Water-use efficiency (WUE), a trait that quantifies the ratio of water loss to carbon gain, is a key ecosystem property that links the global carbon, water and energy budgets. It is also an important trait for understanding ecosystem resilience to drought, as plants respond to water stress by regulating the openness of stomata, which control the release of water from leaves and therefore control WUE. However, how different ecosystems respond to drought stress by regulating their WUE is relatively less well understood. Here, we used 70 flux tower sites with more than 5 years of observations to test how WUE (ratio of gross primary productivity (GPP) over evapotranspiration) is regulated along gradients of vapor pressure deficit (VPD) and soil water content (SWC). The results indicate that WUE decreased with the increase of VPD across different biomes. At the same time, the connection between WUE and VPD was stronger than the WUE-SWC connection, especially when we estimated WUE as GPP over transpiration, indicating a strong influence of VPD on WUE, and raising questions regarding likely plant water use in response to future climate change.