# Vegetation Health in China Is Severely Compromised by Drought, Wet and Heat Stress Events
## 4 Discussion
### Characteristics of Stress Events and Their Impact on Vegetation Health  
Various mechanisms such as drought, heat, and wet stress have physiological and yield impacts on vegetation [46]. We focus on the dynamics of vegetation health under drought, wet, and heat stresses. The results indicate that the probability of simultaneous occurrence of drought and heat stress events has gradually increased [47], and drought stress events have gradually weakened from 2001 to 2020, while wet stress has increased. Climate stress events are shifting toward a warmer and wetter pattern with increasing extremity, particularly in northern regions. Recent studies suggest that extreme heat can enhance precipitation, as warmer air carries and transports 7% more water vapor per degree Celsius, leading to heavier rainfall intensity, especially on hourly to daily scales [48]; this increases the frequency and synchronicity of extreme events. Meanwhile, low CPA leads to frequent occurrences of low VHI in the northern arid and semi-arid regions of China and the Tibetan Plateau despite the frequent extreme heat stress events in the South China and Yangtze River Basin regions [49]. However, they still maintain a high VHI due to ample precipitation. High temperatures can directly affect the efficiency of photosynthesis in plants, and when temperatures exceed a certain threshold, the enzyme activity in plant chloroplasts is inhibited, impacting the formation of photosynthetic products. Heat waves increase the transpiration rate, and if water is not replenished in time, it leads to a decline in plant water potential, significantly affecting cell expansion and growth. In water-scar-drought conditions, vegetation closes stomata to reduce water loss, limiting CO2 intake. Similar to heat stress, if conditions exceed a certain threshold, plants cannot complete carbon absorption, resulting in decreased productivity [50; 51]. However, if water is replenished in a timely manner, vegetation can quickly return to a normal state due to its inherent recovery ability and physiological resilience [52; 53]. The response of vegetation growth to heat stress and drought stress varies among different ecosystem types. Our results generally show that forest ecosystems are less sensitive to stress events compared to non-forest ecosystems, which is likely due to the following mechanisms. Responses of vegetation types to drought stress are closely linked to their water demand and root system structure. MCBF and BF, characterized by high transpiration rates and biomass, exhibit significant sensitivity under drought conditions. Drought reduces soil water availability, suppressing photosynthesis and transpiration, which directly lowers carbon sequestration and increases risks of leaf shedding and tree mortality [54]. In contrast, CF, with its lower transpiration rates and higher water-use efficiency, demonstrates greater tolerance to drought. GL and MD, heavily reliant on surface soil moisture due to their shallow root systems, experience pronounced declines in productivity under drought, thereby weakening their carbon sink capacity [55]. AV, adapted to arid environments with deep root structures and water storage mechanisms, shows robust drought tolerance. In contrast, wet stress causes significant impacts on vegetation health due to excessive soil moisture and oxygen deficiency. BF and MCBF are particularly sensitive to wet stress, as oxygen limitation in saturated soils restricts root respiration and increases vulnerability to root rot, impairing overall vegetation health [56]. Similarly, GL and MD are highly sensitive to wet stress, where excessive moisture promotes the expansion of wet-tolerant species, altering native community structures and reducing ecosystem productivity and stability [57]. While AV is well adapted to extreme conditions, prolonged precipitation under wet stress, especially in specific topographic settings, may result in nutrient leaching and restricted productivity. Notably, heat stress negatively affects all vegetation types but varies in magnitude.