Restriction threshold of vapor pressure deficit for light use efficiency varied with soil water content

Understanding the constraints on light-use efficiency (LUE) created by high evaporative water demand (vapor-pressure deficit; VPD) and restricted water supply (soil moisture content; SMC) is crucial for understanding and simulating vegetation productivity, particularly in arid and semi-arid regions. However, the relative impacts of VPD and SMC on photosynthesis are unclear, as we lack a mechanistic understanding of them and their interactions. In this study, we quantified the relative roles of VPD and SMC in limiting LUE and analyzed the interactions among VPD, SMC, and LUE in China’s Heihe River Basin using data from CO₂ and water flux stations and weather stations along a climatic gradient. We found a threshold for VPD’s restriction of LUE; above the threshold, LUE decreased at only 3.6% to 23.1% of the rate below the threshold. As SMC decreased, however, the VPD threshold increased, and the reduction of LUE caused by VPD decreased significantly, which is more than half lower than that in moister regions. Therefore, both VPD and SMC played essential roles in LUE limitation and the resulting reduction of photosynthesis caused by water stress. A threshold also existed for heat flux and the correlation between SMC and LUE; the strength of the correlation first decreased and then increased with increasing VPD. Our results clarified the relative impacts of VPD and SMC on photosynthesis, and can improve simulation and prediction of plant productivity.