Phenophase‐specific climate impacts and legacy effects govern autumn phenological transitions across northern forest ecosystems

Abstract Autumn phenology plays a crucial role in regulating growing season length and ecosystem carbon and water exchanges, yet its responses to climate change, particularly to climate extremes and their phenophase‐specific carry‐over effects, remain poorly understood. As extreme climatic events become increasingly frequent and intense, understanding the mechanisms governing autumn phenological transitions is essential for predicting forest resilience under climate variability. However, two key questions remain unclear: (i) how phenophase‐specific climatic and developmental carry‐over effects jointly shape autumn phenology, and (ii) how these phenophase‐specific regulatory factors vary across forest management types and climate types. Here, we examined autumn phenological transitions—specifically greenness decrease onset and end of season—across the Northern Hemisphere (30–70° N) from 2013 to 2022 using Visible Infrared Imaging Radiometer Suite (VIIRS) surface phenology products. Temporal shifts in these transitions were analysed across six major Köppen–Geiger climate types and three forest management types, with a focus on phenophase‐specific effects of climate extremes and preceding phase durations together governing autumn phenology. Our results showed that 22% and 26% of regions experienced delayed greenness decrease onset and end of season, while 21% and 10% exhibited advancements, with most areas remaining stable. Partial correlation analysis revealed that the duration of late phenophases and late‐season climate exerted the strongest influence on autumn phenology, followed by temperature extremes, whereas the importance of precipitation extremes increased notably during senescence, especially in cold‐dry climates. Natural forests were more strongly influenced by early‐season extremes, while planted forests were more sensitive to late‐season heat and precipitation extremes, reflecting their limited buffering capacity and structural simplicity. The increasing importance of late‐season heat extremes, especially in planted forests within warm‐humid regions, highlights their growing vulnerability to advanced senescence under continued warming. Synthesis. These findings underscore the importance of incorporating forest management‐type‐specific phenophase sensitivities and phenological carry‐over effects into phenological models to improve predictions of autumn transitions and guide adaptive forest management strategies under ongoing climate change.