Premature leaf senescence in temperate trees is strongly driven by pre-solstice heat and drought while post-solstice effects disappeared over recent decades

Over recent decades, autumn leaf senescence of temperate deciduous trees has generally been delayed. Whilst post-summer solstice warming slows the progression of senescence, pre-solstice warming has been shown to advance senescence onset by increasing developmental rates. Severe heat and drought events, which have been increasing in frequency and intensity, can also advance senescence through stress. Yet, it remains unclear whether premature senescence is primarily driven by faster development or by climatic stress, limiting accurate projections of future premature leaf senescence frequencies. We analysed leaf senescence observations for four dominant deciduous tree species (horse chestnut, silver birch, European beech, and English oak) across >3000 sites in central Europe from 1951–2023. For all species, the proportion of premature senescence events (occurrences within the earliest 5% percentile) has slightly decreased over time (trend: -0.11-0% yr^-1). Pre-solstice climate variables had the largest effects on premature senescence likelihood, with post-solstice effects diminishing over time. Pre-solstice growing degree days were the most influential factor, associated with a 30-40% increase in premature senescence likelihood per standard deviation (sd) increase. Nighttime temperatures were as important as daytime temperatures. Leaf-out date was the next most significant factor (25% increase per sd). Water deficit had smaller effects (5-12% sd^-1), aligning with our experimental results showing that drought conditions do not cause premature senescence when nutrient availability is high. These results suggest that pre-solstice developmental processes exert a larger effect on premature senescence than summer climatic stress. Nevertheless, ongoing early-season warming and the increasing frequency of heatwaves and droughts is likely to intensify premature leaf senescence in European trees. Such shifts could have key impacts on biogeochemical cycles and community interactions within forest ecosystems.