Rear edge populations might be of disproportionate importance for the adaptation of temperate forest tree species to climate change. The study of past demographic dynamics and their effects on extant genetic diversity are therefore keys to understand how to manage these potentially relevant forest genetic resources. Here we combine a comprehensive review of palaeobotanical evidence from peninsular Italy throughout the Quaternary with new genetic data for Picea abies, focusing on all known rear edge populations from the boundary zone between the Alps and the northern Apennines, to shed light on timing and modes of the fragmentation processes leading these populations close to extirpation. Our data show that Picea abies experienced a complex Quaternary history mirrored by a concomitantly complex genetic structure. The population in the southwestern Alps and the two populations living in the northwestern Apennines appear to be the last remnants of a much wider Pleistocene distribution. During the last glacial period and the postglacial they had distinct spatiotemporal dynamics. These peripheral populations are characterized by peculiar genetic features, a substantial pairwise genetic differentiation and general genetic impoverishment, with a ∼ 20% reduction of their allelic richness with respect to other Alpine populations.
These results collectively indicate that the southernmost Italian populations of Picea abies, still present in the northern Apennines and in the southwestern Alps, are extremely vulnerable to extirpation, as already was the case with populations that progressively disappeared from southern and central Italy in the Middle and Upper Pleistocene, respectively, and from the north-eastern Apennines during the late Holocene.
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