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

The quantum yield ( α ) of photosynthesis represents the maximum efficiency of light use as indicated by the initial slope of photosynthetic light response curves. Understanding α is crucial for accurate modeling of photosynthesis and terrestrial carbon cycle. Despite its importance, the spatial and temporal variations in α at large scales remain largely elusive.

We leveraged long-term eddy-covariance observations from 90 sites globally and examined the spatiotemporal variations in α due to climatic 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 and soil moisture variations. Meanwhile, the temporal changes in α are mainly driven by the negative effect of vapor pressure deficit, which weakens the positive effects of elevated CO ₂ and leaf area index.

Our results highlight the dominant role of vapor pressure deficit in controlling the spatiotemporal variations of α as well as the unneglectable impacts of soil water content, CO ₂ , and leaf area on α . Those new results provide insights for improving the representation of α in ecosystem photosynthesis models.