# Impacts of Compound Hot-Dry Events on Vegetation Productivity over Northern East Asia
## 3 Results
### Individual and Compound Effects of Hot and Dry Events
These GPP changes were calculated by subtracting the results of the _SE_Clim_ experiment from the results of the _SE_PROQ_, _SE_RQ_, _SE_TR_, and _SE_R_ experiments, respectively. and Figure S8b). Figure S5 shows the changes in Gc to explore the impacts on stomatal resistance. It suggests that the stomatal resistance increases due to both the precipitation deficit and the heat stress, particularly in the TCZ (Figure S5a,c). The enhanced solar radiation contributes to the decrease in Gc to some degree, also probably due to the stomate closure in cases of light saturation (Figure S5d). Overall, we can find that the terrestrial GPP over this area is impacted firstly by precipitation deficit and secondly by extreme heat during these four CHD events. However, the TCZ covered mainly by grass shows different responses in GPP to solar radiation changes compared to that in the high-latitude forest-dominated area. Firstly, the reduction in sub-surface SM during droughts has a limited effect on forest ecosystems (Figures 5a and S2a). This is probably because tree root systems penetrate deeper than grass into the soil in search of water. In addition, the increase in incoming radiation during drought due to cloudless conditions promotes the photosynthesis of trees. However, the extremely high temperatures during these four CHD events may surpass the maximum carboxylation temperature, reducing enzyme activities and photosynthesis (Figure S6c). Secondly, the VPD increases more due to extreme heat in grasslands than in forests (Figure S3c). The atmospheric dryness is compensated to some degree by ET, and this effect is more prominent in woodlands than in grasslands (Figure S4). Larger VPD increases further induce larger Gc reduction (Figure S5c), reducing GPP more in grasses than in forests. Thirdly, Gc is sensitive to decreases in SM in the TCZ (Figure S5a), and plants control water evaporation by regulating stomatal closure to reduce water loss (Figure S3a). The stomatal closure limits gas exchange, resulting in the inability of plants to access CO2,  
Figure 6: Relative changes in soil moisture (SM), vapor pressure deficit (VPD), stomatal conductance (Gc), maximum stomatal conductance (Vmax), evapotranspiration (ET), and gross primary productivity (GPP) during the four CHD events in grasslands and woodlands. These changes were derived from the _Control_ experiment and sensitivity experiments No. 2 to 8, relative to the _SE_Clim_. The combined effects of multiple factors are denoted as TPRQ (temperature, precipitation, radiation, and humidity), PRQ (precipitation, radiation, and humidity), RQ (radiation and humidity), and TR (temperature and radiation), while individual effects are represented by T (temperature), P (precipitation), R (radiation), and Q (humidity). “Grasslands” and “woodlands” refer to areas with vegetation cover consisting of over 60% grass and over 60% trees and shrubs, respectively. which further inhibits photosynthesis and thus reduces GPP. In addition, physiological changes in vegetation under severe drought conditions reduce leaf area (figure omitted), resulting in a severe disruption of vegetation function and structure. Drought conditions are often accompanied by high incoming radiation. High-intensity radiation could increase the leaf temperature of grassland ecosystems, affecting plant physiological activity by reducing Gc (Figure S5b). To quantitatively estimate the role of each variable in the GPP changes during the CHD events, the averaged changes in GPP are calculated based on the _Control_ experiment and factor-controlled experiments minus the _SE_Clim_ experiment for forests and grasslands, respectively (Figure 7). The impact of the CHD events resulted in an average decrease of \(-1.6\)gC/m\({}^{2}\)/day in GPP. The decline in GPP was higher in grasslands (\(-2.04\) gC/m\({}^{2}\)/day) than in forests (\(-1.50\) gC/m\({}^{2}\)/day).