Earlier end of photosynthesis driven by advanced photosynthetic onset and elevated early-season temperatures

Abstract Climate change has profoundly altered the timing of phenological events, disrupting ecosystem functions and carbon dynamics. However, photosynthetic duration is often overestimated, particularly in autumn, as greenness and photosynthesis tend to decouple. This highlights the need for a more precise understanding of autumn photosynthetic phenology to refine global carbon assessments. In this study, we estimated the end-of-photosynthesis date (EOP) across the Northern Hemisphere (>30° N) from 2001 to 2021 using satellite-derived solar-induced chlorophyll fluorescence and flux tower observations. We then analyzed EOP’s spatiotemporal variations and identified its primary biome-specific drivers. Our results reveal a widespread advancement of EOP, except in water-limited biomes, with the most pronounced shifts in colder regions such as boreal forests and tundra. These shifts are mainly driven by the carryover effects of earlier photosynthetic onset and early-season warming, which are likely influenced by leaf lifespan and sink capacity. Nutrient limitation may further exacerbate sink constraints, particularly in colder biomes. The effects of earlier photosynthetic onset and early-season warming counteract the delaying influence of late-season warming on EOP. The significant advancement of early-stage EOP (EOP 75 ) indicates that early-season drivers have a stronger effect than late-season delays, while late-stage EOP (EOP 25 ) remains nearly unchanged due to a balance of opposing forces. Consequently, the limited photosynthetic gains from an earlier or unchanged EOP may fail to offset respiratory carbon losses, potentially weakening the Northern Hemisphere’s carbon sink capacity and complicating future carbon-climate feedback projections.