## 4 Discussion  
### The Applicability of the CDHI  
The frequent occurrence of CDHEs has severely impacted ecosystems and human societies, attracting widespread attention from scholars worldwide [62]. In the context of global climate change, China, especially its tropical regions, faces an increasingly severe threat from CDHEs, with both the frequency and intensity of these events significantly increasing in recent years [63]. In order to effectively capture the temporal characteristics of CDHEs experienced by the rubber plantations on Hainan Island, we constructed a CDHI for these plantations. As heatwaves are weather disasters occurring on a relatively short time scale [64], this study selected STI-1 as the heatwave indicator for CDHEs. Among the four drought indices, SPEI-3 showed a relatively strong negative correlation with STI-1, proving to be the most suitable for monitoring drought conditions in the rubber plantations on  
Figure 5: Probability of GPP loss in rubber plantations under (**a**) mild, (**b**) moderate, and (**c**) severe heatwave, drought, and compound drought and heatwave scenarios. Statistical differences among scenarios were assessed using the Mann–Whitney U test with Bonferroni correction. Asterisks indicate significance levels: * \(p<0.05\), ** \(p<0.01\), and *** \(p<0.001\); ns indicates no significant difference.  
Hainan Island. This indicates that evapotranspiration is the main cause of drought in this region, consistent with the findings of [65; 66]. Studies have shown that SPI-3 can capture short-term changes in water balance, which aligns with the rapid response of vegetation to soil moisture changes, thus significantly affecting soil humidity and vegetation growth [67]. Therefore, this study selected SPI-3 as the drought indicator for CDHEs. Although SPI-3 and STI-1 exhibited a statistically significant negative correlation (\(p<0.05\)) across rubber plantation areas, the overall correlation coefficient remained relatively low. This reflects fundamental differences in their construction logic and underlying stress mechanisms. Specifically, the two indices represent distinct environmental stress pathways: SPI-3 characterizes drought stress, while STI-1 captures heat stress. As a result, they differ in both temporal scale and variable composition. Moreover, compound drought and heat events are inherently nonlinear in nature, and drought and heatwaves do not necessarily occur simultaneously. Therefore, a strong statistical correlation between the two indices should not be expected. Similar findings of \"significant yet limited correlation\" have also been reported in previous studies [68].  
According to the temporal trend of the constructed CDHI, from January 2001 to December 2020, the CDHI decreased at a rate of 0.0004 per month (Figure 3), indicating a gradual increase in the severity of CDHEs experienced by the rubber plantations on Hainan Island. This trend is consistent with existing research findings that show a significant increase in the frequency and intensity of CDHEs in China's tropical regions [69]. Furthermore, the constructed CDHI effectively captured severe CDHEs in Hainan's rubber plantations in 2001, 2003-2005, 2010, 2012, 2015-2016, and 2020, which largely aligns with the historical drought and heatwave events recorded in the \"Yearbook of Meteorological Disasters in China\" for Hainan Island. This indicates that the CDHI has strong applicability for Hainan's rubber plantations.