Impact of Compound Drought-Hot Stress on the Gross Primary Productivity in Rubber Plantations Based on Identified Optimal Thresholds

Rubber plantations in Southeast Asia and China’s Hainan Island face the threat of increasingly frequent compound drought and hot events (CDHEs). To overcome the biases of traditional meteorological indicators in reflecting ecological stress, we propose herein a method for quantifying environmental stress on the basis of the optimal growth threshold of gross primary productivity (GPP). By constructing three CDHE identification patterns for soil compound drought and hot events (SCDHEs), atmospheric compound drought and hot events (ACDHEs), and ecological compound drought and hot events (ECDHEs), we aimed to quantify the impact of such events on the GPP of rubber plantations on the basis of the degree of deviation from the optimal threshold. The following results were found: (1) The optimal growth threshold of rubber plantations exhibited significant latitudinal gradients and seasonality, with soil moisture thresholds throughout the year being higher and more stable in areas near the equator than in high-latitude regions, the latter of which showed obvious seasonal fluctuations. (2) Both the frequency and intensity of CDHEs showed increasing trends from 1982 to 2018, with ACDHEs exhibiting the highest growth rate (0.0216 events/year) and being mainly concentrated in the inland areas of the Indochina Peninsula. (3) The various CDHE identification patterns differed in their spatial applicability: the compound index of ACDHEs was applicable to the entire region, whereas those of SCDHEs and ECDHEs were primarily applicable to low-latitude regions, with their identification capabilities limited north of 10°N. (4) CDHEs caused significantly higher GPP losses in rubber plantations than did isolated drought or hot events (p < 0.001), with a damaged cell proportion of 70.64%, confirming the synergistic amplification effect of compound events. With regard to the significance of the individual factors, the inhibitory effect of drought stress on GPP was significantly stronger than that of hot stress (p < 0.001), and this difference was most pronounced in the SCDHE pattern. (5) The spatiotemporal differentiations of GPP loss were distinct and characterized by the most severe loss occurring from March to May in the 10°N–24°N region (daily average loss peaking at 1.469 g·C/m2). By contrast, within 10° of both the southern and northern latitudes, the loss peak occurred in August, reaching 48.763 and 55.862 g·C/m2), respectively. In conclusion, the method developed in this study for quantifying the impact of compound stress events on the basis of ecological thresholds can reveal regional variations in different indicators and provides a scientific basis for the climate-adaptive management of rubber plantations in Southeast Asia and China’s Hainan Island.