Soil water content and vapor pressure deficit affect ecosystem water use efficiency through different pathways

Ecosystem water use efficiency (WUE) quantifies the amount of carbon assimilated per unit of water consumed and elucidates the trade-off between water loss and carbon gain at the ecosystem level. Numerous studies have highlighted the significant impact of low soil water content (SWC) and high vapor pressure deficit (VPD) on WUE. However, the underlying mechanisms driving these effects differ markedly. The intensifying coupling between land and atmosphere accentuates the distinctive co-occurrence of low SWC and high VPD, posing challenges in disentangling the respective influences of these two water stressors. Consequently, a clear understanding of the independent responses of ecosystem WUE to SWC and VPD remains elusive. Using meteorological and flux data from FLUXNET's eddy-covariance sites, we investigated the impact of SWC and VPD on ecosystem carbon-water fluxes and proposed potential pathways through which SWC and VPD influence WUE. We deconstructed WUE into intrinsic WUE of vegetation (GPP/T) and the ratio of transpiration to evapotranspiration (T/ET). Our findings revealed that WUE exhibited greater sensitivity to VPD dynamics compared to SWC. The sensitivity of WUE to SWC stemmed from the responsiveness of T/ET to SWC, especially under high VPD conditions. Conversely, the sensitivity of WUE to VPD emanated from the impact on GPP/T. SWC and VPD exerted distinct effects on ecosystem WUE through different pathways.