Drought is one of the most important selection pressures for forest trees in the context of climate change. Yet, the different evolutionary mechanisms, and their environmental drivers, by which certain populations become more drought tolerant than others is still little understood. We studied adaptation to drought in 16 silver fir (Abies alba Mill.) populations from the French Mediterranean Alps by combining observations on seedlings from a greenhouse experiment (N = 8,199) and on adult tress in situ (N = 315). In the greenhouse, we followed half‐sib families for four growing seasons for growth and phenology traits, and tested their water stress response in a "drought until death" experiment. Adult trees in the field were assessed for δ¹³C, a proxy for water use efficiency, and genotyped at 357 SNP loci. SNP data was used to generate a null expectation for seedling trait divergence between populations in order to detect the signature of selection, and 31 environmental variables were used to identify the selective environment. We found that seedlings originating from populations with low soil water capacity grew more slowly, attained a smaller stature, and resisted water stress for a longer period of time in the greenhouse. Additionally, adult trees of these populations exhibited a higher water use efficiency as evidenced by their δ¹³C. These results suggest a correlated evolution of the growth‐drought tolerance trait complex. Population divergence in bud break phenology was adaptive only in the second growing season, and evolved independently from the growth‐drought tolerance trait complex. Adaptive divergence in bud break phenology was principally driven by the inter‐ and intra‐annual variation in temperature at the geographic origin of the population. Our results illustrate the different evolutionary strategies used by populations to cope with drought stress at the range limits across a highly heterogeneous landscape, and can be used to inform assisted migration programs.

中文翻译：

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对干旱的适应伴随着缓慢的生长，但与边缘银杉（Abies alba Mill.）种群的物候无关。


干旱是气候变化背景下林木最重要的选择压力之一。然而，某些种群比其他种群更能耐受干旱的不同进化机制及其环境驱动因素仍然知之甚少。我们通过结合对温室实验幼苗 ( N = 8,199) 和成年树 ( N = 315) 的观察，研究了来自法国地中海阿尔卑斯山的 16 个银冷杉 ( Abies alba Mill.) 种群对干旱的适应。在温室中，我们跟踪了同父异母家庭的四个生长季节的生长和物候特征，并在“干旱直至死亡”实验中测试了他们的水分胁迫反应。对田间成年树木的δ ¹³ C（水分利用效率的代表）进行了评估，并在 357 个 SNP 位点进行了基因分型。 SNP 数据用于生成群体之间幼苗性状差异的零期望，以检测选择特征，并使用 31 个环境变量来识别选择环境。我们发现，来自土壤持水量低的群体的幼苗在温室中生长得更慢，身材更小，并且抵抗水分胁迫的时间更长。此外，这些种群的成年树木表现出更高的水分利用效率，如δ ¹³ C 所证明的那样。这些结果表明生长-耐旱性状复合体的相关进化。芽期物候的种群分化仅在第二个生长季节具有适应性，并且独立于生长-耐旱性状复合体而进化。 芽期物候的适应性分化主要是由种群地理起源的年际和年内温度变化驱动的。我们的结果说明了种群在高度异质性景观范围内应对干旱胁迫所采用的不同进化策略，并可用于为辅助迁移计划提供信息。