Globally constrained forest biophysical cooling benefits under rising atmospheric dryness

Climate change reshapes forest biophysical effects, yet the impact direction and strength remain uncertain. Here we quantify the growing-season land surface temperature between forests and adjacent open land (∆LSTgs ) and show contrasting temporal trends in ∆LSTgs across the globe during 2001–2023. Rising vapour pressure deficit (VPD) has emerged as the primary driver of these contrasting trends, surpassing other common climatic factors. By contrast, plant anisohydricity—an indicator of stomatal regulation behaviour— is the most important forest trait that negatively modulates the strength of the ∆LSTgs response to VPD variability. At low latitudes, forests are more isohydric, and rising VPD has exceeded the hydraulic safety margin, resulting in weakened cooling. Conversely, high-latitude forests are more anisohydric; VPD remains below the safety margin, and rising VPD thus leads to enhanced cooling. These results highlight that the overall climate benefits of global forests may be undermined if global VPD continues to intensify in future.