP and Regional Response Differences  
Our results indicate a significant overall increasing trend in GPP across Xinjiang (Figure 2a), with most regions exhibiting sustained spatial growth (Figure 2b). This pattern aligns with the global vegetation productivity response to climate warming and CO\({}_{2}\) fertilization effects [8]. Notably, [PERSON] et al. (2023) [31] highlighted significant regional differentiation in ecosystem response pathways: in northern areas, changes in aboveground gross primary productivity were primarily driven by leaf area index, whereas southern regions relied more on functional trait regulation. This mechanism may explain the significant increase in VPD observed in the central Tianshan arid zone (\(p<0.05\), Figure 3a) and its inhibitory effect on GPP found in our study. In soil moisture-limited areas like central Tianshan, vegetation photosynthetic capacity is more sensitive to atmospheric drought, with functional trait responses potentially playing a dominant role.  
The inhibitory effect of VPD on GPP involves a dual mechanism. Firstly, elevated VPD directly restricts CO\({}_{2}\) uptake by increasing stomatal resistance, thereby reducing leaf-level light use efficiency [32]. [PERSON] et al. (2024), studying European grasslands, further  
Figure 8: Spatiotemporal dynamics and threshold effects of Vapor Pressure Deficit (VPD) on grassland productivity in Xinjiang (2000–2020). (**a**) Average VPD in Xinjiang from 2000 to 2020, with the horizontal line area indicating the main effect region of VPD. (**b**) Annual variation line chart of the main effect region of VPD. (**c**) Univariate fitting results of ridge regression coefficients. The pink area indicates VPD-promoting regions for grassland productivity, while the blue area indicates VPD-suppressing regions for grassland productivity. The linear regression fit R\({}^{2}\) = 0.9575, and the regression equation is y = \(-0.179\)x + 0.109, with a VPD threshold of 0.61 kPa.  
confirmed a significant positive correlation between increased VPD and vegetation vitality loss, with VPD explaining more variance than PRE alone [33]. This finding is consistent with the observed slowing of GPP growth in high-VPD areas in our study (Figure 8b,c). Secondly, elevated VPD may exacerbate the spatiotemporal coupling of vegetation water stress, especially in arid regions where soil moisture and atmospheric demand imbalance can trigger \"hot drought\" compound events, leading to nonlinear declines in carbon sink function [10].  
Regional heterogeneity analysis revealed that the slowing of GPP growth in relatively humid areas like the Iii River Valley (Figure 2a) contrasts with the findings of [PERSON] et al. (2023) [31], who suggested that vegetation in humid regions typically regulates productivity through functional traits. The significant VPD inhibition on GPP in these relatively humid zones observed in our study (Figure 8c) may stem from unique hydrothermal configurations: rapid increases in growing-season VPD coupled with swift soil moisture depletion. This pattern echoes the \"atmospheric drought-dominated mechanism\" proposed by [PERSON] et al. (2024) for grassland vitality loss [33].