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

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 (VPD_day) and nighttime VPD (VPD_night) is often overlooked. Using multiple long-term remote sensing proxies of vegetation productivity, we reveal distinct effects of VPD_day and VPD_night on growing season vegetation productivity over the extratropical Northern Hemisphere (> 25° N). VPD_day 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 VPD_night 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 VPD_day and VPD_night, while in humid regions, it benefits from increased VPD_night. 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 VPD_night, 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 VPD_day and VPD_night in regulating vegetation productivity and highlight the importance of incorporating both into models to improve predictions of climate change impacts on terrestrial ecosystems.