## 4 Discussion  
### WUE Responses to Phenology  
Our analysis shows that WUE sensitivity to phenology exhibits an obvious difference among different PFTs, and further highlights that the difference varies with the season. During spring, WUE sensitivity to phenology gradually enhances from PFTs in warm climates to those in cold climates. This result is in agreement with the fact that ecological functions seem to be more sensitive in cold-habitat and alpine ecosystems compared to warm-habitat ones [48; 49]; this may be because there are fewer but longer days of growing season at higher latitudes [50]. Although the sensitivities of PFTs in similar climates also show a variance, the amplitude of variance is much less than that in different climates, indicating fundamental similarities in ecosystem functioning across a broad geographic range of climate conditions, with the different sensitivities of different plant types [51]. WUE response to phenology, indeed, is the coupling of GPP and ET responses to phenology. Our analysis shows different response patterns of WUE to phenology, and here are some potential cues to explain how GPP and ET responds to phenology:  
Figure 6: Frequency distribution of response patterns of seasonal WUE to phenological factors (SOS/EOS) at plant functional type (PFT) level. R\({}_{\text{WUE}}\), R\({}_{\text{GPP}}\), R\({}_{\text{ET}}\) indicate the partial correlation coefficient of WUE, GPP, ET with phenological factor after controlling the environmental factors (including temperature, precipitation, insolation and CO\({}_{2}\) concentration). The partial correlation coefficients are calculated in (**a**) spring (March–May); (**b**) summer (June–August) and (**c**) autumn (September–November), respectively. The details regarding the codes of PFTs are provided in Table 1.  
During spring, increases in WUE with earlier SOS are generally associated with increases in GPP with advanced SOS. Warmer spring air temperature can trigger leaf sprouting and unfolding [28]. Thus, increases in productivity are driven by the longer growing season (earlier SOS). Moreover, an earlier spring phenology may result in greater leaf area, enhancing light interception and canopy-level photosynthetic potential and thereby spring GPP [52]. Both negative and positive correlations between ET and SOS are widespread during spring. The spring onsets represents a release from temperature limitation, but it is generally still not warm enough to have large evaporative losses from soil. Transpiration will be the larger contributor to ET at this time of year. On the one hand, the transpiration of the leaves and canopy interception evaporation would increase with the greater leaf area at the beginning of the growing season [53], which could increase ecosystem ET. Meanwhile, warmer spring temperatures could increase soil evaporation during the early growing season, accompanying an earlier spring phenology [54]. On the other hand, an earlier and/or greater display of leaf area can decrease soil evaporation resulting from lower local temperatures [29; 55] and land surface insolation [56]. The offset of these effects determines the relationship between spring ET and SOS, which would imply Pattern I or Pattern II in spring. 2. In summer, an advance in SOS also promotes GPP in humid subtropical climates and subarctic climates, which is probably attributed to higher foliar N or leaf area index resulting from warmer spring temperature [57; 58], which would also imply Pattern I or Pattern II. However, positive correlations between summer GPP and SOS are mainly observed during summer, especially in dry climates and continental climates. These positive relationships (that is, earlier SOS reducing summer GPP) may be attributed to water stress resulting from the preseason environmental condition [54; 59] and/or from the enhanced plant activity during the warm spring [1; 54].