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 betweennative and introduced status can be particularly difficult (and at the same time, is mostimportant) 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 Europeanriparian tree species whose populations have been fragmented by human activity and isprotected wherever it is considered native. Small populations of this species are locatedin Iberia, where they are unprotected because they are considered introductions due totheir rarity. However, Iberia and neighbouring regions in southwestern France have beenshown to support discrete glacial refuge populations of many European trees, and thepossibility remains that Iberian white elms are native relicts. We used chloroplast RFLPsand nuclear microsatellites to establish the relationship between populations in Iberiaand the Central European core distribution. Bayesian approaches revealed significantspatial structure across populations. Those in Iberia and southwestern France sharedalleles absent from Central Europe, and showed spatial population structure within Iberiacommon in recognized native taxa. Iberian populations show a demographic signature ofancient population bottlenecks, while those in Central European show a signature of recentpopulation bottlenecks. These patterns are not consistent with historical introduction ofwhite elm to Iberia, and instead strongly support native status, arguing for immediateimplementation of conservation measures for white elm populations in Spain and contiguousareas of southern France.