Data from: Extending glacial refugia for a European tree: genetic markers show that Iberian populations of white elm are native relicts and not introductions

Conservation policies usually focus on in situ protection of native populations, a priority that requires accurate assessment of population status. Distinction between native and introduced status can be particularly difficult (and at the same time, is most important) for species whose natural habitat has become both rare and highly fragmented. Here we address the status of the white elm (Ulmus laevis Pallas), a European riparian tree species whose populations have been fragmented by human activity and is protected wherever it is considered native. Small populations of this species are located in Iberia, where they are unprotected because they are considered introductions due to their rarity. However, Iberia and neighbouring regions in south-western France have been shown to support discrete glacial refuge populations of many European trees, and the possibility remains that Iberian white elms are native relicts. We used chloroplast RFLPs and nuclear microsatellites to establish the relationship between populations in Iberia and the Central European core distribution. Bayesian approaches revealed significant spatial structure across populations. Those in Iberia and south-western France shared alleles absent from Central Europe, and showed spatial population structure within Iberia common in recognised native taxa. Iberian populations show a demographic signature of ancient population bottlenecks, while those in Central European show a signature of recent population bottlenecks. These patterns are not consistent with historical introduction of white elm to Iberia, and instead strongly support native status, arguing for immediate implementation of conservation measures for white elm populations in Spain and contiguous areas of southern France.

保护政策通常聚焦于原生种群的原位保护，而这一优先级工作的开展有赖于对种群现状的精准评估。对于自然栖息地已极度濒危且高度破碎化的物种而言，区分其原生与引种身份尤为困难，但同时也最为关键。本研究聚焦欧洲白榆（Ulmus laevis Pallas）的种群现状：该物种为欧洲河岸树种，其种群因人类活动而破碎化，在被认定为原生分布的区域均受到保护。该物种在伊比利亚半岛存在小型种群，但由于其种群数量稀少而被认定为引种物种，因此未获保护。然而已有研究表明，伊比利亚半岛及法国西南部邻近区域曾是众多欧洲树木的独立冰期避难所，因此伊比利亚白榆有可能为原生孑遗种群。本研究采用叶绿体限制性片段长度多态性（chloroplast RFLPs）与核微卫星标记，解析伊比利亚半岛种群与中欧核心分布区种群之间的亲缘关系。贝叶斯分析法揭示了种群间显著的空间遗传结构：伊比利亚半岛与法国西南部的种群拥有中欧种群缺失的等位基因，且在伊比利亚半岛内部呈现出公认原生类群所特有的空间种群结构。伊比利亚种群呈现出古老种群瓶颈效应的人口统计学特征，而中欧种群则表现出近期种群瓶颈效应的特征。上述遗传模式与白榆被历史引种至伊比利亚半岛的假说并不相符，反而有力支持其原生身份，因此呼吁立即为西班牙及法国南部邻近区域的白榆种群实施保护措施。