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The timing of phenological events exerts a strong control over ecosystem function and leads to multiple feedbacks to the climate system. Phenology is inherently sensitive to temperature (although the exact sensitivity is disputed) and recent warming is reported to have led to earlier spring, later autumn^, and increased vegetation activity^,. Such greening could be expected to enhance ecosystem carbon uptake^,, although reports also suggest decreased uptake for boreal forests^,. Here we assess changes in phenology of temperate forests over the eastern US during the past two decades, and quantify the resulting changes in forest carbon storage. We combine long-term ground observations of phenology, satellite indices, and ecosystem-scale carbon dioxide flux measurements, along with 18 terrestrial biosphere models. We observe a strong trend of earlier spring and later autumn. In contrast to previous suggestions …