## 4 Discussion  
A deep understanding of the interactions between global climate change, particularly for drought extremes, and terrestrial ecosystems is critical for predicting the future trends of terrestrial carbon sequestration. With the increase in air temperature and changes in relative humidity, the changing climate has altered the VPD dynamics in recent decades [27; 40]. As a result of the increased VPD, recent researchers have projected that Earth is facing a severe increase in atmospheric dryness in the next few decades [41; 42], which importantly contributes to the CO\({}_{2}\) and water vapour fluxes between the land surface and atmosphere. Thus, the VPD is a vital environmental driver for vegetation structures and functions. The variability in ecosystem evapotranspiration owing to the VPD will then affect soil moisture dynamics [43; 44]. But plant stomatal conductance is directly related to the VPD, which leads to stomata closure in order to prevent excessive water consumption [45; 46]. Meanwhile, the increase in VPD will severely limit the stomatal conductance and constrain vegetation photosynthetic activity, resulting in a decline in vegetation productivity and  
Figure 8: Differences in resistance (Rt) and resilience (Rs) of GPP, Re, and NEE at the forest (CH-Dav) and grassland (CH-Cha) ecosystem to the 2018 drought. Figure 7: Differences in resistance (Rt) and resilience (Rs) of GPP, Re, and NEE at the forest (CH-Dav) and grassland (CH-Oe1) ecosystem to the 2003 drought. carbon uptake [42; 47]. Our study also demonstrated the severe consequences of drought events on the carbon sequestration of two different ecosystems (Tables 2 and 3). The forest ecosystem changed from carbon sink to carbon source, and the carbon sequestration capacity of the grassland ecosystem was also severely altered. Meanwhile, we noted that the optimal response relationship was quadratic between the VPD and the NEE for both forest and grassland ecosystems. Below the threshold, the carbon uptakes of these two different ecosystems increased with the increase in the VPD. But when the VPD exceeded the thresholds, the carbon sequestration capacity of both forest and grassland gradually decreased (Figure 6). [PERSON] et al. (2020) also indicated that, in most parts of the wet Amazon rainforest, the ecosystem GPP tended to enhance together with the VPD increase owing to the changes in canopy characteristics [48]. Moreover, the studies based on various drought indexes revealed similar findings indicating that vegetation carbon sinks exhibited nonlinear relationships with the severity of the drought [49; 50]. Recent evidence has highlighted the necessity to account for the feedback between soil moisture and atmospheric dryness (VPD) when evaluating the response mechanism of global carbon cycles to the changing climate, and when investigating the field-scale response of specific ecosystems to drought events [51; 52]. Moreover, most of the inter-annual variations in global terrestrial carbon sequestration were driven by the effect of the VPD regulated by soil moisture status [44]. When extreme drought events occurred, low soil water content hindered ecosystem evapotranspiration, which is the most efficient cooling effect of land surface [53]. The alterations in energy balance raised air temperature, lowered the relative humidity, and essentially increased the VPD [54]. We further examined the relationships between the VPD and soil moisture at the grassland sites. As shown in Figure 9, we found inverse seasonal patterns in the VPD and SWC over each weekly interval. With the decrease in the SWC during the growing season, the VPD exhibited an upward trend throughout the periods of 2002 and 2005, as well as in 2017 and 2020. Meanwhile, it was noted that the lowest SWC was 17.2% as the VPD reached up to 19.1 during the 2003 drought, which was over twice the VPD during the normal years.