How hydrothermal factors and CO2 concentration affect vegetation carbon sink over time and elevation gradient

How hydrothermal factors and CO2 concentration affect the vegetation carbon sink over time and across elevation gradients is crucial for carbon sequestration and enhancement, which remains unclear. Here, the temporal changes of the interannual relationship of net primary productivity (NPP) with hydrothermal factors and CO2 concentration over elevation were revealed using ensemble empirical mode decomposition (EEMD) method and moving window-based multiple regression analysis during 1981–2018, in a case study of six provinces along the ‘Maritime Silk Road’ of China. We found that: (1) except for low-elevation farmland, the interannual relationship of vegetation NPP with CO2 (RNPP-CO2), precipitation accumulation (RNPP-PA), soil moisture (RNPP-SM), downward surface shortwave radiation (RNPP-SR) and minimum temperature (RNPP-TMN) is mainly positive, while the relationship with maximum temperature (TMAX, RNPP-TMX) and vapor pressure deficit (VPD, RNPP-VPD) is mainly negative, indicating that most hydrothermal factors can increase NPP except TMAX and VPD. Most of the interannual relationship is strengthening, especially for thermal factor, indicating the strengthened impacts of hydrothermal factors and CO2, while the weakening relationship with hydrological factors accounted for large areas, which cannot be ignored. (2) With increasing elevation, the positive RNPP-CO2 diminished, while the positive RNPP-SM and the negative RNPP-VPD amplified, indicating the positive impacts from CO2 decreased, while those from SM and VPD increased. Meanwhile, RNPP-SR, RNPP-TMN, and RNPP-PA reversed from positive to negative, while RNPP-TMX reversals from negative to positive, indicating reversed impacts. (3) With increasing elevation, the temporal changes of positive RNPP-CO2 and RNPP-SM varied from weakening to strengthening and from strengthening to weakening, respectively, while those of RNPP-SR and RNPP-TMN together with RNPP-PA varied from strengthening positive to strengthening negative and weakening negative, respectively. Meanwhile, the strengthening negative of RNPP-TMX, and RNPP-VPD, changed to weakening positive and weakening negative, respectively. Our study highlights the importance of the divergent trends of the interannual relationship of NPP with hydrothermal factors and CO2 concentration across elevation for accurate prediction of future carbon sink potential under global change.