## 5 Conclusions  
This paper explored the spatiotemporal variation of GPP and WUE in China from 2001 to 2021. China was divided into four arid-humid zones based on the aridity index, and the effects of climate change on GPP and WUE in different arid-humid zones and multiple vegetation types therein were analyzed. In our investigations, we found the following:  
1. From 2001 to 2021, the increasing trend of WUE in the four arid-humid zones of China was less pronounced than GPP. The GPP value gradually decreased from the humid to arid zone, while WUE in the arid zone was slightly higher than that in semi-arid zone due to drought stress. The \(CV\) of GPP and WUE increased gradually from the humid zone to the arid zone, and the relatively high \(CV\) values of both were mainly concentrated in the central-eastern part of the semi-arid zone and the southwestern part of the humid and semi-humid zones.
2. In all arid-humid zones, forest and cropland possess higher values of GPP and WUE. For individual zones and vegetation, shrubland in the semi-humid zone and wetland in the arid and semi-arid zone showed higher GPP and WUE values, which may be strongly related to the regional environment as well as to the structure of the vegetation itself. Across China, all arid-humid zones and vegetation types exhibited an upward trend for GPP, while shrubland and wetland WUE showed a downward trend.
3. Among the five climatic factors, Ta and Pr trended upward from 2001 to 2021, while RH, Rn, and WS trended downward in most parts of China. Ta and Pr were the main climatic factors responsible for the increase in GPP for each vegetation type in different aridity gradients. Ta had a stronger positive correlation than Pr, but the positive correlation of Pr gradually increased with the transition to the arid zone. WUE showed obvious positive and negative correlations with thermal factors (Ta and Rn) and moisture factors (Pr and RH), this pattern being more pronounced in humid and semi-humid zones. The decrease in Rn and increase in Pr may be the main climatic factors contributing to the weaker upward trend in WUE across the arid-humid zones, while the decrease in shrubland and wetland WUE may be related to RH and Pr. Overall, climate change may affect GPP and WUE differently along the aridity gradient, causing cross-gradient differences among ecosystems.  
The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/rs15061563/s1](https://www.mdpi.com/article/10.3390/rs15061563/s1)]([https://www.mdpi.com/article/10.3390/rs15061563/s1](https://www.mdpi.com/article/10.3390/rs15061563/s1)), Figure S1: Linear trends of GPP, WUE, and five climatic factors from 2001 to 2021. Table S1: Area proportion of each vegetation type in four arid-humid zones. Table S2: Area proportion of changing trend in climatic factors across China. Table S3: Average correlation coefficients between GPP and climatic factors for each vegetation type in four arid-humid zones from 2001 to 2021. Table S4: Average correlation coefficients between WUE and climatic factors for each vegetation type in four arid-humid zones from 2001 to 2021.  
**Author Contributions:** X.L.: Methodology, Investigation, Software, Visualization, Writing--Original draft preparation. L.Z.: Methodology, Conceptualization, Supervision, Writing--Review & Editing, Resources. J.X.: Writing--Review & Editing, Data curation. F.W.: Writing--Review & Editing. H.L.: Writing--Review & Editing. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the National Natural Science Foundation of China (No. 42101043, 41890822), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA23040304) and the Project funded by China Postdoctoral Science Foundation (2021M703178).  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.  
**Conflicts of Interest:** The authors declare no conflict of interest.