Ecological niche modelling of pedunculate oak (Quercus robur) supports the ‘expansion–contraction’ model of Pleistocene biogeography

During the Pleistocene glacial periods, the ranges of many temperate species in Europe contracted southwards to refugia near the Mediterranean Basin. This study tested the ‘expansion–contraction’ model by applying the ecological niche modelling approach to one widespread temperate tree species, pedunculate oak, Quercus robur L., distributed across Europe and Anatolia. We projected the distribution of Q. robur under climatic conditions of the Present, the mid-Holocene [HOL, c. 6000 years before present (YBP)], the Last Glacial Maximum (LGM, c. 22000 YBP) and the Last Interglacial (LIG, c. 130000 YBP). Quercus robur was at equilibrium with the climate as the model prediction was highly compatible with the its known distribution range under present bioclimatic conditions. The LIG and the HOL predictions gave a much broader distribution range than that during the LGM, suggesting its range contracted towards favourable areas in southern Europe and Anatolia. The results indicate that the glacial refugia hypothesis, based mainly on the ‘expansion–contraction’ model, applies to Q. robur, although possible extra-Mediterranean refugia in northern, western and southern France are also identified. This study therefore supports claims that European biogeography is significantly more complex than previously thought.