## Discussion
### Vulnerability of Rubber Ecosystem GPP to STFD  
STFD in April led to the highest relative loss in monthly GPP mainly because April is the early part of the rapid rubber-growing season on Hainan Island [57]. The vulnerability of the rubber ecosystem to drought is highest during this period because the trees need to regrow their leaves and the dry season has just ended, depleting the soil and requiring water replenishment [58], thus amplifying the effect of the lack of water. On the contrary, STFD in September causes a lower relative GPP loss, mainly because September is deep into the rainy season so the water shortage in the dry season has been effectively replenished. The water resources in September are extremely abundant and much higher than the available moisture, which is not a dominant limiting factor for rubber growth, even if the RH is much lower than the climatic average. It has also been confirmed by the research on water requirements of rubber trees that, from December to April, the requirements are always larger than the effective precipitation, and the cumulative difference between supply and demand is highest up to April. However, from May, the water demand begins to be less than the effective precipitation, and the difference between requirements and effective precipitation reaches the maximum in September [26]. The relatively low loss in January is mainly due to the fact that rubber in Hainan Island is in the deciduous period from December to January, not during the growing season, and the GPP is very low. Secondly, the fact that the air temperature is lower than the optimal temperature becomes the dominant limiting factor instead of water demand. Regarding on the spatial pattern of vulnerability, except for the earlier period of the dry season (November and December), the dry season is mainly dominated by east-west direction differences, while the rainy season is dominated by north-south differences. This is mainly due to the fact that there is less rainfall in the dry season, so the vulnerability is dominated by the precipitation pattern; while the rainfall in the rainy season is sufficient not a limiting factor, so it is mainly regulated by the temperature distribution pattern [59].  
The vulnerability of GPP to STFD in the rubber ecosystem in this study was not severe on the whole; even under extreme SRHI conditions, only a 10% relative GPP loss was generated. The reasons are as follows: first, the limiting of radiation on GPP in the tropical region is more significant than that of moisture and temperature [60]; second, RH and Ta determine VPD jointly in the real world [53], but in this case study Ta was assumed to remain unchanged and only the atmospheric water demand changed, resulting in the interaction between temperature and moisture did not reflect. In EC-LUE, Ta and VPD are the direct limiting factors; the effect of drought from RH on GPP is simulated through VPD, which is also controlled by the unchanged Ta, since the influences of drought have been scaled. Last but not least, in this study, vulnerability was assessed by drought scenarios in which RH was controlled according to the optimal meteorological conditions, but the reduction in RH was calculated through the real value over a historical period. Therefore, using the optimal meteorological scenario as the basis input data for vulnerability assessment, the impact of drought would be lower than the true value, but the relative loss rate of GPP in each drought scenario would not be affected.