## 4 Discussion  
### The Diverse Responses of Carbon-Water Process to Summer Drought  
Different from most research results, our results suggest that the responses of GPP and ET to summer drought differ substantially among biome types (Figure 5), which agrees  
Figure 6: The average and changes of summer GPP and ET among different dryness levels and biomes: mean GPP (**a**) and ET (**b**), the difference of GPP (**c**) and ET (**d**) between total normal and dry summer; GPP (**e**) and ET (**f**) shifts from normal to different dryness levels, (**g**) GPP shifts from normal to different dryness levels among different grassland types.  
with [PERSON] and [PERSON] [56]. The diverse response might result from the different main limited factors for ecosystems. For instance, water plays a key role in the functions and activities of semi-arid grassland regions dominated by herbaceous plants [57; 58; 59]. During drought, plants reduce enzymatic activities as well as stomatal closure to prevent water loss [60], causing the reduction of the transpiration and photosynthesis rate in grassland (Figure 5), whereas energy supply (or temperature) drives the variation of ecosystem functions largely in high latitude [61] and alpine regions [62]. Less precipitation and cloud cover with more coming solar radiation accelerate the carbon-water process [63; 64], which could account for the negative response of GPP and ET to summer drought in mixed forests in cold regions and alpine meadows in Qinghai (Figure 5). The negative effects of drought on productivity for boreal regions are also supported by [PERSON] et al. [61].  
Notably, with significant correlation between GPP (or ET) and SPEI, semi-arid grassland present larger sensitivity to drought than arid regions (Figure 5), which also are found in North America [65] and southeastern Australia [25]. As [PERSON] et al. [66] suggested, the drought timescale determines the sensitivity of biomes to drought, and largely, arid biomes respond to drought at short timescales, while semiarid and sub-humid biomes respond at long timescales due to different root systems. After comparing the relationship between GPP (or ET) and the 1-month scale SPEI value (Figure 7), the sensitivity value gets higher, but the significant area is almost similar with Figure 4. It means that other compensation mechanisms except for drought timescale weaken the impacts of drought in insignificant correlated regions, like different vegetation species physiological responses. For example, on the one hand, the arid desert ecosystems in central Asia dominated by some drought-tolerant species (i.e., _Tamarix ramossissima_) are more resistant to drought with lengthy root systems to tap deep soil water [67; 68]. On the other hand, the ephemerals and annual plants with shallow roots just reserve seeds during drought periods, while flowers grow fast after one or several precipitation events [69], which is difficult to reveal via 1- or 3-month SPEI data [66]. Moreover, some drought-tolerant species with shallow root systems (i.e., _Haloxylon ammodendron_) maintain photosynthesis but reduce transpiration during drought via effective morphological adjustment (i.e., shrinking shoot size) [64], which explains the different response of GPP and ET to drought in Alashan. In barren areas with very sparse vegetation, like northern Tarim Basin, drought mainly induces the increased soil evaporation rather than vegetation dynamics, which suggest the negative response of ET to drought in these regions. Overall, the insignificant sensitivity to drought in arid ecosystems hinders some compensation mechanisms (i.e., physiological changes of vegetation species to resistant drought), which should be paid much more attention to.  
Figure 7: The linear slope between 1-month scale SPEI and (**a**) GPP anomaly, (**b**) ET anomaly for 2003–2015.