Adaptation to local climate in a multi-trait space: evidence from silver fir (Abies alba Mill.) populations across a heterogeneous environment

Heterogeneous environments, such as mountainous landscapes, create spatially varying selection pressure that potentially affects several traits simultaneously across different life stages, yet little is known about the general patterns and drivers of adaptation in such complex settings. We studied silver fir (Abies alba Mill.) populations across Switzerland and characterized their mountainous landscape using downscaled historical climate data. We sampled 387 trees from 19 populations and genotyped them at 374 single-nucleotide polymorphisms (SNPs) to estimate their demographic distances. Seedling morphology, growth and phenology traits were recorded in a common garden, and a proxy for water use efficiency was estimated for adult trees. We tested whether populations have more strongly diverged at quantitative traits than expected based on genetic drift alone in a multi-trait framework, and identified potential environmental drivers of selection. We found two main responses to selection: (i) populations from warmer and more thermally stable locations have evolved towards a taller stature, and (ii) the growth timing of populations evolved towards two extreme strategies, "start early and grow slowly" or "start late and grow fast", driven by precipitation seasonality. Populations following the "start early and grow slowly" strategy had higher water use efficiency and came from inner Alpine valleys characterized by pronounced summer droughts. Our results suggest that contrasting adaptive life-history strategies exist in silver fir across different life stages (seedling to adult), and that some of the characterized populations may provide suitable seed sources for tree growth under future climatic conditions.

异质环境（如山地景观）可形成空间异质性的选择压力，可能在不同生命阶段同时影响多个性状，但目前学界对这类复杂环境中生物适应的普遍模式与驱动因子仍知之甚少。本研究以瑞士全境的欧洲银杉（Abies alba Mill.）种群为研究对象，利用降尺度历史气候数据对其山地生境景观进行表征。我们从19个种群中采集了387株树木样本，通过374个单核苷酸多态性（single-nucleotide polymorphisms, SNPs）位点完成基因分型，以估算种群间的谱系分化距离。在同质种植园中记录了幼苗的形态、生长与物候性状，并对成年植株的水分利用效率替代指标进行了测算。本研究在多性状框架下检验了种群在数量性状上的分化程度是否高于仅由遗传漂变预期的水平，并鉴定了潜在的选择环境驱动因子。研究结果发现两类主要的选择响应：其一，来自更温暖且热环境更稳定区域的种群，演化出更高的株高；其二，在降水季节格局的驱动下，种群的生长物候演化出两种极端策略——"早萌发慢生长"与"晚萌发快生长"。采用"早萌发慢生长"策略的种群具有更高的水分利用效率，其原生境为以显著夏季干旱为特征的阿尔卑斯内陆河谷。本研究结果表明，欧洲银杉在不同生命阶段（幼苗至成年个体）存在截然不同的适应性生活史策略，且部分已表征的种群可作为未来气候条件下林木生长的适宜种源。