Deciduous forests form the dominant natural vegetation of Europe today,
but were restricted to small refugia during Pleistocene cold stages,
implying an evolutionary past shaped by recurrent range contractions and
expansions. Cold-stage forest refugia were probably widespread in
southern and central Europe, with the northwestern Balkan Peninsula
being of particular importance. However, the actual number and location
of deciduous forest refugia, as well as the connections between them,
remain disputed. Here, we address the evolutionary dynamics of the
deciduous forest understorey species Euphorbia carniolica as a proxy for
past forest dynamics. To do so, we obtained genomic and morphometric
data from populations representing the species' entire range,
investigated phylogenetic position and intraspecific genetic variation,
tested explicit demographic scenarios and applied species distribution
models. Our data support two disjoint groups linked to separate refugia
on the northwestern and central Balkan Peninsula. We find that genetic
differentiation between groups started in the early Pleistocene via
vicariance, suggesting a larger distribution in the past. Both refugia
acted as sources for founder events to the southeastern Alps and the
Carpathians, whereas the latter were likely colonized before the last
cold stage. In line with traditional views on the pre-Pleistocene origin
of many southeastern European deciduous forest species, the origin of E.
carniolica was dated to the late Pliocene. The fact that E. carniolica
evolved at a time when a period of continuous forestation was ending in
much of Eurasia provides an interesting biogeographical perspective on
the past links between Eurasian deciduous forests and their biota.