The spatiotemporal variations in ecosystem photosynthetic quantum yield and their drivers

· There was no widespread increase in quantum yield of ecosystem photosynthesis ( α ) from 1980 to 2020. · The spatial variation in α was driven mainly by soil moisture and vapor pressure deficit (VPD). · The temporal variation in α was dominated by VPD, which dampened the positive effects of CO 2 fertilization and enhanced leaf area index on α . The quantum yield ( α ) of photosynthesis represents the maximum light use efficiency (LUE) as indicated by the initial slope of photosynthetic light response curves. α is an important variable in LUE-based models which are widely used to simulate gross primary productivity (GPP) from regional to global scales. However, the spatiotemporal variations in α at the ecosystem scale remain elusive despite its importance. Here, we leveraged long-term eddy-covariance observations from 90 sites globally and examined the spatiotemporal variations in α and their drivers, using statistical and machine learning approaches. We found significant spatial variability in α across and within biomes, primarily driven by atmospheric vapor pressure deficit (VPD) and soil moisture variations. Meanwhile, the temporal changes in α are primarily driven by the negative effect of VPD, which weakens the positive effects of elevated CO 2 and leaf area index (LAI). Our results highlight the dominant role of VPD in controlling the spatiotemporal variations of α and the unneglectable impacts of soil moisture, CO 2 , and LAI on α . These new results provide insights for improving the representation of α in LUE-based models for GPP simulations.