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 southwestern 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 southwestern France shared alleles absent from Central Europe, and showed spatial population structure within Iberia common in recognized 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.