Global convergence in the response of terrestrial gross primary production to atmospheric vapour pressure deficit

With climate warming, atmospheric vapor pressure deficit (VPD) shows an increasing trend, which may restrict plant growth. However, there is still uncertainty regarding the response mechanisms of plant transpiration and photosynthesis to VPD, soil moisture, and their interactions. This uncertainty leads to significant discrepancies among different Earth system models when simulating the impact of atmospheric drought on terrestrial ecosystem productivity, and it constitutes a crucial source of uncertainty in predicting the global carbon balance of land ecosystems in the future. In this study, through analyzing field measurements, satellite-derived data, and Earth system model (ESM) simulations, we reveal a similar threshold response pattern of GPP to VPD for most ecosystem types, where GPP initially increases and then decreases with increasing VPD. When VPD exceeds these thresholds, increased soil moisture loss and atmospheric drought stress lead to reduced stomatal conductance and lowered light saturation point in plant leaves, decreasing terrestrial ecosystems' productivity. Existing Earth system models emphasize the influence of CO2 fertilization on land ecosystem productivity and predict a continuous increase in global terrestrial GPP throughout the 21st century. However, these models also indicate a significant reduction in GPP of low-latitude land ecosystems when VPD exceeds the threshold. This finding highlights the impact of climate warming on VPD and implies potential limitations on future land ecosystem productivity due to increased atmospheric water demand. This study suggests incorporating the interactions among VPD, soil moisture, and canopy conductance into Earth system models to enhance the predictive capacity for the response of land ecosystems to climate change.