Spring greening reduces autumnal runoff across high northern latitudes

Spring greening increasing leaf area has strengthened vegetation transpiration and increased the risk of carry-over summer soil moisture depletion across the Northern Hemisphere ( > 45° N) ^1-4 . The persistence in Autumn and impact on runoff of these effects, however, remain poorly quantified ^5,6 . We combined satellite-derived vegetation indices, runoff observations from 297 watersheds, and experiments using the LPJ-GUESS dynamic vegetation model, to demonstrate that spring greening reduced autumnal runoff by an average of 1.88-7.34% relative to the baseline from 1982 to 2022, through cross-seasonal carry over effects (CSE) where vegetation growth in one season reduces freshwater availability in subsequent seasons. The CSEs lasted longer in regions dominated by mixed or evergreen forests (until November) than in regions with green vegetation in summer (until October), primarily due to longer foliar longevity. Our findings underscore the need to integrate cross-seasonal interactions between vegetation and hydrology in assessing climate change impacts on water availability and in the planning of sustainable water resources. They also suggest that amplified soil drying in autumn may accelerate the transition of high-latitude ecosystems from carbon sinks to sources.