Overlooked Effects of Nighttime Vapor Pressure Deficit on Extratropical Northern Hemisphere Vegetation Productivity

Journal article, 2026

The impact of elevated atmospheric vapor pressure deficit (VPD) on vegetation productivity is well-documented at monthly and annual scales. However, the influence of daytime VPD (VPDday) and nighttime VPD (VPDnight) is often overlooked. Using multiple long-term remote sensing proxies of vegetation productivity, we reveal distinct effects of VPDday and VPDnight on growing season vegetation productivity over the extratropical Northern Hemisphere (> 25° N). VPDday was negatively associated with vegetation productivity in 73.2% of vegetated pixels, and robustness analyses across alternative datasets and methods yielded a range of 67.4%–75.6%. By contrast, positive effects of VPDnight were detected in 51.8% of vegetated pixels, with corresponding estimates ranging from 36.5% to 55.7% across robustness analyses. This contrast was strongly related to aridity conditions. Vegetation productivity in drylands is more vulnerable to the double negative effects of high VPDday and VPDnight, while in humid regions, it benefits from increased VPDnight. Sap-flow observations helped explain this contrast from the perspective of plant hydraulic transport. In humid regions, relatively ample soil moisture allowed nocturnal water transport to be maintained under elevated VPDnight, helping restore plant water status overnight and providing favorable hydraulic conditions for daytime carbon uptake and vegetation productivity. In drylands, sap flow declined more strongly under high atmospheric demand and limited moisture during both daytime and nighttime, suggesting stronger hydraulic limitation and reduced overnight recovery, and thereby creating less favorable hydraulic conditions for vegetation productivity. These findings underscore the different roles of VPDday and VPDnight in regulating vegetation productivity and highlight the importance of incorporating both into models to improve predictions of climate change impacts on terrestrial ecosystems.