## 4 Discussion
### Distribution and Dynamics of GPP and WUE  
This study found that the spatial distribution of GPP and WUE across China was in good agreement with the delineation of each arid-humid zone, suggesting that the aridity gradient has a significant impact on GPP and WUE across China (Figure 2). GPP decreased steadily from the humid zone to the arid zones, while the WUE was slightly higher in the arid zone than the semi-arid zone. In general, the humid zone is well-watered, allowing plants to perform adequate photosynthesis, resulting in higher GPP values. Conversely, the arid zone receives less precipitation and tends to have lower soil moisture levels, which may limit plant growth and reduce GPP. This phenomenon has been well documented in previous studies [43]. In addition, [PERSON] et al. [44] indicated a slight improvement in vegetation WUE under drought stress, which can lead to a higher WUE in the arid zone than the semi-arid zone. The main vegetation types in each arid-humid zone contributed most of the GPP and WUE values (Figures 2, 6 and 7); forest with a larger leaf area, higher canopy structure, and deeper root distribution had higher GPP and WUE than other vegetation types [45]. China's cropland is vast and dominated by irrigated production, contributing to relatively high GPP and WUE [46]. The GPP and WUE of forest and cropland were prominent in different arid-humid zones, but shrubland also showed higher values in semi-humid zones, which was similar to the study of [PERSON] et al. [22]. This study inferred that the semi-humid zone, which lies between the arid and humid zones, usually has a moderate amount of  
Figure 14: Average correlation coefficients between WUE and climatic factors for each vegetation type in four arid-humid zones from 2001 to 2021. (**a**) Relative Humidity (RH); (**b**) Net radiation (Rn); (**c**) Average temperature (Ta); (**d**) Precipitation (Pr); (**e**) Wind Speed (WS). F: forest; S: shrubland; G: grassland; W: wetland; C: cropland. rainfall, which is not sufficient to support the growth of tall plant species. This means that shrubs adapted to these conditions can be more efficient than forest in using water and nutrients to maintain high levels of GPP and WUE [47]. In addition, shrubland in the semi-humid zone usually has small, thick leaves and long root whiskers, which allow shrubland to maintain high levels of photosynthesis while minimizing water loss, further increasing its GPP and WUE [48]. Furthermore, in semi-arid and arid zones, the wetland tended to have stronger GPP and WUE than the common substrate. Wetland in arid zones is often characterized by low water availability, and wetland plants and microorganisms may have adapted to use water more efficiently. This may result in higher photosynthetic rates as well as higher water use efficiency [49]. Despite their small individual size, grassland had a large distribution in each arid-humid zone, which caused their overall average GPP to be similar to that of shrubland and wetland, while WUE was higher than that of shrubland and wetland. GPP and WUE exhibited an upward trend from 2001 to 2021 across the country as well as in each arid-humid zone, but it is also found that China's ET showed an upward trend during this period [12], which may be the direct reason for the weaker rise in WUE (Figures 3-5). We summarized the ET variation patterns in recent years across China to further investigate the mechanisms of GPP and WUE changes. It was found that the central, southwestern and northeastern parts of the semi-humid and semi-arid zones have been the main regions of increased ET in China since the 21 st century [50], with a similar spatial distribution to the GPP significant increase regions obtained in this study. Among these regions, WUE also exhibited increasing trends to different degrees. We inferred that implementing ecological restoration projects and related agricultural production modification in these regions led to an increase in GPP, WUE and ET [51; 52].