Three Mediterranean oak species were considered in this study: one evergreen oak (Quercus suber, the most tolerant to drought) and two winter-deciduous oaks (Q. faginea, which shows intermediate tolerance, and Q. pyrenaica, with the lowest tolerance and highest soil water requirements). These three species were sampled, covering wide age and diameter gradients, in two National Parks located in central (Cabañeros; cooler site) and south-western (Monfragüe; warmer site) Spain. Their radial growth and the evolution over time of their intrinsic water-use efficiency (iWUE) and wood δ18O were obtained. Our specific objectives were to analyze: (i) growth and iWUE dynamics in recent decades, (ii) climate-growth/iWUE relationships, and (iii) the effect of tree size and age on growth, iWUE and the sensitivity of growth to climate, including resistance to drought. Our results show that, although the three species started from very different growth performance, with Q. suber and Q. faginea having the highest and lowest growth rates, respectively, in recent years their growth rates have converged possibly due to hydrological (topographic and edaphic) niche differentiation. This makes the species behave in a more resilient and stable way than might be expected according to regional climate trends. Q. suber was the most sensitive species to climate variability and showed a growth decline due, at least in part, to lower carbon uptake resulting from stomatal closure to limit water loss through leaves as temperature rises. In contrast, Q. faginea had a more limited stomata control, which allowed it to control transpiration without limiting growth. Finally, Q. pyrenaica did not show an effective stomata control and showed stable growth. Tree size and age were important factors explaining individual differences in growth, climate sensitivity and growth resistance and resilience to drought.

中文翻译：

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水文生态位和树木大小的差异解释了三种地中海橡树对干旱的生长适应能力


本研究考虑了三种地中海栎树种：一种常绿橡树（Quercus suber，最耐旱）和两种冬季落叶橡树（Q. faginea，表现出中等耐受性，和 Q. pyrenaica，具有最低的耐受性和最高的土壤水分需求）。这三个物种在位于西班牙中部（Cabañeros，较凉爽的地方）和西南部（Monfragüe，较温暖的地方）的两个国家公园中进行了采样，涵盖了较宽的年龄和直径梯度。获得了它们的径向生长及其固有水分利用效率 （iWUE） 和木材 δ18O 随时间的变化。我们的具体目标是分析：（i） 近几十年的生长和 iWUE 动态，（ii） 气候-生长/iWUE 关系，以及 （iii） 树木大小和年龄对生长、iWUE 和生长对气候的敏感性（包括抗旱性）的影响。我们的结果表明，尽管这三个物种的生长表现非常不同，其中 Q. suber 和 Q. faginea 的增长率分别为最高和最低，但近年来它们的增长率趋同，这可能是由于水文（地形和 edaphic）生态位分化。这使得该物种的行为方式比根据区域气候趋势预期的更具弹性和稳定性。栎 suber 是对气候变化最敏感的物种，并且显示出生长下降，至少部分是由于气孔关闭导致碳吸收降低，以限制温度升高时通过叶子的水分流失。相比之下，栎 （Q. faginea） 的气孔控制更有限，这使得它能够在不限制生长的情况下控制蒸腾作用。最后，栎 没有显示出有效的气孔控制，并显示出稳定的生长。 树木大小和年龄是解释生长、气候敏感性和生长抵抗力和抗旱能力个体差异的重要因素。