Decoupled driving forces of variabilities of transpiration in Chinese subtropical vegetation based on remote sensing data

Transpiration (Tc) is a critical component of the global water cycle. Soil moisture (SM) and vapor pressure deficit (VPD) are key regulators of Tc, and exploring their contributions to changes in Tc can deepen our understanding of the mechanisms of water cycling in terrestrial ecosystems. However, the driving roles of VPD and SM in Tc changes remain debated because of the coupling of SM and VPD through land-atmosphere interactions which restrict the quantification of the independent effects of SM and VPD on Tc. By decoupling the correlations between SM and VPD using a novel binning approach, this study analyzed the dominant drivers of vegetation transpiration in subtropical China from 2003 to 2018 based on multi-source data, including meteorological reanalysis, remotely sensed soil moisture, transpiration, and land cover data. The results show that Tc first increased and then remained stable with an increase in SM across the study area but changed slightly with increasing VPD. Overall, the relative contribution of SM to the change in Tc was approximately five times that of VPD. The sensitivities of Tc to SM and VPD differed among vegetation types. Although the sensitivity of Tc to SM was greater than that of VPD for all four vegetation types, the thresholds of Tc in response to SM were different, with the lowest threshold (approximately 35%) for the other forests and the highest threshold (approximately 55%) for short wood vegetation. We infer that this is associated with the differences in ecological strategies. To verify the reliability of our conclusions, we used solar- induced chlorophyll fluorescence (SIF) data as a proxy for Tc based on the tight coupling between photosynthesis and transpiration. Consistent results were obtained by repeating the analyses. The results of this study, in which the impacts of SM and VPD on Tc were decoupled, are beneficial for further understanding the critical processes involved in water cycling in terrestrial ecosystems in response to climate change.