Abstract: Aim: To reconstruct the colonisation dynamics of silver fir in the Pyrenees during the Holocene using genetic and paleoecological data. Location: Abies alba forests in the Pyrenees and Northeastern Iberia. Taxon: Silver fir (Abies alba Mill.). Methods: Palaeoecological data from 43 pollen diagrams were used to elucidate the appearance and persistence of silver fir during the Holocene. Additionally, 43 forests were sampled in the southern Pyrenees to perform genetic analyses using 65 SNPs derived from a total of 273 SNPs sourced from a transcriptome assembly. Linear regressions of pairwise population statistics (FST) against geographical distances were performed to examine the presence of isolation by distance. An assumption-free approach was also employed to explore the genetic structure of the populations and coalescence analyses were carried out to infer past demography history. Results: We identified Pyrenean eastern and western genetic groups, which originated from an ancestral population located in the eastern Pyrenees. From this population, a westward colonisation took place, during which isolation by distance led to genetic divergence among populations. The eastern group diverged from a common ancestor shared with Alpine populations approximately 110,250 years (3150 generations) ago. The divergence between the eastern and western groups began around 40,250 years ago, coinciding with a reduction in effective population size to 1/2 of its ancestral value. Subsequently, a secondary contact occurred approximately 8575 years ago, resulting in admixed populations in the central Pyrenees. Main Conclusions: We identified a single eastern origin for Pyrenean silver fir populations, followed by westward expansion and spatial differentiation, with recent gene flow between previously isolated genetic groups. This scenario aligns with palaeoecological evidence and shows how the Pyrenees acted as a longitudinal corridor for postglacial colonisation. The resilience and genetic diversity of Pyrenean silver fir populations underscore their importance for the conservation of the species in Europe.

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