Vapor pressure deficit dominates the spatiotemporal variations in ecosystem photosynthetic quantum yield

O_LIThe quantum yield () of photosynthesis represents the maximum efficiency of light use as indicated by the initial slope of photosynthetic light response curves. Understanding is crucial for accurate modeling of photosynthesis and terrestrial carbon cycle. Despite its importance, the spatial and temporal variations in at large scales remain largely elusive. C_LIO_LIWe leveraged long-term eddy-covariance observations from 90 sites globally and examined the spatiotemporal variations in due to climatic drivers, using statistical and machine learning approaches. C_LIO_LIWe found significant spatial variability in across and within biomes, primarily driven by atmospheric vapor pressure deficit and soil moisture variations. Meanwhile, the temporal changes in are mainly driven by the negative effect of vapor pressure deficit, which weakens the positive effects of elevated CO2 and leaf area index. C_LIO_LIOur results highlight the dominant role of vapor pressure deficit in controlling the spatiotemporal variations of as well as the unneglectable impacts of soil water content, CO2, and leaf area on . Those new results provide insights for improving the representation of in ecosystem photosynthesis models. C_LI