Drought stress has induced dieback episodes affecting many forest types and tree species worldwide. However, there is scarce information regarding drought-triggered growth decline and canopy dieback in Mediterranean deciduous oaks. These species face summer drought but have to form new foliage every spring which can make them vulnerable to hotter and drier conditions during that season. Here, we investigated two stands dominated by Quercus frainetto Ten. and Quercus canariensis Willd. and situated in southern Italy and Spain, respectively, showing drought-induced dieback since the 2000s. We analyzed how radial growth and its responses to climate differed between non-declining (ND) and declining (D) trees, showing different crown defoliation and coexisting in each stand by: (i) characterizing growth variability and its responsiveness to climate and drought through time, and (ii) simulating growth responses to soil moisture and temperature thresholds using the Vaganov–Shashkin VS-lite model. Our results show how growth responsiveness to climate and drought was higher in D trees for both oak species. Growth has become increasingly limited by warmer-drier climate and decreasing soil moisture availability since the 1990s. These conditions preceded growth drops in D trees indicating they were more vulnerable to warming and aridification trends. Extremely warm and dry conditions during the early growing season trigger dieback. Changes in the seasonal timing of water limitations caused contrasting effects on long-term growth trends of D trees after the 1980s in Q. frainetto and during the 1990s in Q. canariensis. Using growth models allows identifying early-warning signals of vulnerability, which can be compared with shifts in the growth responses to warmer and drier conditions. Our approach facilitates establishing drought-vulnerability thresholds by combining growth models with field records of dieback.

中文翻译：

---

生长对气候变化的响应以及超过干旱易受害性阈值的变化是两种地中海落叶橡树死亡的特征

干旱胁迫已导致枯萎病发作，影响了全世界许多森林类型和树木种类。但是，很少有关于干旱引起的生长下降和地中海落叶橡树冠层枯死的信息。这些物种面临夏季干旱，但每年春季都必须形成新的叶子，这会使它们在该季节中更容易遭受高温和干燥的条件。在这里，我们调查了两个以栎栎十种为主的林分 。和加拿大栎威尔德。分别位于意大利南部和西班牙，自2000年代以来由于干旱而死亡。我们分析了非下降（ND）树和下降（D）树之间的径向生长及其对气候的响应如何不同，从而显示了不同林冠的落叶和共存情况，方法是：（i）通过以下方法表征生长的变异性及其对气候和干旱的响应能力时间；（ii）使用Vaganov–Shashkin VS-lite模型模拟对土壤水分和温度阈值的生长响应。我们的结果表明，两种橡树的D树对气候和干旱的生长响应能力如何更高。自1990年代以来，由于气候变暖和土壤水分供应减少，增长变得越来越受到限制。这些条件先于D树的生长下降，表明它们更容易受暖化和干旱化趋势的影响。在生长季节的早期，极端温暖和干燥的条件会导致死亡。限水季节的时机变化对1980年代后D树的长期生长趋势产生了相反的影响。Q. frainetto并在20世纪90年代Q.海枣。使用增长模型可以识别脆弱性的预警信号，可以将其与对温暖和干燥条件的增长响应变化进行比较。我们的方法通过将增长模型与枯萎病的现场记录相结合来促进建立干旱脆弱性阈值。