Effect of climate warming on the timing of autumn leaf senescence reverses after the summer solstice来自 https://www.science.org/doi/10.1126/science.adf5098## Abstract:Structured AbstractINTRODUCTIONOngoing climate changeis causing rapid shifts in plant phenology, with far-reaching effects on the terrestrial carbon cycle and biodiversity. While advances in spring leaf-out dates in temperate and boreal forests are well documented, the effects on autumn leaf senescence are less clear. This is becauseleaf senescence is not only affected by temperature but also by day length and vegetation activity early in the season in ways that are poorly understood.Accurately predicting future growing-season lengths and plant photosynthesis requires a better understanding of these interacting mechanisms at broad spatial scales.RATIONALELocal observations and experiments suggest thatearly-season warming, causing earlier spring leaf-out and faster plant development, tends to advance autumn senescence dates. Conversely, late-season warming tends to delay autumn senescence. If true more generally, then climate warming has opposing effects at the start and end of the growing season, with a reversal of effects somewhere in between. To test the generality of the opposing effects of climate warming on leaf senescence in Northern Hemisphere forests, we used satellite, ground, and carbon flux data, as well as controlled experiments.RESULTSOur results revealed that warming early and late in the growing season indeed has contrasting effects on leaf senescence, with a reversal occurring after the summer solstice. Across 84% of the northern forest area, we found that warmer temperatures and increased vegetation activity before the solstice advance the onset of senescence by 1.9 ± 0.1 days per °C, whereas warmer post-solstice temperatures slow the progression of senescence by 2.6 ± 0.1 days per °C. Between 1966 and 2015, the earlier onset of senescence has led to advances of 0.20 ± 0.07 days per year of the date at which autumn temperature starts to drive senescence progression. By contrast, mid-senescence continues to occur slightly later by 0.04 ± 0.01 days per year, leading to a lengthening of the autumnal senescence period.In our experiments, warmer pre-solstice temperatures also led to earlier primary growth cessation (bud set), demonstrating that the impact of a warmer pre-solstice period extends beyond leaf development and life span. This highlights the crucial role of overall plant development and sink activity before the summer solstice in determining growing-season length.CONCLUSIONWe have developed a unified explanatory framework for predicting autumn phenology, showing that leaf senescence now tends to begin earlier, because of increased pre-solstice vegetation activity, but progresses more slowly, such that the end of senescence occurs later. The reversal in trees’ responsiveness to warming after the summer solstice likely is triggered by changes in day length and allows them to initiate the physiological processes of leaf senescence and nutrient resorption in a fine-tuned balance between source and sink dynamics. Our results demonstrate the impact of developmental constraints (from cell and tissue growth) on autumn leaf senescence and forest productivity, affecting trends in growing-season length across the entire Northern Hemisphere. These insights provide a better understanding of the changes in growing seasons and carbon uptake of temperate andboreal forests under climate change.## Intro:## Para1The phenological cycles of treesexert a strong control on the structure and functioning of ecosystems(1, 2) and global carbon, water, and nutrient cycles (3–5).Anthropogenic climate changehas resulted inshifts in the growing seasons of temperate and boreal trees, with the start of the season today occurring, on average, two weeks earlier than it did during the 19th and 20th centuries (6) and the end of the season (EOS) being delayed (4, 7, 8). Each day of a longer growing season may increase net ecosystem carbon uptake by 3.0 to 9.8 gC m−2 (4). Yet, owing to the complex and interacting effects of growing-season climate and the annual day-length cycle, the direction of EOS changes in response to climate change and the cascading effects on ecosystem productivity remain highly uncertain (9–13).## Para2Characterizing the interplay among the environmental drivers of EOS at broad spatial scales is integral to improving our understanding of growing-season length and tree growth.Cell division, tissue formation, and growth in northern trees are highest at the beginning of the season and decline with shortening days(14–18), the adaptive reason being the limited time remaining for tissue maturation and bud set before the first frost (19). Local observations and experiments have shown that early-season warming, causing earlier spring leaf-out and faster growth and tissue maturation, tends to advance EOS dates (9, 11, 20, 21), whereas late-season warming has the opposite effect, delaying the EOS (22–24). Increased temperatures and physiological activity in the beginning of the season might drive earlier autumn senescence through a variety of possible mechanisms, including developmental and nutrient constraints (9, 25, 26), seasonal buildup of water stress (27, 28), and radiation-induced leaf aging (29). In contrast, later in the season, a direct effect of temperature (cooling) is likely to drive the timing of autumn senescence (10, 24, 30). If these trends are correct, then climate warming has opposing effects at the start and end of the growing season, with a reversal of effects somewhere in between.## Results:Effect of temperature and vegetation activity on senescence dates reverses after the summer solstice