Increasing risk of vapor pressure deficit exceeding critical thresholds for tree growth.

With global warming, rising atmospheric vapor pressure deficit (VPD) has emerged as a critical driver of vegetation productivity and the terrestrial carbon sink. Yet, the thresholds beyond which VPD constrains tree growth remain poorly quantified. Using 2,953 tree-ring sites and combining generalized additive models with threshold regression models, we demonstrate that growth-related VPD thresholds are widespread. Approximately 63% of the studied species groups exhibited identifiable thresholds, which were generally lower in cool, humid regions and higher in warm, dry environments. For the same tree species occurring under contrasting aridity conditions, thresholds also differed substantially. As VPD has risen, the proportion of sites exceeding these thresholds has increased rapidly since about 1970 and is projected to rise further under different emissions scenarios. Under SSP5-8.5 in particular, approximately 79.8% of the sites are projected to be exposed to VPD above their thresholds by the end of the century. Although potential upward threshold shifts associated with acclimation or adaptation could partly reduce future exceedance, this buffering effect becomes increasingly limited under stronger climate forcing. Without effective climate mitigation, rising VPD will increasingly exceed the tolerance thresholds of trees, placing more trees under sustained growth constraints and undermining terrestrial carbon sequestration.