## 4 Discussion
### Response of GPP and WUE to Drought across Different Climate and Biomes  
The Koppen climate classification and FAO land cover data were employed in our study to investigate the regional connections between ecosystem-scale sGPP and sWUE and drought. In arid and seasonally dry regions (classified as BW and BS), we observed only a minor reduction in sGPP. Additionally, our analysis unveiled distinct responses of WUE to drought in dry versus semi-arid/sub-humid environments, as depicted in Appendix A, Figure 12. In arid ecosystems, WUE fluctuations are primarily driven by physical processes such as evaporation, whereas in semi-arid or sub-humid environments, WUE variability is predominantly influenced by biological activities such as assimilation [37; 95; 115]. Our findings align with existing literature emphasizing the regional relationships between plant abundance and climatic patterns that shape ecosystems [37; 96; 97]. Moreover, vegetation systems adapted to arid regions demonstrate greater drought tolerance and lower sensitivity of GPP [37; 97; 116]. However, the association between ecosystem WUE and drought exhibits notable variation across different vegetation types and environmental contexts. Studies have revealed contrasting responses in various regions. For instance, in Northeast China and central Inner Mongolia, annual WUE increased during drought periods, whereas in Central China, it decreased [38]. In the Eurasian temperate grasslands, located in the central Eurasian continent, drought led to an enhancement of ecosystem WUE in over 65% of the region [117]. [PERSON] et al. [37] conducted a global synthesis using two observational WUE datasets, revealing contrasting WUE responses to drought across ecosystems. They observed that WUE tends to increase with drought in arid ecosystems but decreases in semi-arid/sub-humid ecosystems, indicating varying sensitivities of ecosystem processes to changes in hydroclimatic conditions [37]. Moreover, drought severities exhibited divergent impacts across WUE of climate patterns and land cover for our study area (see Figure A2). This somewhat aligns with some studies; for example, [PERSON] and [PERSON] [118] found that ecosystem WUE in the United States tended to increase under moderate drought but decrease under severe drought. In North China, slight and moderate drought enhanced ecosystem WUE, while severe and extreme drought resulted in WUE reductions regardless of different hydroclimatic conditions and biomes [119]. For South China, however, moderate and extreme drought reduced annual WUE, and severe drought slightly increased annual WUE, suggesting a 'turning point' in the WUE-drought relationship [38]. In contrast, positive WUE responses to severe and extreme drought occurred across land biomes in California [120] and in the forest ecosystem in Southwest China [121]. Spatially coherent increases in intrinsic WUE were also observed over the Northern Hemisphere during severe droughts that affected Europe, Russia, and the United States in 2001-2011 [42]. Notably, the negative effects of drought resulting in declining WUE were most prominent in the transition zone between arid and semi-arid lands [119]. [PERSON] et al. [110] analyzed the effects of drought on WUE for various vegetation types and illustrated that croplands had a peak WUE value under moderate drought events, while forests and grasslands reached their peak WUE under slight drought. In addition to these findings, our study indicated that both bare soil and shrub-covered areas showed a positive sensitivity to drought intensity in terms of WUE, while croplands and grasslands exhibited a negative sensitivity to drought intensity in terms of GPP. Areas with rapidly and seasonally growing vegetation that are sensitive to water demand have relatively thin and shallow root structures, limiting their drought tolerance ability. As a result, soil water deficits can lead to critical reductions in productivity in such regions [115; 122; 123].