## 3 Results
### Spatial Pattern of Vulnerability of Rubber Plantation to STFDs  
An assessment of the vulnerability of GPP in the Hainan Island rubber ecosystem to STFD indicates that the center of gravity of vulnerability shifts from the southwest to the northeast and then back to the southwest with the month when STFD occurs (natural year from January to December), from a dynamic spatial perspective (Figure 5). The spatial distribution of responses of the rubber ecosystem in Hainan Island to STFD in May to August is similar. The vulnerability of the rubber ecosystem to STFD in February and March moved westward as the drought intensity increased, while for the rest of the months, with the intensification of STFD, the center of gravity of the effects gradually moved southward (the first half of the year is biased to the southeast, and the second half of the year to the southwest).  
Figure 4: Vulnerability curves of rubber plantations’ GPP to different drought intensities and the month the drought occurs.  
Figure 5: Spatial pattern changing tracks of rubber plantations’ GPP vulnerability with drought intensities and drought occurrence month over Hainan Island. Arrow represents the changing tracks of rubber plantations’ GPP vulnerability with SRHI from \(-0.25\) to \(-2.00\); Red means the dry season, and blue means the wet season.  
The extreme and light STFDs occurring in April and September were chosen as example scenarios to analyze the spatial pattern of the vulnerability of rubber ecosystem GPP to STFD, since they are the months when STFD occurred and GPP loss was the greatest or least in terms of the overall numerical performance. The overall pattern (Figure 6) showed that rubber plantations in Northern Hainan Island are more vulnerable to STFD than in the southern part; rubber plantations in Western Hainan Island are more vulnerable to STFD than in the eastern part. The spatial distribution of rubber plantations' GPP losses caused by STFDs of the same intensity occurring in different months was not consistent. The extreme drought in April had the most significant and spatially contiguous effect on the GPP of rubber plantations in northwestern Hainan Island; the drought of the same intensity that occurred in September had a more obvious effect on the GPP of rubber plantations in the northern part than in other parts of Hainan Island. The effect of the light drought in April on the GPP of rubber plantations in Hainan Island showed a decreasing pattern from northwest to southeast, while the vulnerability of rubber ecosystem GPP to a drought of the same intensity as the one from April but occurring in September had the distribution characteristics of increased vulnerability from northeast to southwest. The effect of STFD in April on the GPP of the rubber ecosystem was more concentrated in spatial pattern than the drought in September.  
The GPP losses caused by the light drought in April and the extreme drought in September at rubber plantations in Hainan Island were relatively close in value (0.32 gC/m\({}^{2}\)/day light STFD in April vs. 0.36 gC/m\({}^{2}\)/day extreme STFD in September), but the spatial distribution of GPP loss behaved differently (Figure 6). The rubber ecosystem GPP in the northern part, especially the northwestern part,  
Figure 6: The spatial distribution of a rubber plantation’s GPP loss for different drought months and intensities: (**a**) extreme drought in April; (**b**) light drought in April; (**c**) extreme drought in September; (**d**) light drought in September.  
relatively high vulnerability to the light drought in April, while the extreme drought that occurred in September had a greater impact on the central and eastern part of Hainan Island. For the spatial distribution of the vulnerability of the rubber ecosystem in Hainan Island with different STFD intensities and different timing, see the Supplementary Materials.