## 4 Discussion  
In our study, we estimated multiyear-averaged WUE (1.55 g C kg\({}^{-1}\) H\({}_{2}\)O) and found large spatial differences of mean annual WUE in nine sub-regions due to the heterogeneities in climate, soils and plant physiological characteristics [20; 23], which agreed mostly with [PERSON] et al. (2018) and [PERSON] et al. (2019) [31; 32]. The highest annual WUE in Northeast China is probably due to its large areas of deciduous forests (DBF and DNF) [23; 30] while the higher annual WUE in Central China, North China, Southeast China and Southwest China may be because of their suitable temperature, sufficient water supply [23] or large amount areas of crop lands [51; 52]. In previous studies, the  
Figure 6: The spatial distribution of ecosystem resilience to drought in China (**a**) during the period 1982–2015 (the white color indicates non-vegetated areas, and the black and red lines show the boundaries of sub-regions and eco-climatic zones). The regional mean \(\text{R}_{\text{d}}\) for different sub-regions and vegetation types (CRO: croplands, EBF: evergreen broadleaf forests, DBF: deciduous broadleaf forests, ENF: evergreen needle-leaf forests, DNF: deciduous needle-leaf forests, MF: mixed forests, SHR: shrub lands and GRA: grasslands) are exhibited in (**b**,**c**). WUE of forests was usually higher than croplands, and grassland was usually the lowest [29; 53; 54]. However, the WUE in this study showed the following order: deciduous broadleaf forests \(>\) deciduous needle leaf forests \(>\) evergreen broadleaf forests \(>\) croplands \(>\) shrub lands \(>\) evergreen needle leaf forests \(>\) mixed forests \(>\) grasslands, which were similar to [PERSON] et al. (2015) [23] except for croplands. The discrepancy in those studies could be partly attributed to the different environmental conditions, plant species, planting areas and cultivation methods. Moreover, the significant increasing trend (\(8.5\times 10^{-3}\) g C kg\({}^{-1}\) H\({}_{2}\)O per year) of annual WUE found in this study which may be due to the increased atmospheric CO\({}_{2}\) concentration [17; 55; 56]. However, the WUE trend was much higher than [PERSON] et al. (2018) (\(7.32\times 10^{-4}\) g C kg\({}^{-1}\) H\({}_{2}\)O per year) [30] which may induced by their different data sources. The significant increasing WUE in some regions of North China, Northeast China, Northwest China and Inner Mongolia may, to some extent, be owing to the implementation of ecological restoration projects (e.g., the 'Three North' Shelterbelt Development Program, the Grain-for-Green Project and the Beijing--Tianjin Sand Source Control Program) [57; 58; 59; 60]. The relationship between WUE and SPEI in our study was positive in semi-arid which was consistent with [PERSON] et al. (2016) [26]. The WUE in the Northeast China responded negatively with an increase in SPEI but [PERSON] et al. (2016) showed positive correlations between WUE and drought in this region [26]. Moreover, our findings indicated that the relationship between WUE and SPEI in DBF was negative whereas was positive in [PERSON] et al. (2016) [26] and [PERSON] et al. (2017) [20]. These divergences could be due to differences in drought index, study areas and data sources. The different sensitivities of GPP and ET to droughts controlled the WUE-drought relationships across various sub-regions and biomasses [23; 26]. For example, water supply played an important role for vegetation productivity [61] in arid and semi-arid regions (e.g., most parts in Inner Mongolia and Tibetan Plateau, Northwest China) with sparse vegetation (e.g., grasslands and croplands), and increase in precipitation (wetter) would mitigate water limitations and thus improve plant productivity in these regions [21; 62], precipitation maybe was also consumed by soil evaporation, leading to an increase in ET [26; 63]. However, the increased GPP was higher than ET, resulting in an increase in WUE.