## 4 Discussion
### Comparison of the Probability of GPP Loss Under Different Scenarios  
Heatwaves, droughts, and CDHEs all lead to GPP losses in rubber plantations, but the probability of loss varies due to differences in monthly temperature and hydrological characteristics. As shown in Figure 5, under compound drought and heatwave scenarios, the probability of GPP loss in rubber plantations is not always higher than in single drought or heatwave scenarios and is sometimes even lower than both.  
In May and July, the probability of GPP loss in rubber plantations under compound drought and heatwave scenarios is higher than in single drought or heatwave scenarios. As shown in Figure S3, May marks the beginning of the rainy season on Hainan Island. Rainfall increases to 164 mm, and temperatures reach up to 36.1 \({}^{\circ}\)C. Although rainfall increases in May, the newly grown leaves of young rubber trees struggle to offset the heat stress from heatwaves. At the same time, there is not enough shade cover for the soil, leading to intense soil evaporation. This situation is similar to the phenomenon observed under meteorological drought conditions, where high temperatures and increased solar radiation can lead to reduced water availability for vegetation [25]. In June, rubber tree leaves are almost fully open, but their protective organs are not fully formed. High temperatures increase leaf water loss, making June more sensitive to heatwaves. August to October is the rainy season, with high rainfall in rubber plantations. Despite high temperatures, plenty of rain greatly reduces the negative effects of heatwaves. Studies showed that atmospheric dryness usually slows down photosynthesis, lowering GPP. However, when the soil has enough water, atmospheric dryness can help photosynthesis and increase GPP [70].  
In July, rainfall significantly increases to 264 mm. However, continuous high temperatures cause rapid water evaporation. Rubber plantations still face significant water shortages and heatwave stress during the hottest parts of the day. The dual pressure of high temperatures and temporarily insufficient rainfall results in a high probability of GPP loss. Additionally, during the rainy season, rubber plantations act as \"water pumps\". They absorb large amounts of water from the soil and release it into the air through their leaves [34; 71].  
From August to October, despite heatwaves increasing atmospheric dryness, abundant rainfall and soil moisture still contribute to increased GPP. April and November are part of the dry season. In April, rubber trees have not finished growing their leaves, while in November, they are losing their leaves. However, the rainfall during these months is still more than what rubber trees need. So, even though heatwaves increase the chance of GPP loss in rubber trees, there is enough water to help reduce this loss. This phenomenon is similar to research findings in energy-limited ecosystems such as tropical rainforests, where GPP may still increase even as soil moisture decreases [72; 73; 74].  
Spatially, as shown in Figure S4, regardless of the month or compound drought-heat scenario, the probability of GPP loss in rubber plantations in northwestern Hainan Island is always higher than in other regions. This is because the central mountainous area, affected by the terrain uplift effect, blocks the humid sea breeze from the southeast, resulting in less precipitation in the northwest, making this area more prone to drought. Less cloud cover and precipitation mean more direct sunlight and reduced evaporative cooling, further leading to heatwaves [75].