## 4 Discussion  
### Spatial and Temporal Dynamic and Distribution of CUE and WUE in NMR  
Based on remote sensing observations and reanalysis data, this paper analyzed the CUE and WUE of vegetation in the NMR from 2001 to 2023. The results revealed that the average value of vegetation CUE and WUE on the NMR was 0.47 and 0.82 g C-m\({}^{-2}\)-mm\({}^{-1}\) for the 23-year period, respectively. In Chinese terrestrial ecosystems, the overall vegetation CUE spanned from 0.44 to 0.53, and the average value, which remained relatively stable, was 0.5 [44]. [PERSON] et al. discovered the average CUE for open shrulands to be significantly higher compared to other grassland categories worldwide [45]. The findings of the aforementioned studies align closely with what has been observed in this research. The study identified a significant overall decline in both CUE and WUE (\(p<0.05\)) in the NMR from 2001 to 2023, primarily driven by the faster increases in GPP and ET compared to NPP, which resulted in reductions in the long-term averages of CUE and WUE. Globally, CUE exhibited a slight reduction from 2000 to 2009 [46], while WUE demonstrated a negligible decline from 2000 to 2012 (\(p>0.05\)) [47]. In the Chinese terrestrial ecosystem, the annual CUE of vegetation showed a slight overall increasing trend from 2007 to 2013 [17], and the annual WUE of vegetation, based on remote sensing-driven analytical models, rose markedly at a pace of 0.021 g C mm\({}^{-1}\) H\({}_{2}\)O y\({}^{-1}\) from 2000 to 2016 (\(p<0.01\)) [48]. Additionally, [PERSON] et al. [18] reported a slight annual decline in WUE across Central Asia from 2001 to 2021 (\(p>0.05\)), whereas the yearly mean WUE of vegetation in China showed an upward trend of 0.003 g C-m\({}^{-2}\)-mm\({}^{-1}\)-a\({}^{-1}\). Despite this, some researchers have observed increasing trends in vegetation CUE and WUE within specific study regions over certain periods. For instance, vegetation CUE on the Mongolian Plateau showed a notable rise from 2000 to 2019, growing at an annual pace of 0.2% [23], while vegetation ecosystem WUE on the Loess Plateau also exhibited a marked increase between 2000 and 2014, rising by 0.02 g C-kg\({}^{-1}\) H\({}_{2}\)O annually (\(p<0.001\)) [21]. This indicates that the distribution of vegetation CUE and WUE exhibits significant geographical variation, influenced by differences in study regions and time periods. Such variations highlight the complex interactions between vegetation productivity, water use efficiency, and regional climatic and ecological factors, underscoring the need for spatially and temporally explicit analyses to better understand ecosystem processes and their responses to environmental changes. In terms of spatial distribution, both vegetation CUE and WUE exhibit distinct spatial heterogeneity, generally following a trend in smaller values in the northwest and greater values in the southeast. Studies have shown that topographic factors, such as higher altitudes, often lead to an increase in vegetation CUE. In higher elevation areas, cooler temperatures reduce autotrophic respiration, which in turn enhances carbon sequestration by vegetation, resulting in a higher CUE [25]. However, the relationship linking vegetation CUE to elevation in this research is not significant, with higher-altitude areas not exhibiting higher CUE. Similarly, vegetation WUE follows a pattern comparable to CUE, suggesting that a variety of factors play a role in shaping the spatial patterns of both CUE and WUE, rather than strictly following the expected patterns. The investigation into future tendencies and the stability of vegetation CUE in the NMR indicates notable patterns, with 90.03% of the study area experiencing a decline. This pattern is consistent with long-term trends in vegetation CUE in South China, as well as predictions for future changes [17]. Likewise, areas with higher vegetation CUE in the study region exhibit lower coefficients of variation,indicating low fluctuation, which aligns with observed CUE changes in vegetation across China [17].