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.

保护政策通常将本土种群的原位保护（in situ protection）作为核心目标，而这一优先级工作需要对种群现状开展精准评估。对于自然栖息地既已稀缺且高度破碎化的物种而言，区分其本土与引入种群的身份尤为困难，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，其种群因人类活动发生破碎化，且在被认定为本土分布的区域均受到保护。伊比利亚半岛分布有该物种的小型种群，但因种群极为稀有而被判定为引入种群，因此未获得相应保护。然而已有研究表明，伊比利亚半岛与法国西南部的邻近区域曾孕育诸多欧洲树木的独立冰期避难所种群，因此伊比利亚半岛的欧洲白榆有可能是本土孑遗种群。本研究借助叶绿体限制性片段长度多态性（chloroplast RFLPs）与核微卫星（nuclear microsatellites）分析，厘清了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。贝叶斯方法（Bayesian approaches）分析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携带有中欧种群所缺失的等位基因，且伊比利亚半岛内部的种群空间结构符合公认本土类群的特征。伊比利亚种群呈现出古老种群瓶颈的人口统计学特征，而中欧种群则表现出近期种群瓶颈的信号。这些格局与欧洲白榆被人为引入伊比利亚半岛的假说并不相符，反而强有力地支持了其本土种群身份，因此呼吁立即为西班牙及法国南部毗邻区域的欧洲白榆种群实施保护措施。