What it is
Using 2,953 tree-ring sites and combining generalized additive models with threshold regression, the authors show that thresholds beyond which vapor pressure deficit (VPD) constrains tree growth 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, and even the same species differed substantially in threshold across contrasting aridity. The share of sites exceeding these thresholds has risen rapidly since about 1970.
Why it matters
Rising VPD is a recognized driver of vegetation productivity and the terrestrial carbon sink, but the growth-limiting thresholds had been poorly quantified, leaving the risk to forests hard to bound. Quantifying these thresholds across thousands of sites, and projecting that under SSP5-8.5 approximately 79.8% of sites would face VPD above their thresholds by the end of the century, turns a general concern into a specific, mappable exposure that grows with emissions.
Underlined numbers link to their source. Every metric and quoted figure is listed under Sources and data below.
Filed underPlant Water Relations and Carbon Dynamics, Tree-ring climate responses, Plant responses to elevated CO2