# Impacts of Compound Hot-Dry Events on Vegetation Productivity over Northern East Asia
## 4 Discussion  
### The Roles of Drought and Heat in GPP Changes during CHD Events  
Precipitation deficits have decreased GPP, with grasslands being significantly more sensitive to such events than forests. This finding is consistent with previous research results [43; 44; 45]. In contrast to grasslands, trees typically have deeper root systems, allowing them to access deeper soil moisture levels, thus enabling them to sustain their physiological activities during the early stages of drought [44; 46; 47; 48]. On the other hand, grassland plants have shallower root systems that are primarily concentrated in the upper soil layers, limiting their ability to access water during dry conditions [45]. Due to water limitations, this may lead to stomatal closure, thereby reducing photosynthesis and transpiration. Plant leaves may be damaged in severe and prolonged drought [49], directly leading to a significant decrease in GPP. Under non-drought conditions, extremely high temperatures also inhibit GPP, serving as the second-largest factor influencing grassland GPP changes and a key driver behind the simulated decline in high-latitude forest GPP. While studies by [PERSON] et al. [13] suggest  
Figure 8: Contributions of various climatic factors to GPP changes for the four CHD events derived from the comparison between sensitivity experiments with one abnormal variable and with all abnormal variables. Marks of minus and plus represent the increase and decrease in variables. “Grasslands” and “woodlands” refer to grid cells with over 60% grass and over 60% trees and shrubs, respectively. that extreme temperatures without accompanying drought have minimal to no negative impact on GPP in most cases, other research indicates that heatwaves can indirectly cause water stress, leading to GPP reductions [50; 51]. Moreover, prolonged and intense heatwave events may significantly affect GPP negatively [13], and the seasonality of heatwaves also plays a crucial role in their impact [52]. Extreme high temperatures can restrict GPP in two ways. On the one hand, they may directly impact enzyme activity, leading to a decrease in the activity of the key enzyme Rubisco involved in photosynthesis [53], resulting in a decrease in \(\mathrm{V_{cmax}}/\mathrm{J_{max}}\), thereby causing a reduction in GPP. On the other hand, extremely high temperatures can also cause abnormally high VPD. If this situation persists for a long time, even with soil moisture present, plants may be forced to reduce \(\mathrm{G_{c}}\) to prevent embolisms in the xylem [54]. Further analysis in this study reveals that excluding the influence of high temperatures, the combined effects of other factors appear to promote GPP in high-latitude forests. However, only when considering the impact of high temperatures do we observe the negative GPP response that is consistent with actual observations. While the models may overestimate the adverse effects of extremely high temperatures, this analysis, to some extent, highlights the significant threat posed by extremely high temperatures to GPP in high-latitude forests during CHD events. There is a close connection between heatwaves and drought [55]. Heatwaves typically lead to abnormally high temperatures, exacerbating soil moisture evaporation and vegetation transpiration rates, and intensifying drought severity and scope. The unusually high VPD triggered by heatwaves accelerates soil moisture loss, leading to soil dryness and worsening the impact of drought. On the other hand, drought itself can also cause temperature increases as the lack of vegetation transpiration reduces atmospheric moisture content, making surface temperatures more liable to rise. This positive feedback loop exacerbates both heatwaves and drought, forming a vicious cycle. Extreme temperatures and severe drought can both damage plants. Still, in cases where they are combined, plants need to balance strategies of closing stomata to prevent dehydration and keeping stomata open to enhance evaporative cooling [56].