## 3 Results
### Effects of Climate Factors on Ecosystem Carbon Fluxes  
The Pearson correlation analysis was used to examine the relationships between climate factors and ecosystem carbon fluxes at the forest and grassland sites for each weekly period, respectively (Table 1). We found that all climate factors, namely Rg, Ta, VPD, and Pre, were strongly and positively correlated with GPP, Re, and the carbon sequestration capacity (the absolute value of NEE) of the forest ecosystem. In contrast, the main components, i.e., both GPP and Re, showed strong correlations with these climate factors except Pre, which may be ascribed to the multiple having throughout one year at the grassland sites. We also noticed that the grassland NEE only exhibited a strong negative correlation with Rg (\(r=-0.212\), \(p<0.01\)). Meanwhile, with regard to the forest, Ta was the dominant climate factor of GPP (\(r=0.864\), \(p<0.01\)) and Re (\(r=0.728\), \(p<0.01\)), whereas Rg dominated NEE (\(r=-0.632\), \(p<0.01\)). However, the variabilities of GPP and NEE were mainly dominated by Rg, and Ta exerted the largest effect on Re (\(r=0.635\), \(p<0.01\)) for the grassland ecosystem. Therefore, all climate factors jointly affected the seasonal dynamics  
Figure 5: Seasonal dynamics of carbon fluxes, namely gross primary productivity (GPP) (**a,b**), ecosystem respiration (Re) (**c,d**), and net ecosystem exchange (NEE) (**e,f**), at the forest (left: CH-Dav) and grassland (right: CH-Cha) sites for each weekly period, respectively. The blue and red lines represent multi-year means during the adjacent normal years and the drought year 2018, respectively. The error bars represent \(\pm\) 1 standard deviation. For the \(x\)-axis, the data refer to the week of the year (WOY) ranging from 1 to 52.  
in the NEE of the forest ecosystem, and only Rg dominated the variability in the NEE of grassland. Given that the VPD was a good proxy for atmospheric dryness, we especially explored the response relationships between the VPD and carbon fluxes, namely GPP, Re, and NEE, at the forest and grassland ecosystems, respectively. Multiple different methods, namely linear, quadratic, and exponential, were used for analysis. As shown in Figure 6, there existed contrasting response relationships to the variability in the VPD for forest and grassland ecosystems, particularly for GPP and Re. Specifically, both GPP and Re rose with the increase in VPD during the early stage in the grassland ecosystem and then tended to decline. However, there was not a downward trend for the forest ecosystem, and a linear relationship was observed between the VPD and GPP or Re. Nevertheless, both forest and grassland NEE showed a quadratic polynomial form, with the thresholds being 5.46 and 5.84, respectively. Once the VPD exceeded such thresholds, the carbon sequestration capacity of forest and grassland ecosystems reduced gradually. We also examined the relative deviations of changes in climate and ecosystem carbon fluxes between the multi-year means during the adjacent normal years and the two extreme drought events at the forest and grassland sites. As shown in Tables 2 and 3, owing to larger Ta and less Pre in 2003 and 2018, the VPD increased by 28.1% and 15.0% compared with the normal years in the forest ecosystem, which led to a severe decrease in NEE from carbon sink to carbon source. Nevertheless, we found that both GPP and Re increased in this period, but Re increased more than GPP and resulted in an alteration in forest carbon sequestration function. Meanwhile, the drought extremes exerted a more significant effect on the VPD by increasing 60.4% and 38.8% at the CH-Oe1 and CH-Cha grassland sites, respectively, which was mainly ascribed to the sharp decrease in Pre. Contrastingly, both GPP and Re at this site decreased substantially. The larger reduction in GPP led to a sharp decrease in NEE by approximately \(-64.3\%\) during the drought in 2003 (Table 2), and the carbon release more than doubled during the drought in 2018 (Table 3), respectively.