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AbstractGlobal climate change is expected to further raise the frequency and severity of extreme events, such as droughts. The effects of extreme droughts on trees are difficult to disentangle given the inherent complexity of drought events (frequency, severity, duration, and timing during the growing season). Besides, drought effects might be modulated by trees’ phenotypic variability, which is, in turn, affected by long‐term local selective pressures and management legacies. Here we investigated the magnitude and the temporal changes of tree‐level resilience (i.e., resistance, recovery, and resilience) to extreme droughts. Moreover, we assessed the tree‐, site‐, and drought‐related factors and their interactions driving the tree‐level resilience to extreme droughts. We used a tree‐ring network of the widely distributed Scots pine (Pinus sylvestris) along a 2,800 km latitudinal gradient from southern Spain to northern Germany. We found that the resilience to extreme drought decreased in mid‐elevation and low productivity sites from 1980–1999 to 2000–2011 likely due to more frequent and severe droughts in the later period. Our study showed that the impact of drought on tree‐level resilience was not dependent on its latitudinal location, but rather on the type of sites trees were growing at and on their growth performances (i.e., magnitude and variability of growth) during the predrought period. We found significant interactive effects between drought duration and tree growth prior to drought, suggesting that Scots pine trees with higher magnitude and variability of growth in the long term are more vulnerable to long and severe droughts. Moreover, our results indicate that Scots pine trees that experienced more frequent droughts over the long‐term were less resistant to extreme droughts. We, therefore, conclude that the physiological resilience to extreme droughts might be constrained by their growth prior to drought, and that more frequent and longer drought periods may overstrain their potential for acclimation.

AbstractClimate warming should result in hotter droughts of unprecedented severity in this century. Such droughts have been linked with massive tree mortality, and data suggest that warming interacts with drought to aggravate plant performance. Yet how forests will respond to hotter droughts remains unclear, as does the suite of mechanisms trees use to deal with hot droughts. We used an ecosystem‐scale manipulation of precipitation and temperature on piñon pine (Pinus edulis) and juniper (Juniperus monosperma) trees to investigate nitrogen (N) cycling‐induced mitigation processes related to hotter droughts. We found that while negative impacts on plant carbon and water balance are manifest after prolonged drought, performance reductions were not amplified by warmer temperatures. Rather, increased temperatures for 5 years stimulated soil N cycling under piñon trees and modified tree N allocation for both species, resulting in mitigation of hotter drought impacts on tree water and carbon functions. These findings suggest that adjustments in N cycling are likely after multi‐year warming conditions and that such changes may buffer reductions in tree performance during hotter droughts. The results highlight our incomplete understanding of trees' ability to acclimate to climate change, raising fundamental questions about the resistance potential of forests to long‐term, compound climatic stresses.

Forests play a major role in the global carbon cycle. Previous studies on the capacity of forests to sequester atmospheric CO 2 have mostly focused on carbon uptake, but the roles of carbon turnover time and its spatiotemporal changes remain poorly understood. Here, we used long-term inventory data (1955 to 2018) from 695 mature forest plots to quantify temporal trends in living vegetation carbon turnover time across tropical, temperate, and cold climate zones, and compared plot data to 8 Earth system models (ESMs). Long-term plots consistently showed decreases in living vegetation carbon turnover time, likely driven by increased tree mortality across all major climate zones. Changes in living vegetation carbon turnover time were negatively correlated with CO 2 enrichment in both forest plot data and ESM simulations. However, plot-based correlations between living vegetation carbon turnover time and climate drivers such as precipitation and temperature diverged from those of ESM simulations. Our analyses suggest that forest carbon sinks are likely to be constrained by a decrease in living vegetation carbon turnover time, and accurate projections of forest carbon sink dynamics will require an improved representation of tree mortality processes and their sensitivity to climate in ESMs.

Summary The timing of diel stem growth of mature forest trees is still largely unknown, as empirical data with high temporal resolution have not been available so far. Consequently, the effects of day–night conditions on tree growth remained uncertain. Here we present the first comprehensive field study of hourly‐resolved radial stem growth of seven temperate tree species, based on 57 million underlying data points over a period of up to 8 yr. We show that trees grow mainly at night, with a peak after midnight, when the vapour pressure deficit (VPD) is among the lowest. A high VPD strictly limits radial stem growth and allows little growth during daylight hours, except in the early morning. Surprisingly, trees also grow in moderately dry soil when the VPD is low. Species‐specific differences in diel growth dynamics show that species able to grow earlier during the night are associated with the highest number of hours with growth per year and the largest annual growth increment. We conclude that species with the ability to overcome daily water deficits faster have greater growth potential. Furthermore, we conclude that growth is more sensitive than carbon uptake to dry air, as growth stops before stomata are known to close.

Les forêts subissent des risques croissants de mortalité des arbres induite par la sécheresse. Les schémas de remplacement des espèces après la mortalité peuvent avoir un impact significatif sur le cycle mondial du carbone. Parmi les principaux feuillus, les chênes à feuilles caduques (Quercus spp.) sont de plus en plus signalés comme remplaçant les conifères mourants dans l'hémisphère Nord. Pourtant, nos connaissances sur les réponses de croissance de ces chênes à la sécheresse sont incomplètes, en particulier en ce qui concerne les effets de l'héritage post-sécheresse. Les objectifs de cette étude étaient de déterminer l'occurrence, la durée et l'ampleur des effets hérités des sécheresses extrêmes et comment cela varie selon les espèces, les sites et les caractéristiques de la sécheresse. Les effets hérités ont été quantifiés par la déviation des indices de croissance radiale observés par rapport aux indices de croissance radiale attendus au cours de la période 1940–2016. Nous avons utilisé des chronologies au niveau des peuplements provenant de 458 sites et de 21 espèces de chênes provenant principalement d'Europe, d'Amérique du Nord-Est et d'Asie orientale. Nous avons constaté que les effets hérités des sécheresses pouvaient durer de 1 à 5 ans après la sécheresse et étaient plus prolongés dans les sites secs. Les effets négatifs sur l'héritage (c.-à-d. une croissance plus faible que prévu) étaient plus fréquents après des sécheresses répétitives dans les sites secs. L'effet de la sécheresse répétitive était plus fort chez les chênes méditerranéens, en particulier chez Quercus faginea. Les analyses spécifiques aux espèces ont révélé que Q. petraea et Q. macrocarpa des sites secs étaient plus négativement affectés par les sécheresses tandis que la croissance de plusieurs espèces de chênes des sites mésiques augmentait pendant les années post-sécheresse. Les sites montrant des corrélations positives avec la température hivernale ont montré peu ou pas de dépression de croissance après la sécheresse, tandis que les sites avec une corrélation positive avec le bilan hydrique estival précédent ont montré une diminution de la croissance. Cela peut indiquer que, bien que le réchauffement hivernal favorise la croissance des arbres pendant les sécheresses, les précipitations estivales de l'année précédente peuvent prédisposer les chênes aux sécheresses extrêmes de l'année en cours. Nos résultats ont révélé un rôle massif des sécheresses répétitives dans la détermination des effets patrimoniaux et ont mis en évidence comment la sensibilité de la croissance au climat, la saisonnalité de la sécheresse et les traits spécifiques aux espèces déterminent les effets patrimoniaux chez les espèces de chênes à feuilles caduques. Los bosques corren un riesgo creciente de mortalidad arbórea inducida por la sequía. Los patrones de reemplazo de especies después de la mortalidad pueden tener un impacto significativo en el ciclo global del carbono. Entre las principales maderas duras, se informa cada vez más que los robles caducifolios (Quercus spp.) reemplazan a las coníferas moribundas en todo el hemisferio norte. Sin embargo, nuestro conocimiento sobre las respuestas de crecimiento de estos robles a la sequía es incompleto, especialmente con respecto a los efectos heredados posteriores a la sequía. Los objetivos de este estudio fueron determinar la ocurrencia, duración y magnitud de los efectos heredados de las sequías extremas y cómo varían según las especies, los sitios y las características de la sequía. Los efectos heredados se cuantificaron por la desviación de los índices de crecimiento radial esperados observados en el período 1940–2016. Utilizamos cronologías a nivel de stand de 458 sitios y 21 especies de roble, principalmente de Europa, el noreste de América y el este de Asia. Descubrimos que los efectos heredados de las sequías podían durar de 1 a 5 años después de la sequía y eran más prolongados en sitios secos. Los efectos negativos heredados (es decir, un crecimiento menor de lo esperado) fueron más frecuentes después de sequías repetitivas en sitios secos. El efecto de la sequía repetitiva fue más fuerte en los robles mediterráneos, especialmente en Quercus faginea. Los análisis específicos de la especie revelaron que Q. petraea y Q. macrocarpa de sitios secos se vieron más negativamente afectados por las sequías, mientras que el crecimiento de varias especies de roble de sitios mesicos aumentó durante los años posteriores a la sequía. Los sitios que mostraron correlaciones positivas con la temperatura invernal mostraron poca o ninguna depresión del crecimiento después de la sequía, mientras que los sitios con una correlación positiva con el balance hídrico del verano anterior mostraron un crecimiento disminuido. Esto puede indicar que, aunque el calentamiento invernal favorece el crecimiento de los árboles durante las sequías, las precipitaciones de verano del año anterior pueden predisponer a los robles a las sequías extremas del año en curso. Nuestros resultados revelaron un papel masivo de las sequías repetitivas en la determinación de los efectos heredados y destacaron cómo la sensibilidad del crecimiento al clima, la estacionalidad de la sequía y los rasgos específicos de las especies impulsan los efectos heredados en las especies de roble caducifolio. Forests are undergoing increasing risks of drought-induced tree mortality. Species replacement patterns following mortality may have a significant impact on the global carbon cycle. Among major hardwoods, deciduous oaks (Quercus spp.) are increasingly reported as replacing dying conifers across the Northern Hemisphere. Yet, our knowledge on the growth responses of these oaks to drought is incomplete, especially regarding post-drought legacy effects. The objectives of this study were to determine the occurrence, duration, and magnitude of legacy effects of extreme droughts and how that vary across species, sites, and drought characteristics. The legacy effects were quantified by the deviation of observed from expected radial growth indices in the period 1940–2016. We used stand-level chronologies from 458 sites and 21 oak species primarily from Europe, north-eastern America, and eastern Asia. We found that legacy effects of droughts could last from 1 to 5 years after the drought and were more prolonged in dry sites. Negative legacy effects (i.e., lower growth than expected) were more prevalent after repetitive droughts in dry sites. The effect of repetitive drought was stronger in Mediterranean oaks especially of Quercus faginea. Species-specific analyses revealed that Q. petraea and Q. macrocarpa from dry sites were more negatively affected by the droughts while growth of several oak species from mesic sites increased during post-drought years. Sites showing positive correlations to winter temperature showed little to no growth depression after drought, whereas sites with a positive correlation to previous summer water balance showed decreased growth. This may indicate that although winter warming favors tree growth during droughts, previous-year summer precipitation may predispose oak trees to current-year extreme droughts. Our results revealed a massive role of repetitive droughts in determining legacy effects and highlighted how growth sensitivity to climate, drought seasonality and species-specific traits drive the legacy effects in deciduous oak species. تتعرض الغابات لمخاطر متزايدة من نفوق الأشجار بسبب الجفاف. قد يكون لأنماط استبدال الأنواع بعد الوفاة تأثير كبير على دورة الكربون العالمية. من بين الأخشاب الصلبة الرئيسية، يتم الإبلاغ بشكل متزايد عن أن أشجار البلوط المتساقطة (Quercus spp.) تحل محل الصنوبريات الميتة في جميع أنحاء نصف الكرة الشمالي. ومع ذلك، فإن معرفتنا باستجابات النمو لهذه البلوط للجفاف غير مكتملة، خاصة فيما يتعلق بآثار إرث ما بعد الجفاف. كانت أهداف هذه الدراسة هي تحديد حدوث ومدة وحجم الآثار القديمة للجفاف الشديد وكيف يختلف ذلك عبر الأنواع والمواقع وخصائص الجفاف. تم قياس الآثار القديمة من خلال الانحراف الملحوظ عن مؤشرات النمو الشعاعي المتوقعة في الفترة 1940–2016. استخدمنا التسلسل الزمني على مستوى الوقوف من 458 موقعًا و 21 نوعًا من البلوط في المقام الأول من أوروبا وشمال شرق أمريكا وشرق آسيا. وجدنا أن الآثار القديمة للجفاف يمكن أن تستمر من 1 إلى 5 سنوات بعد الجفاف وكانت أطول في المواقع الجافة. كانت الآثار القديمة السلبية (أي نمو أقل مما كان متوقعًا) أكثر انتشارًا بعد الجفاف المتكرر في المواقع الجافة. كان تأثير الجفاف المتكرر أقوى في أشجار البلوط المتوسطية خاصةً من سعف البحر الأبيض المتوسط. كشفت التحليلات الخاصة بالأنواع أن Q. petraea و Q. macrocarpa من المواقع الجافة تأثرا سلبًا أكثر بالجفاف بينما زاد نمو العديد من أنواع البلوط من المواقع الإنسية خلال سنوات ما بعد الجفاف. أظهرت المواقع التي تظهر ارتباطات إيجابية لدرجة حرارة الشتاء انخفاضًا طفيفًا أو معدومًا في النمو بعد الجفاف، في حين أظهرت المواقع ذات الارتباط الإيجابي بتوازن المياه في الصيف السابق انخفاضًا في النمو. قد يشير هذا إلى أنه على الرغم من أن الاحترار الشتوي يفضل نمو الأشجار أثناء الجفاف، إلا أن هطول الأمطار في الصيف في العام السابق قد يهيئ أشجار البلوط للجفاف الشديد في العام الحالي. كشفت نتائجنا عن دور هائل للجفاف المتكرر في تحديد الآثار القديمة وأبرزت كيف أن حساسية النمو للمناخ وموسمية الجفاف والسمات الخاصة بالأنواع تدفع الآثار القديمة في أنواع البلوط المتساقطة.

Seasonal variation in environmental constraints (vapor pressure deficit – VPD, air temperature, and soil moisture) on tree growth for the potential distribution range of seven widespread Central European tree species. We simulated environmental constraints on growth fusing 3-PG model or the species’ potential distribution range within the forested area of Switzerland on a 1×1 km grid for seven dominant tree species: _Larix decidua_, _Picea abies_, _Abies alba_, _Fagus sylvatica_, _Acer pseudoplatanus_, _Pinus sylvestris_, and _Quercus robur_. For this purpose, we simulated the growth of these tree species in monocultures with the average climate observed during 1961–1990 or 1991-2018. The stands were initialized as 2-year-old plantations with an initial density of 2,500 trees ha-1 and simulated until the age of 30 years. For each simulated month, we obtained the relative contribution of environmental constraints (VPD, temperature, and soil water) on tree growth.